Adding upstream version 1.22.1.upstream/1.22.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

16 files changed, 6346 insertions, 0 deletions

diff --git a/src/strings/builder.go b/src/strings/builder.go
new file mode 100644
index 0000000..189dadb
--- /dev/null
+++ b/src/strings/builder.go
@@ -0,0 +1,118 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+import (
+	"internal/bytealg"
+	"unicode/utf8"
+	"unsafe"
+)
+
+// A Builder is used to efficiently build a string using [Builder.Write] methods.
+// It minimizes memory copying. The zero value is ready to use.
+// Do not copy a non-zero Builder.
+type Builder struct {
+	addr *Builder // of receiver, to detect copies by value
+	buf  []byte
+}
+
+// noescape hides a pointer from escape analysis. It is the identity function
+// but escape analysis doesn't think the output depends on the input.
+// noescape is inlined and currently compiles down to zero instructions.
+// USE CAREFULLY!
+// This was copied from the runtime; see issues 23382 and 7921.
+//
+//go:nosplit
+//go:nocheckptr
+func noescape(p unsafe.Pointer) unsafe.Pointer {
+	x := uintptr(p)
+	return unsafe.Pointer(x ^ 0)
+}
+
+func (b *Builder) copyCheck() {
+	if b.addr == nil {
+		// This hack works around a failing of Go's escape analysis
+		// that was causing b to escape and be heap allocated.
+		// See issue 23382.
+		// TODO: once issue 7921 is fixed, this should be reverted to
+		// just "b.addr = b".
+		b.addr = (*Builder)(noescape(unsafe.Pointer(b)))
+	} else if b.addr != b {
+		panic("strings: illegal use of non-zero Builder copied by value")
+	}
+}
+
+// String returns the accumulated string.
+func (b *Builder) String() string {
+	return unsafe.String(unsafe.SliceData(b.buf), len(b.buf))
+}
+
+// Len returns the number of accumulated bytes; b.Len() == len(b.String()).
+func (b *Builder) Len() int { return len(b.buf) }
+
+// Cap returns the capacity of the builder's underlying byte slice. It is the
+// total space allocated for the string being built and includes any bytes
+// already written.
+func (b *Builder) Cap() int { return cap(b.buf) }
+
+// Reset resets the [Builder] to be empty.
+func (b *Builder) Reset() {
+	b.addr = nil
+	b.buf = nil
+}
+
+// grow copies the buffer to a new, larger buffer so that there are at least n
+// bytes of capacity beyond len(b.buf).
+func (b *Builder) grow(n int) {
+	buf := bytealg.MakeNoZero(2*cap(b.buf) + n)[:len(b.buf)]
+	copy(buf, b.buf)
+	b.buf = buf
+}
+
+// Grow grows b's capacity, if necessary, to guarantee space for
+// another n bytes. After Grow(n), at least n bytes can be written to b
+// without another allocation. If n is negative, Grow panics.
+func (b *Builder) Grow(n int) {
+	b.copyCheck()
+	if n < 0 {
+		panic("strings.Builder.Grow: negative count")
+	}
+	if cap(b.buf)-len(b.buf) < n {
+		b.grow(n)
+	}
+}
+
+// Write appends the contents of p to b's buffer.
+// Write always returns len(p), nil.
+func (b *Builder) Write(p []byte) (int, error) {
+	b.copyCheck()
+	b.buf = append(b.buf, p...)
+	return len(p), nil
+}
+
+// WriteByte appends the byte c to b's buffer.
+// The returned error is always nil.
+func (b *Builder) WriteByte(c byte) error {
+	b.copyCheck()
+	b.buf = append(b.buf, c)
+	return nil
+}
+
+// WriteRune appends the UTF-8 encoding of Unicode code point r to b's buffer.
+// It returns the length of r and a nil error.
+func (b *Builder) WriteRune(r rune) (int, error) {
+	b.copyCheck()
+	n := len(b.buf)
+	b.buf = utf8.AppendRune(b.buf, r)
+	return len(b.buf) - n, nil
+}
+
+// WriteString appends the contents of s to b's buffer.
+// It returns the length of s and a nil error.
+func (b *Builder) WriteString(s string) (int, error) {
+	b.copyCheck()
+	b.buf = append(b.buf, s...)
+	return len(s), nil
+}
diff --git a/src/strings/builder_test.go b/src/strings/builder_test.go
new file mode 100644
index 0000000..c3c627e
--- /dev/null
+++ b/src/strings/builder_test.go
@@ -0,0 +1,387 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	"bytes"
+	. "strings"
+	"testing"
+	"unicode/utf8"
+)
+
+func check(t *testing.T, b *Builder, want string) {
+	t.Helper()
+	got := b.String()
+	if got != want {
+		t.Errorf("String: got %#q; want %#q", got, want)
+		return
+	}
+	if n := b.Len(); n != len(got) {
+		t.Errorf("Len: got %d; but len(String()) is %d", n, len(got))
+	}
+	if n := b.Cap(); n < len(got) {
+		t.Errorf("Cap: got %d; but len(String()) is %d", n, len(got))
+	}
+}
+
+func TestBuilder(t *testing.T) {
+	var b Builder
+	check(t, &b, "")
+	n, err := b.WriteString("hello")
+	if err != nil || n != 5 {
+		t.Errorf("WriteString: got %d,%s; want 5,nil", n, err)
+	}
+	check(t, &b, "hello")
+	if err = b.WriteByte(' '); err != nil {
+		t.Errorf("WriteByte: %s", err)
+	}
+	check(t, &b, "hello ")
+	n, err = b.WriteString("world")
+	if err != nil || n != 5 {
+		t.Errorf("WriteString: got %d,%s; want 5,nil", n, err)
+	}
+	check(t, &b, "hello world")
+}
+
+func TestBuilderString(t *testing.T) {
+	var b Builder
+	b.WriteString("alpha")
+	check(t, &b, "alpha")
+	s1 := b.String()
+	b.WriteString("beta")
+	check(t, &b, "alphabeta")
+	s2 := b.String()
+	b.WriteString("gamma")
+	check(t, &b, "alphabetagamma")
+	s3 := b.String()
+
+	// Check that subsequent operations didn't change the returned strings.
+	if want := "alpha"; s1 != want {
+		t.Errorf("first String result is now %q; want %q", s1, want)
+	}
+	if want := "alphabeta"; s2 != want {
+		t.Errorf("second String result is now %q; want %q", s2, want)
+	}
+	if want := "alphabetagamma"; s3 != want {
+		t.Errorf("third String result is now %q; want %q", s3, want)
+	}
+}
+
+func TestBuilderReset(t *testing.T) {
+	var b Builder
+	check(t, &b, "")
+	b.WriteString("aaa")
+	s := b.String()
+	check(t, &b, "aaa")
+	b.Reset()
+	check(t, &b, "")
+
+	// Ensure that writing after Reset doesn't alter
+	// previously returned strings.
+	b.WriteString("bbb")
+	check(t, &b, "bbb")
+	if want := "aaa"; s != want {
+		t.Errorf("previous String result changed after Reset: got %q; want %q", s, want)
+	}
+}
+
+func TestBuilderGrow(t *testing.T) {
+	for _, growLen := range []int{0, 100, 1000, 10000, 100000} {
+		p := bytes.Repeat([]byte{'a'}, growLen)
+		allocs := testing.AllocsPerRun(100, func() {
+			var b Builder
+			b.Grow(growLen) // should be only alloc, when growLen > 0
+			if b.Cap() < growLen {
+				t.Fatalf("growLen=%d: Cap() is lower than growLen", growLen)
+			}
+			b.Write(p)
+			if b.String() != string(p) {
+				t.Fatalf("growLen=%d: bad data written after Grow", growLen)
+			}
+		})
+		wantAllocs := 1
+		if growLen == 0 {
+			wantAllocs = 0
+		}
+		if g, w := int(allocs), wantAllocs; g != w {
+			t.Errorf("growLen=%d: got %d allocs during Write; want %v", growLen, g, w)
+		}
+	}
+	// when growLen < 0, should panic
+	var a Builder
+	n := -1
+	defer func() {
+		if r := recover(); r == nil {
+			t.Errorf("a.Grow(%d) should panic()", n)
+		}
+	}()
+	a.Grow(n)
+}
+
+func TestBuilderWrite2(t *testing.T) {
+	const s0 = "hello 世界"
+	for _, tt := range []struct {
+		name string
+		fn   func(b *Builder) (int, error)
+		n    int
+		want string
+	}{
+		{
+			"Write",
+			func(b *Builder) (int, error) { return b.Write([]byte(s0)) },
+			len(s0),
+			s0,
+		},
+		{
+			"WriteRune",
+			func(b *Builder) (int, error) { return b.WriteRune('a') },
+			1,
+			"a",
+		},
+		{
+			"WriteRuneWide",
+			func(b *Builder) (int, error) { return b.WriteRune('世') },
+			3,
+			"世",
+		},
+		{
+			"WriteString",
+			func(b *Builder) (int, error) { return b.WriteString(s0) },
+			len(s0),
+			s0,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			var b Builder
+			n, err := tt.fn(&b)
+			if err != nil {
+				t.Fatalf("first call: got %s", err)
+			}
+			if n != tt.n {
+				t.Errorf("first call: got n=%d; want %d", n, tt.n)
+			}
+			check(t, &b, tt.want)
+
+			n, err = tt.fn(&b)
+			if err != nil {
+				t.Fatalf("second call: got %s", err)
+			}
+			if n != tt.n {
+				t.Errorf("second call: got n=%d; want %d", n, tt.n)
+			}
+			check(t, &b, tt.want+tt.want)
+		})
+	}
+}
+
+func TestBuilderWriteByte(t *testing.T) {
+	var b Builder
+	if err := b.WriteByte('a'); err != nil {
+		t.Error(err)
+	}
+	if err := b.WriteByte(0); err != nil {
+		t.Error(err)
+	}
+	check(t, &b, "a\x00")
+}
+
+func TestBuilderAllocs(t *testing.T) {
+	// Issue 23382; verify that copyCheck doesn't force the
+	// Builder to escape and be heap allocated.
+	n := testing.AllocsPerRun(10000, func() {
+		var b Builder
+		b.Grow(5)
+		b.WriteString("abcde")
+		_ = b.String()
+	})
+	if n != 1 {
+		t.Errorf("Builder allocs = %v; want 1", n)
+	}
+}
+
+func TestBuilderCopyPanic(t *testing.T) {
+	tests := []struct {
+		name      string
+		fn        func()
+		wantPanic bool
+	}{
+		{
+			name:      "String",
+			wantPanic: false,
+			fn: func() {
+				var a Builder
+				a.WriteByte('x')
+				b := a
+				_ = b.String() // appease vet
+			},
+		},
+		{
+			name:      "Len",
+			wantPanic: false,
+			fn: func() {
+				var a Builder
+				a.WriteByte('x')
+				b := a
+				b.Len()
+			},
+		},
+		{
+			name:      "Cap",
+			wantPanic: false,
+			fn: func() {
+				var a Builder
+				a.WriteByte('x')
+				b := a
+				b.Cap()
+			},
+		},
+		{
+			name:      "Reset",
+			wantPanic: false,
+			fn: func() {
+				var a Builder
+				a.WriteByte('x')
+				b := a
+				b.Reset()
+				b.WriteByte('y')
+			},
+		},
+		{
+			name:      "Write",
+			wantPanic: true,
+			fn: func() {
+				var a Builder
+				a.Write([]byte("x"))
+				b := a
+				b.Write([]byte("y"))
+			},
+		},
+		{
+			name:      "WriteByte",
+			wantPanic: true,
+			fn: func() {
+				var a Builder
+				a.WriteByte('x')
+				b := a
+				b.WriteByte('y')
+			},
+		},
+		{
+			name:      "WriteString",
+			wantPanic: true,
+			fn: func() {
+				var a Builder
+				a.WriteString("x")
+				b := a
+				b.WriteString("y")
+			},
+		},
+		{
+			name:      "WriteRune",
+			wantPanic: true,
+			fn: func() {
+				var a Builder
+				a.WriteRune('x')
+				b := a
+				b.WriteRune('y')
+			},
+		},
+		{
+			name:      "Grow",
+			wantPanic: true,
+			fn: func() {
+				var a Builder
+				a.Grow(1)
+				b := a
+				b.Grow(2)
+			},
+		},
+	}
+	for _, tt := range tests {
+		didPanic := make(chan bool)
+		go func() {
+			defer func() { didPanic <- recover() != nil }()
+			tt.fn()
+		}()
+		if got := <-didPanic; got != tt.wantPanic {
+			t.Errorf("%s: panicked = %v; want %v", tt.name, got, tt.wantPanic)
+		}
+	}
+}
+
+func TestBuilderWriteInvalidRune(t *testing.T) {
+	// Invalid runes, including negative ones, should be written as
+	// utf8.RuneError.
+	for _, r := range []rune{-1, utf8.MaxRune + 1} {
+		var b Builder
+		b.WriteRune(r)
+		check(t, &b, "\uFFFD")
+	}
+}
+
+var someBytes = []byte("some bytes sdljlk jsklj3lkjlk djlkjw")
+
+var sinkS string
+
+func benchmarkBuilder(b *testing.B, f func(b *testing.B, numWrite int, grow bool)) {
+	b.Run("1Write_NoGrow", func(b *testing.B) {
+		b.ReportAllocs()
+		f(b, 1, false)
+	})
+	b.Run("3Write_NoGrow", func(b *testing.B) {
+		b.ReportAllocs()
+		f(b, 3, false)
+	})
+	b.Run("3Write_Grow", func(b *testing.B) {
+		b.ReportAllocs()
+		f(b, 3, true)
+	})
+}
+
+func BenchmarkBuildString_Builder(b *testing.B) {
+	benchmarkBuilder(b, func(b *testing.B, numWrite int, grow bool) {
+		for i := 0; i < b.N; i++ {
+			var buf Builder
+			if grow {
+				buf.Grow(len(someBytes) * numWrite)
+			}
+			for i := 0; i < numWrite; i++ {
+				buf.Write(someBytes)
+			}
+			sinkS = buf.String()
+		}
+	})
+}
+
+func BenchmarkBuildString_WriteString(b *testing.B) {
+	someString := string(someBytes)
+	benchmarkBuilder(b, func(b *testing.B, numWrite int, grow bool) {
+		for i := 0; i < b.N; i++ {
+			var buf Builder
+			if grow {
+				buf.Grow(len(someString) * numWrite)
+			}
+			for i := 0; i < numWrite; i++ {
+				buf.WriteString(someString)
+			}
+			sinkS = buf.String()
+		}
+	})
+}
+
+func BenchmarkBuildString_ByteBuffer(b *testing.B) {
+	benchmarkBuilder(b, func(b *testing.B, numWrite int, grow bool) {
+		for i := 0; i < b.N; i++ {
+			var buf bytes.Buffer
+			if grow {
+				buf.Grow(len(someBytes) * numWrite)
+			}
+			for i := 0; i < numWrite; i++ {
+				buf.Write(someBytes)
+			}
+			sinkS = buf.String()
+		}
+	})
+}
diff --git a/src/strings/clone.go b/src/strings/clone.go
new file mode 100644
index 0000000..d14df11
--- /dev/null
+++ b/src/strings/clone.go
@@ -0,0 +1,28 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+import (
+	"unsafe"
+)
+
+// Clone returns a fresh copy of s.
+// It guarantees to make a copy of s into a new allocation,
+// which can be important when retaining only a small substring
+// of a much larger string. Using Clone can help such programs
+// use less memory. Of course, since using Clone makes a copy,
+// overuse of Clone can make programs use more memory.
+// Clone should typically be used only rarely, and only when
+// profiling indicates that it is needed.
+// For strings of length zero the string "" will be returned
+// and no allocation is made.
+func Clone(s string) string {
+	if len(s) == 0 {
+		return ""
+	}
+	b := make([]byte, len(s))
+	copy(b, s)
+	return unsafe.String(&b[0], len(b))
+}
diff --git a/src/strings/clone_test.go b/src/strings/clone_test.go
new file mode 100644
index 0000000..64f2760
--- /dev/null
+++ b/src/strings/clone_test.go
@@ -0,0 +1,45 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	"strings"
+	"testing"
+	"unsafe"
+)
+
+var emptyString string
+
+func TestClone(t *testing.T) {
+	var cloneTests = []string{
+		"",
+		strings.Clone(""),
+		strings.Repeat("a", 42)[:0],
+		"short",
+		strings.Repeat("a", 42),
+	}
+	for _, input := range cloneTests {
+		clone := strings.Clone(input)
+		if clone != input {
+			t.Errorf("Clone(%q) = %q; want %q", input, clone, input)
+		}
+
+		if len(input) != 0 && unsafe.StringData(clone) == unsafe.StringData(input) {
+			t.Errorf("Clone(%q) return value should not reference inputs backing memory.", input)
+		}
+
+		if len(input) == 0 && unsafe.StringData(clone) != unsafe.StringData(emptyString) {
+			t.Errorf("Clone(%#v) return value should be equal to empty string.", unsafe.StringData(input))
+		}
+	}
+}
+
+func BenchmarkClone(b *testing.B) {
+	var str = strings.Repeat("a", 42)
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		stringSink = strings.Clone(str)
+	}
+}
diff --git a/src/strings/compare.go b/src/strings/compare.go
new file mode 100644
index 0000000..2bd4a24
--- /dev/null
+++ b/src/strings/compare.go
@@ -0,0 +1,28 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+// Compare returns an integer comparing two strings lexicographically.
+// The result will be 0 if a == b, -1 if a < b, and +1 if a > b.
+//
+// Compare is included only for symmetry with package bytes.
+// It is usually clearer and always faster to use the built-in
+// string comparison operators ==, <, >, and so on.
+func Compare(a, b string) int {
+	// NOTE(rsc): This function does NOT call the runtime cmpstring function,
+	// because we do not want to provide any performance justification for
+	// using strings.Compare. Basically no one should use strings.Compare.
+	// As the comment above says, it is here only for symmetry with package bytes.
+	// If performance is important, the compiler should be changed to recognize
+	// the pattern so that all code doing three-way comparisons, not just code
+	// using strings.Compare, can benefit.
+	if a == b {
+		return 0
+	}
+	if a < b {
+		return -1
+	}
+	return +1
+}
diff --git a/src/strings/compare_test.go b/src/strings/compare_test.go
new file mode 100644
index 0000000..a435784
--- /dev/null
+++ b/src/strings/compare_test.go
@@ -0,0 +1,119 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+// Derived from bytes/compare_test.go.
+// Benchmarks omitted since the underlying implementation is identical.
+
+import (
+	"internal/testenv"
+	. "strings"
+	"testing"
+	"unsafe"
+)
+
+var compareTests = []struct {
+	a, b string
+	i    int
+}{
+	{"", "", 0},
+	{"a", "", 1},
+	{"", "a", -1},
+	{"abc", "abc", 0},
+	{"ab", "abc", -1},
+	{"abc", "ab", 1},
+	{"x", "ab", 1},
+	{"ab", "x", -1},
+	{"x", "a", 1},
+	{"b", "x", -1},
+	// test runtime·memeq's chunked implementation
+	{"abcdefgh", "abcdefgh", 0},
+	{"abcdefghi", "abcdefghi", 0},
+	{"abcdefghi", "abcdefghj", -1},
+}
+
+func TestCompare(t *testing.T) {
+	for _, tt := range compareTests {
+		cmp := Compare(tt.a, tt.b)
+		if cmp != tt.i {
+			t.Errorf(`Compare(%q, %q) = %v`, tt.a, tt.b, cmp)
+		}
+	}
+}
+
+func TestCompareIdenticalString(t *testing.T) {
+	var s = "Hello Gophers!"
+	if Compare(s, s) != 0 {
+		t.Error("s != s")
+	}
+	if Compare(s, s[:1]) != 1 {
+		t.Error("s > s[:1] failed")
+	}
+}
+
+func TestCompareStrings(t *testing.T) {
+	// unsafeString converts a []byte to a string with no allocation.
+	// The caller must not modify b while the result string is in use.
+	unsafeString := func(b []byte) string {
+		return unsafe.String(unsafe.SliceData(b), len(b))
+	}
+
+	lengths := make([]int, 0) // lengths to test in ascending order
+	for i := 0; i <= 128; i++ {
+		lengths = append(lengths, i)
+	}
+	lengths = append(lengths, 256, 512, 1024, 1333, 4095, 4096, 4097)
+
+	if !testing.Short() || testenv.Builder() != "" {
+		lengths = append(lengths, 65535, 65536, 65537, 99999)
+	}
+
+	n := lengths[len(lengths)-1]
+	a := make([]byte, n+1)
+	b := make([]byte, n+1)
+	lastLen := 0
+	for _, len := range lengths {
+		// randomish but deterministic data. No 0 or 255.
+		for i := 0; i < len; i++ {
+			a[i] = byte(1 + 31*i%254)
+			b[i] = byte(1 + 31*i%254)
+		}
+		// data past the end is different
+		for i := len; i <= n; i++ {
+			a[i] = 8
+			b[i] = 9
+		}
+
+		sa, sb := unsafeString(a), unsafeString(b)
+		cmp := Compare(sa[:len], sb[:len])
+		if cmp != 0 {
+			t.Errorf(`CompareIdentical(%d) = %d`, len, cmp)
+		}
+		if len > 0 {
+			cmp = Compare(sa[:len-1], sb[:len])
+			if cmp != -1 {
+				t.Errorf(`CompareAshorter(%d) = %d`, len, cmp)
+			}
+			cmp = Compare(sa[:len], sb[:len-1])
+			if cmp != 1 {
+				t.Errorf(`CompareBshorter(%d) = %d`, len, cmp)
+			}
+		}
+		for k := lastLen; k < len; k++ {
+			b[k] = a[k] - 1
+			cmp = Compare(unsafeString(a[:len]), unsafeString(b[:len]))
+			if cmp != 1 {
+				t.Errorf(`CompareAbigger(%d,%d) = %d`, len, k, cmp)
+			}
+			b[k] = a[k] + 1
+			cmp = Compare(unsafeString(a[:len]), unsafeString(b[:len]))
+			if cmp != -1 {
+				t.Errorf(`CompareBbigger(%d,%d) = %d`, len, k, cmp)
+			}
+			b[k] = a[k]
+		}
+		lastLen = len
+	}
+}
diff --git a/src/strings/example_test.go b/src/strings/example_test.go
new file mode 100644
index 0000000..bdab7ae
--- /dev/null
+++ b/src/strings/example_test.go
@@ -0,0 +1,460 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	"fmt"
+	"strings"
+	"unicode"
+	"unsafe"
+)
+
+func ExampleClone() {
+	s := "abc"
+	clone := strings.Clone(s)
+	fmt.Println(s == clone)
+	fmt.Println(unsafe.StringData(s) == unsafe.StringData(clone))
+	// Output:
+	// true
+	// false
+}
+
+func ExampleBuilder() {
+	var b strings.Builder
+	for i := 3; i >= 1; i-- {
+		fmt.Fprintf(&b, "%d...", i)
+	}
+	b.WriteString("ignition")
+	fmt.Println(b.String())
+
+	// Output: 3...2...1...ignition
+}
+
+func ExampleCompare() {
+	fmt.Println(strings.Compare("a", "b"))
+	fmt.Println(strings.Compare("a", "a"))
+	fmt.Println(strings.Compare("b", "a"))
+	// Output:
+	// -1
+	// 0
+	// 1
+}
+
+func ExampleContains() {
+	fmt.Println(strings.Contains("seafood", "foo"))
+	fmt.Println(strings.Contains("seafood", "bar"))
+	fmt.Println(strings.Contains("seafood", ""))
+	fmt.Println(strings.Contains("", ""))
+	// Output:
+	// true
+	// false
+	// true
+	// true
+}
+
+func ExampleContainsAny() {
+	fmt.Println(strings.ContainsAny("team", "i"))
+	fmt.Println(strings.ContainsAny("fail", "ui"))
+	fmt.Println(strings.ContainsAny("ure", "ui"))
+	fmt.Println(strings.ContainsAny("failure", "ui"))
+	fmt.Println(strings.ContainsAny("foo", ""))
+	fmt.Println(strings.ContainsAny("", ""))
+	// Output:
+	// false
+	// true
+	// true
+	// true
+	// false
+	// false
+}
+
+func ExampleContainsRune() {
+	// Finds whether a string contains a particular Unicode code point.
+	// The code point for the lowercase letter "a", for example, is 97.
+	fmt.Println(strings.ContainsRune("aardvark", 97))
+	fmt.Println(strings.ContainsRune("timeout", 97))
+	// Output:
+	// true
+	// false
+}
+
+func ExampleContainsFunc() {
+	f := func(r rune) bool {
+		return r == 'a' || r == 'e' || r == 'i' || r == 'o' || r == 'u'
+	}
+	fmt.Println(strings.ContainsFunc("hello", f))
+	fmt.Println(strings.ContainsFunc("rhythms", f))
+	// Output:
+	// true
+	// false
+}
+
+func ExampleCount() {
+	fmt.Println(strings.Count("cheese", "e"))
+	fmt.Println(strings.Count("five", "")) // before & after each rune
+	// Output:
+	// 3
+	// 5
+}
+
+func ExampleCut() {
+	show := func(s, sep string) {
+		before, after, found := strings.Cut(s, sep)
+		fmt.Printf("Cut(%q, %q) = %q, %q, %v\n", s, sep, before, after, found)
+	}
+	show("Gopher", "Go")
+	show("Gopher", "ph")
+	show("Gopher", "er")
+	show("Gopher", "Badger")
+	// Output:
+	// Cut("Gopher", "Go") = "", "pher", true
+	// Cut("Gopher", "ph") = "Go", "er", true
+	// Cut("Gopher", "er") = "Goph", "", true
+	// Cut("Gopher", "Badger") = "Gopher", "", false
+}
+
+func ExampleCutPrefix() {
+	show := func(s, sep string) {
+		after, found := strings.CutPrefix(s, sep)
+		fmt.Printf("CutPrefix(%q, %q) = %q, %v\n", s, sep, after, found)
+	}
+	show("Gopher", "Go")
+	show("Gopher", "ph")
+	// Output:
+	// CutPrefix("Gopher", "Go") = "pher", true
+	// CutPrefix("Gopher", "ph") = "Gopher", false
+}
+
+func ExampleCutSuffix() {
+	show := func(s, sep string) {
+		before, found := strings.CutSuffix(s, sep)
+		fmt.Printf("CutSuffix(%q, %q) = %q, %v\n", s, sep, before, found)
+	}
+	show("Gopher", "Go")
+	show("Gopher", "er")
+	// Output:
+	// CutSuffix("Gopher", "Go") = "Gopher", false
+	// CutSuffix("Gopher", "er") = "Goph", true
+}
+
+func ExampleEqualFold() {
+	fmt.Println(strings.EqualFold("Go", "go"))
+	fmt.Println(strings.EqualFold("AB", "ab")) // true because comparison uses simple case-folding
+	fmt.Println(strings.EqualFold("ß", "ss"))  // false because comparison does not use full case-folding
+	// Output:
+	// true
+	// true
+	// false
+}
+
+func ExampleFields() {
+	fmt.Printf("Fields are: %q", strings.Fields("  foo bar  baz   "))
+	// Output: Fields are: ["foo" "bar" "baz"]
+}
+
+func ExampleFieldsFunc() {
+	f := func(c rune) bool {
+		return !unicode.IsLetter(c) && !unicode.IsNumber(c)
+	}
+	fmt.Printf("Fields are: %q", strings.FieldsFunc("  foo1;bar2,baz3...", f))
+	// Output: Fields are: ["foo1" "bar2" "baz3"]
+}
+
+func ExampleHasPrefix() {
+	fmt.Println(strings.HasPrefix("Gopher", "Go"))
+	fmt.Println(strings.HasPrefix("Gopher", "C"))
+	fmt.Println(strings.HasPrefix("Gopher", ""))
+	// Output:
+	// true
+	// false
+	// true
+}
+
+func ExampleHasSuffix() {
+	fmt.Println(strings.HasSuffix("Amigo", "go"))
+	fmt.Println(strings.HasSuffix("Amigo", "O"))
+	fmt.Println(strings.HasSuffix("Amigo", "Ami"))
+	fmt.Println(strings.HasSuffix("Amigo", ""))
+	// Output:
+	// true
+	// false
+	// false
+	// true
+}
+
+func ExampleIndex() {
+	fmt.Println(strings.Index("chicken", "ken"))
+	fmt.Println(strings.Index("chicken", "dmr"))
+	// Output:
+	// 4
+	// -1
+}
+
+func ExampleIndexFunc() {
+	f := func(c rune) bool {
+		return unicode.Is(unicode.Han, c)
+	}
+	fmt.Println(strings.IndexFunc("Hello, 世界", f))
+	fmt.Println(strings.IndexFunc("Hello, world", f))
+	// Output:
+	// 7
+	// -1
+}
+
+func ExampleIndexAny() {
+	fmt.Println(strings.IndexAny("chicken", "aeiouy"))
+	fmt.Println(strings.IndexAny("crwth", "aeiouy"))
+	// Output:
+	// 2
+	// -1
+}
+
+func ExampleIndexByte() {
+	fmt.Println(strings.IndexByte("golang", 'g'))
+	fmt.Println(strings.IndexByte("gophers", 'h'))
+	fmt.Println(strings.IndexByte("golang", 'x'))
+	// Output:
+	// 0
+	// 3
+	// -1
+}
+func ExampleIndexRune() {
+	fmt.Println(strings.IndexRune("chicken", 'k'))
+	fmt.Println(strings.IndexRune("chicken", 'd'))
+	// Output:
+	// 4
+	// -1
+}
+
+func ExampleLastIndex() {
+	fmt.Println(strings.Index("go gopher", "go"))
+	fmt.Println(strings.LastIndex("go gopher", "go"))
+	fmt.Println(strings.LastIndex("go gopher", "rodent"))
+	// Output:
+	// 0
+	// 3
+	// -1
+}
+
+func ExampleLastIndexAny() {
+	fmt.Println(strings.LastIndexAny("go gopher", "go"))
+	fmt.Println(strings.LastIndexAny("go gopher", "rodent"))
+	fmt.Println(strings.LastIndexAny("go gopher", "fail"))
+	// Output:
+	// 4
+	// 8
+	// -1
+}
+
+func ExampleLastIndexByte() {
+	fmt.Println(strings.LastIndexByte("Hello, world", 'l'))
+	fmt.Println(strings.LastIndexByte("Hello, world", 'o'))
+	fmt.Println(strings.LastIndexByte("Hello, world", 'x'))
+	// Output:
+	// 10
+	// 8
+	// -1
+}
+
+func ExampleLastIndexFunc() {
+	fmt.Println(strings.LastIndexFunc("go 123", unicode.IsNumber))
+	fmt.Println(strings.LastIndexFunc("123 go", unicode.IsNumber))
+	fmt.Println(strings.LastIndexFunc("go", unicode.IsNumber))
+	// Output:
+	// 5
+	// 2
+	// -1
+}
+
+func ExampleJoin() {
+	s := []string{"foo", "bar", "baz"}
+	fmt.Println(strings.Join(s, ", "))
+	// Output: foo, bar, baz
+}
+
+func ExampleRepeat() {
+	fmt.Println("ba" + strings.Repeat("na", 2))
+	// Output: banana
+}
+
+func ExampleReplace() {
+	fmt.Println(strings.Replace("oink oink oink", "k", "ky", 2))
+	fmt.Println(strings.Replace("oink oink oink", "oink", "moo", -1))
+	// Output:
+	// oinky oinky oink
+	// moo moo moo
+}
+
+func ExampleReplaceAll() {
+	fmt.Println(strings.ReplaceAll("oink oink oink", "oink", "moo"))
+	// Output:
+	// moo moo moo
+}
+
+func ExampleSplit() {
+	fmt.Printf("%q\n", strings.Split("a,b,c", ","))
+	fmt.Printf("%q\n", strings.Split("a man a plan a canal panama", "a "))
+	fmt.Printf("%q\n", strings.Split(" xyz ", ""))
+	fmt.Printf("%q\n", strings.Split("", "Bernardo O'Higgins"))
+	// Output:
+	// ["a" "b" "c"]
+	// ["" "man " "plan " "canal panama"]
+	// [" " "x" "y" "z" " "]
+	// [""]
+}
+
+func ExampleSplitN() {
+	fmt.Printf("%q\n", strings.SplitN("a,b,c", ",", 2))
+	z := strings.SplitN("a,b,c", ",", 0)
+	fmt.Printf("%q (nil = %v)\n", z, z == nil)
+	// Output:
+	// ["a" "b,c"]
+	// [] (nil = true)
+}
+
+func ExampleSplitAfter() {
+	fmt.Printf("%q\n", strings.SplitAfter("a,b,c", ","))
+	// Output: ["a," "b," "c"]
+}
+
+func ExampleSplitAfterN() {
+	fmt.Printf("%q\n", strings.SplitAfterN("a,b,c", ",", 2))
+	// Output: ["a," "b,c"]
+}
+
+func ExampleTitle() {
+	// Compare this example to the ToTitle example.
+	fmt.Println(strings.Title("her royal highness"))
+	fmt.Println(strings.Title("loud noises"))
+	fmt.Println(strings.Title("хлеб"))
+	// Output:
+	// Her Royal Highness
+	// Loud Noises
+	// Хлеб
+}
+
+func ExampleToTitle() {
+	// Compare this example to the Title example.
+	fmt.Println(strings.ToTitle("her royal highness"))
+	fmt.Println(strings.ToTitle("loud noises"))
+	fmt.Println(strings.ToTitle("хлеб"))
+	// Output:
+	// HER ROYAL HIGHNESS
+	// LOUD NOISES
+	// ХЛЕБ
+}
+
+func ExampleToTitleSpecial() {
+	fmt.Println(strings.ToTitleSpecial(unicode.TurkishCase, "dünyanın ilk borsa yapısı Aizonai kabul edilir"))
+	// Output:
+	// DÜNYANIN İLK BORSA YAPISI AİZONAİ KABUL EDİLİR
+}
+
+func ExampleMap() {
+	rot13 := func(r rune) rune {
+		switch {
+		case r >= 'A' && r <= 'Z':
+			return 'A' + (r-'A'+13)%26
+		case r >= 'a' && r <= 'z':
+			return 'a' + (r-'a'+13)%26
+		}
+		return r
+	}
+	fmt.Println(strings.Map(rot13, "'Twas brillig and the slithy gopher..."))
+	// Output: 'Gjnf oevyyvt naq gur fyvgul tbcure...
+}
+
+func ExampleNewReplacer() {
+	r := strings.NewReplacer("<", "&lt;", ">", "&gt;")
+	fmt.Println(r.Replace("This is <b>HTML</b>!"))
+	// Output: This is &lt;b&gt;HTML&lt;/b&gt;!
+}
+
+func ExampleToUpper() {
+	fmt.Println(strings.ToUpper("Gopher"))
+	// Output: GOPHER
+}
+
+func ExampleToUpperSpecial() {
+	fmt.Println(strings.ToUpperSpecial(unicode.TurkishCase, "örnek iş"))
+	// Output: ÖRNEK İŞ
+}
+
+func ExampleToLower() {
+	fmt.Println(strings.ToLower("Gopher"))
+	// Output: gopher
+}
+
+func ExampleToLowerSpecial() {
+	fmt.Println(strings.ToLowerSpecial(unicode.TurkishCase, "Önnek İş"))
+	// Output: önnek iş
+}
+
+func ExampleTrim() {
+	fmt.Print(strings.Trim("¡¡¡Hello, Gophers!!!", "!¡"))
+	// Output: Hello, Gophers
+}
+
+func ExampleTrimSpace() {
+	fmt.Println(strings.TrimSpace(" \t\n Hello, Gophers \n\t\r\n"))
+	// Output: Hello, Gophers
+}
+
+func ExampleTrimPrefix() {
+	var s = "¡¡¡Hello, Gophers!!!"
+	s = strings.TrimPrefix(s, "¡¡¡Hello, ")
+	s = strings.TrimPrefix(s, "¡¡¡Howdy, ")
+	fmt.Print(s)
+	// Output: Gophers!!!
+}
+
+func ExampleTrimSuffix() {
+	var s = "¡¡¡Hello, Gophers!!!"
+	s = strings.TrimSuffix(s, ", Gophers!!!")
+	s = strings.TrimSuffix(s, ", Marmots!!!")
+	fmt.Print(s)
+	// Output: ¡¡¡Hello
+}
+
+func ExampleTrimFunc() {
+	fmt.Print(strings.TrimFunc("¡¡¡Hello, Gophers!!!", func(r rune) bool {
+		return !unicode.IsLetter(r) && !unicode.IsNumber(r)
+	}))
+	// Output: Hello, Gophers
+}
+
+func ExampleTrimLeft() {
+	fmt.Print(strings.TrimLeft("¡¡¡Hello, Gophers!!!", "!¡"))
+	// Output: Hello, Gophers!!!
+}
+
+func ExampleTrimLeftFunc() {
+	fmt.Print(strings.TrimLeftFunc("¡¡¡Hello, Gophers!!!", func(r rune) bool {
+		return !unicode.IsLetter(r) && !unicode.IsNumber(r)
+	}))
+	// Output: Hello, Gophers!!!
+}
+
+func ExampleTrimRight() {
+	fmt.Print(strings.TrimRight("¡¡¡Hello, Gophers!!!", "!¡"))
+	// Output: ¡¡¡Hello, Gophers
+}
+
+func ExampleTrimRightFunc() {
+	fmt.Print(strings.TrimRightFunc("¡¡¡Hello, Gophers!!!", func(r rune) bool {
+		return !unicode.IsLetter(r) && !unicode.IsNumber(r)
+	}))
+	// Output: ¡¡¡Hello, Gophers
+}
+
+func ExampleToValidUTF8() {
+	fmt.Printf("%s\n", strings.ToValidUTF8("abc", "\uFFFD"))
+	fmt.Printf("%s\n", strings.ToValidUTF8("a\xffb\xC0\xAFc\xff", ""))
+	fmt.Printf("%s\n", strings.ToValidUTF8("\xed\xa0\x80", "abc"))
+	// Output:
+	// abc
+	// abc
+	// abc
+}
diff --git a/src/strings/export_test.go b/src/strings/export_test.go
new file mode 100644
index 0000000..81d1cab
--- /dev/null
+++ b/src/strings/export_test.go
@@ -0,0 +1,47 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+func (r *Replacer) Replacer() any {
+	r.once.Do(r.buildOnce)
+	return r.r
+}
+
+func (r *Replacer) PrintTrie() string {
+	r.once.Do(r.buildOnce)
+	gen := r.r.(*genericReplacer)
+	return gen.printNode(&gen.root, 0)
+}
+
+func (r *genericReplacer) printNode(t *trieNode, depth int) (s string) {
+	if t.priority > 0 {
+		s += "+"
+	} else {
+		s += "-"
+	}
+	s += "\n"
+
+	if t.prefix != "" {
+		s += Repeat(".", depth) + t.prefix
+		s += r.printNode(t.next, depth+len(t.prefix))
+	} else if t.table != nil {
+		for b, m := range r.mapping {
+			if int(m) != r.tableSize && t.table[m] != nil {
+				s += Repeat(".", depth) + string([]byte{byte(b)})
+				s += r.printNode(t.table[m], depth+1)
+			}
+		}
+	}
+	return
+}
+
+func StringFind(pattern, text string) int {
+	return makeStringFinder(pattern).next(text)
+}
+
+func DumpTables(pattern string) ([]int, []int) {
+	finder := makeStringFinder(pattern)
+	return finder.badCharSkip[:], finder.goodSuffixSkip
+}
diff --git a/src/strings/reader.go b/src/strings/reader.go
new file mode 100644
index 0000000..497ffb7
--- /dev/null
+++ b/src/strings/reader.go
@@ -0,0 +1,160 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+import (
+	"errors"
+	"io"
+	"unicode/utf8"
+)
+
+// A Reader implements the [io.Reader], [io.ReaderAt], [io.ByteReader], [io.ByteScanner],
+// [io.RuneReader], [io.RuneScanner], [io.Seeker], and [io.WriterTo] interfaces by reading
+// from a string.
+// The zero value for Reader operates like a Reader of an empty string.
+type Reader struct {
+	s        string
+	i        int64 // current reading index
+	prevRune int   // index of previous rune; or < 0
+}
+
+// Len returns the number of bytes of the unread portion of the
+// string.
+func (r *Reader) Len() int {
+	if r.i >= int64(len(r.s)) {
+		return 0
+	}
+	return int(int64(len(r.s)) - r.i)
+}
+
+// Size returns the original length of the underlying string.
+// Size is the number of bytes available for reading via [Reader.ReadAt].
+// The returned value is always the same and is not affected by calls
+// to any other method.
+func (r *Reader) Size() int64 { return int64(len(r.s)) }
+
+// Read implements the [io.Reader] interface.
+func (r *Reader) Read(b []byte) (n int, err error) {
+	if r.i >= int64(len(r.s)) {
+		return 0, io.EOF
+	}
+	r.prevRune = -1
+	n = copy(b, r.s[r.i:])
+	r.i += int64(n)
+	return
+}
+
+// ReadAt implements the [io.ReaderAt] interface.
+func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
+	// cannot modify state - see io.ReaderAt
+	if off < 0 {
+		return 0, errors.New("strings.Reader.ReadAt: negative offset")
+	}
+	if off >= int64(len(r.s)) {
+		return 0, io.EOF
+	}
+	n = copy(b, r.s[off:])
+	if n < len(b) {
+		err = io.EOF
+	}
+	return
+}
+
+// ReadByte implements the [io.ByteReader] interface.
+func (r *Reader) ReadByte() (byte, error) {
+	r.prevRune = -1
+	if r.i >= int64(len(r.s)) {
+		return 0, io.EOF
+	}
+	b := r.s[r.i]
+	r.i++
+	return b, nil
+}
+
+// UnreadByte implements the [io.ByteScanner] interface.
+func (r *Reader) UnreadByte() error {
+	if r.i <= 0 {
+		return errors.New("strings.Reader.UnreadByte: at beginning of string")
+	}
+	r.prevRune = -1
+	r.i--
+	return nil
+}
+
+// ReadRune implements the [io.RuneReader] interface.
+func (r *Reader) ReadRune() (ch rune, size int, err error) {
+	if r.i >= int64(len(r.s)) {
+		r.prevRune = -1
+		return 0, 0, io.EOF
+	}
+	r.prevRune = int(r.i)
+	if c := r.s[r.i]; c < utf8.RuneSelf {
+		r.i++
+		return rune(c), 1, nil
+	}
+	ch, size = utf8.DecodeRuneInString(r.s[r.i:])
+	r.i += int64(size)
+	return
+}
+
+// UnreadRune implements the [io.RuneScanner] interface.
+func (r *Reader) UnreadRune() error {
+	if r.i <= 0 {
+		return errors.New("strings.Reader.UnreadRune: at beginning of string")
+	}
+	if r.prevRune < 0 {
+		return errors.New("strings.Reader.UnreadRune: previous operation was not ReadRune")
+	}
+	r.i = int64(r.prevRune)
+	r.prevRune = -1
+	return nil
+}
+
+// Seek implements the [io.Seeker] interface.
+func (r *Reader) Seek(offset int64, whence int) (int64, error) {
+	r.prevRune = -1
+	var abs int64
+	switch whence {
+	case io.SeekStart:
+		abs = offset
+	case io.SeekCurrent:
+		abs = r.i + offset
+	case io.SeekEnd:
+		abs = int64(len(r.s)) + offset
+	default:
+		return 0, errors.New("strings.Reader.Seek: invalid whence")
+	}
+	if abs < 0 {
+		return 0, errors.New("strings.Reader.Seek: negative position")
+	}
+	r.i = abs
+	return abs, nil
+}
+
+// WriteTo implements the [io.WriterTo] interface.
+func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
+	r.prevRune = -1
+	if r.i >= int64(len(r.s)) {
+		return 0, nil
+	}
+	s := r.s[r.i:]
+	m, err := io.WriteString(w, s)
+	if m > len(s) {
+		panic("strings.Reader.WriteTo: invalid WriteString count")
+	}
+	r.i += int64(m)
+	n = int64(m)
+	if m != len(s) && err == nil {
+		err = io.ErrShortWrite
+	}
+	return
+}
+
+// Reset resets the [Reader] to be reading from s.
+func (r *Reader) Reset(s string) { *r = Reader{s, 0, -1} }
+
+// NewReader returns a new [Reader] reading from s.
+// It is similar to [bytes.NewBufferString] but more efficient and non-writable.
+func NewReader(s string) *Reader { return &Reader{s, 0, -1} }
diff --git a/src/strings/reader_test.go b/src/strings/reader_test.go
new file mode 100644
index 0000000..dc99f9c
--- /dev/null
+++ b/src/strings/reader_test.go
@@ -0,0 +1,233 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"strings"
+	"sync"
+	"testing"
+)
+
+func TestReader(t *testing.T) {
+	r := strings.NewReader("0123456789")
+	tests := []struct {
+		off     int64
+		seek    int
+		n       int
+		want    string
+		wantpos int64
+		readerr error
+		seekerr string
+	}{
+		{seek: io.SeekStart, off: 0, n: 20, want: "0123456789"},
+		{seek: io.SeekStart, off: 1, n: 1, want: "1"},
+		{seek: io.SeekCurrent, off: 1, wantpos: 3, n: 2, want: "34"},
+		{seek: io.SeekStart, off: -1, seekerr: "strings.Reader.Seek: negative position"},
+		{seek: io.SeekStart, off: 1 << 33, wantpos: 1 << 33, readerr: io.EOF},
+		{seek: io.SeekCurrent, off: 1, wantpos: 1<<33 + 1, readerr: io.EOF},
+		{seek: io.SeekStart, n: 5, want: "01234"},
+		{seek: io.SeekCurrent, n: 5, want: "56789"},
+		{seek: io.SeekEnd, off: -1, n: 1, wantpos: 9, want: "9"},
+	}
+
+	for i, tt := range tests {
+		pos, err := r.Seek(tt.off, tt.seek)
+		if err == nil && tt.seekerr != "" {
+			t.Errorf("%d. want seek error %q", i, tt.seekerr)
+			continue
+		}
+		if err != nil && err.Error() != tt.seekerr {
+			t.Errorf("%d. seek error = %q; want %q", i, err.Error(), tt.seekerr)
+			continue
+		}
+		if tt.wantpos != 0 && tt.wantpos != pos {
+			t.Errorf("%d. pos = %d, want %d", i, pos, tt.wantpos)
+		}
+		buf := make([]byte, tt.n)
+		n, err := r.Read(buf)
+		if err != tt.readerr {
+			t.Errorf("%d. read = %v; want %v", i, err, tt.readerr)
+			continue
+		}
+		got := string(buf[:n])
+		if got != tt.want {
+			t.Errorf("%d. got %q; want %q", i, got, tt.want)
+		}
+	}
+}
+
+func TestReadAfterBigSeek(t *testing.T) {
+	r := strings.NewReader("0123456789")
+	if _, err := r.Seek(1<<31+5, io.SeekStart); err != nil {
+		t.Fatal(err)
+	}
+	if n, err := r.Read(make([]byte, 10)); n != 0 || err != io.EOF {
+		t.Errorf("Read = %d, %v; want 0, EOF", n, err)
+	}
+}
+
+func TestReaderAt(t *testing.T) {
+	r := strings.NewReader("0123456789")
+	tests := []struct {
+		off     int64
+		n       int
+		want    string
+		wanterr any
+	}{
+		{0, 10, "0123456789", nil},
+		{1, 10, "123456789", io.EOF},
+		{1, 9, "123456789", nil},
+		{11, 10, "", io.EOF},
+		{0, 0, "", nil},
+		{-1, 0, "", "strings.Reader.ReadAt: negative offset"},
+	}
+	for i, tt := range tests {
+		b := make([]byte, tt.n)
+		rn, err := r.ReadAt(b, tt.off)
+		got := string(b[:rn])
+		if got != tt.want {
+			t.Errorf("%d. got %q; want %q", i, got, tt.want)
+		}
+		if fmt.Sprintf("%v", err) != fmt.Sprintf("%v", tt.wanterr) {
+			t.Errorf("%d. got error = %v; want %v", i, err, tt.wanterr)
+		}
+	}
+}
+
+func TestReaderAtConcurrent(t *testing.T) {
+	// Test for the race detector, to verify ReadAt doesn't mutate
+	// any state.
+	r := strings.NewReader("0123456789")
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			var buf [1]byte
+			r.ReadAt(buf[:], int64(i))
+		}(i)
+	}
+	wg.Wait()
+}
+
+func TestEmptyReaderConcurrent(t *testing.T) {
+	// Test for the race detector, to verify a Read that doesn't yield any bytes
+	// is okay to use from multiple goroutines. This was our historic behavior.
+	// See golang.org/issue/7856
+	r := strings.NewReader("")
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			var buf [1]byte
+			r.Read(buf[:])
+		}()
+		go func() {
+			defer wg.Done()
+			r.Read(nil)
+		}()
+	}
+	wg.Wait()
+}
+
+func TestWriteTo(t *testing.T) {
+	const str = "0123456789"
+	for i := 0; i <= len(str); i++ {
+		s := str[i:]
+		r := strings.NewReader(s)
+		var b bytes.Buffer
+		n, err := r.WriteTo(&b)
+		if expect := int64(len(s)); n != expect {
+			t.Errorf("got %v; want %v", n, expect)
+		}
+		if err != nil {
+			t.Errorf("for length %d: got error = %v; want nil", len(s), err)
+		}
+		if b.String() != s {
+			t.Errorf("got string %q; want %q", b.String(), s)
+		}
+		if r.Len() != 0 {
+			t.Errorf("reader contains %v bytes; want 0", r.Len())
+		}
+	}
+}
+
+// tests that Len is affected by reads, but Size is not.
+func TestReaderLenSize(t *testing.T) {
+	r := strings.NewReader("abc")
+	io.CopyN(io.Discard, r, 1)
+	if r.Len() != 2 {
+		t.Errorf("Len = %d; want 2", r.Len())
+	}
+	if r.Size() != 3 {
+		t.Errorf("Size = %d; want 3", r.Size())
+	}
+}
+
+func TestReaderReset(t *testing.T) {
+	r := strings.NewReader("世界")
+	if _, _, err := r.ReadRune(); err != nil {
+		t.Errorf("ReadRune: unexpected error: %v", err)
+	}
+
+	const want = "abcdef"
+	r.Reset(want)
+	if err := r.UnreadRune(); err == nil {
+		t.Errorf("UnreadRune: expected error, got nil")
+	}
+	buf, err := io.ReadAll(r)
+	if err != nil {
+		t.Errorf("ReadAll: unexpected error: %v", err)
+	}
+	if got := string(buf); got != want {
+		t.Errorf("ReadAll: got %q, want %q", got, want)
+	}
+}
+
+func TestReaderZero(t *testing.T) {
+	if l := (&strings.Reader{}).Len(); l != 0 {
+		t.Errorf("Len: got %d, want 0", l)
+	}
+
+	if n, err := (&strings.Reader{}).Read(nil); n != 0 || err != io.EOF {
+		t.Errorf("Read: got %d, %v; want 0, io.EOF", n, err)
+	}
+
+	if n, err := (&strings.Reader{}).ReadAt(nil, 11); n != 0 || err != io.EOF {
+		t.Errorf("ReadAt: got %d, %v; want 0, io.EOF", n, err)
+	}
+
+	if b, err := (&strings.Reader{}).ReadByte(); b != 0 || err != io.EOF {
+		t.Errorf("ReadByte: got %d, %v; want 0, io.EOF", b, err)
+	}
+
+	if ch, size, err := (&strings.Reader{}).ReadRune(); ch != 0 || size != 0 || err != io.EOF {
+		t.Errorf("ReadRune: got %d, %d, %v; want 0, 0, io.EOF", ch, size, err)
+	}
+
+	if offset, err := (&strings.Reader{}).Seek(11, io.SeekStart); offset != 11 || err != nil {
+		t.Errorf("Seek: got %d, %v; want 11, nil", offset, err)
+	}
+
+	if s := (&strings.Reader{}).Size(); s != 0 {
+		t.Errorf("Size: got %d, want 0", s)
+	}
+
+	if (&strings.Reader{}).UnreadByte() == nil {
+		t.Errorf("UnreadByte: got nil, want error")
+	}
+
+	if (&strings.Reader{}).UnreadRune() == nil {
+		t.Errorf("UnreadRune: got nil, want error")
+	}
+
+	if n, err := (&strings.Reader{}).WriteTo(io.Discard); n != 0 || err != nil {
+		t.Errorf("WriteTo: got %d, %v; want 0, nil", n, err)
+	}
+}
diff --git a/src/strings/replace.go b/src/strings/replace.go
new file mode 100644
index 0000000..3b17a55
--- /dev/null
+++ b/src/strings/replace.go
@@ -0,0 +1,578 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+import (
+	"io"
+	"sync"
+)
+
+// Replacer replaces a list of strings with replacements.
+// It is safe for concurrent use by multiple goroutines.
+type Replacer struct {
+	once   sync.Once // guards buildOnce method
+	r      replacer
+	oldnew []string
+}
+
+// replacer is the interface that a replacement algorithm needs to implement.
+type replacer interface {
+	Replace(s string) string
+	WriteString(w io.Writer, s string) (n int, err error)
+}
+
+// NewReplacer returns a new [Replacer] from a list of old, new string
+// pairs. Replacements are performed in the order they appear in the
+// target string, without overlapping matches. The old string
+// comparisons are done in argument order.
+//
+// NewReplacer panics if given an odd number of arguments.
+func NewReplacer(oldnew ...string) *Replacer {
+	if len(oldnew)%2 == 1 {
+		panic("strings.NewReplacer: odd argument count")
+	}
+	return &Replacer{oldnew: append([]string(nil), oldnew...)}
+}
+
+func (r *Replacer) buildOnce() {
+	r.r = r.build()
+	r.oldnew = nil
+}
+
+func (b *Replacer) build() replacer {
+	oldnew := b.oldnew
+	if len(oldnew) == 2 && len(oldnew[0]) > 1 {
+		return makeSingleStringReplacer(oldnew[0], oldnew[1])
+	}
+
+	allNewBytes := true
+	for i := 0; i < len(oldnew); i += 2 {
+		if len(oldnew[i]) != 1 {
+			return makeGenericReplacer(oldnew)
+		}
+		if len(oldnew[i+1]) != 1 {
+			allNewBytes = false
+		}
+	}
+
+	if allNewBytes {
+		r := byteReplacer{}
+		for i := range r {
+			r[i] = byte(i)
+		}
+		// The first occurrence of old->new map takes precedence
+		// over the others with the same old string.
+		for i := len(oldnew) - 2; i >= 0; i -= 2 {
+			o := oldnew[i][0]
+			n := oldnew[i+1][0]
+			r[o] = n
+		}
+		return &r
+	}
+
+	r := byteStringReplacer{toReplace: make([]string, 0, len(oldnew)/2)}
+	// The first occurrence of old->new map takes precedence
+	// over the others with the same old string.
+	for i := len(oldnew) - 2; i >= 0; i -= 2 {
+		o := oldnew[i][0]
+		n := oldnew[i+1]
+		// To avoid counting repetitions multiple times.
+		if r.replacements[o] == nil {
+			// We need to use string([]byte{o}) instead of string(o),
+			// to avoid utf8 encoding of o.
+			// E. g. byte(150) produces string of length 2.
+			r.toReplace = append(r.toReplace, string([]byte{o}))
+		}
+		r.replacements[o] = []byte(n)
+
+	}
+	return &r
+}
+
+// Replace returns a copy of s with all replacements performed.
+func (r *Replacer) Replace(s string) string {
+	r.once.Do(r.buildOnce)
+	return r.r.Replace(s)
+}
+
+// WriteString writes s to w with all replacements performed.
+func (r *Replacer) WriteString(w io.Writer, s string) (n int, err error) {
+	r.once.Do(r.buildOnce)
+	return r.r.WriteString(w, s)
+}
+
+// trieNode is a node in a lookup trie for prioritized key/value pairs. Keys
+// and values may be empty. For example, the trie containing keys "ax", "ay",
+// "bcbc", "x" and "xy" could have eight nodes:
+//
+//	n0  -
+//	n1  a-
+//	n2  .x+
+//	n3  .y+
+//	n4  b-
+//	n5  .cbc+
+//	n6  x+
+//	n7  .y+
+//
+// n0 is the root node, and its children are n1, n4 and n6; n1's children are
+// n2 and n3; n4's child is n5; n6's child is n7. Nodes n0, n1 and n4 (marked
+// with a trailing "-") are partial keys, and nodes n2, n3, n5, n6 and n7
+// (marked with a trailing "+") are complete keys.
+type trieNode struct {
+	// value is the value of the trie node's key/value pair. It is empty if
+	// this node is not a complete key.
+	value string
+	// priority is the priority (higher is more important) of the trie node's
+	// key/value pair; keys are not necessarily matched shortest- or longest-
+	// first. Priority is positive if this node is a complete key, and zero
+	// otherwise. In the example above, positive/zero priorities are marked
+	// with a trailing "+" or "-".
+	priority int
+
+	// A trie node may have zero, one or more child nodes:
+	//  * if the remaining fields are zero, there are no children.
+	//  * if prefix and next are non-zero, there is one child in next.
+	//  * if table is non-zero, it defines all the children.
+	//
+	// Prefixes are preferred over tables when there is one child, but the
+	// root node always uses a table for lookup efficiency.
+
+	// prefix is the difference in keys between this trie node and the next.
+	// In the example above, node n4 has prefix "cbc" and n4's next node is n5.
+	// Node n5 has no children and so has zero prefix, next and table fields.
+	prefix string
+	next   *trieNode
+
+	// table is a lookup table indexed by the next byte in the key, after
+	// remapping that byte through genericReplacer.mapping to create a dense
+	// index. In the example above, the keys only use 'a', 'b', 'c', 'x' and
+	// 'y', which remap to 0, 1, 2, 3 and 4. All other bytes remap to 5, and
+	// genericReplacer.tableSize will be 5. Node n0's table will be
+	// []*trieNode{ 0:n1, 1:n4, 3:n6 }, where the 0, 1 and 3 are the remapped
+	// 'a', 'b' and 'x'.
+	table []*trieNode
+}
+
+func (t *trieNode) add(key, val string, priority int, r *genericReplacer) {
+	if key == "" {
+		if t.priority == 0 {
+			t.value = val
+			t.priority = priority
+		}
+		return
+	}
+
+	if t.prefix != "" {
+		// Need to split the prefix among multiple nodes.
+		var n int // length of the longest common prefix
+		for ; n < len(t.prefix) && n < len(key); n++ {
+			if t.prefix[n] != key[n] {
+				break
+			}
+		}
+		if n == len(t.prefix) {
+			t.next.add(key[n:], val, priority, r)
+		} else if n == 0 {
+			// First byte differs, start a new lookup table here. Looking up
+			// what is currently t.prefix[0] will lead to prefixNode, and
+			// looking up key[0] will lead to keyNode.
+			var prefixNode *trieNode
+			if len(t.prefix) == 1 {
+				prefixNode = t.next
+			} else {
+				prefixNode = &trieNode{
+					prefix: t.prefix[1:],
+					next:   t.next,
+				}
+			}
+			keyNode := new(trieNode)
+			t.table = make([]*trieNode, r.tableSize)
+			t.table[r.mapping[t.prefix[0]]] = prefixNode
+			t.table[r.mapping[key[0]]] = keyNode
+			t.prefix = ""
+			t.next = nil
+			keyNode.add(key[1:], val, priority, r)
+		} else {
+			// Insert new node after the common section of the prefix.
+			next := &trieNode{
+				prefix: t.prefix[n:],
+				next:   t.next,
+			}
+			t.prefix = t.prefix[:n]
+			t.next = next
+			next.add(key[n:], val, priority, r)
+		}
+	} else if t.table != nil {
+		// Insert into existing table.
+		m := r.mapping[key[0]]
+		if t.table[m] == nil {
+			t.table[m] = new(trieNode)
+		}
+		t.table[m].add(key[1:], val, priority, r)
+	} else {
+		t.prefix = key
+		t.next = new(trieNode)
+		t.next.add("", val, priority, r)
+	}
+}
+
+func (r *genericReplacer) lookup(s string, ignoreRoot bool) (val string, keylen int, found bool) {
+	// Iterate down the trie to the end, and grab the value and keylen with
+	// the highest priority.
+	bestPriority := 0
+	node := &r.root
+	n := 0
+	for node != nil {
+		if node.priority > bestPriority && !(ignoreRoot && node == &r.root) {
+			bestPriority = node.priority
+			val = node.value
+			keylen = n
+			found = true
+		}
+
+		if s == "" {
+			break
+		}
+		if node.table != nil {
+			index := r.mapping[s[0]]
+			if int(index) == r.tableSize {
+				break
+			}
+			node = node.table[index]
+			s = s[1:]
+			n++
+		} else if node.prefix != "" && HasPrefix(s, node.prefix) {
+			n += len(node.prefix)
+			s = s[len(node.prefix):]
+			node = node.next
+		} else {
+			break
+		}
+	}
+	return
+}
+
+// genericReplacer is the fully generic algorithm.
+// It's used as a fallback when nothing faster can be used.
+type genericReplacer struct {
+	root trieNode
+	// tableSize is the size of a trie node's lookup table. It is the number
+	// of unique key bytes.
+	tableSize int
+	// mapping maps from key bytes to a dense index for trieNode.table.
+	mapping [256]byte
+}
+
+func makeGenericReplacer(oldnew []string) *genericReplacer {
+	r := new(genericReplacer)
+	// Find each byte used, then assign them each an index.
+	for i := 0; i < len(oldnew); i += 2 {
+		key := oldnew[i]
+		for j := 0; j < len(key); j++ {
+			r.mapping[key[j]] = 1
+		}
+	}
+
+	for _, b := range r.mapping {
+		r.tableSize += int(b)
+	}
+
+	var index byte
+	for i, b := range r.mapping {
+		if b == 0 {
+			r.mapping[i] = byte(r.tableSize)
+		} else {
+			r.mapping[i] = index
+			index++
+		}
+	}
+	// Ensure root node uses a lookup table (for performance).
+	r.root.table = make([]*trieNode, r.tableSize)
+
+	for i := 0; i < len(oldnew); i += 2 {
+		r.root.add(oldnew[i], oldnew[i+1], len(oldnew)-i, r)
+	}
+	return r
+}
+
+type appendSliceWriter []byte
+
+// Write writes to the buffer to satisfy io.Writer.
+func (w *appendSliceWriter) Write(p []byte) (int, error) {
+	*w = append(*w, p...)
+	return len(p), nil
+}
+
+// WriteString writes to the buffer without string->[]byte->string allocations.
+func (w *appendSliceWriter) WriteString(s string) (int, error) {
+	*w = append(*w, s...)
+	return len(s), nil
+}
+
+type stringWriter struct {
+	w io.Writer
+}
+
+func (w stringWriter) WriteString(s string) (int, error) {
+	return w.w.Write([]byte(s))
+}
+
+func getStringWriter(w io.Writer) io.StringWriter {
+	sw, ok := w.(io.StringWriter)
+	if !ok {
+		sw = stringWriter{w}
+	}
+	return sw
+}
+
+func (r *genericReplacer) Replace(s string) string {
+	buf := make(appendSliceWriter, 0, len(s))
+	r.WriteString(&buf, s)
+	return string(buf)
+}
+
+func (r *genericReplacer) WriteString(w io.Writer, s string) (n int, err error) {
+	sw := getStringWriter(w)
+	var last, wn int
+	var prevMatchEmpty bool
+	for i := 0; i <= len(s); {
+		// Fast path: s[i] is not a prefix of any pattern.
+		if i != len(s) && r.root.priority == 0 {
+			index := int(r.mapping[s[i]])
+			if index == r.tableSize || r.root.table[index] == nil {
+				i++
+				continue
+			}
+		}
+
+		// Ignore the empty match iff the previous loop found the empty match.
+		val, keylen, match := r.lookup(s[i:], prevMatchEmpty)
+		prevMatchEmpty = match && keylen == 0
+		if match {
+			wn, err = sw.WriteString(s[last:i])
+			n += wn
+			if err != nil {
+				return
+			}
+			wn, err = sw.WriteString(val)
+			n += wn
+			if err != nil {
+				return
+			}
+			i += keylen
+			last = i
+			continue
+		}
+		i++
+	}
+	if last != len(s) {
+		wn, err = sw.WriteString(s[last:])
+		n += wn
+	}
+	return
+}
+
+// singleStringReplacer is the implementation that's used when there is only
+// one string to replace (and that string has more than one byte).
+type singleStringReplacer struct {
+	finder *stringFinder
+	// value is the new string that replaces that pattern when it's found.
+	value string
+}
+
+func makeSingleStringReplacer(pattern string, value string) *singleStringReplacer {
+	return &singleStringReplacer{finder: makeStringFinder(pattern), value: value}
+}
+
+func (r *singleStringReplacer) Replace(s string) string {
+	var buf Builder
+	i, matched := 0, false
+	for {
+		match := r.finder.next(s[i:])
+		if match == -1 {
+			break
+		}
+		matched = true
+		buf.Grow(match + len(r.value))
+		buf.WriteString(s[i : i+match])
+		buf.WriteString(r.value)
+		i += match + len(r.finder.pattern)
+	}
+	if !matched {
+		return s
+	}
+	buf.WriteString(s[i:])
+	return buf.String()
+}
+
+func (r *singleStringReplacer) WriteString(w io.Writer, s string) (n int, err error) {
+	sw := getStringWriter(w)
+	var i, wn int
+	for {
+		match := r.finder.next(s[i:])
+		if match == -1 {
+			break
+		}
+		wn, err = sw.WriteString(s[i : i+match])
+		n += wn
+		if err != nil {
+			return
+		}
+		wn, err = sw.WriteString(r.value)
+		n += wn
+		if err != nil {
+			return
+		}
+		i += match + len(r.finder.pattern)
+	}
+	wn, err = sw.WriteString(s[i:])
+	n += wn
+	return
+}
+
+// byteReplacer is the implementation that's used when all the "old"
+// and "new" values are single ASCII bytes.
+// The array contains replacement bytes indexed by old byte.
+type byteReplacer [256]byte
+
+func (r *byteReplacer) Replace(s string) string {
+	var buf []byte // lazily allocated
+	for i := 0; i < len(s); i++ {
+		b := s[i]
+		if r[b] != b {
+			if buf == nil {
+				buf = []byte(s)
+			}
+			buf[i] = r[b]
+		}
+	}
+	if buf == nil {
+		return s
+	}
+	return string(buf)
+}
+
+func (r *byteReplacer) WriteString(w io.Writer, s string) (n int, err error) {
+	sw := getStringWriter(w)
+	last := 0
+	for i := 0; i < len(s); i++ {
+		b := s[i]
+		if r[b] == b {
+			continue
+		}
+		if last != i {
+			wn, err := sw.WriteString(s[last:i])
+			n += wn
+			if err != nil {
+				return n, err
+			}
+		}
+		last = i + 1
+		nw, err := w.Write(r[b : int(b)+1])
+		n += nw
+		if err != nil {
+			return n, err
+		}
+	}
+	if last != len(s) {
+		nw, err := sw.WriteString(s[last:])
+		n += nw
+		if err != nil {
+			return n, err
+		}
+	}
+	return n, nil
+}
+
+// byteStringReplacer is the implementation that's used when all the
+// "old" values are single ASCII bytes but the "new" values vary in size.
+type byteStringReplacer struct {
+	// replacements contains replacement byte slices indexed by old byte.
+	// A nil []byte means that the old byte should not be replaced.
+	replacements [256][]byte
+	// toReplace keeps a list of bytes to replace. Depending on length of toReplace
+	// and length of target string it may be faster to use Count, or a plain loop.
+	// We store single byte as a string, because Count takes a string.
+	toReplace []string
+}
+
+// countCutOff controls the ratio of a string length to a number of replacements
+// at which (*byteStringReplacer).Replace switches algorithms.
+// For strings with higher ration of length to replacements than that value,
+// we call Count, for each replacement from toReplace.
+// For strings, with a lower ratio we use simple loop, because of Count overhead.
+// countCutOff is an empirically determined overhead multiplier.
+// TODO(tocarip) revisit once we have register-based abi/mid-stack inlining.
+const countCutOff = 8
+
+func (r *byteStringReplacer) Replace(s string) string {
+	newSize := len(s)
+	anyChanges := false
+	// Is it faster to use Count?
+	if len(r.toReplace)*countCutOff <= len(s) {
+		for _, x := range r.toReplace {
+			if c := Count(s, x); c != 0 {
+				// The -1 is because we are replacing 1 byte with len(replacements[b]) bytes.
+				newSize += c * (len(r.replacements[x[0]]) - 1)
+				anyChanges = true
+			}
+
+		}
+	} else {
+		for i := 0; i < len(s); i++ {
+			b := s[i]
+			if r.replacements[b] != nil {
+				// See above for explanation of -1
+				newSize += len(r.replacements[b]) - 1
+				anyChanges = true
+			}
+		}
+	}
+	if !anyChanges {
+		return s
+	}
+	buf := make([]byte, newSize)
+	j := 0
+	for i := 0; i < len(s); i++ {
+		b := s[i]
+		if r.replacements[b] != nil {
+			j += copy(buf[j:], r.replacements[b])
+		} else {
+			buf[j] = b
+			j++
+		}
+	}
+	return string(buf)
+}
+
+func (r *byteStringReplacer) WriteString(w io.Writer, s string) (n int, err error) {
+	sw := getStringWriter(w)
+	last := 0
+	for i := 0; i < len(s); i++ {
+		b := s[i]
+		if r.replacements[b] == nil {
+			continue
+		}
+		if last != i {
+			nw, err := sw.WriteString(s[last:i])
+			n += nw
+			if err != nil {
+				return n, err
+			}
+		}
+		last = i + 1
+		nw, err := w.Write(r.replacements[b])
+		n += nw
+		if err != nil {
+			return n, err
+		}
+	}
+	if last != len(s) {
+		var nw int
+		nw, err = sw.WriteString(s[last:])
+		n += nw
+	}
+	return
+}
diff --git a/src/strings/replace_test.go b/src/strings/replace_test.go
new file mode 100644
index 0000000..34b5bad
--- /dev/null
+++ b/src/strings/replace_test.go
@@ -0,0 +1,583 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	"bytes"
+	"fmt"
+	. "strings"
+	"testing"
+)
+
+var htmlEscaper = NewReplacer(
+	"&", "&amp;",
+	"<", "&lt;",
+	">", "&gt;",
+	`"`, "&quot;",
+	"'", "&apos;",
+)
+
+var htmlUnescaper = NewReplacer(
+	"&amp;", "&",
+	"&lt;", "<",
+	"&gt;", ">",
+	"&quot;", `"`,
+	"&apos;", "'",
+)
+
+// The http package's old HTML escaping function.
+func oldHTMLEscape(s string) string {
+	s = Replace(s, "&", "&amp;", -1)
+	s = Replace(s, "<", "&lt;", -1)
+	s = Replace(s, ">", "&gt;", -1)
+	s = Replace(s, `"`, "&quot;", -1)
+	s = Replace(s, "'", "&apos;", -1)
+	return s
+}
+
+var capitalLetters = NewReplacer("a", "A", "b", "B")
+
+// TestReplacer tests the replacer implementations.
+func TestReplacer(t *testing.T) {
+	type testCase struct {
+		r       *Replacer
+		in, out string
+	}
+	var testCases []testCase
+
+	// str converts 0xff to "\xff". This isn't just string(b) since that converts to UTF-8.
+	str := func(b byte) string {
+		return string([]byte{b})
+	}
+	var s []string
+
+	// inc maps "\x00"->"\x01", ..., "a"->"b", "b"->"c", ..., "\xff"->"\x00".
+	s = nil
+	for i := 0; i < 256; i++ {
+		s = append(s, str(byte(i)), str(byte(i+1)))
+	}
+	inc := NewReplacer(s...)
+
+	// Test cases with 1-byte old strings, 1-byte new strings.
+	testCases = append(testCases,
+		testCase{capitalLetters, "brad", "BrAd"},
+		testCase{capitalLetters, Repeat("a", (32<<10)+123), Repeat("A", (32<<10)+123)},
+		testCase{capitalLetters, "", ""},
+
+		testCase{inc, "brad", "csbe"},
+		testCase{inc, "\x00\xff", "\x01\x00"},
+		testCase{inc, "", ""},
+
+		testCase{NewReplacer("a", "1", "a", "2"), "brad", "br1d"},
+	)
+
+	// repeat maps "a"->"a", "b"->"bb", "c"->"ccc", ...
+	s = nil
+	for i := 0; i < 256; i++ {
+		n := i + 1 - 'a'
+		if n < 1 {
+			n = 1
+		}
+		s = append(s, str(byte(i)), Repeat(str(byte(i)), n))
+	}
+	repeat := NewReplacer(s...)
+
+	// Test cases with 1-byte old strings, variable length new strings.
+	testCases = append(testCases,
+		testCase{htmlEscaper, "No changes", "No changes"},
+		testCase{htmlEscaper, "I <3 escaping & stuff", "I &lt;3 escaping &amp; stuff"},
+		testCase{htmlEscaper, "&&&", "&amp;&amp;&amp;"},
+		testCase{htmlEscaper, "", ""},
+
+		testCase{repeat, "brad", "bbrrrrrrrrrrrrrrrrrradddd"},
+		testCase{repeat, "abba", "abbbba"},
+		testCase{repeat, "", ""},
+
+		testCase{NewReplacer("a", "11", "a", "22"), "brad", "br11d"},
+	)
+
+	// The remaining test cases have variable length old strings.
+
+	testCases = append(testCases,
+		testCase{htmlUnescaper, "&amp;amp;", "&amp;"},
+		testCase{htmlUnescaper, "&lt;b&gt;HTML&apos;s neat&lt;/b&gt;", "<b>HTML's neat</b>"},
+		testCase{htmlUnescaper, "", ""},
+
+		testCase{NewReplacer("a", "1", "a", "2", "xxx", "xxx"), "brad", "br1d"},
+
+		testCase{NewReplacer("a", "1", "aa", "2", "aaa", "3"), "aaaa", "1111"},
+
+		testCase{NewReplacer("aaa", "3", "aa", "2", "a", "1"), "aaaa", "31"},
+	)
+
+	// gen1 has multiple old strings of variable length. There is no
+	// overall non-empty common prefix, but some pairwise common prefixes.
+	gen1 := NewReplacer(
+		"aaa", "3[aaa]",
+		"aa", "2[aa]",
+		"a", "1[a]",
+		"i", "i",
+		"longerst", "most long",
+		"longer", "medium",
+		"long", "short",
+		"xx", "xx",
+		"x", "X",
+		"X", "Y",
+		"Y", "Z",
+	)
+	testCases = append(testCases,
+		testCase{gen1, "fooaaabar", "foo3[aaa]b1[a]r"},
+		testCase{gen1, "long, longerst, longer", "short, most long, medium"},
+		testCase{gen1, "xxxxx", "xxxxX"},
+		testCase{gen1, "XiX", "YiY"},
+		testCase{gen1, "", ""},
+	)
+
+	// gen2 has multiple old strings with no pairwise common prefix.
+	gen2 := NewReplacer(
+		"roses", "red",
+		"violets", "blue",
+		"sugar", "sweet",
+	)
+	testCases = append(testCases,
+		testCase{gen2, "roses are red, violets are blue...", "red are red, blue are blue..."},
+		testCase{gen2, "", ""},
+	)
+
+	// gen3 has multiple old strings with an overall common prefix.
+	gen3 := NewReplacer(
+		"abracadabra", "poof",
+		"abracadabrakazam", "splat",
+		"abraham", "lincoln",
+		"abrasion", "scrape",
+		"abraham", "isaac",
+	)
+	testCases = append(testCases,
+		testCase{gen3, "abracadabrakazam abraham", "poofkazam lincoln"},
+		testCase{gen3, "abrasion abracad", "scrape abracad"},
+		testCase{gen3, "abba abram abrasive", "abba abram abrasive"},
+		testCase{gen3, "", ""},
+	)
+
+	// foo{1,2,3,4} have multiple old strings with an overall common prefix
+	// and 1- or 2- byte extensions from the common prefix.
+	foo1 := NewReplacer(
+		"foo1", "A",
+		"foo2", "B",
+		"foo3", "C",
+	)
+	foo2 := NewReplacer(
+		"foo1", "A",
+		"foo2", "B",
+		"foo31", "C",
+		"foo32", "D",
+	)
+	foo3 := NewReplacer(
+		"foo11", "A",
+		"foo12", "B",
+		"foo31", "C",
+		"foo32", "D",
+	)
+	foo4 := NewReplacer(
+		"foo12", "B",
+		"foo32", "D",
+	)
+	testCases = append(testCases,
+		testCase{foo1, "fofoofoo12foo32oo", "fofooA2C2oo"},
+		testCase{foo1, "", ""},
+
+		testCase{foo2, "fofoofoo12foo32oo", "fofooA2Doo"},
+		testCase{foo2, "", ""},
+
+		testCase{foo3, "fofoofoo12foo32oo", "fofooBDoo"},
+		testCase{foo3, "", ""},
+
+		testCase{foo4, "fofoofoo12foo32oo", "fofooBDoo"},
+		testCase{foo4, "", ""},
+	)
+
+	// genAll maps "\x00\x01\x02...\xfe\xff" to "[all]", amongst other things.
+	allBytes := make([]byte, 256)
+	for i := range allBytes {
+		allBytes[i] = byte(i)
+	}
+	allString := string(allBytes)
+	genAll := NewReplacer(
+		allString, "[all]",
+		"\xff", "[ff]",
+		"\x00", "[00]",
+	)
+	testCases = append(testCases,
+		testCase{genAll, allString, "[all]"},
+		testCase{genAll, "a\xff" + allString + "\x00", "a[ff][all][00]"},
+		testCase{genAll, "", ""},
+	)
+
+	// Test cases with empty old strings.
+
+	blankToX1 := NewReplacer("", "X")
+	blankToX2 := NewReplacer("", "X", "", "")
+	blankHighPriority := NewReplacer("", "X", "o", "O")
+	blankLowPriority := NewReplacer("o", "O", "", "X")
+	blankNoOp1 := NewReplacer("", "")
+	blankNoOp2 := NewReplacer("", "", "", "A")
+	blankFoo := NewReplacer("", "X", "foobar", "R", "foobaz", "Z")
+	testCases = append(testCases,
+		testCase{blankToX1, "foo", "XfXoXoX"},
+		testCase{blankToX1, "", "X"},
+
+		testCase{blankToX2, "foo", "XfXoXoX"},
+		testCase{blankToX2, "", "X"},
+
+		testCase{blankHighPriority, "oo", "XOXOX"},
+		testCase{blankHighPriority, "ii", "XiXiX"},
+		testCase{blankHighPriority, "oiio", "XOXiXiXOX"},
+		testCase{blankHighPriority, "iooi", "XiXOXOXiX"},
+		testCase{blankHighPriority, "", "X"},
+
+		testCase{blankLowPriority, "oo", "OOX"},
+		testCase{blankLowPriority, "ii", "XiXiX"},
+		testCase{blankLowPriority, "oiio", "OXiXiOX"},
+		testCase{blankLowPriority, "iooi", "XiOOXiX"},
+		testCase{blankLowPriority, "", "X"},
+
+		testCase{blankNoOp1, "foo", "foo"},
+		testCase{blankNoOp1, "", ""},
+
+		testCase{blankNoOp2, "foo", "foo"},
+		testCase{blankNoOp2, "", ""},
+
+		testCase{blankFoo, "foobarfoobaz", "XRXZX"},
+		testCase{blankFoo, "foobar-foobaz", "XRX-XZX"},
+		testCase{blankFoo, "", "X"},
+	)
+
+	// single string replacer
+
+	abcMatcher := NewReplacer("abc", "[match]")
+
+	testCases = append(testCases,
+		testCase{abcMatcher, "", ""},
+		testCase{abcMatcher, "ab", "ab"},
+		testCase{abcMatcher, "abc", "[match]"},
+		testCase{abcMatcher, "abcd", "[match]d"},
+		testCase{abcMatcher, "cabcabcdabca", "c[match][match]d[match]a"},
+	)
+
+	// Issue 6659 cases (more single string replacer)
+
+	noHello := NewReplacer("Hello", "")
+	testCases = append(testCases,
+		testCase{noHello, "Hello", ""},
+		testCase{noHello, "Hellox", "x"},
+		testCase{noHello, "xHello", "x"},
+		testCase{noHello, "xHellox", "xx"},
+	)
+
+	// No-arg test cases.
+
+	nop := NewReplacer()
+	testCases = append(testCases,
+		testCase{nop, "abc", "abc"},
+		testCase{nop, "", ""},
+	)
+
+	// Run the test cases.
+
+	for i, tc := range testCases {
+		if s := tc.r.Replace(tc.in); s != tc.out {
+			t.Errorf("%d. Replace(%q) = %q, want %q", i, tc.in, s, tc.out)
+		}
+		var buf bytes.Buffer
+		n, err := tc.r.WriteString(&buf, tc.in)
+		if err != nil {
+			t.Errorf("%d. WriteString: %v", i, err)
+			continue
+		}
+		got := buf.String()
+		if got != tc.out {
+			t.Errorf("%d. WriteString(%q) wrote %q, want %q", i, tc.in, got, tc.out)
+			continue
+		}
+		if n != len(tc.out) {
+			t.Errorf("%d. WriteString(%q) wrote correct string but reported %d bytes; want %d (%q)",
+				i, tc.in, n, len(tc.out), tc.out)
+		}
+	}
+}
+
+var algorithmTestCases = []struct {
+	r    *Replacer
+	want string
+}{
+	{capitalLetters, "*strings.byteReplacer"},
+	{htmlEscaper, "*strings.byteStringReplacer"},
+	{NewReplacer("12", "123"), "*strings.singleStringReplacer"},
+	{NewReplacer("1", "12"), "*strings.byteStringReplacer"},
+	{NewReplacer("", "X"), "*strings.genericReplacer"},
+	{NewReplacer("a", "1", "b", "12", "cde", "123"), "*strings.genericReplacer"},
+}
+
+// TestPickAlgorithm tests that NewReplacer picks the correct algorithm.
+func TestPickAlgorithm(t *testing.T) {
+	for i, tc := range algorithmTestCases {
+		got := fmt.Sprintf("%T", tc.r.Replacer())
+		if got != tc.want {
+			t.Errorf("%d. algorithm = %s, want %s", i, got, tc.want)
+		}
+	}
+}
+
+type errWriter struct{}
+
+func (errWriter) Write(p []byte) (n int, err error) {
+	return 0, fmt.Errorf("unwritable")
+}
+
+// TestWriteStringError tests that WriteString returns an error
+// received from the underlying io.Writer.
+func TestWriteStringError(t *testing.T) {
+	for i, tc := range algorithmTestCases {
+		n, err := tc.r.WriteString(errWriter{}, "abc")
+		if n != 0 || err == nil || err.Error() != "unwritable" {
+			t.Errorf("%d. WriteStringError = %d, %v, want 0, unwritable", i, n, err)
+		}
+	}
+}
+
+// TestGenericTrieBuilding verifies the structure of the generated trie. There
+// is one node per line, and the key ending with the current line is in the
+// trie if it ends with a "+".
+func TestGenericTrieBuilding(t *testing.T) {
+	testCases := []struct{ in, out string }{
+		{"abc;abdef;abdefgh;xx;xy;z", `-
+			a-
+			.b-
+			..c+
+			..d-
+			...ef+
+			.....gh+
+			x-
+			.x+
+			.y+
+			z+
+			`},
+		{"abracadabra;abracadabrakazam;abraham;abrasion", `-
+			a-
+			.bra-
+			....c-
+			.....adabra+
+			...........kazam+
+			....h-
+			.....am+
+			....s-
+			.....ion+
+			`},
+		{"aaa;aa;a;i;longerst;longer;long;xx;x;X;Y", `-
+			X+
+			Y+
+			a+
+			.a+
+			..a+
+			i+
+			l-
+			.ong+
+			....er+
+			......st+
+			x+
+			.x+
+			`},
+		{"foo;;foo;foo1", `+
+			f-
+			.oo+
+			...1+
+			`},
+	}
+
+	for _, tc := range testCases {
+		keys := Split(tc.in, ";")
+		args := make([]string, len(keys)*2)
+		for i, key := range keys {
+			args[i*2] = key
+		}
+
+		got := NewReplacer(args...).PrintTrie()
+		// Remove tabs from tc.out
+		wantbuf := make([]byte, 0, len(tc.out))
+		for i := 0; i < len(tc.out); i++ {
+			if tc.out[i] != '\t' {
+				wantbuf = append(wantbuf, tc.out[i])
+			}
+		}
+		want := string(wantbuf)
+
+		if got != want {
+			t.Errorf("PrintTrie(%q)\ngot\n%swant\n%s", tc.in, got, want)
+		}
+	}
+}
+
+func BenchmarkGenericNoMatch(b *testing.B) {
+	str := Repeat("A", 100) + Repeat("B", 100)
+	generic := NewReplacer("a", "A", "b", "B", "12", "123") // varying lengths forces generic
+	for i := 0; i < b.N; i++ {
+		generic.Replace(str)
+	}
+}
+
+func BenchmarkGenericMatch1(b *testing.B) {
+	str := Repeat("a", 100) + Repeat("b", 100)
+	generic := NewReplacer("a", "A", "b", "B", "12", "123")
+	for i := 0; i < b.N; i++ {
+		generic.Replace(str)
+	}
+}
+
+func BenchmarkGenericMatch2(b *testing.B) {
+	str := Repeat("It&apos;s &lt;b&gt;HTML&lt;/b&gt;!", 100)
+	for i := 0; i < b.N; i++ {
+		htmlUnescaper.Replace(str)
+	}
+}
+
+func benchmarkSingleString(b *testing.B, pattern, text string) {
+	r := NewReplacer(pattern, "[match]")
+	b.SetBytes(int64(len(text)))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		r.Replace(text)
+	}
+}
+
+func BenchmarkSingleMaxSkipping(b *testing.B) {
+	benchmarkSingleString(b, Repeat("b", 25), Repeat("a", 10000))
+}
+
+func BenchmarkSingleLongSuffixFail(b *testing.B) {
+	benchmarkSingleString(b, "b"+Repeat("a", 500), Repeat("a", 1002))
+}
+
+func BenchmarkSingleMatch(b *testing.B) {
+	benchmarkSingleString(b, "abcdef", Repeat("abcdefghijklmno", 1000))
+}
+
+func BenchmarkByteByteNoMatch(b *testing.B) {
+	str := Repeat("A", 100) + Repeat("B", 100)
+	for i := 0; i < b.N; i++ {
+		capitalLetters.Replace(str)
+	}
+}
+
+func BenchmarkByteByteMatch(b *testing.B) {
+	str := Repeat("a", 100) + Repeat("b", 100)
+	for i := 0; i < b.N; i++ {
+		capitalLetters.Replace(str)
+	}
+}
+
+func BenchmarkByteStringMatch(b *testing.B) {
+	str := "<" + Repeat("a", 99) + Repeat("b", 99) + ">"
+	for i := 0; i < b.N; i++ {
+		htmlEscaper.Replace(str)
+	}
+}
+
+func BenchmarkHTMLEscapeNew(b *testing.B) {
+	str := "I <3 to escape HTML & other text too."
+	for i := 0; i < b.N; i++ {
+		htmlEscaper.Replace(str)
+	}
+}
+
+func BenchmarkHTMLEscapeOld(b *testing.B) {
+	str := "I <3 to escape HTML & other text too."
+	for i := 0; i < b.N; i++ {
+		oldHTMLEscape(str)
+	}
+}
+
+func BenchmarkByteStringReplacerWriteString(b *testing.B) {
+	str := Repeat("I <3 to escape HTML & other text too.", 100)
+	buf := new(bytes.Buffer)
+	for i := 0; i < b.N; i++ {
+		htmlEscaper.WriteString(buf, str)
+		buf.Reset()
+	}
+}
+
+func BenchmarkByteReplacerWriteString(b *testing.B) {
+	str := Repeat("abcdefghijklmnopqrstuvwxyz", 100)
+	buf := new(bytes.Buffer)
+	for i := 0; i < b.N; i++ {
+		capitalLetters.WriteString(buf, str)
+		buf.Reset()
+	}
+}
+
+// BenchmarkByteByteReplaces compares byteByteImpl against multiple Replaces.
+func BenchmarkByteByteReplaces(b *testing.B) {
+	str := Repeat("a", 100) + Repeat("b", 100)
+	for i := 0; i < b.N; i++ {
+		Replace(Replace(str, "a", "A", -1), "b", "B", -1)
+	}
+}
+
+// BenchmarkByteByteMap compares byteByteImpl against Map.
+func BenchmarkByteByteMap(b *testing.B) {
+	str := Repeat("a", 100) + Repeat("b", 100)
+	fn := func(r rune) rune {
+		switch r {
+		case 'a':
+			return 'A'
+		case 'b':
+			return 'B'
+		}
+		return r
+	}
+	for i := 0; i < b.N; i++ {
+		Map(fn, str)
+	}
+}
+
+var mapdata = []struct{ name, data string }{
+	{"ASCII", "a b c d e f g h i j k l m n o p q r s t u v w x y z"},
+	{"Greek", "α β γ δ ε ζ η θ ι κ λ μ ν ξ ο π ρ ς σ τ υ φ χ ψ ω"},
+}
+
+func BenchmarkMap(b *testing.B) {
+	mapidentity := func(r rune) rune {
+		return r
+	}
+
+	b.Run("identity", func(b *testing.B) {
+		for _, md := range mapdata {
+			b.Run(md.name, func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					Map(mapidentity, md.data)
+				}
+			})
+		}
+	})
+
+	mapchange := func(r rune) rune {
+		if 'a' <= r && r <= 'z' {
+			return r + 'A' - 'a'
+		}
+		if 'α' <= r && r <= 'ω' {
+			return r + 'Α' - 'α'
+		}
+		return r
+	}
+
+	b.Run("change", func(b *testing.B) {
+		for _, md := range mapdata {
+			b.Run(md.name, func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					Map(mapchange, md.data)
+				}
+			})
+		}
+	})
+}
diff --git a/src/strings/search.go b/src/strings/search.go
new file mode 100644
index 0000000..e1ace3e
--- /dev/null
+++ b/src/strings/search.go
@@ -0,0 +1,117 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+// stringFinder efficiently finds strings in a source text. It's implemented
+// using the Boyer-Moore string search algorithm:
+// https://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm
+// https://www.cs.utexas.edu/~moore/publications/fstrpos.pdf (note: this aged
+// document uses 1-based indexing)
+type stringFinder struct {
+	// pattern is the string that we are searching for in the text.
+	pattern string
+
+	// badCharSkip[b] contains the distance between the last byte of pattern
+	// and the rightmost occurrence of b in pattern. If b is not in pattern,
+	// badCharSkip[b] is len(pattern).
+	//
+	// Whenever a mismatch is found with byte b in the text, we can safely
+	// shift the matching frame at least badCharSkip[b] until the next time
+	// the matching char could be in alignment.
+	badCharSkip [256]int
+
+	// goodSuffixSkip[i] defines how far we can shift the matching frame given
+	// that the suffix pattern[i+1:] matches, but the byte pattern[i] does
+	// not. There are two cases to consider:
+	//
+	// 1. The matched suffix occurs elsewhere in pattern (with a different
+	// byte preceding it that we might possibly match). In this case, we can
+	// shift the matching frame to align with the next suffix chunk. For
+	// example, the pattern "mississi" has the suffix "issi" next occurring
+	// (in right-to-left order) at index 1, so goodSuffixSkip[3] ==
+	// shift+len(suffix) == 3+4 == 7.
+	//
+	// 2. If the matched suffix does not occur elsewhere in pattern, then the
+	// matching frame may share part of its prefix with the end of the
+	// matching suffix. In this case, goodSuffixSkip[i] will contain how far
+	// to shift the frame to align this portion of the prefix to the
+	// suffix. For example, in the pattern "abcxxxabc", when the first
+	// mismatch from the back is found to be in position 3, the matching
+	// suffix "xxabc" is not found elsewhere in the pattern. However, its
+	// rightmost "abc" (at position 6) is a prefix of the whole pattern, so
+	// goodSuffixSkip[3] == shift+len(suffix) == 6+5 == 11.
+	goodSuffixSkip []int
+}
+
+func makeStringFinder(pattern string) *stringFinder {
+	f := &stringFinder{
+		pattern:        pattern,
+		goodSuffixSkip: make([]int, len(pattern)),
+	}
+	// last is the index of the last character in the pattern.
+	last := len(pattern) - 1
+
+	// Build bad character table.
+	// Bytes not in the pattern can skip one pattern's length.
+	for i := range f.badCharSkip {
+		f.badCharSkip[i] = len(pattern)
+	}
+	// The loop condition is < instead of <= so that the last byte does not
+	// have a zero distance to itself. Finding this byte out of place implies
+	// that it is not in the last position.
+	for i := 0; i < last; i++ {
+		f.badCharSkip[pattern[i]] = last - i
+	}
+
+	// Build good suffix table.
+	// First pass: set each value to the next index which starts a prefix of
+	// pattern.
+	lastPrefix := last
+	for i := last; i >= 0; i-- {
+		if HasPrefix(pattern, pattern[i+1:]) {
+			lastPrefix = i + 1
+		}
+		// lastPrefix is the shift, and (last-i) is len(suffix).
+		f.goodSuffixSkip[i] = lastPrefix + last - i
+	}
+	// Second pass: find repeats of pattern's suffix starting from the front.
+	for i := 0; i < last; i++ {
+		lenSuffix := longestCommonSuffix(pattern, pattern[1:i+1])
+		if pattern[i-lenSuffix] != pattern[last-lenSuffix] {
+			// (last-i) is the shift, and lenSuffix is len(suffix).
+			f.goodSuffixSkip[last-lenSuffix] = lenSuffix + last - i
+		}
+	}
+
+	return f
+}
+
+func longestCommonSuffix(a, b string) (i int) {
+	for ; i < len(a) && i < len(b); i++ {
+		if a[len(a)-1-i] != b[len(b)-1-i] {
+			break
+		}
+	}
+	return
+}
+
+// next returns the index in text of the first occurrence of the pattern. If
+// the pattern is not found, it returns -1.
+func (f *stringFinder) next(text string) int {
+	i := len(f.pattern) - 1
+	for i < len(text) {
+		// Compare backwards from the end until the first unmatching character.
+		j := len(f.pattern) - 1
+		for j >= 0 && text[i] == f.pattern[j] {
+			i--
+			j--
+		}
+		if j < 0 {
+			return i + 1 // match
+		}
+		i += max(f.badCharSkip[text[i]], f.goodSuffixSkip[j])
+	}
+	return -1
+}
diff --git a/src/strings/search_test.go b/src/strings/search_test.go
new file mode 100644
index 0000000..c01a393
--- /dev/null
+++ b/src/strings/search_test.go
@@ -0,0 +1,90 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	"reflect"
+	. "strings"
+	"testing"
+)
+
+func TestFinderNext(t *testing.T) {
+	testCases := []struct {
+		pat, text string
+		index     int
+	}{
+		{"", "", 0},
+		{"", "abc", 0},
+		{"abc", "", -1},
+		{"abc", "abc", 0},
+		{"d", "abcdefg", 3},
+		{"nan", "banana", 2},
+		{"pan", "anpanman", 2},
+		{"nnaaman", "anpanmanam", -1},
+		{"abcd", "abc", -1},
+		{"abcd", "bcd", -1},
+		{"bcd", "abcd", 1},
+		{"abc", "acca", -1},
+		{"aa", "aaa", 0},
+		{"baa", "aaaaa", -1},
+		{"at that", "which finally halts.  at that point", 22},
+	}
+
+	for _, tc := range testCases {
+		got := StringFind(tc.pat, tc.text)
+		want := tc.index
+		if got != want {
+			t.Errorf("stringFind(%q, %q) got %d, want %d\n", tc.pat, tc.text, got, want)
+		}
+	}
+}
+
+func TestFinderCreation(t *testing.T) {
+	testCases := []struct {
+		pattern string
+		bad     [256]int
+		suf     []int
+	}{
+		{
+			"abc",
+			[256]int{'a': 2, 'b': 1, 'c': 3},
+			[]int{5, 4, 1},
+		},
+		{
+			"mississi",
+			[256]int{'i': 3, 'm': 7, 's': 1},
+			[]int{15, 14, 13, 7, 11, 10, 7, 1},
+		},
+		// From https://www.cs.utexas.edu/~moore/publications/fstrpos.pdf
+		{
+			"abcxxxabc",
+			[256]int{'a': 2, 'b': 1, 'c': 6, 'x': 3},
+			[]int{14, 13, 12, 11, 10, 9, 11, 10, 1},
+		},
+		{
+			"abyxcdeyx",
+			[256]int{'a': 8, 'b': 7, 'c': 4, 'd': 3, 'e': 2, 'y': 1, 'x': 5},
+			[]int{17, 16, 15, 14, 13, 12, 7, 10, 1},
+		},
+	}
+
+	for _, tc := range testCases {
+		bad, good := DumpTables(tc.pattern)
+
+		for i, got := range bad {
+			want := tc.bad[i]
+			if want == 0 {
+				want = len(tc.pattern)
+			}
+			if got != want {
+				t.Errorf("boyerMoore(%q) bad['%c']: got %d want %d", tc.pattern, i, got, want)
+			}
+		}
+
+		if !reflect.DeepEqual(good, tc.suf) {
+			t.Errorf("boyerMoore(%q) got %v want %v", tc.pattern, good, tc.suf)
+		}
+	}
+}
diff --git a/src/strings/strings.go b/src/strings/strings.go
new file mode 100644
index 0000000..f3f0723
--- /dev/null
+++ b/src/strings/strings.go
@@ -0,0 +1,1300 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package strings implements simple functions to manipulate UTF-8 encoded strings.
+//
+// For information about UTF-8 strings in Go, see https://blog.golang.org/strings.
+package strings
+
+import (
+	"internal/bytealg"
+	"unicode"
+	"unicode/utf8"
+)
+
+const maxInt = int(^uint(0) >> 1)
+
+// explode splits s into a slice of UTF-8 strings,
+// one string per Unicode character up to a maximum of n (n < 0 means no limit).
+// Invalid UTF-8 bytes are sliced individually.
+func explode(s string, n int) []string {
+	l := utf8.RuneCountInString(s)
+	if n < 0 || n > l {
+		n = l
+	}
+	a := make([]string, n)
+	for i := 0; i < n-1; i++ {
+		_, size := utf8.DecodeRuneInString(s)
+		a[i] = s[:size]
+		s = s[size:]
+	}
+	if n > 0 {
+		a[n-1] = s
+	}
+	return a
+}
+
+// Count counts the number of non-overlapping instances of substr in s.
+// If substr is an empty string, Count returns 1 + the number of Unicode code points in s.
+func Count(s, substr string) int {
+	// special case
+	if len(substr) == 0 {
+		return utf8.RuneCountInString(s) + 1
+	}
+	if len(substr) == 1 {
+		return bytealg.CountString(s, substr[0])
+	}
+	n := 0
+	for {
+		i := Index(s, substr)
+		if i == -1 {
+			return n
+		}
+		n++
+		s = s[i+len(substr):]
+	}
+}
+
+// Contains reports whether substr is within s.
+func Contains(s, substr string) bool {
+	return Index(s, substr) >= 0
+}
+
+// ContainsAny reports whether any Unicode code points in chars are within s.
+func ContainsAny(s, chars string) bool {
+	return IndexAny(s, chars) >= 0
+}
+
+// ContainsRune reports whether the Unicode code point r is within s.
+func ContainsRune(s string, r rune) bool {
+	return IndexRune(s, r) >= 0
+}
+
+// ContainsFunc reports whether any Unicode code points r within s satisfy f(r).
+func ContainsFunc(s string, f func(rune) bool) bool {
+	return IndexFunc(s, f) >= 0
+}
+
+// LastIndex returns the index of the last instance of substr in s, or -1 if substr is not present in s.
+func LastIndex(s, substr string) int {
+	n := len(substr)
+	switch {
+	case n == 0:
+		return len(s)
+	case n == 1:
+		return bytealg.LastIndexByteString(s, substr[0])
+	case n == len(s):
+		if substr == s {
+			return 0
+		}
+		return -1
+	case n > len(s):
+		return -1
+	}
+	// Rabin-Karp search from the end of the string
+	hashss, pow := bytealg.HashStrRev(substr)
+	last := len(s) - n
+	var h uint32
+	for i := len(s) - 1; i >= last; i-- {
+		h = h*bytealg.PrimeRK + uint32(s[i])
+	}
+	if h == hashss && s[last:] == substr {
+		return last
+	}
+	for i := last - 1; i >= 0; i-- {
+		h *= bytealg.PrimeRK
+		h += uint32(s[i])
+		h -= pow * uint32(s[i+n])
+		if h == hashss && s[i:i+n] == substr {
+			return i
+		}
+	}
+	return -1
+}
+
+// IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
+func IndexByte(s string, c byte) int {
+	return bytealg.IndexByteString(s, c)
+}
+
+// IndexRune returns the index of the first instance of the Unicode code point
+// r, or -1 if rune is not present in s.
+// If r is utf8.RuneError, it returns the first instance of any
+// invalid UTF-8 byte sequence.
+func IndexRune(s string, r rune) int {
+	switch {
+	case 0 <= r && r < utf8.RuneSelf:
+		return IndexByte(s, byte(r))
+	case r == utf8.RuneError:
+		for i, r := range s {
+			if r == utf8.RuneError {
+				return i
+			}
+		}
+		return -1
+	case !utf8.ValidRune(r):
+		return -1
+	default:
+		return Index(s, string(r))
+	}
+}
+
+// IndexAny returns the index of the first instance of any Unicode code point
+// from chars in s, or -1 if no Unicode code point from chars is present in s.
+func IndexAny(s, chars string) int {
+	if chars == "" {
+		// Avoid scanning all of s.
+		return -1
+	}
+	if len(chars) == 1 {
+		// Avoid scanning all of s.
+		r := rune(chars[0])
+		if r >= utf8.RuneSelf {
+			r = utf8.RuneError
+		}
+		return IndexRune(s, r)
+	}
+	if len(s) > 8 {
+		if as, isASCII := makeASCIISet(chars); isASCII {
+			for i := 0; i < len(s); i++ {
+				if as.contains(s[i]) {
+					return i
+				}
+			}
+			return -1
+		}
+	}
+	for i, c := range s {
+		if IndexRune(chars, c) >= 0 {
+			return i
+		}
+	}
+	return -1
+}
+
+// LastIndexAny returns the index of the last instance of any Unicode code
+// point from chars in s, or -1 if no Unicode code point from chars is
+// present in s.
+func LastIndexAny(s, chars string) int {
+	if chars == "" {
+		// Avoid scanning all of s.
+		return -1
+	}
+	if len(s) == 1 {
+		rc := rune(s[0])
+		if rc >= utf8.RuneSelf {
+			rc = utf8.RuneError
+		}
+		if IndexRune(chars, rc) >= 0 {
+			return 0
+		}
+		return -1
+	}
+	if len(s) > 8 {
+		if as, isASCII := makeASCIISet(chars); isASCII {
+			for i := len(s) - 1; i >= 0; i-- {
+				if as.contains(s[i]) {
+					return i
+				}
+			}
+			return -1
+		}
+	}
+	if len(chars) == 1 {
+		rc := rune(chars[0])
+		if rc >= utf8.RuneSelf {
+			rc = utf8.RuneError
+		}
+		for i := len(s); i > 0; {
+			r, size := utf8.DecodeLastRuneInString(s[:i])
+			i -= size
+			if rc == r {
+				return i
+			}
+		}
+		return -1
+	}
+	for i := len(s); i > 0; {
+		r, size := utf8.DecodeLastRuneInString(s[:i])
+		i -= size
+		if IndexRune(chars, r) >= 0 {
+			return i
+		}
+	}
+	return -1
+}
+
+// LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
+func LastIndexByte(s string, c byte) int {
+	return bytealg.LastIndexByteString(s, c)
+}
+
+// Generic split: splits after each instance of sep,
+// including sepSave bytes of sep in the subarrays.
+func genSplit(s, sep string, sepSave, n int) []string {
+	if n == 0 {
+		return nil
+	}
+	if sep == "" {
+		return explode(s, n)
+	}
+	if n < 0 {
+		n = Count(s, sep) + 1
+	}
+
+	if n > len(s)+1 {
+		n = len(s) + 1
+	}
+	a := make([]string, n)
+	n--
+	i := 0
+	for i < n {
+		m := Index(s, sep)
+		if m < 0 {
+			break
+		}
+		a[i] = s[:m+sepSave]
+		s = s[m+len(sep):]
+		i++
+	}
+	a[i] = s
+	return a[:i+1]
+}
+
+// SplitN slices s into substrings separated by sep and returns a slice of
+// the substrings between those separators.
+//
+// The count determines the number of substrings to return:
+//
+//	n > 0: at most n substrings; the last substring will be the unsplit remainder.
+//	n == 0: the result is nil (zero substrings)
+//	n < 0: all substrings
+//
+// Edge cases for s and sep (for example, empty strings) are handled
+// as described in the documentation for [Split].
+//
+// To split around the first instance of a separator, see Cut.
+func SplitN(s, sep string, n int) []string { return genSplit(s, sep, 0, n) }
+
+// SplitAfterN slices s into substrings after each instance of sep and
+// returns a slice of those substrings.
+//
+// The count determines the number of substrings to return:
+//
+//	n > 0: at most n substrings; the last substring will be the unsplit remainder.
+//	n == 0: the result is nil (zero substrings)
+//	n < 0: all substrings
+//
+// Edge cases for s and sep (for example, empty strings) are handled
+// as described in the documentation for SplitAfter.
+func SplitAfterN(s, sep string, n int) []string {
+	return genSplit(s, sep, len(sep), n)
+}
+
+// Split slices s into all substrings separated by sep and returns a slice of
+// the substrings between those separators.
+//
+// If s does not contain sep and sep is not empty, Split returns a
+// slice of length 1 whose only element is s.
+//
+// If sep is empty, Split splits after each UTF-8 sequence. If both s
+// and sep are empty, Split returns an empty slice.
+//
+// It is equivalent to [SplitN] with a count of -1.
+//
+// To split around the first instance of a separator, see Cut.
+func Split(s, sep string) []string { return genSplit(s, sep, 0, -1) }
+
+// SplitAfter slices s into all substrings after each instance of sep and
+// returns a slice of those substrings.
+//
+// If s does not contain sep and sep is not empty, SplitAfter returns
+// a slice of length 1 whose only element is s.
+//
+// If sep is empty, SplitAfter splits after each UTF-8 sequence. If
+// both s and sep are empty, SplitAfter returns an empty slice.
+//
+// It is equivalent to [SplitAfterN] with a count of -1.
+func SplitAfter(s, sep string) []string {
+	return genSplit(s, sep, len(sep), -1)
+}
+
+var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
+
+// Fields splits the string s around each instance of one or more consecutive white space
+// characters, as defined by unicode.IsSpace, returning a slice of substrings of s or an
+// empty slice if s contains only white space.
+func Fields(s string) []string {
+	// First count the fields.
+	// This is an exact count if s is ASCII, otherwise it is an approximation.
+	n := 0
+	wasSpace := 1
+	// setBits is used to track which bits are set in the bytes of s.
+	setBits := uint8(0)
+	for i := 0; i < len(s); i++ {
+		r := s[i]
+		setBits |= r
+		isSpace := int(asciiSpace[r])
+		n += wasSpace & ^isSpace
+		wasSpace = isSpace
+	}
+
+	if setBits >= utf8.RuneSelf {
+		// Some runes in the input string are not ASCII.
+		return FieldsFunc(s, unicode.IsSpace)
+	}
+	// ASCII fast path
+	a := make([]string, n)
+	na := 0
+	fieldStart := 0
+	i := 0
+	// Skip spaces in the front of the input.
+	for i < len(s) && asciiSpace[s[i]] != 0 {
+		i++
+	}
+	fieldStart = i
+	for i < len(s) {
+		if asciiSpace[s[i]] == 0 {
+			i++
+			continue
+		}
+		a[na] = s[fieldStart:i]
+		na++
+		i++
+		// Skip spaces in between fields.
+		for i < len(s) && asciiSpace[s[i]] != 0 {
+			i++
+		}
+		fieldStart = i
+	}
+	if fieldStart < len(s) { // Last field might end at EOF.
+		a[na] = s[fieldStart:]
+	}
+	return a
+}
+
+// FieldsFunc splits the string s at each run of Unicode code points c satisfying f(c)
+// and returns an array of slices of s. If all code points in s satisfy f(c) or the
+// string is empty, an empty slice is returned.
+//
+// FieldsFunc makes no guarantees about the order in which it calls f(c)
+// and assumes that f always returns the same value for a given c.
+func FieldsFunc(s string, f func(rune) bool) []string {
+	// A span is used to record a slice of s of the form s[start:end].
+	// The start index is inclusive and the end index is exclusive.
+	type span struct {
+		start int
+		end   int
+	}
+	spans := make([]span, 0, 32)
+
+	// Find the field start and end indices.
+	// Doing this in a separate pass (rather than slicing the string s
+	// and collecting the result substrings right away) is significantly
+	// more efficient, possibly due to cache effects.
+	start := -1 // valid span start if >= 0
+	for end, rune := range s {
+		if f(rune) {
+			if start >= 0 {
+				spans = append(spans, span{start, end})
+				// Set start to a negative value.
+				// Note: using -1 here consistently and reproducibly
+				// slows down this code by a several percent on amd64.
+				start = ^start
+			}
+		} else {
+			if start < 0 {
+				start = end
+			}
+		}
+	}
+
+	// Last field might end at EOF.
+	if start >= 0 {
+		spans = append(spans, span{start, len(s)})
+	}
+
+	// Create strings from recorded field indices.
+	a := make([]string, len(spans))
+	for i, span := range spans {
+		a[i] = s[span.start:span.end]
+	}
+
+	return a
+}
+
+// Join concatenates the elements of its first argument to create a single string. The separator
+// string sep is placed between elements in the resulting string.
+func Join(elems []string, sep string) string {
+	switch len(elems) {
+	case 0:
+		return ""
+	case 1:
+		return elems[0]
+	}
+
+	var n int
+	if len(sep) > 0 {
+		if len(sep) >= maxInt/(len(elems)-1) {
+			panic("strings: Join output length overflow")
+		}
+		n += len(sep) * (len(elems) - 1)
+	}
+	for _, elem := range elems {
+		if len(elem) > maxInt-n {
+			panic("strings: Join output length overflow")
+		}
+		n += len(elem)
+	}
+
+	var b Builder
+	b.Grow(n)
+	b.WriteString(elems[0])
+	for _, s := range elems[1:] {
+		b.WriteString(sep)
+		b.WriteString(s)
+	}
+	return b.String()
+}
+
+// HasPrefix reports whether the string s begins with prefix.
+func HasPrefix(s, prefix string) bool {
+	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
+}
+
+// HasSuffix reports whether the string s ends with suffix.
+func HasSuffix(s, suffix string) bool {
+	return len(s) >= len(suffix) && s[len(s)-len(suffix):] == suffix
+}
+
+// Map returns a copy of the string s with all its characters modified
+// according to the mapping function. If mapping returns a negative value, the character is
+// dropped from the string with no replacement.
+func Map(mapping func(rune) rune, s string) string {
+	// In the worst case, the string can grow when mapped, making
+	// things unpleasant. But it's so rare we barge in assuming it's
+	// fine. It could also shrink but that falls out naturally.
+
+	// The output buffer b is initialized on demand, the first
+	// time a character differs.
+	var b Builder
+
+	for i, c := range s {
+		r := mapping(c)
+		if r == c && c != utf8.RuneError {
+			continue
+		}
+
+		var width int
+		if c == utf8.RuneError {
+			c, width = utf8.DecodeRuneInString(s[i:])
+			if width != 1 && r == c {
+				continue
+			}
+		} else {
+			width = utf8.RuneLen(c)
+		}
+
+		b.Grow(len(s) + utf8.UTFMax)
+		b.WriteString(s[:i])
+		if r >= 0 {
+			b.WriteRune(r)
+		}
+
+		s = s[i+width:]
+		break
+	}
+
+	// Fast path for unchanged input
+	if b.Cap() == 0 { // didn't call b.Grow above
+		return s
+	}
+
+	for _, c := range s {
+		r := mapping(c)
+
+		if r >= 0 {
+			// common case
+			// Due to inlining, it is more performant to determine if WriteByte should be
+			// invoked rather than always call WriteRune
+			if r < utf8.RuneSelf {
+				b.WriteByte(byte(r))
+			} else {
+				// r is not an ASCII rune.
+				b.WriteRune(r)
+			}
+		}
+	}
+
+	return b.String()
+}
+
+// Repeat returns a new string consisting of count copies of the string s.
+//
+// It panics if count is negative or if the result of (len(s) * count)
+// overflows.
+func Repeat(s string, count int) string {
+	switch count {
+	case 0:
+		return ""
+	case 1:
+		return s
+	}
+
+	// Since we cannot return an error on overflow,
+	// we should panic if the repeat will generate an overflow.
+	// See golang.org/issue/16237.
+	if count < 0 {
+		panic("strings: negative Repeat count")
+	}
+	if len(s) >= maxInt/count {
+		panic("strings: Repeat output length overflow")
+	}
+	n := len(s) * count
+
+	if len(s) == 0 {
+		return ""
+	}
+
+	// Past a certain chunk size it is counterproductive to use
+	// larger chunks as the source of the write, as when the source
+	// is too large we are basically just thrashing the CPU D-cache.
+	// So if the result length is larger than an empirically-found
+	// limit (8KB), we stop growing the source string once the limit
+	// is reached and keep reusing the same source string - that
+	// should therefore be always resident in the L1 cache - until we
+	// have completed the construction of the result.
+	// This yields significant speedups (up to +100%) in cases where
+	// the result length is large (roughly, over L2 cache size).
+	const chunkLimit = 8 * 1024
+	chunkMax := n
+	if n > chunkLimit {
+		chunkMax = chunkLimit / len(s) * len(s)
+		if chunkMax == 0 {
+			chunkMax = len(s)
+		}
+	}
+
+	var b Builder
+	b.Grow(n)
+	b.WriteString(s)
+	for b.Len() < n {
+		chunk := n - b.Len()
+		if chunk > b.Len() {
+			chunk = b.Len()
+		}
+		if chunk > chunkMax {
+			chunk = chunkMax
+		}
+		b.WriteString(b.String()[:chunk])
+	}
+	return b.String()
+}
+
+// ToUpper returns s with all Unicode letters mapped to their upper case.
+func ToUpper(s string) string {
+	isASCII, hasLower := true, false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			isASCII = false
+			break
+		}
+		hasLower = hasLower || ('a' <= c && c <= 'z')
+	}
+
+	if isASCII { // optimize for ASCII-only strings.
+		if !hasLower {
+			return s
+		}
+		var (
+			b   Builder
+			pos int
+		)
+		b.Grow(len(s))
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if 'a' <= c && c <= 'z' {
+				c -= 'a' - 'A'
+				if pos < i {
+					b.WriteString(s[pos:i])
+				}
+				b.WriteByte(c)
+				pos = i + 1
+			}
+		}
+		if pos < len(s) {
+			b.WriteString(s[pos:])
+		}
+		return b.String()
+	}
+	return Map(unicode.ToUpper, s)
+}
+
+// ToLower returns s with all Unicode letters mapped to their lower case.
+func ToLower(s string) string {
+	isASCII, hasUpper := true, false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			isASCII = false
+			break
+		}
+		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
+	}
+
+	if isASCII { // optimize for ASCII-only strings.
+		if !hasUpper {
+			return s
+		}
+		var (
+			b   Builder
+			pos int
+		)
+		b.Grow(len(s))
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if 'A' <= c && c <= 'Z' {
+				c += 'a' - 'A'
+				if pos < i {
+					b.WriteString(s[pos:i])
+				}
+				b.WriteByte(c)
+				pos = i + 1
+			}
+		}
+		if pos < len(s) {
+			b.WriteString(s[pos:])
+		}
+		return b.String()
+	}
+	return Map(unicode.ToLower, s)
+}
+
+// ToTitle returns a copy of the string s with all Unicode letters mapped to
+// their Unicode title case.
+func ToTitle(s string) string { return Map(unicode.ToTitle, s) }
+
+// ToUpperSpecial returns a copy of the string s with all Unicode letters mapped to their
+// upper case using the case mapping specified by c.
+func ToUpperSpecial(c unicode.SpecialCase, s string) string {
+	return Map(c.ToUpper, s)
+}
+
+// ToLowerSpecial returns a copy of the string s with all Unicode letters mapped to their
+// lower case using the case mapping specified by c.
+func ToLowerSpecial(c unicode.SpecialCase, s string) string {
+	return Map(c.ToLower, s)
+}
+
+// ToTitleSpecial returns a copy of the string s with all Unicode letters mapped to their
+// Unicode title case, giving priority to the special casing rules.
+func ToTitleSpecial(c unicode.SpecialCase, s string) string {
+	return Map(c.ToTitle, s)
+}
+
+// ToValidUTF8 returns a copy of the string s with each run of invalid UTF-8 byte sequences
+// replaced by the replacement string, which may be empty.
+func ToValidUTF8(s, replacement string) string {
+	var b Builder
+
+	for i, c := range s {
+		if c != utf8.RuneError {
+			continue
+		}
+
+		_, wid := utf8.DecodeRuneInString(s[i:])
+		if wid == 1 {
+			b.Grow(len(s) + len(replacement))
+			b.WriteString(s[:i])
+			s = s[i:]
+			break
+		}
+	}
+
+	// Fast path for unchanged input
+	if b.Cap() == 0 { // didn't call b.Grow above
+		return s
+	}
+
+	invalid := false // previous byte was from an invalid UTF-8 sequence
+	for i := 0; i < len(s); {
+		c := s[i]
+		if c < utf8.RuneSelf {
+			i++
+			invalid = false
+			b.WriteByte(c)
+			continue
+		}
+		_, wid := utf8.DecodeRuneInString(s[i:])
+		if wid == 1 {
+			i++
+			if !invalid {
+				invalid = true
+				b.WriteString(replacement)
+			}
+			continue
+		}
+		invalid = false
+		b.WriteString(s[i : i+wid])
+		i += wid
+	}
+
+	return b.String()
+}
+
+// isSeparator reports whether the rune could mark a word boundary.
+// TODO: update when package unicode captures more of the properties.
+func isSeparator(r rune) bool {
+	// ASCII alphanumerics and underscore are not separators
+	if r <= 0x7F {
+		switch {
+		case '0' <= r && r <= '9':
+			return false
+		case 'a' <= r && r <= 'z':
+			return false
+		case 'A' <= r && r <= 'Z':
+			return false
+		case r == '_':
+			return false
+		}
+		return true
+	}
+	// Letters and digits are not separators
+	if unicode.IsLetter(r) || unicode.IsDigit(r) {
+		return false
+	}
+	// Otherwise, all we can do for now is treat spaces as separators.
+	return unicode.IsSpace(r)
+}
+
+// Title returns a copy of the string s with all Unicode letters that begin words
+// mapped to their Unicode title case.
+//
+// Deprecated: The rule Title uses for word boundaries does not handle Unicode
+// punctuation properly. Use golang.org/x/text/cases instead.
+func Title(s string) string {
+	// Use a closure here to remember state.
+	// Hackish but effective. Depends on Map scanning in order and calling
+	// the closure once per rune.
+	prev := ' '
+	return Map(
+		func(r rune) rune {
+			if isSeparator(prev) {
+				prev = r
+				return unicode.ToTitle(r)
+			}
+			prev = r
+			return r
+		},
+		s)
+}
+
+// TrimLeftFunc returns a slice of the string s with all leading
+// Unicode code points c satisfying f(c) removed.
+func TrimLeftFunc(s string, f func(rune) bool) string {
+	i := indexFunc(s, f, false)
+	if i == -1 {
+		return ""
+	}
+	return s[i:]
+}
+
+// TrimRightFunc returns a slice of the string s with all trailing
+// Unicode code points c satisfying f(c) removed.
+func TrimRightFunc(s string, f func(rune) bool) string {
+	i := lastIndexFunc(s, f, false)
+	if i >= 0 && s[i] >= utf8.RuneSelf {
+		_, wid := utf8.DecodeRuneInString(s[i:])
+		i += wid
+	} else {
+		i++
+	}
+	return s[0:i]
+}
+
+// TrimFunc returns a slice of the string s with all leading
+// and trailing Unicode code points c satisfying f(c) removed.
+func TrimFunc(s string, f func(rune) bool) string {
+	return TrimRightFunc(TrimLeftFunc(s, f), f)
+}
+
+// IndexFunc returns the index into s of the first Unicode
+// code point satisfying f(c), or -1 if none do.
+func IndexFunc(s string, f func(rune) bool) int {
+	return indexFunc(s, f, true)
+}
+
+// LastIndexFunc returns the index into s of the last
+// Unicode code point satisfying f(c), or -1 if none do.
+func LastIndexFunc(s string, f func(rune) bool) int {
+	return lastIndexFunc(s, f, true)
+}
+
+// indexFunc is the same as IndexFunc except that if
+// truth==false, the sense of the predicate function is
+// inverted.
+func indexFunc(s string, f func(rune) bool, truth bool) int {
+	for i, r := range s {
+		if f(r) == truth {
+			return i
+		}
+	}
+	return -1
+}
+
+// lastIndexFunc is the same as LastIndexFunc except that if
+// truth==false, the sense of the predicate function is
+// inverted.
+func lastIndexFunc(s string, f func(rune) bool, truth bool) int {
+	for i := len(s); i > 0; {
+		r, size := utf8.DecodeLastRuneInString(s[0:i])
+		i -= size
+		if f(r) == truth {
+			return i
+		}
+	}
+	return -1
+}
+
+// asciiSet is a 32-byte value, where each bit represents the presence of a
+// given ASCII character in the set. The 128-bits of the lower 16 bytes,
+// starting with the least-significant bit of the lowest word to the
+// most-significant bit of the highest word, map to the full range of all
+// 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
+// ensuring that any non-ASCII character will be reported as not in the set.
+// This allocates a total of 32 bytes even though the upper half
+// is unused to avoid bounds checks in asciiSet.contains.
+type asciiSet [8]uint32
+
+// makeASCIISet creates a set of ASCII characters and reports whether all
+// characters in chars are ASCII.
+func makeASCIISet(chars string) (as asciiSet, ok bool) {
+	for i := 0; i < len(chars); i++ {
+		c := chars[i]
+		if c >= utf8.RuneSelf {
+			return as, false
+		}
+		as[c/32] |= 1 << (c % 32)
+	}
+	return as, true
+}
+
+// contains reports whether c is inside the set.
+func (as *asciiSet) contains(c byte) bool {
+	return (as[c/32] & (1 << (c % 32))) != 0
+}
+
+// Trim returns a slice of the string s with all leading and
+// trailing Unicode code points contained in cutset removed.
+func Trim(s, cutset string) string {
+	if s == "" || cutset == "" {
+		return s
+	}
+	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
+		return trimLeftByte(trimRightByte(s, cutset[0]), cutset[0])
+	}
+	if as, ok := makeASCIISet(cutset); ok {
+		return trimLeftASCII(trimRightASCII(s, &as), &as)
+	}
+	return trimLeftUnicode(trimRightUnicode(s, cutset), cutset)
+}
+
+// TrimLeft returns a slice of the string s with all leading
+// Unicode code points contained in cutset removed.
+//
+// To remove a prefix, use [TrimPrefix] instead.
+func TrimLeft(s, cutset string) string {
+	if s == "" || cutset == "" {
+		return s
+	}
+	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
+		return trimLeftByte(s, cutset[0])
+	}
+	if as, ok := makeASCIISet(cutset); ok {
+		return trimLeftASCII(s, &as)
+	}
+	return trimLeftUnicode(s, cutset)
+}
+
+func trimLeftByte(s string, c byte) string {
+	for len(s) > 0 && s[0] == c {
+		s = s[1:]
+	}
+	return s
+}
+
+func trimLeftASCII(s string, as *asciiSet) string {
+	for len(s) > 0 {
+		if !as.contains(s[0]) {
+			break
+		}
+		s = s[1:]
+	}
+	return s
+}
+
+func trimLeftUnicode(s, cutset string) string {
+	for len(s) > 0 {
+		r, n := rune(s[0]), 1
+		if r >= utf8.RuneSelf {
+			r, n = utf8.DecodeRuneInString(s)
+		}
+		if !ContainsRune(cutset, r) {
+			break
+		}
+		s = s[n:]
+	}
+	return s
+}
+
+// TrimRight returns a slice of the string s, with all trailing
+// Unicode code points contained in cutset removed.
+//
+// To remove a suffix, use [TrimSuffix] instead.
+func TrimRight(s, cutset string) string {
+	if s == "" || cutset == "" {
+		return s
+	}
+	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
+		return trimRightByte(s, cutset[0])
+	}
+	if as, ok := makeASCIISet(cutset); ok {
+		return trimRightASCII(s, &as)
+	}
+	return trimRightUnicode(s, cutset)
+}
+
+func trimRightByte(s string, c byte) string {
+	for len(s) > 0 && s[len(s)-1] == c {
+		s = s[:len(s)-1]
+	}
+	return s
+}
+
+func trimRightASCII(s string, as *asciiSet) string {
+	for len(s) > 0 {
+		if !as.contains(s[len(s)-1]) {
+			break
+		}
+		s = s[:len(s)-1]
+	}
+	return s
+}
+
+func trimRightUnicode(s, cutset string) string {
+	for len(s) > 0 {
+		r, n := rune(s[len(s)-1]), 1
+		if r >= utf8.RuneSelf {
+			r, n = utf8.DecodeLastRuneInString(s)
+		}
+		if !ContainsRune(cutset, r) {
+			break
+		}
+		s = s[:len(s)-n]
+	}
+	return s
+}
+
+// TrimSpace returns a slice of the string s, with all leading
+// and trailing white space removed, as defined by Unicode.
+func TrimSpace(s string) string {
+	// Fast path for ASCII: look for the first ASCII non-space byte
+	start := 0
+	for ; start < len(s); start++ {
+		c := s[start]
+		if c >= utf8.RuneSelf {
+			// If we run into a non-ASCII byte, fall back to the
+			// slower unicode-aware method on the remaining bytes
+			return TrimFunc(s[start:], unicode.IsSpace)
+		}
+		if asciiSpace[c] == 0 {
+			break
+		}
+	}
+
+	// Now look for the first ASCII non-space byte from the end
+	stop := len(s)
+	for ; stop > start; stop-- {
+		c := s[stop-1]
+		if c >= utf8.RuneSelf {
+			// start has been already trimmed above, should trim end only
+			return TrimRightFunc(s[start:stop], unicode.IsSpace)
+		}
+		if asciiSpace[c] == 0 {
+			break
+		}
+	}
+
+	// At this point s[start:stop] starts and ends with an ASCII
+	// non-space bytes, so we're done. Non-ASCII cases have already
+	// been handled above.
+	return s[start:stop]
+}
+
+// TrimPrefix returns s without the provided leading prefix string.
+// If s doesn't start with prefix, s is returned unchanged.
+func TrimPrefix(s, prefix string) string {
+	if HasPrefix(s, prefix) {
+		return s[len(prefix):]
+	}
+	return s
+}
+
+// TrimSuffix returns s without the provided trailing suffix string.
+// If s doesn't end with suffix, s is returned unchanged.
+func TrimSuffix(s, suffix string) string {
+	if HasSuffix(s, suffix) {
+		return s[:len(s)-len(suffix)]
+	}
+	return s
+}
+
+// Replace returns a copy of the string s with the first n
+// non-overlapping instances of old replaced by new.
+// If old is empty, it matches at the beginning of the string
+// and after each UTF-8 sequence, yielding up to k+1 replacements
+// for a k-rune string.
+// If n < 0, there is no limit on the number of replacements.
+func Replace(s, old, new string, n int) string {
+	if old == new || n == 0 {
+		return s // avoid allocation
+	}
+
+	// Compute number of replacements.
+	if m := Count(s, old); m == 0 {
+		return s // avoid allocation
+	} else if n < 0 || m < n {
+		n = m
+	}
+
+	// Apply replacements to buffer.
+	var b Builder
+	b.Grow(len(s) + n*(len(new)-len(old)))
+	start := 0
+	for i := 0; i < n; i++ {
+		j := start
+		if len(old) == 0 {
+			if i > 0 {
+				_, wid := utf8.DecodeRuneInString(s[start:])
+				j += wid
+			}
+		} else {
+			j += Index(s[start:], old)
+		}
+		b.WriteString(s[start:j])
+		b.WriteString(new)
+		start = j + len(old)
+	}
+	b.WriteString(s[start:])
+	return b.String()
+}
+
+// ReplaceAll returns a copy of the string s with all
+// non-overlapping instances of old replaced by new.
+// If old is empty, it matches at the beginning of the string
+// and after each UTF-8 sequence, yielding up to k+1 replacements
+// for a k-rune string.
+func ReplaceAll(s, old, new string) string {
+	return Replace(s, old, new, -1)
+}
+
+// EqualFold reports whether s and t, interpreted as UTF-8 strings,
+// are equal under simple Unicode case-folding, which is a more general
+// form of case-insensitivity.
+func EqualFold(s, t string) bool {
+	// ASCII fast path
+	i := 0
+	for ; i < len(s) && i < len(t); i++ {
+		sr := s[i]
+		tr := t[i]
+		if sr|tr >= utf8.RuneSelf {
+			goto hasUnicode
+		}
+
+		// Easy case.
+		if tr == sr {
+			continue
+		}
+
+		// Make sr < tr to simplify what follows.
+		if tr < sr {
+			tr, sr = sr, tr
+		}
+		// ASCII only, sr/tr must be upper/lower case
+		if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
+			continue
+		}
+		return false
+	}
+	// Check if we've exhausted both strings.
+	return len(s) == len(t)
+
+hasUnicode:
+	s = s[i:]
+	t = t[i:]
+	for _, sr := range s {
+		// If t is exhausted the strings are not equal.
+		if len(t) == 0 {
+			return false
+		}
+
+		// Extract first rune from second string.
+		var tr rune
+		if t[0] < utf8.RuneSelf {
+			tr, t = rune(t[0]), t[1:]
+		} else {
+			r, size := utf8.DecodeRuneInString(t)
+			tr, t = r, t[size:]
+		}
+
+		// If they match, keep going; if not, return false.
+
+		// Easy case.
+		if tr == sr {
+			continue
+		}
+
+		// Make sr < tr to simplify what follows.
+		if tr < sr {
+			tr, sr = sr, tr
+		}
+		// Fast check for ASCII.
+		if tr < utf8.RuneSelf {
+			// ASCII only, sr/tr must be upper/lower case
+			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
+				continue
+			}
+			return false
+		}
+
+		// General case. SimpleFold(x) returns the next equivalent rune > x
+		// or wraps around to smaller values.
+		r := unicode.SimpleFold(sr)
+		for r != sr && r < tr {
+			r = unicode.SimpleFold(r)
+		}
+		if r == tr {
+			continue
+		}
+		return false
+	}
+
+	// First string is empty, so check if the second one is also empty.
+	return len(t) == 0
+}
+
+// Index returns the index of the first instance of substr in s, or -1 if substr is not present in s.
+func Index(s, substr string) int {
+	n := len(substr)
+	switch {
+	case n == 0:
+		return 0
+	case n == 1:
+		return IndexByte(s, substr[0])
+	case n == len(s):
+		if substr == s {
+			return 0
+		}
+		return -1
+	case n > len(s):
+		return -1
+	case n <= bytealg.MaxLen:
+		// Use brute force when s and substr both are small
+		if len(s) <= bytealg.MaxBruteForce {
+			return bytealg.IndexString(s, substr)
+		}
+		c0 := substr[0]
+		c1 := substr[1]
+		i := 0
+		t := len(s) - n + 1
+		fails := 0
+		for i < t {
+			if s[i] != c0 {
+				// IndexByte is faster than bytealg.IndexString, so use it as long as
+				// we're not getting lots of false positives.
+				o := IndexByte(s[i+1:t], c0)
+				if o < 0 {
+					return -1
+				}
+				i += o + 1
+			}
+			if s[i+1] == c1 && s[i:i+n] == substr {
+				return i
+			}
+			fails++
+			i++
+			// Switch to bytealg.IndexString when IndexByte produces too many false positives.
+			if fails > bytealg.Cutover(i) {
+				r := bytealg.IndexString(s[i:], substr)
+				if r >= 0 {
+					return r + i
+				}
+				return -1
+			}
+		}
+		return -1
+	}
+	c0 := substr[0]
+	c1 := substr[1]
+	i := 0
+	t := len(s) - n + 1
+	fails := 0
+	for i < t {
+		if s[i] != c0 {
+			o := IndexByte(s[i+1:t], c0)
+			if o < 0 {
+				return -1
+			}
+			i += o + 1
+		}
+		if s[i+1] == c1 && s[i:i+n] == substr {
+			return i
+		}
+		i++
+		fails++
+		if fails >= 4+i>>4 && i < t {
+			// See comment in ../bytes/bytes.go.
+			j := bytealg.IndexRabinKarp(s[i:], substr)
+			if j < 0 {
+				return -1
+			}
+			return i + j
+		}
+	}
+	return -1
+}
+
+// Cut slices s around the first instance of sep,
+// returning the text before and after sep.
+// The found result reports whether sep appears in s.
+// If sep does not appear in s, cut returns s, "", false.
+func Cut(s, sep string) (before, after string, found bool) {
+	if i := Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, "", false
+}
+
+// CutPrefix returns s without the provided leading prefix string
+// and reports whether it found the prefix.
+// If s doesn't start with prefix, CutPrefix returns s, false.
+// If prefix is the empty string, CutPrefix returns s, true.
+func CutPrefix(s, prefix string) (after string, found bool) {
+	if !HasPrefix(s, prefix) {
+		return s, false
+	}
+	return s[len(prefix):], true
+}
+
+// CutSuffix returns s without the provided ending suffix string
+// and reports whether it found the suffix.
+// If s doesn't end with suffix, CutSuffix returns s, false.
+// If suffix is the empty string, CutSuffix returns s, true.
+func CutSuffix(s, suffix string) (before string, found bool) {
+	if !HasSuffix(s, suffix) {
+		return s, false
+	}
+	return s[:len(s)-len(suffix)], true
+}
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
new file mode 100644
index 0000000..f93cf68
--- /dev/null
+++ b/src/strings/strings_test.go
@@ -0,0 +1,2053 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings_test
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"math"
+	"math/rand"
+	"reflect"
+	"strconv"
+	. "strings"
+	"testing"
+	"unicode"
+	"unicode/utf8"
+	"unsafe"
+)
+
+func eq(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := 0; i < len(a); i++ {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+var abcd = "abcd"
+var faces = "☺☻☹"
+var commas = "1,2,3,4"
+var dots = "1....2....3....4"
+
+type IndexTest struct {
+	s   string
+	sep string
+	out int
+}
+
+var indexTests = []IndexTest{
+	{"", "", 0},
+	{"", "a", -1},
+	{"", "foo", -1},
+	{"fo", "foo", -1},
+	{"foo", "foo", 0},
+	{"oofofoofooo", "f", 2},
+	{"oofofoofooo", "foo", 4},
+	{"barfoobarfoo", "foo", 3},
+	{"foo", "", 0},
+	{"foo", "o", 1},
+	{"abcABCabc", "A", 3},
+	{"jrzm6jjhorimglljrea4w3rlgosts0w2gia17hno2td4qd1jz", "jz", 47},
+	{"ekkuk5oft4eq0ocpacknhwouic1uua46unx12l37nioq9wbpnocqks6", "ks6", 52},
+	{"999f2xmimunbuyew5vrkla9cpwhmxan8o98ec", "98ec", 33},
+	{"9lpt9r98i04k8bz6c6dsrthb96bhi", "96bhi", 24},
+	{"55u558eqfaod2r2gu42xxsu631xf0zobs5840vl", "5840vl", 33},
+	// cases with one byte strings - test special case in Index()
+	{"", "a", -1},
+	{"x", "a", -1},
+	{"x", "x", 0},
+	{"abc", "a", 0},
+	{"abc", "b", 1},
+	{"abc", "c", 2},
+	{"abc", "x", -1},
+	// test special cases in Index() for short strings
+	{"", "ab", -1},
+	{"bc", "ab", -1},
+	{"ab", "ab", 0},
+	{"xab", "ab", 1},
+	{"xab"[:2], "ab", -1},
+	{"", "abc", -1},
+	{"xbc", "abc", -1},
+	{"abc", "abc", 0},
+	{"xabc", "abc", 1},
+	{"xabc"[:3], "abc", -1},
+	{"xabxc", "abc", -1},
+	{"", "abcd", -1},
+	{"xbcd", "abcd", -1},
+	{"abcd", "abcd", 0},
+	{"xabcd", "abcd", 1},
+	{"xyabcd"[:5], "abcd", -1},
+	{"xbcqq", "abcqq", -1},
+	{"abcqq", "abcqq", 0},
+	{"xabcqq", "abcqq", 1},
+	{"xyabcqq"[:6], "abcqq", -1},
+	{"xabxcqq", "abcqq", -1},
+	{"xabcqxq", "abcqq", -1},
+	{"", "01234567", -1},
+	{"32145678", "01234567", -1},
+	{"01234567", "01234567", 0},
+	{"x01234567", "01234567", 1},
+	{"x0123456x01234567", "01234567", 9},
+	{"xx01234567"[:9], "01234567", -1},
+	{"", "0123456789", -1},
+	{"3214567844", "0123456789", -1},
+	{"0123456789", "0123456789", 0},
+	{"x0123456789", "0123456789", 1},
+	{"x012345678x0123456789", "0123456789", 11},
+	{"xyz0123456789"[:12], "0123456789", -1},
+	{"x01234567x89", "0123456789", -1},
+	{"", "0123456789012345", -1},
+	{"3214567889012345", "0123456789012345", -1},
+	{"0123456789012345", "0123456789012345", 0},
+	{"x0123456789012345", "0123456789012345", 1},
+	{"x012345678901234x0123456789012345", "0123456789012345", 17},
+	{"", "01234567890123456789", -1},
+	{"32145678890123456789", "01234567890123456789", -1},
+	{"01234567890123456789", "01234567890123456789", 0},
+	{"x01234567890123456789", "01234567890123456789", 1},
+	{"x0123456789012345678x01234567890123456789", "01234567890123456789", 21},
+	{"xyz01234567890123456789"[:22], "01234567890123456789", -1},
+	{"", "0123456789012345678901234567890", -1},
+	{"321456788901234567890123456789012345678911", "0123456789012345678901234567890", -1},
+	{"0123456789012345678901234567890", "0123456789012345678901234567890", 0},
+	{"x0123456789012345678901234567890", "0123456789012345678901234567890", 1},
+	{"x012345678901234567890123456789x0123456789012345678901234567890", "0123456789012345678901234567890", 32},
+	{"xyz0123456789012345678901234567890"[:33], "0123456789012345678901234567890", -1},
+	{"", "01234567890123456789012345678901", -1},
+	{"32145678890123456789012345678901234567890211", "01234567890123456789012345678901", -1},
+	{"01234567890123456789012345678901", "01234567890123456789012345678901", 0},
+	{"x01234567890123456789012345678901", "01234567890123456789012345678901", 1},
+	{"x0123456789012345678901234567890x01234567890123456789012345678901", "01234567890123456789012345678901", 33},
+	{"xyz01234567890123456789012345678901"[:34], "01234567890123456789012345678901", -1},
+	{"xxxxxx012345678901234567890123456789012345678901234567890123456789012", "012345678901234567890123456789012345678901234567890123456789012", 6},
+	{"", "0123456789012345678901234567890123456789", -1},
+	{"xx012345678901234567890123456789012345678901234567890123456789012", "0123456789012345678901234567890123456789", 2},
+	{"xx012345678901234567890123456789012345678901234567890123456789012"[:41], "0123456789012345678901234567890123456789", -1},
+	{"xx012345678901234567890123456789012345678901234567890123456789012", "0123456789012345678901234567890123456xxx", -1},
+	{"xx0123456789012345678901234567890123456789012345678901234567890120123456789012345678901234567890123456xxx", "0123456789012345678901234567890123456xxx", 65},
+	// test fallback to Rabin-Karp.
+	{"oxoxoxoxoxoxoxoxoxoxoxoy", "oy", 22},
+	{"oxoxoxoxoxoxoxoxoxoxoxox", "oy", -1},
+}
+
+var lastIndexTests = []IndexTest{
+	{"", "", 0},
+	{"", "a", -1},
+	{"", "foo", -1},
+	{"fo", "foo", -1},
+	{"foo", "foo", 0},
+	{"foo", "f", 0},
+	{"oofofoofooo", "f", 7},
+	{"oofofoofooo", "foo", 7},
+	{"barfoobarfoo", "foo", 9},
+	{"foo", "", 3},
+	{"foo", "o", 2},
+	{"abcABCabc", "A", 3},
+	{"abcABCabc", "a", 6},
+}
+
+var indexAnyTests = []IndexTest{
+	{"", "", -1},
+	{"", "a", -1},
+	{"", "abc", -1},
+	{"a", "", -1},
+	{"a", "a", 0},
+	{"\x80", "\xffb", 0},
+	{"aaa", "a", 0},
+	{"abc", "xyz", -1},
+	{"abc", "xcz", 2},
+	{"ab☺c", "x☺yz", 2},
+	{"a☺b☻c☹d", "cx", len("a☺b☻")},
+	{"a☺b☻c☹d", "uvw☻xyz", len("a☺b")},
+	{"aRegExp*", ".(|)*+?^$[]", 7},
+	{dots + dots + dots, " ", -1},
+	{"012abcba210", "\xffb", 4},
+	{"012\x80bcb\x80210", "\xffb", 3},
+	{"0123456\xcf\x80abc", "\xcfb\x80", 10},
+}
+
+var lastIndexAnyTests = []IndexTest{
+	{"", "", -1},
+	{"", "a", -1},
+	{"", "abc", -1},
+	{"a", "", -1},
+	{"a", "a", 0},
+	{"\x80", "\xffb", 0},
+	{"aaa", "a", 2},
+	{"abc", "xyz", -1},
+	{"abc", "ab", 1},
+	{"ab☺c", "x☺yz", 2},
+	{"a☺b☻c☹d", "cx", len("a☺b☻")},
+	{"a☺b☻c☹d", "uvw☻xyz", len("a☺b")},
+	{"a.RegExp*", ".(|)*+?^$[]", 8},
+	{dots + dots + dots, " ", -1},
+	{"012abcba210", "\xffb", 6},
+	{"012\x80bcb\x80210", "\xffb", 7},
+	{"0123456\xcf\x80abc", "\xcfb\x80", 10},
+}
+
+// Execute f on each test case.  funcName should be the name of f; it's used
+// in failure reports.
+func runIndexTests(t *testing.T, f func(s, sep string) int, funcName string, testCases []IndexTest) {
+	for _, test := range testCases {
+		actual := f(test.s, test.sep)
+		if actual != test.out {
+			t.Errorf("%s(%q,%q) = %v; want %v", funcName, test.s, test.sep, actual, test.out)
+		}
+	}
+}
+
+func TestIndex(t *testing.T)     { runIndexTests(t, Index, "Index", indexTests) }
+func TestLastIndex(t *testing.T) { runIndexTests(t, LastIndex, "LastIndex", lastIndexTests) }
+func TestIndexAny(t *testing.T)  { runIndexTests(t, IndexAny, "IndexAny", indexAnyTests) }
+func TestLastIndexAny(t *testing.T) {
+	runIndexTests(t, LastIndexAny, "LastIndexAny", lastIndexAnyTests)
+}
+
+func TestIndexByte(t *testing.T) {
+	for _, tt := range indexTests {
+		if len(tt.sep) != 1 {
+			continue
+		}
+		pos := IndexByte(tt.s, tt.sep[0])
+		if pos != tt.out {
+			t.Errorf(`IndexByte(%q, %q) = %v; want %v`, tt.s, tt.sep[0], pos, tt.out)
+		}
+	}
+}
+
+func TestLastIndexByte(t *testing.T) {
+	testCases := []IndexTest{
+		{"", "q", -1},
+		{"abcdef", "q", -1},
+		{"abcdefabcdef", "a", len("abcdef")},      // something in the middle
+		{"abcdefabcdef", "f", len("abcdefabcde")}, // last byte
+		{"zabcdefabcdef", "z", 0},                 // first byte
+		{"a☺b☻c☹d", "b", len("a☺")},               // non-ascii
+	}
+	for _, test := range testCases {
+		actual := LastIndexByte(test.s, test.sep[0])
+		if actual != test.out {
+			t.Errorf("LastIndexByte(%q,%c) = %v; want %v", test.s, test.sep[0], actual, test.out)
+		}
+	}
+}
+
+func simpleIndex(s, sep string) int {
+	n := len(sep)
+	for i := n; i <= len(s); i++ {
+		if s[i-n:i] == sep {
+			return i - n
+		}
+	}
+	return -1
+}
+
+func TestIndexRandom(t *testing.T) {
+	const chars = "abcdefghijklmnopqrstuvwxyz0123456789"
+	for times := 0; times < 10; times++ {
+		for strLen := 5 + rand.Intn(5); strLen < 140; strLen += 10 { // Arbitrary
+			s1 := make([]byte, strLen)
+			for i := range s1 {
+				s1[i] = chars[rand.Intn(len(chars))]
+			}
+			s := string(s1)
+			for i := 0; i < 50; i++ {
+				begin := rand.Intn(len(s) + 1)
+				end := begin + rand.Intn(len(s)+1-begin)
+				sep := s[begin:end]
+				if i%4 == 0 {
+					pos := rand.Intn(len(sep) + 1)
+					sep = sep[:pos] + "A" + sep[pos:]
+				}
+				want := simpleIndex(s, sep)
+				res := Index(s, sep)
+				if res != want {
+					t.Errorf("Index(%s,%s) = %d; want %d", s, sep, res, want)
+				}
+			}
+		}
+	}
+}
+
+func TestIndexRune(t *testing.T) {
+	tests := []struct {
+		in   string
+		rune rune
+		want int
+	}{
+		{"", 'a', -1},
+		{"", '☺', -1},
+		{"foo", '☹', -1},
+		{"foo", 'o', 1},
+		{"foo☺bar", '☺', 3},
+		{"foo☺☻☹bar", '☹', 9},
+		{"a A x", 'A', 2},
+		{"some_text=some_value", '=', 9},
+		{"☺a", 'a', 3},
+		{"a☻☺b", '☺', 4},
+
+		// RuneError should match any invalid UTF-8 byte sequence.
+		{"�", '�', 0},
+		{"\xff", '�', 0},
+		{"☻x�", '�', len("☻x")},
+		{"☻x\xe2\x98", '�', len("☻x")},
+		{"☻x\xe2\x98�", '�', len("☻x")},
+		{"☻x\xe2\x98x", '�', len("☻x")},
+
+		// Invalid rune values should never match.
+		{"a☺b☻c☹d\xe2\x98�\xff�\xed\xa0\x80", -1, -1},
+		{"a☺b☻c☹d\xe2\x98�\xff�\xed\xa0\x80", 0xD800, -1}, // Surrogate pair
+		{"a☺b☻c☹d\xe2\x98�\xff�\xed\xa0\x80", utf8.MaxRune + 1, -1},
+	}
+	for _, tt := range tests {
+		if got := IndexRune(tt.in, tt.rune); got != tt.want {
+			t.Errorf("IndexRune(%q, %d) = %v; want %v", tt.in, tt.rune, got, tt.want)
+		}
+	}
+
+	haystack := "test世界"
+	allocs := testing.AllocsPerRun(1000, func() {
+		if i := IndexRune(haystack, 's'); i != 2 {
+			t.Fatalf("'s' at %d; want 2", i)
+		}
+		if i := IndexRune(haystack, '世'); i != 4 {
+			t.Fatalf("'世' at %d; want 4", i)
+		}
+	})
+	if allocs != 0 && testing.CoverMode() == "" {
+		t.Errorf("expected no allocations, got %f", allocs)
+	}
+}
+
+const benchmarkString = "some_text=some☺value"
+
+func BenchmarkIndexRune(b *testing.B) {
+	if got := IndexRune(benchmarkString, '☺'); got != 14 {
+		b.Fatalf("wrong index: expected 14, got=%d", got)
+	}
+	for i := 0; i < b.N; i++ {
+		IndexRune(benchmarkString, '☺')
+	}
+}
+
+var benchmarkLongString = Repeat(" ", 100) + benchmarkString
+
+func BenchmarkIndexRuneLongString(b *testing.B) {
+	if got := IndexRune(benchmarkLongString, '☺'); got != 114 {
+		b.Fatalf("wrong index: expected 114, got=%d", got)
+	}
+	for i := 0; i < b.N; i++ {
+		IndexRune(benchmarkLongString, '☺')
+	}
+}
+
+func BenchmarkIndexRuneFastPath(b *testing.B) {
+	if got := IndexRune(benchmarkString, 'v'); got != 17 {
+		b.Fatalf("wrong index: expected 17, got=%d", got)
+	}
+	for i := 0; i < b.N; i++ {
+		IndexRune(benchmarkString, 'v')
+	}
+}
+
+func BenchmarkIndex(b *testing.B) {
+	if got := Index(benchmarkString, "v"); got != 17 {
+		b.Fatalf("wrong index: expected 17, got=%d", got)
+	}
+	for i := 0; i < b.N; i++ {
+		Index(benchmarkString, "v")
+	}
+}
+
+func BenchmarkLastIndex(b *testing.B) {
+	if got := Index(benchmarkString, "v"); got != 17 {
+		b.Fatalf("wrong index: expected 17, got=%d", got)
+	}
+	for i := 0; i < b.N; i++ {
+		LastIndex(benchmarkString, "v")
+	}
+}
+
+func BenchmarkIndexByte(b *testing.B) {
+	if got := IndexByte(benchmarkString, 'v'); got != 17 {
+		b.Fatalf("wrong index: expected 17, got=%d", got)
+	}
+	for i := 0; i < b.N; i++ {
+		IndexByte(benchmarkString, 'v')
+	}
+}
+
+type SplitTest struct {
+	s   string
+	sep string
+	n   int
+	a   []string
+}
+
+var splittests = []SplitTest{
+	{"", "", -1, []string{}},
+	{abcd, "", 2, []string{"a", "bcd"}},
+	{abcd, "", 4, []string{"a", "b", "c", "d"}},
+	{abcd, "", -1, []string{"a", "b", "c", "d"}},
+	{faces, "", -1, []string{"☺", "☻", "☹"}},
+	{faces, "", 3, []string{"☺", "☻", "☹"}},
+	{faces, "", 17, []string{"☺", "☻", "☹"}},
+	{"☺�☹", "", -1, []string{"☺", "�", "☹"}},
+	{abcd, "a", 0, nil},
+	{abcd, "a", -1, []string{"", "bcd"}},
+	{abcd, "z", -1, []string{"abcd"}},
+	{commas, ",", -1, []string{"1", "2", "3", "4"}},
+	{dots, "...", -1, []string{"1", ".2", ".3", ".4"}},
+	{faces, "☹", -1, []string{"☺☻", ""}},
+	{faces, "~", -1, []string{faces}},
+	{"1 2 3 4", " ", 3, []string{"1", "2", "3 4"}},
+	{"1 2", " ", 3, []string{"1", "2"}},
+	{"", "T", math.MaxInt / 4, []string{""}},
+	{"\xff-\xff", "", -1, []string{"\xff", "-", "\xff"}},
+	{"\xff-\xff", "-", -1, []string{"\xff", "\xff"}},
+}
+
+func TestSplit(t *testing.T) {
+	for _, tt := range splittests {
+		a := SplitN(tt.s, tt.sep, tt.n)
+		if !eq(a, tt.a) {
+			t.Errorf("Split(%q, %q, %d) = %v; want %v", tt.s, tt.sep, tt.n, a, tt.a)
+			continue
+		}
+		if tt.n == 0 {
+			continue
+		}
+		s := Join(a, tt.sep)
+		if s != tt.s {
+			t.Errorf("Join(Split(%q, %q, %d), %q) = %q", tt.s, tt.sep, tt.n, tt.sep, s)
+		}
+		if tt.n < 0 {
+			b := Split(tt.s, tt.sep)
+			if !reflect.DeepEqual(a, b) {
+				t.Errorf("Split disagrees with SplitN(%q, %q, %d) = %v; want %v", tt.s, tt.sep, tt.n, b, a)
+			}
+		}
+	}
+}
+
+var splitaftertests = []SplitTest{
+	{abcd, "a", -1, []string{"a", "bcd"}},
+	{abcd, "z", -1, []string{"abcd"}},
+	{abcd, "", -1, []string{"a", "b", "c", "d"}},
+	{commas, ",", -1, []string{"1,", "2,", "3,", "4"}},
+	{dots, "...", -1, []string{"1...", ".2...", ".3...", ".4"}},
+	{faces, "☹", -1, []string{"☺☻☹", ""}},
+	{faces, "~", -1, []string{faces}},
+	{faces, "", -1, []string{"☺", "☻", "☹"}},
+	{"1 2 3 4", " ", 3, []string{"1 ", "2 ", "3 4"}},
+	{"1 2 3", " ", 3, []string{"1 ", "2 ", "3"}},
+	{"1 2", " ", 3, []string{"1 ", "2"}},
+	{"123", "", 2, []string{"1", "23"}},
+	{"123", "", 17, []string{"1", "2", "3"}},
+}
+
+func TestSplitAfter(t *testing.T) {
+	for _, tt := range splitaftertests {
+		a := SplitAfterN(tt.s, tt.sep, tt.n)
+		if !eq(a, tt.a) {
+			t.Errorf(`Split(%q, %q, %d) = %v; want %v`, tt.s, tt.sep, tt.n, a, tt.a)
+			continue
+		}
+		s := Join(a, "")
+		if s != tt.s {
+			t.Errorf(`Join(Split(%q, %q, %d), %q) = %q`, tt.s, tt.sep, tt.n, tt.sep, s)
+		}
+		if tt.n < 0 {
+			b := SplitAfter(tt.s, tt.sep)
+			if !reflect.DeepEqual(a, b) {
+				t.Errorf("SplitAfter disagrees with SplitAfterN(%q, %q, %d) = %v; want %v", tt.s, tt.sep, tt.n, b, a)
+			}
+		}
+	}
+}
+
+type FieldsTest struct {
+	s string
+	a []string
+}
+
+var fieldstests = []FieldsTest{
+	{"", []string{}},
+	{" ", []string{}},
+	{" \t ", []string{}},
+	{"\u2000", []string{}},
+	{"  abc  ", []string{"abc"}},
+	{"1 2 3 4", []string{"1", "2", "3", "4"}},
+	{"1  2  3  4", []string{"1", "2", "3", "4"}},
+	{"1\t\t2\t\t3\t4", []string{"1", "2", "3", "4"}},
+	{"1\u20002\u20013\u20024", []string{"1", "2", "3", "4"}},
+	{"\u2000\u2001\u2002", []string{}},
+	{"\n™\t™\n", []string{"™", "™"}},
+	{"\n\u20001™2\u2000 \u2001 ™", []string{"1™2", "™"}},
+	{"\n1\uFFFD \uFFFD2\u20003\uFFFD4", []string{"1\uFFFD", "\uFFFD2", "3\uFFFD4"}},
+	{"1\xFF\u2000\xFF2\xFF \xFF", []string{"1\xFF", "\xFF2\xFF", "\xFF"}},
+	{faces, []string{faces}},
+}
+
+func TestFields(t *testing.T) {
+	for _, tt := range fieldstests {
+		a := Fields(tt.s)
+		if !eq(a, tt.a) {
+			t.Errorf("Fields(%q) = %v; want %v", tt.s, a, tt.a)
+			continue
+		}
+	}
+}
+
+var FieldsFuncTests = []FieldsTest{
+	{"", []string{}},
+	{"XX", []string{}},
+	{"XXhiXXX", []string{"hi"}},
+	{"aXXbXXXcX", []string{"a", "b", "c"}},
+}
+
+func TestFieldsFunc(t *testing.T) {
+	for _, tt := range fieldstests {
+		a := FieldsFunc(tt.s, unicode.IsSpace)
+		if !eq(a, tt.a) {
+			t.Errorf("FieldsFunc(%q, unicode.IsSpace) = %v; want %v", tt.s, a, tt.a)
+			continue
+		}
+	}
+	pred := func(c rune) bool { return c == 'X' }
+	for _, tt := range FieldsFuncTests {
+		a := FieldsFunc(tt.s, pred)
+		if !eq(a, tt.a) {
+			t.Errorf("FieldsFunc(%q) = %v, want %v", tt.s, a, tt.a)
+		}
+	}
+}
+
+// Test case for any function which accepts and returns a single string.
+type StringTest struct {
+	in, out string
+}
+
+// Execute f on each test case.  funcName should be the name of f; it's used
+// in failure reports.
+func runStringTests(t *testing.T, f func(string) string, funcName string, testCases []StringTest) {
+	for _, tc := range testCases {
+		actual := f(tc.in)
+		if actual != tc.out {
+			t.Errorf("%s(%q) = %q; want %q", funcName, tc.in, actual, tc.out)
+		}
+	}
+}
+
+var upperTests = []StringTest{
+	{"", ""},
+	{"ONLYUPPER", "ONLYUPPER"},
+	{"abc", "ABC"},
+	{"AbC123", "ABC123"},
+	{"azAZ09_", "AZAZ09_"},
+	{"longStrinGwitHmixofsmaLLandcAps", "LONGSTRINGWITHMIXOFSMALLANDCAPS"},
+	{"RENAN BASTOS 93 AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO", "RENAN BASTOS 93 AOSDAJDJAIDJAIDAJIAIDSJJAIDIJADSJIADJIOOKKO"},
+	{"long\u0250string\u0250with\u0250nonascii\u2C6Fchars", "LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS"},
+	{"\u0250\u0250\u0250\u0250\u0250", "\u2C6F\u2C6F\u2C6F\u2C6F\u2C6F"}, // grows one byte per char
+	{"a\u0080\U0010FFFF", "A\u0080\U0010FFFF"},                           // test utf8.RuneSelf and utf8.MaxRune
+}
+
+var lowerTests = []StringTest{
+	{"", ""},
+	{"abc", "abc"},
+	{"AbC123", "abc123"},
+	{"azAZ09_", "azaz09_"},
+	{"longStrinGwitHmixofsmaLLandcAps", "longstringwithmixofsmallandcaps"},
+	{"renan bastos 93 AOSDAJDJAIDJAIDAJIaidsjjaidijadsjiadjiOOKKO", "renan bastos 93 aosdajdjaidjaidajiaidsjjaidijadsjiadjiookko"},
+	{"LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS", "long\u0250string\u0250with\u0250nonascii\u0250chars"},
+	{"\u2C6D\u2C6D\u2C6D\u2C6D\u2C6D", "\u0251\u0251\u0251\u0251\u0251"}, // shrinks one byte per char
+	{"A\u0080\U0010FFFF", "a\u0080\U0010FFFF"},                           // test utf8.RuneSelf and utf8.MaxRune
+}
+
+const space = "\t\v\r\f\n\u0085\u00a0\u2000\u3000"
+
+var trimSpaceTests = []StringTest{
+	{"", ""},
+	{"abc", "abc"},
+	{space + "abc" + space, "abc"},
+	{" ", ""},
+	{" \t\r\n \t\t\r\r\n\n ", ""},
+	{" \t\r\n x\t\t\r\r\n\n ", "x"},
+	{" \u2000\t\r\n x\t\t\r\r\ny\n \u3000", "x\t\t\r\r\ny"},
+	{"1 \t\r\n2", "1 \t\r\n2"},
+	{" x\x80", "x\x80"},
+	{" x\xc0", "x\xc0"},
+	{"x \xc0\xc0 ", "x \xc0\xc0"},
+	{"x \xc0", "x \xc0"},
+	{"x \xc0 ", "x \xc0"},
+	{"x \xc0\xc0 ", "x \xc0\xc0"},
+	{"x ☺\xc0\xc0 ", "x ☺\xc0\xc0"},
+	{"x ☺ ", "x ☺"},
+}
+
+func tenRunes(ch rune) string {
+	r := make([]rune, 10)
+	for i := range r {
+		r[i] = ch
+	}
+	return string(r)
+}
+
+// User-defined self-inverse mapping function
+func rot13(r rune) rune {
+	step := rune(13)
+	if r >= 'a' && r <= 'z' {
+		return ((r - 'a' + step) % 26) + 'a'
+	}
+	if r >= 'A' && r <= 'Z' {
+		return ((r - 'A' + step) % 26) + 'A'
+	}
+	return r
+}
+
+func TestMap(t *testing.T) {
+	// Run a couple of awful growth/shrinkage tests
+	a := tenRunes('a')
+	// 1.  Grow. This triggers two reallocations in Map.
+	maxRune := func(rune) rune { return unicode.MaxRune }
+	m := Map(maxRune, a)
+	expect := tenRunes(unicode.MaxRune)
+	if m != expect {
+		t.Errorf("growing: expected %q got %q", expect, m)
+	}
+
+	// 2. Shrink
+	minRune := func(rune) rune { return 'a' }
+	m = Map(minRune, tenRunes(unicode.MaxRune))
+	expect = a
+	if m != expect {
+		t.Errorf("shrinking: expected %q got %q", expect, m)
+	}
+
+	// 3. Rot13
+	m = Map(rot13, "a to zed")
+	expect = "n gb mrq"
+	if m != expect {
+		t.Errorf("rot13: expected %q got %q", expect, m)
+	}
+
+	// 4. Rot13^2
+	m = Map(rot13, Map(rot13, "a to zed"))
+	expect = "a to zed"
+	if m != expect {
+		t.Errorf("rot13: expected %q got %q", expect, m)
+	}
+
+	// 5. Drop
+	dropNotLatin := func(r rune) rune {
+		if unicode.Is(unicode.Latin, r) {
+			return r
+		}
+		return -1
+	}
+	m = Map(dropNotLatin, "Hello, 세계")
+	expect = "Hello"
+	if m != expect {
+		t.Errorf("drop: expected %q got %q", expect, m)
+	}
+
+	// 6. Identity
+	identity := func(r rune) rune {
+		return r
+	}
+	orig := "Input string that we expect not to be copied."
+	m = Map(identity, orig)
+	if unsafe.StringData(orig) != unsafe.StringData(m) {
+		t.Error("unexpected copy during identity map")
+	}
+
+	// 7. Handle invalid UTF-8 sequence
+	replaceNotLatin := func(r rune) rune {
+		if unicode.Is(unicode.Latin, r) {
+			return r
+		}
+		return utf8.RuneError
+	}
+	m = Map(replaceNotLatin, "Hello\255World")
+	expect = "Hello\uFFFDWorld"
+	if m != expect {
+		t.Errorf("replace invalid sequence: expected %q got %q", expect, m)
+	}
+
+	// 8. Check utf8.RuneSelf and utf8.MaxRune encoding
+	encode := func(r rune) rune {
+		switch r {
+		case utf8.RuneSelf:
+			return unicode.MaxRune
+		case unicode.MaxRune:
+			return utf8.RuneSelf
+		}
+		return r
+	}
+	s := string(rune(utf8.RuneSelf)) + string(utf8.MaxRune)
+	r := string(utf8.MaxRune) + string(rune(utf8.RuneSelf)) // reverse of s
+	m = Map(encode, s)
+	if m != r {
+		t.Errorf("encoding not handled correctly: expected %q got %q", r, m)
+	}
+	m = Map(encode, r)
+	if m != s {
+		t.Errorf("encoding not handled correctly: expected %q got %q", s, m)
+	}
+
+	// 9. Check mapping occurs in the front, middle and back
+	trimSpaces := func(r rune) rune {
+		if unicode.IsSpace(r) {
+			return -1
+		}
+		return r
+	}
+	m = Map(trimSpaces, "   abc    123   ")
+	expect = "abc123"
+	if m != expect {
+		t.Errorf("trimSpaces: expected %q got %q", expect, m)
+	}
+}
+
+func TestToUpper(t *testing.T) { runStringTests(t, ToUpper, "ToUpper", upperTests) }
+
+func TestToLower(t *testing.T) { runStringTests(t, ToLower, "ToLower", lowerTests) }
+
+var toValidUTF8Tests = []struct {
+	in   string
+	repl string
+	out  string
+}{
+	{"", "\uFFFD", ""},
+	{"abc", "\uFFFD", "abc"},
+	{"\uFDDD", "\uFFFD", "\uFDDD"},
+	{"a\xffb", "\uFFFD", "a\uFFFDb"},
+	{"a\xffb\uFFFD", "X", "aXb\uFFFD"},
+	{"a☺\xffb☺\xC0\xAFc☺\xff", "", "a☺b☺c☺"},
+	{"a☺\xffb☺\xC0\xAFc☺\xff", "日本語", "a☺日本語b☺日本語c☺日本語"},
+	{"\xC0\xAF", "\uFFFD", "\uFFFD"},
+	{"\xE0\x80\xAF", "\uFFFD", "\uFFFD"},
+	{"\xed\xa0\x80", "abc", "abc"},
+	{"\xed\xbf\xbf", "\uFFFD", "\uFFFD"},
+	{"\xF0\x80\x80\xaf", "☺", "☺"},
+	{"\xF8\x80\x80\x80\xAF", "\uFFFD", "\uFFFD"},
+	{"\xFC\x80\x80\x80\x80\xAF", "\uFFFD", "\uFFFD"},
+}
+
+func TestToValidUTF8(t *testing.T) {
+	for _, tc := range toValidUTF8Tests {
+		got := ToValidUTF8(tc.in, tc.repl)
+		if got != tc.out {
+			t.Errorf("ToValidUTF8(%q, %q) = %q; want %q", tc.in, tc.repl, got, tc.out)
+		}
+	}
+}
+
+func BenchmarkToUpper(b *testing.B) {
+	for _, tc := range upperTests {
+		b.Run(tc.in, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				actual := ToUpper(tc.in)
+				if actual != tc.out {
+					b.Errorf("ToUpper(%q) = %q; want %q", tc.in, actual, tc.out)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkToLower(b *testing.B) {
+	for _, tc := range lowerTests {
+		b.Run(tc.in, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				actual := ToLower(tc.in)
+				if actual != tc.out {
+					b.Errorf("ToLower(%q) = %q; want %q", tc.in, actual, tc.out)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkMapNoChanges(b *testing.B) {
+	identity := func(r rune) rune {
+		return r
+	}
+	for i := 0; i < b.N; i++ {
+		Map(identity, "Some string that won't be modified.")
+	}
+}
+
+func TestSpecialCase(t *testing.T) {
+	lower := "abcçdefgğhıijklmnoöprsştuüvyz"
+	upper := "ABCÇDEFGĞHIİJKLMNOÖPRSŞTUÜVYZ"
+	u := ToUpperSpecial(unicode.TurkishCase, upper)
+	if u != upper {
+		t.Errorf("Upper(upper) is %s not %s", u, upper)
+	}
+	u = ToUpperSpecial(unicode.TurkishCase, lower)
+	if u != upper {
+		t.Errorf("Upper(lower) is %s not %s", u, upper)
+	}
+	l := ToLowerSpecial(unicode.TurkishCase, lower)
+	if l != lower {
+		t.Errorf("Lower(lower) is %s not %s", l, lower)
+	}
+	l = ToLowerSpecial(unicode.TurkishCase, upper)
+	if l != lower {
+		t.Errorf("Lower(upper) is %s not %s", l, lower)
+	}
+}
+
+func TestTrimSpace(t *testing.T) { runStringTests(t, TrimSpace, "TrimSpace", trimSpaceTests) }
+
+var trimTests = []struct {
+	f            string
+	in, arg, out string
+}{
+	{"Trim", "abba", "a", "bb"},
+	{"Trim", "abba", "ab", ""},
+	{"TrimLeft", "abba", "ab", ""},
+	{"TrimRight", "abba", "ab", ""},
+	{"TrimLeft", "abba", "a", "bba"},
+	{"TrimLeft", "abba", "b", "abba"},
+	{"TrimRight", "abba", "a", "abb"},
+	{"TrimRight", "abba", "b", "abba"},
+	{"Trim", "<tag>", "<>", "tag"},
+	{"Trim", "* listitem", " *", "listitem"},
+	{"Trim", `"quote"`, `"`, "quote"},
+	{"Trim", "\u2C6F\u2C6F\u0250\u0250\u2C6F\u2C6F", "\u2C6F", "\u0250\u0250"},
+	{"Trim", "\x80test\xff", "\xff", "test"},
+	{"Trim", " Ġ ", " ", "Ġ"},
+	{"Trim", " Ġİ0", "0 ", "Ġİ"},
+	//empty string tests
+	{"Trim", "abba", "", "abba"},
+	{"Trim", "", "123", ""},
+	{"Trim", "", "", ""},
+	{"TrimLeft", "abba", "", "abba"},
+	{"TrimLeft", "", "123", ""},
+	{"TrimLeft", "", "", ""},
+	{"TrimRight", "abba", "", "abba"},
+	{"TrimRight", "", "123", ""},
+	{"TrimRight", "", "", ""},
+	{"TrimRight", "☺\xc0", "☺", "☺\xc0"},
+	{"TrimPrefix", "aabb", "a", "abb"},
+	{"TrimPrefix", "aabb", "b", "aabb"},
+	{"TrimSuffix", "aabb", "a", "aabb"},
+	{"TrimSuffix", "aabb", "b", "aab"},
+}
+
+func TestTrim(t *testing.T) {
+	for _, tc := range trimTests {
+		name := tc.f
+		var f func(string, string) string
+		switch name {
+		case "Trim":
+			f = Trim
+		case "TrimLeft":
+			f = TrimLeft
+		case "TrimRight":
+			f = TrimRight
+		case "TrimPrefix":
+			f = TrimPrefix
+		case "TrimSuffix":
+			f = TrimSuffix
+		default:
+			t.Errorf("Undefined trim function %s", name)
+		}
+		actual := f(tc.in, tc.arg)
+		if actual != tc.out {
+			t.Errorf("%s(%q, %q) = %q; want %q", name, tc.in, tc.arg, actual, tc.out)
+		}
+	}
+}
+
+func BenchmarkTrim(b *testing.B) {
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		for _, tc := range trimTests {
+			name := tc.f
+			var f func(string, string) string
+			switch name {
+			case "Trim":
+				f = Trim
+			case "TrimLeft":
+				f = TrimLeft
+			case "TrimRight":
+				f = TrimRight
+			case "TrimPrefix":
+				f = TrimPrefix
+			case "TrimSuffix":
+				f = TrimSuffix
+			default:
+				b.Errorf("Undefined trim function %s", name)
+			}
+			actual := f(tc.in, tc.arg)
+			if actual != tc.out {
+				b.Errorf("%s(%q, %q) = %q; want %q", name, tc.in, tc.arg, actual, tc.out)
+			}
+		}
+	}
+}
+
+func BenchmarkToValidUTF8(b *testing.B) {
+	tests := []struct {
+		name  string
+		input string
+	}{
+		{"Valid", "typical"},
+		{"InvalidASCII", "foo\xffbar"},
+		{"InvalidNonASCII", "日本語\xff日本語"},
+	}
+	replacement := "\uFFFD"
+	b.ResetTimer()
+	for _, test := range tests {
+		b.Run(test.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				ToValidUTF8(test.input, replacement)
+			}
+		})
+	}
+}
+
+type predicate struct {
+	f    func(rune) bool
+	name string
+}
+
+var isSpace = predicate{unicode.IsSpace, "IsSpace"}
+var isDigit = predicate{unicode.IsDigit, "IsDigit"}
+var isUpper = predicate{unicode.IsUpper, "IsUpper"}
+var isValidRune = predicate{
+	func(r rune) bool {
+		return r != utf8.RuneError
+	},
+	"IsValidRune",
+}
+
+func not(p predicate) predicate {
+	return predicate{
+		func(r rune) bool {
+			return !p.f(r)
+		},
+		"not " + p.name,
+	}
+}
+
+var trimFuncTests = []struct {
+	f        predicate
+	in       string
+	trimOut  string
+	leftOut  string
+	rightOut string
+}{
+	{isSpace, space + " hello " + space,
+		"hello",
+		"hello " + space,
+		space + " hello"},
+	{isDigit, "\u0e50\u0e5212hello34\u0e50\u0e51",
+		"hello",
+		"hello34\u0e50\u0e51",
+		"\u0e50\u0e5212hello"},
+	{isUpper, "\u2C6F\u2C6F\u2C6F\u2C6FABCDhelloEF\u2C6F\u2C6FGH\u2C6F\u2C6F",
+		"hello",
+		"helloEF\u2C6F\u2C6FGH\u2C6F\u2C6F",
+		"\u2C6F\u2C6F\u2C6F\u2C6FABCDhello"},
+	{not(isSpace), "hello" + space + "hello",
+		space,
+		space + "hello",
+		"hello" + space},
+	{not(isDigit), "hello\u0e50\u0e521234\u0e50\u0e51helo",
+		"\u0e50\u0e521234\u0e50\u0e51",
+		"\u0e50\u0e521234\u0e50\u0e51helo",
+		"hello\u0e50\u0e521234\u0e50\u0e51"},
+	{isValidRune, "ab\xc0a\xc0cd",
+		"\xc0a\xc0",
+		"\xc0a\xc0cd",
+		"ab\xc0a\xc0"},
+	{not(isValidRune), "\xc0a\xc0",
+		"a",
+		"a\xc0",
+		"\xc0a"},
+	{isSpace, "",
+		"",
+		"",
+		""},
+	{isSpace, " ",
+		"",
+		"",
+		""},
+}
+
+func TestTrimFunc(t *testing.T) {
+	for _, tc := range trimFuncTests {
+		trimmers := []struct {
+			name string
+			trim func(s string, f func(r rune) bool) string
+			out  string
+		}{
+			{"TrimFunc", TrimFunc, tc.trimOut},
+			{"TrimLeftFunc", TrimLeftFunc, tc.leftOut},
+			{"TrimRightFunc", TrimRightFunc, tc.rightOut},
+		}
+		for _, trimmer := range trimmers {
+			actual := trimmer.trim(tc.in, tc.f.f)
+			if actual != trimmer.out {
+				t.Errorf("%s(%q, %q) = %q; want %q", trimmer.name, tc.in, tc.f.name, actual, trimmer.out)
+			}
+		}
+	}
+}
+
+var indexFuncTests = []struct {
+	in          string
+	f           predicate
+	first, last int
+}{
+	{"", isValidRune, -1, -1},
+	{"abc", isDigit, -1, -1},
+	{"0123", isDigit, 0, 3},
+	{"a1b", isDigit, 1, 1},
+	{space, isSpace, 0, len(space) - 3}, // last rune in space is 3 bytes
+	{"\u0e50\u0e5212hello34\u0e50\u0e51", isDigit, 0, 18},
+	{"\u2C6F\u2C6F\u2C6F\u2C6FABCDhelloEF\u2C6F\u2C6FGH\u2C6F\u2C6F", isUpper, 0, 34},
+	{"12\u0e50\u0e52hello34\u0e50\u0e51", not(isDigit), 8, 12},
+
+	// tests of invalid UTF-8
+	{"\x801", isDigit, 1, 1},
+	{"\x80abc", isDigit, -1, -1},
+	{"\xc0a\xc0", isValidRune, 1, 1},
+	{"\xc0a\xc0", not(isValidRune), 0, 2},
+	{"\xc0☺\xc0", not(isValidRune), 0, 4},
+	{"\xc0☺\xc0\xc0", not(isValidRune), 0, 5},
+	{"ab\xc0a\xc0cd", not(isValidRune), 2, 4},
+	{"a\xe0\x80cd", not(isValidRune), 1, 2},
+	{"\x80\x80\x80\x80", not(isValidRune), 0, 3},
+}
+
+func TestIndexFunc(t *testing.T) {
+	for _, tc := range indexFuncTests {
+		first := IndexFunc(tc.in, tc.f.f)
+		if first != tc.first {
+			t.Errorf("IndexFunc(%q, %s) = %d; want %d", tc.in, tc.f.name, first, tc.first)
+		}
+		last := LastIndexFunc(tc.in, tc.f.f)
+		if last != tc.last {
+			t.Errorf("LastIndexFunc(%q, %s) = %d; want %d", tc.in, tc.f.name, last, tc.last)
+		}
+	}
+}
+
+func equal(m string, s1, s2 string, t *testing.T) bool {
+	if s1 == s2 {
+		return true
+	}
+	e1 := Split(s1, "")
+	e2 := Split(s2, "")
+	for i, c1 := range e1 {
+		if i >= len(e2) {
+			break
+		}
+		r1, _ := utf8.DecodeRuneInString(c1)
+		r2, _ := utf8.DecodeRuneInString(e2[i])
+		if r1 != r2 {
+			t.Errorf("%s diff at %d: U+%04X U+%04X", m, i, r1, r2)
+		}
+	}
+	return false
+}
+
+func TestCaseConsistency(t *testing.T) {
+	// Make a string of all the runes.
+	numRunes := int(unicode.MaxRune + 1)
+	if testing.Short() {
+		numRunes = 1000
+	}
+	a := make([]rune, numRunes)
+	for i := range a {
+		a[i] = rune(i)
+	}
+	s := string(a)
+	// convert the cases.
+	upper := ToUpper(s)
+	lower := ToLower(s)
+
+	// Consistency checks
+	if n := utf8.RuneCountInString(upper); n != numRunes {
+		t.Error("rune count wrong in upper:", n)
+	}
+	if n := utf8.RuneCountInString(lower); n != numRunes {
+		t.Error("rune count wrong in lower:", n)
+	}
+	if !equal("ToUpper(upper)", ToUpper(upper), upper, t) {
+		t.Error("ToUpper(upper) consistency fail")
+	}
+	if !equal("ToLower(lower)", ToLower(lower), lower, t) {
+		t.Error("ToLower(lower) consistency fail")
+	}
+	/*
+		  These fail because of non-one-to-oneness of the data, such as multiple
+		  upper case 'I' mapping to 'i'.  We comment them out but keep them for
+		  interest.
+		  For instance: CAPITAL LETTER I WITH DOT ABOVE:
+			unicode.ToUpper(unicode.ToLower('\u0130')) != '\u0130'
+
+		if !equal("ToUpper(lower)", ToUpper(lower), upper, t) {
+			t.Error("ToUpper(lower) consistency fail");
+		}
+		if !equal("ToLower(upper)", ToLower(upper), lower, t) {
+			t.Error("ToLower(upper) consistency fail");
+		}
+	*/
+}
+
+var longString = "a" + string(make([]byte, 1<<16)) + "z"
+
+var RepeatTests = []struct {
+	in, out string
+	count   int
+}{
+	{"", "", 0},
+	{"", "", 1},
+	{"", "", 2},
+	{"-", "", 0},
+	{"-", "-", 1},
+	{"-", "----------", 10},
+	{"abc ", "abc abc abc ", 3},
+	// Tests for results over the chunkLimit
+	{string(rune(0)), string(make([]byte, 1<<16)), 1 << 16},
+	{longString, longString + longString, 2},
+}
+
+func TestRepeat(t *testing.T) {
+	for _, tt := range RepeatTests {
+		a := Repeat(tt.in, tt.count)
+		if !equal("Repeat(s)", a, tt.out, t) {
+			t.Errorf("Repeat(%v, %d) = %v; want %v", tt.in, tt.count, a, tt.out)
+			continue
+		}
+	}
+}
+
+func repeat(s string, count int) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			switch v := r.(type) {
+			case error:
+				err = v
+			default:
+				err = fmt.Errorf("%s", v)
+			}
+		}
+	}()
+
+	Repeat(s, count)
+
+	return
+}
+
+// See Issue golang.org/issue/16237
+func TestRepeatCatchesOverflow(t *testing.T) {
+	tests := [...]struct {
+		s      string
+		count  int
+		errStr string
+	}{
+		0: {"--", -2147483647, "negative"},
+		1: {"", int(^uint(0) >> 1), ""},
+		2: {"-", 10, ""},
+		3: {"gopher", 0, ""},
+		4: {"-", -1, "negative"},
+		5: {"--", -102, "negative"},
+		6: {string(make([]byte, 255)), int((^uint(0))/255 + 1), "overflow"},
+	}
+
+	for i, tt := range tests {
+		err := repeat(tt.s, tt.count)
+		if tt.errStr == "" {
+			if err != nil {
+				t.Errorf("#%d panicked %v", i, err)
+			}
+			continue
+		}
+
+		if err == nil || !Contains(err.Error(), tt.errStr) {
+			t.Errorf("#%d expected %q got %q", i, tt.errStr, err)
+		}
+	}
+}
+
+func runesEqual(a, b []rune) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i, r := range a {
+		if r != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+var RunesTests = []struct {
+	in    string
+	out   []rune
+	lossy bool
+}{
+	{"", []rune{}, false},
+	{" ", []rune{32}, false},
+	{"ABC", []rune{65, 66, 67}, false},
+	{"abc", []rune{97, 98, 99}, false},
+	{"\u65e5\u672c\u8a9e", []rune{26085, 26412, 35486}, false},
+	{"ab\x80c", []rune{97, 98, 0xFFFD, 99}, true},
+	{"ab\xc0c", []rune{97, 98, 0xFFFD, 99}, true},
+}
+
+func TestRunes(t *testing.T) {
+	for _, tt := range RunesTests {
+		a := []rune(tt.in)
+		if !runesEqual(a, tt.out) {
+			t.Errorf("[]rune(%q) = %v; want %v", tt.in, a, tt.out)
+			continue
+		}
+		if !tt.lossy {
+			// can only test reassembly if we didn't lose information
+			s := string(a)
+			if s != tt.in {
+				t.Errorf("string([]rune(%q)) = %x; want %x", tt.in, s, tt.in)
+			}
+		}
+	}
+}
+
+func TestReadByte(t *testing.T) {
+	testStrings := []string{"", abcd, faces, commas}
+	for _, s := range testStrings {
+		reader := NewReader(s)
+		if e := reader.UnreadByte(); e == nil {
+			t.Errorf("Unreading %q at beginning: expected error", s)
+		}
+		var res bytes.Buffer
+		for {
+			b, e := reader.ReadByte()
+			if e == io.EOF {
+				break
+			}
+			if e != nil {
+				t.Errorf("Reading %q: %s", s, e)
+				break
+			}
+			res.WriteByte(b)
+			// unread and read again
+			e = reader.UnreadByte()
+			if e != nil {
+				t.Errorf("Unreading %q: %s", s, e)
+				break
+			}
+			b1, e := reader.ReadByte()
+			if e != nil {
+				t.Errorf("Reading %q after unreading: %s", s, e)
+				break
+			}
+			if b1 != b {
+				t.Errorf("Reading %q after unreading: want byte %q, got %q", s, b, b1)
+				break
+			}
+		}
+		if res.String() != s {
+			t.Errorf("Reader(%q).ReadByte() produced %q", s, res.String())
+		}
+	}
+}
+
+func TestReadRune(t *testing.T) {
+	testStrings := []string{"", abcd, faces, commas}
+	for _, s := range testStrings {
+		reader := NewReader(s)
+		if e := reader.UnreadRune(); e == nil {
+			t.Errorf("Unreading %q at beginning: expected error", s)
+		}
+		res := ""
+		for {
+			r, z, e := reader.ReadRune()
+			if e == io.EOF {
+				break
+			}
+			if e != nil {
+				t.Errorf("Reading %q: %s", s, e)
+				break
+			}
+			res += string(r)
+			// unread and read again
+			e = reader.UnreadRune()
+			if e != nil {
+				t.Errorf("Unreading %q: %s", s, e)
+				break
+			}
+			r1, z1, e := reader.ReadRune()
+			if e != nil {
+				t.Errorf("Reading %q after unreading: %s", s, e)
+				break
+			}
+			if r1 != r {
+				t.Errorf("Reading %q after unreading: want rune %q, got %q", s, r, r1)
+				break
+			}
+			if z1 != z {
+				t.Errorf("Reading %q after unreading: want size %d, got %d", s, z, z1)
+				break
+			}
+		}
+		if res != s {
+			t.Errorf("Reader(%q).ReadRune() produced %q", s, res)
+		}
+	}
+}
+
+var UnreadRuneErrorTests = []struct {
+	name string
+	f    func(*Reader)
+}{
+	{"Read", func(r *Reader) { r.Read([]byte{0}) }},
+	{"ReadByte", func(r *Reader) { r.ReadByte() }},
+	{"UnreadRune", func(r *Reader) { r.UnreadRune() }},
+	{"Seek", func(r *Reader) { r.Seek(0, io.SeekCurrent) }},
+	{"WriteTo", func(r *Reader) { r.WriteTo(&bytes.Buffer{}) }},
+}
+
+func TestUnreadRuneError(t *testing.T) {
+	for _, tt := range UnreadRuneErrorTests {
+		reader := NewReader("0123456789")
+		if _, _, err := reader.ReadRune(); err != nil {
+			// should not happen
+			t.Fatal(err)
+		}
+		tt.f(reader)
+		err := reader.UnreadRune()
+		if err == nil {
+			t.Errorf("Unreading after %s: expected error", tt.name)
+		}
+	}
+}
+
+var ReplaceTests = []struct {
+	in       string
+	old, new string
+	n        int
+	out      string
+}{
+	{"hello", "l", "L", 0, "hello"},
+	{"hello", "l", "L", -1, "heLLo"},
+	{"hello", "x", "X", -1, "hello"},
+	{"", "x", "X", -1, ""},
+	{"radar", "r", "<r>", -1, "<r>ada<r>"},
+	{"", "", "<>", -1, "<>"},
+	{"banana", "a", "<>", -1, "b<>n<>n<>"},
+	{"banana", "a", "<>", 1, "b<>nana"},
+	{"banana", "a", "<>", 1000, "b<>n<>n<>"},
+	{"banana", "an", "<>", -1, "b<><>a"},
+	{"banana", "ana", "<>", -1, "b<>na"},
+	{"banana", "", "<>", -1, "<>b<>a<>n<>a<>n<>a<>"},
+	{"banana", "", "<>", 10, "<>b<>a<>n<>a<>n<>a<>"},
+	{"banana", "", "<>", 6, "<>b<>a<>n<>a<>n<>a"},
+	{"banana", "", "<>", 5, "<>b<>a<>n<>a<>na"},
+	{"banana", "", "<>", 1, "<>banana"},
+	{"banana", "a", "a", -1, "banana"},
+	{"banana", "a", "a", 1, "banana"},
+	{"☺☻☹", "", "<>", -1, "<>☺<>☻<>☹<>"},
+}
+
+func TestReplace(t *testing.T) {
+	for _, tt := range ReplaceTests {
+		if s := Replace(tt.in, tt.old, tt.new, tt.n); s != tt.out {
+			t.Errorf("Replace(%q, %q, %q, %d) = %q, want %q", tt.in, tt.old, tt.new, tt.n, s, tt.out)
+		}
+		if tt.n == -1 {
+			s := ReplaceAll(tt.in, tt.old, tt.new)
+			if s != tt.out {
+				t.Errorf("ReplaceAll(%q, %q, %q) = %q, want %q", tt.in, tt.old, tt.new, s, tt.out)
+			}
+		}
+	}
+}
+
+var TitleTests = []struct {
+	in, out string
+}{
+	{"", ""},
+	{"a", "A"},
+	{" aaa aaa aaa ", " Aaa Aaa Aaa "},
+	{" Aaa Aaa Aaa ", " Aaa Aaa Aaa "},
+	{"123a456", "123a456"},
+	{"double-blind", "Double-Blind"},
+	{"ÿøû", "Ÿøû"},
+	{"with_underscore", "With_underscore"},
+	{"unicode \xe2\x80\xa8 line separator", "Unicode \xe2\x80\xa8 Line Separator"},
+}
+
+func TestTitle(t *testing.T) {
+	for _, tt := range TitleTests {
+		if s := Title(tt.in); s != tt.out {
+			t.Errorf("Title(%q) = %q, want %q", tt.in, s, tt.out)
+		}
+	}
+}
+
+var ContainsTests = []struct {
+	str, substr string
+	expected    bool
+}{
+	{"abc", "bc", true},
+	{"abc", "bcd", false},
+	{"abc", "", true},
+	{"", "a", false},
+
+	// cases to cover code in runtime/asm_amd64.s:indexShortStr
+	// 2-byte needle
+	{"xxxxxx", "01", false},
+	{"01xxxx", "01", true},
+	{"xx01xx", "01", true},
+	{"xxxx01", "01", true},
+	{"01xxxxx"[1:], "01", false},
+	{"xxxxx01"[:6], "01", false},
+	// 3-byte needle
+	{"xxxxxxx", "012", false},
+	{"012xxxx", "012", true},
+	{"xx012xx", "012", true},
+	{"xxxx012", "012", true},
+	{"012xxxxx"[1:], "012", false},
+	{"xxxxx012"[:7], "012", false},
+	// 4-byte needle
+	{"xxxxxxxx", "0123", false},
+	{"0123xxxx", "0123", true},
+	{"xx0123xx", "0123", true},
+	{"xxxx0123", "0123", true},
+	{"0123xxxxx"[1:], "0123", false},
+	{"xxxxx0123"[:8], "0123", false},
+	// 5-7-byte needle
+	{"xxxxxxxxx", "01234", false},
+	{"01234xxxx", "01234", true},
+	{"xx01234xx", "01234", true},
+	{"xxxx01234", "01234", true},
+	{"01234xxxxx"[1:], "01234", false},
+	{"xxxxx01234"[:9], "01234", false},
+	// 8-byte needle
+	{"xxxxxxxxxxxx", "01234567", false},
+	{"01234567xxxx", "01234567", true},
+	{"xx01234567xx", "01234567", true},
+	{"xxxx01234567", "01234567", true},
+	{"01234567xxxxx"[1:], "01234567", false},
+	{"xxxxx01234567"[:12], "01234567", false},
+	// 9-15-byte needle
+	{"xxxxxxxxxxxxx", "012345678", false},
+	{"012345678xxxx", "012345678", true},
+	{"xx012345678xx", "012345678", true},
+	{"xxxx012345678", "012345678", true},
+	{"012345678xxxxx"[1:], "012345678", false},
+	{"xxxxx012345678"[:13], "012345678", false},
+	// 16-byte needle
+	{"xxxxxxxxxxxxxxxxxxxx", "0123456789ABCDEF", false},
+	{"0123456789ABCDEFxxxx", "0123456789ABCDEF", true},
+	{"xx0123456789ABCDEFxx", "0123456789ABCDEF", true},
+	{"xxxx0123456789ABCDEF", "0123456789ABCDEF", true},
+	{"0123456789ABCDEFxxxxx"[1:], "0123456789ABCDEF", false},
+	{"xxxxx0123456789ABCDEF"[:20], "0123456789ABCDEF", false},
+	// 17-31-byte needle
+	{"xxxxxxxxxxxxxxxxxxxxx", "0123456789ABCDEFG", false},
+	{"0123456789ABCDEFGxxxx", "0123456789ABCDEFG", true},
+	{"xx0123456789ABCDEFGxx", "0123456789ABCDEFG", true},
+	{"xxxx0123456789ABCDEFG", "0123456789ABCDEFG", true},
+	{"0123456789ABCDEFGxxxxx"[1:], "0123456789ABCDEFG", false},
+	{"xxxxx0123456789ABCDEFG"[:21], "0123456789ABCDEFG", false},
+
+	// partial match cases
+	{"xx01x", "012", false},                             // 3
+	{"xx0123x", "01234", false},                         // 5-7
+	{"xx01234567x", "012345678", false},                 // 9-15
+	{"xx0123456789ABCDEFx", "0123456789ABCDEFG", false}, // 17-31, issue 15679
+}
+
+func TestContains(t *testing.T) {
+	for _, ct := range ContainsTests {
+		if Contains(ct.str, ct.substr) != ct.expected {
+			t.Errorf("Contains(%s, %s) = %v, want %v",
+				ct.str, ct.substr, !ct.expected, ct.expected)
+		}
+	}
+}
+
+var ContainsAnyTests = []struct {
+	str, substr string
+	expected    bool
+}{
+	{"", "", false},
+	{"", "a", false},
+	{"", "abc", false},
+	{"a", "", false},
+	{"a", "a", true},
+	{"aaa", "a", true},
+	{"abc", "xyz", false},
+	{"abc", "xcz", true},
+	{"a☺b☻c☹d", "uvw☻xyz", true},
+	{"aRegExp*", ".(|)*+?^$[]", true},
+	{dots + dots + dots, " ", false},
+}
+
+func TestContainsAny(t *testing.T) {
+	for _, ct := range ContainsAnyTests {
+		if ContainsAny(ct.str, ct.substr) != ct.expected {
+			t.Errorf("ContainsAny(%s, %s) = %v, want %v",
+				ct.str, ct.substr, !ct.expected, ct.expected)
+		}
+	}
+}
+
+var ContainsRuneTests = []struct {
+	str      string
+	r        rune
+	expected bool
+}{
+	{"", 'a', false},
+	{"a", 'a', true},
+	{"aaa", 'a', true},
+	{"abc", 'y', false},
+	{"abc", 'c', true},
+	{"a☺b☻c☹d", 'x', false},
+	{"a☺b☻c☹d", '☻', true},
+	{"aRegExp*", '*', true},
+}
+
+func TestContainsRune(t *testing.T) {
+	for _, ct := range ContainsRuneTests {
+		if ContainsRune(ct.str, ct.r) != ct.expected {
+			t.Errorf("ContainsRune(%q, %q) = %v, want %v",
+				ct.str, ct.r, !ct.expected, ct.expected)
+		}
+	}
+}
+
+func TestContainsFunc(t *testing.T) {
+	for _, ct := range ContainsRuneTests {
+		if ContainsFunc(ct.str, func(r rune) bool {
+			return ct.r == r
+		}) != ct.expected {
+			t.Errorf("ContainsFunc(%q, func(%q)) = %v, want %v",
+				ct.str, ct.r, !ct.expected, ct.expected)
+		}
+	}
+}
+
+var EqualFoldTests = []struct {
+	s, t string
+	out  bool
+}{
+	{"abc", "abc", true},
+	{"ABcd", "ABcd", true},
+	{"123abc", "123ABC", true},
+	{"αβδ", "ΑΒΔ", true},
+	{"abc", "xyz", false},
+	{"abc", "XYZ", false},
+	{"abcdefghijk", "abcdefghijX", false},
+	{"abcdefghijk", "abcdefghij\u212A", true},
+	{"abcdefghijK", "abcdefghij\u212A", true},
+	{"abcdefghijkz", "abcdefghij\u212Ay", false},
+	{"abcdefghijKz", "abcdefghij\u212Ay", false},
+	{"1", "2", false},
+	{"utf-8", "US-ASCII", false},
+}
+
+func TestEqualFold(t *testing.T) {
+	for _, tt := range EqualFoldTests {
+		if out := EqualFold(tt.s, tt.t); out != tt.out {
+			t.Errorf("EqualFold(%#q, %#q) = %v, want %v", tt.s, tt.t, out, tt.out)
+		}
+		if out := EqualFold(tt.t, tt.s); out != tt.out {
+			t.Errorf("EqualFold(%#q, %#q) = %v, want %v", tt.t, tt.s, out, tt.out)
+		}
+	}
+}
+
+func BenchmarkEqualFold(b *testing.B) {
+	b.Run("Tests", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			for _, tt := range EqualFoldTests {
+				if out := EqualFold(tt.s, tt.t); out != tt.out {
+					b.Fatal("wrong result")
+				}
+			}
+		}
+	})
+
+	const s1 = "abcdefghijKz"
+	const s2 = "abcDefGhijKz"
+
+	b.Run("ASCII", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			EqualFold(s1, s2)
+		}
+	})
+
+	b.Run("UnicodePrefix", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			EqualFold("αβδ"+s1, "ΑΒΔ"+s2)
+		}
+	})
+
+	b.Run("UnicodeSuffix", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			EqualFold(s1+"αβδ", s2+"ΑΒΔ")
+		}
+	})
+}
+
+var CountTests = []struct {
+	s, sep string
+	num    int
+}{
+	{"", "", 1},
+	{"", "notempty", 0},
+	{"notempty", "", 9},
+	{"smaller", "not smaller", 0},
+	{"12345678987654321", "6", 2},
+	{"611161116", "6", 3},
+	{"notequal", "NotEqual", 0},
+	{"equal", "equal", 1},
+	{"abc1231231123q", "123", 3},
+	{"11111", "11", 2},
+}
+
+func TestCount(t *testing.T) {
+	for _, tt := range CountTests {
+		if num := Count(tt.s, tt.sep); num != tt.num {
+			t.Errorf("Count(%q, %q) = %d, want %d", tt.s, tt.sep, num, tt.num)
+		}
+	}
+}
+
+var cutTests = []struct {
+	s, sep        string
+	before, after string
+	found         bool
+}{
+	{"abc", "b", "a", "c", true},
+	{"abc", "a", "", "bc", true},
+	{"abc", "c", "ab", "", true},
+	{"abc", "abc", "", "", true},
+	{"abc", "", "", "abc", true},
+	{"abc", "d", "abc", "", false},
+	{"", "d", "", "", false},
+	{"", "", "", "", true},
+}
+
+func TestCut(t *testing.T) {
+	for _, tt := range cutTests {
+		if before, after, found := Cut(tt.s, tt.sep); before != tt.before || after != tt.after || found != tt.found {
+			t.Errorf("Cut(%q, %q) = %q, %q, %v, want %q, %q, %v", tt.s, tt.sep, before, after, found, tt.before, tt.after, tt.found)
+		}
+	}
+}
+
+var cutPrefixTests = []struct {
+	s, sep string
+	after  string
+	found  bool
+}{
+	{"abc", "a", "bc", true},
+	{"abc", "abc", "", true},
+	{"abc", "", "abc", true},
+	{"abc", "d", "abc", false},
+	{"", "d", "", false},
+	{"", "", "", true},
+}
+
+func TestCutPrefix(t *testing.T) {
+	for _, tt := range cutPrefixTests {
+		if after, found := CutPrefix(tt.s, tt.sep); after != tt.after || found != tt.found {
+			t.Errorf("CutPrefix(%q, %q) = %q, %v, want %q, %v", tt.s, tt.sep, after, found, tt.after, tt.found)
+		}
+	}
+}
+
+var cutSuffixTests = []struct {
+	s, sep string
+	before string
+	found  bool
+}{
+	{"abc", "bc", "a", true},
+	{"abc", "abc", "", true},
+	{"abc", "", "abc", true},
+	{"abc", "d", "abc", false},
+	{"", "d", "", false},
+	{"", "", "", true},
+}
+
+func TestCutSuffix(t *testing.T) {
+	for _, tt := range cutSuffixTests {
+		if before, found := CutSuffix(tt.s, tt.sep); before != tt.before || found != tt.found {
+			t.Errorf("CutSuffix(%q, %q) = %q, %v, want %q, %v", tt.s, tt.sep, before, found, tt.before, tt.found)
+		}
+	}
+}
+
+func makeBenchInputHard() string {
+	tokens := [...]string{
+		"<a>", "<p>", "<b>", "<strong>",
+		"</a>", "</p>", "</b>", "</strong>",
+		"hello", "world",
+	}
+	x := make([]byte, 0, 1<<20)
+	for {
+		i := rand.Intn(len(tokens))
+		if len(x)+len(tokens[i]) >= 1<<20 {
+			break
+		}
+		x = append(x, tokens[i]...)
+	}
+	return string(x)
+}
+
+var benchInputHard = makeBenchInputHard()
+
+func benchmarkIndexHard(b *testing.B, sep string) {
+	for i := 0; i < b.N; i++ {
+		Index(benchInputHard, sep)
+	}
+}
+
+func benchmarkLastIndexHard(b *testing.B, sep string) {
+	for i := 0; i < b.N; i++ {
+		LastIndex(benchInputHard, sep)
+	}
+}
+
+func benchmarkCountHard(b *testing.B, sep string) {
+	for i := 0; i < b.N; i++ {
+		Count(benchInputHard, sep)
+	}
+}
+
+func BenchmarkIndexHard1(b *testing.B) { benchmarkIndexHard(b, "<>") }
+func BenchmarkIndexHard2(b *testing.B) { benchmarkIndexHard(b, "</pre>") }
+func BenchmarkIndexHard3(b *testing.B) { benchmarkIndexHard(b, "<b>hello world</b>") }
+func BenchmarkIndexHard4(b *testing.B) {
+	benchmarkIndexHard(b, "<pre><b>hello</b><strong>world</strong></pre>")
+}
+
+func BenchmarkLastIndexHard1(b *testing.B) { benchmarkLastIndexHard(b, "<>") }
+func BenchmarkLastIndexHard2(b *testing.B) { benchmarkLastIndexHard(b, "</pre>") }
+func BenchmarkLastIndexHard3(b *testing.B) { benchmarkLastIndexHard(b, "<b>hello world</b>") }
+
+func BenchmarkCountHard1(b *testing.B) { benchmarkCountHard(b, "<>") }
+func BenchmarkCountHard2(b *testing.B) { benchmarkCountHard(b, "</pre>") }
+func BenchmarkCountHard3(b *testing.B) { benchmarkCountHard(b, "<b>hello world</b>") }
+
+var benchInputTorture = Repeat("ABC", 1<<10) + "123" + Repeat("ABC", 1<<10)
+var benchNeedleTorture = Repeat("ABC", 1<<10+1)
+
+func BenchmarkIndexTorture(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Index(benchInputTorture, benchNeedleTorture)
+	}
+}
+
+func BenchmarkCountTorture(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Count(benchInputTorture, benchNeedleTorture)
+	}
+}
+
+func BenchmarkCountTortureOverlapping(b *testing.B) {
+	A := Repeat("ABC", 1<<20)
+	B := Repeat("ABC", 1<<10)
+	for i := 0; i < b.N; i++ {
+		Count(A, B)
+	}
+}
+
+func BenchmarkCountByte(b *testing.B) {
+	indexSizes := []int{10, 32, 4 << 10, 4 << 20, 64 << 20}
+	benchStr := Repeat(benchmarkString,
+		(indexSizes[len(indexSizes)-1]+len(benchmarkString)-1)/len(benchmarkString))
+	benchFunc := func(b *testing.B, benchStr string) {
+		b.SetBytes(int64(len(benchStr)))
+		for i := 0; i < b.N; i++ {
+			Count(benchStr, "=")
+		}
+	}
+	for _, size := range indexSizes {
+		b.Run(fmt.Sprintf("%d", size), func(b *testing.B) {
+			benchFunc(b, benchStr[:size])
+		})
+	}
+
+}
+
+var makeFieldsInput = func() string {
+	x := make([]byte, 1<<20)
+	// Input is ~10% space, ~10% 2-byte UTF-8, rest ASCII non-space.
+	for i := range x {
+		switch rand.Intn(10) {
+		case 0:
+			x[i] = ' '
+		case 1:
+			if i > 0 && x[i-1] == 'x' {
+				copy(x[i-1:], "χ")
+				break
+			}
+			fallthrough
+		default:
+			x[i] = 'x'
+		}
+	}
+	return string(x)
+}
+
+var makeFieldsInputASCII = func() string {
+	x := make([]byte, 1<<20)
+	// Input is ~10% space, rest ASCII non-space.
+	for i := range x {
+		if rand.Intn(10) == 0 {
+			x[i] = ' '
+		} else {
+			x[i] = 'x'
+		}
+	}
+	return string(x)
+}
+
+var stringdata = []struct{ name, data string }{
+	{"ASCII", makeFieldsInputASCII()},
+	{"Mixed", makeFieldsInput()},
+}
+
+func BenchmarkFields(b *testing.B) {
+	for _, sd := range stringdata {
+		b.Run(sd.name, func(b *testing.B) {
+			for j := 1 << 4; j <= 1<<20; j <<= 4 {
+				b.Run(fmt.Sprintf("%d", j), func(b *testing.B) {
+					b.ReportAllocs()
+					b.SetBytes(int64(j))
+					data := sd.data[:j]
+					for i := 0; i < b.N; i++ {
+						Fields(data)
+					}
+				})
+			}
+		})
+	}
+}
+
+func BenchmarkFieldsFunc(b *testing.B) {
+	for _, sd := range stringdata {
+		b.Run(sd.name, func(b *testing.B) {
+			for j := 1 << 4; j <= 1<<20; j <<= 4 {
+				b.Run(fmt.Sprintf("%d", j), func(b *testing.B) {
+					b.ReportAllocs()
+					b.SetBytes(int64(j))
+					data := sd.data[:j]
+					for i := 0; i < b.N; i++ {
+						FieldsFunc(data, unicode.IsSpace)
+					}
+				})
+			}
+		})
+	}
+}
+
+func BenchmarkSplitEmptySeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, "")
+	}
+}
+
+func BenchmarkSplitSingleByteSeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, "/")
+	}
+}
+
+func BenchmarkSplitMultiByteSeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, "hello")
+	}
+}
+
+func BenchmarkSplitNSingleByteSeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		SplitN(benchInputHard, "/", 10)
+	}
+}
+
+func BenchmarkSplitNMultiByteSeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		SplitN(benchInputHard, "hello", 10)
+	}
+}
+
+func BenchmarkRepeat(b *testing.B) {
+	s := "0123456789"
+	for _, n := range []int{5, 10} {
+		for _, c := range []int{0, 1, 2, 6} {
+			b.Run(fmt.Sprintf("%dx%d", n, c), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					Repeat(s[:n], c)
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkRepeatLarge(b *testing.B) {
+	s := Repeat("@", 8*1024)
+	for j := 8; j <= 30; j++ {
+		for _, k := range []int{1, 16, 4097} {
+			s := s[:k]
+			n := (1 << j) / k
+			if n == 0 {
+				continue
+			}
+			b.Run(fmt.Sprintf("%d/%d", 1<<j, k), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					Repeat(s, n)
+				}
+				b.SetBytes(int64(n * len(s)))
+			})
+		}
+	}
+}
+
+func BenchmarkIndexAnyASCII(b *testing.B) {
+	x := Repeat("#", 2048) // Never matches set
+	cs := "0123456789abcdefghijklmnopqrstuvwxyz0123456789abcdefghijklmnopqrstuvwxyz"
+	for k := 1; k <= 2048; k <<= 4 {
+		for j := 1; j <= 64; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					IndexAny(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkIndexAnyUTF8(b *testing.B) {
+	x := Repeat("#", 2048) // Never matches set
+	cs := "你好世界, hello world. 你好世界, hello world. 你好世界, hello world."
+	for k := 1; k <= 2048; k <<= 4 {
+		for j := 1; j <= 64; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					IndexAny(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkLastIndexAnyASCII(b *testing.B) {
+	x := Repeat("#", 2048) // Never matches set
+	cs := "0123456789abcdefghijklmnopqrstuvwxyz0123456789abcdefghijklmnopqrstuvwxyz"
+	for k := 1; k <= 2048; k <<= 4 {
+		for j := 1; j <= 64; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					LastIndexAny(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkLastIndexAnyUTF8(b *testing.B) {
+	x := Repeat("#", 2048) // Never matches set
+	cs := "你好世界, hello world. 你好世界, hello world. 你好世界, hello world."
+	for k := 1; k <= 2048; k <<= 4 {
+		for j := 1; j <= 64; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					LastIndexAny(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkTrimASCII(b *testing.B) {
+	cs := "0123456789abcdef"
+	for k := 1; k <= 4096; k <<= 4 {
+		for j := 1; j <= 16; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				x := Repeat(cs[:j], k) // Always matches set
+				for i := 0; i < b.N; i++ {
+					Trim(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkTrimByte(b *testing.B) {
+	x := "  the quick brown fox   "
+	for i := 0; i < b.N; i++ {
+		Trim(x, " ")
+	}
+}
+
+func BenchmarkIndexPeriodic(b *testing.B) {
+	key := "aa"
+	for _, skip := range [...]int{2, 4, 8, 16, 32, 64} {
+		b.Run(fmt.Sprintf("IndexPeriodic%d", skip), func(b *testing.B) {
+			s := Repeat("a"+Repeat(" ", skip-1), 1<<16/skip)
+			for i := 0; i < b.N; i++ {
+				Index(s, key)
+			}
+		})
+	}
+}
+
+func BenchmarkJoin(b *testing.B) {
+	vals := []string{"red", "yellow", "pink", "green", "purple", "orange", "blue"}
+	for l := 0; l <= len(vals); l++ {
+		b.Run(strconv.Itoa(l), func(b *testing.B) {
+			b.ReportAllocs()
+			vals := vals[:l]
+			for i := 0; i < b.N; i++ {
+				Join(vals, " and ")
+			}
+		})
+	}
+}
+
+func BenchmarkTrimSpace(b *testing.B) {
+	tests := []struct{ name, input string }{
+		{"NoTrim", "typical"},
+		{"ASCII", "  foo bar  "},
+		{"SomeNonASCII", "    \u2000\t\r\n x\t\t\r\r\ny\n \u3000    "},
+		{"JustNonASCII", "\u2000\u2000\u2000☺☺☺☺\u3000\u3000\u3000"},
+	}
+	for _, test := range tests {
+		b.Run(test.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				TrimSpace(test.input)
+			}
+		})
+	}
+}
+
+var stringSink string
+
+func BenchmarkReplaceAll(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		stringSink = ReplaceAll("banana", "a", "<>")
+	}
+}