1 files changed, 160 insertions, 0 deletions
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} }