2 files changed, 218 insertions, 0 deletions
diff --git a/src/internal/coverage/slicereader/slicereader.go b/src/internal/coverage/slicereader/slicereader.go
new file mode 100644
index 0000000..d9f2a7e
--- /dev/null
+++ b/src/internal/coverage/slicereader/slicereader.go
@@ -0,0 +1,123 @@
+// 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 slicereader
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"unsafe"
+)
+
+// This file contains the helper "SliceReader", a utility for
+// reading values from a byte slice that may or may not be backed
+// by a read-only mmap'd region.
+
+type Reader struct {
+	b        []byte
+	readonly bool
+	off      int64
+}
+
+func NewReader(b []byte, readonly bool) *Reader {
+	r := Reader{
+		b:        b,
+		readonly: readonly,
+	}
+	return &r
+}
+
+func (r *Reader) Read(b []byte) (int, error) {
+	amt := len(b)
+	toread := r.b[r.off:]
+	if len(toread) < amt {
+		amt = len(toread)
+	}
+	copy(b, toread)
+	r.off += int64(amt)
+	return amt, nil
+}
+
+func (r *Reader) Seek(offset int64, whence int) (ret int64, err error) {
+	switch whence {
+	case io.SeekStart:
+		if offset < 0 || offset > int64(len(r.b)) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", offset, len(r.b))
+		}
+		r.off = offset
+		return offset, nil
+	case io.SeekCurrent:
+		newoff := r.off + offset
+		if newoff < 0 || newoff > int64(len(r.b)) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(r.b))
+		}
+		r.off = newoff
+		return r.off, nil
+	case io.SeekEnd:
+		newoff := int64(len(r.b)) + offset
+		if newoff < 0 || newoff > int64(len(r.b)) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(r.b))
+		}
+		r.off = newoff
+		return r.off, nil
+	}
+	// other modes are not supported
+	return 0, fmt.Errorf("unsupported seek mode %d", whence)
+}
+
+func (r *Reader) Offset() int64 {
+	return r.off
+}
+
+func (r *Reader) ReadUint8() uint8 {
+	rv := uint8(r.b[int(r.off)])
+	r.off += 1
+	return rv
+}
+
+func (r *Reader) ReadUint32() uint32 {
+	end := int(r.off) + 4
+	rv := binary.LittleEndian.Uint32(r.b[int(r.off):end:end])
+	r.off += 4
+	return rv
+}
+
+func (r *Reader) ReadUint64() uint64 {
+	end := int(r.off) + 8
+	rv := binary.LittleEndian.Uint64(r.b[int(r.off):end:end])
+	r.off += 8
+	return rv
+}
+
+func (r *Reader) ReadULEB128() (value uint64) {
+	var shift uint
+
+	for {
+		b := r.b[r.off]
+		r.off++
+		value |= (uint64(b&0x7F) << shift)
+		if b&0x80 == 0 {
+			break
+		}
+		shift += 7
+	}
+	return
+}
+
+func (r *Reader) ReadString(len int64) string {
+	b := r.b[r.off : r.off+len]
+	r.off += len
+	if r.readonly {
+		return toString(b) // backed by RO memory, ok to make unsafe string
+	}
+	return string(b)
+}
+
+func toString(b []byte) string {
+	if len(b) == 0 {
+		return ""
+	}
+	return unsafe.String(&b[0], len(b))
+}
diff --git a/src/internal/coverage/slicereader/slr_test.go b/src/internal/coverage/slicereader/slr_test.go
new file mode 100644
index 0000000..461436d
--- /dev/null
+++ b/src/internal/coverage/slicereader/slr_test.go
@@ -0,0 +1,95 @@
+// 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 slicereader
+
+import (
+	"encoding/binary"
+	"io"
+	"testing"
+)
+
+func TestSliceReader(t *testing.T) {
+	b := []byte{}
+
+	bt := make([]byte, 4)
+	e32 := uint32(1030507)
+	binary.LittleEndian.PutUint32(bt, e32)
+	b = append(b, bt...)
+
+	bt = make([]byte, 8)
+	e64 := uint64(907050301)
+	binary.LittleEndian.PutUint64(bt, e64)
+	b = append(b, bt...)
+
+	b = appendUleb128(b, uint(e32))
+	b = appendUleb128(b, uint(e64))
+	b = appendUleb128(b, 6)
+	s1 := "foobar"
+	s1b := []byte(s1)
+	b = append(b, s1b...)
+	b = appendUleb128(b, 9)
+	s2 := "bazbasher"
+	s2b := []byte(s2)
+	b = append(b, s2b...)
+
+	readStr := func(slr *Reader) string {
+		len := slr.ReadULEB128()
+		return slr.ReadString(int64(len))
+	}
+
+	for i := 0; i < 2; i++ {
+		slr := NewReader(b, i == 0)
+		g32 := slr.ReadUint32()
+		if g32 != e32 {
+			t.Fatalf("slr.ReadUint32() got %d want %d", g32, e32)
+		}
+		g64 := slr.ReadUint64()
+		if g64 != e64 {
+			t.Fatalf("slr.ReadUint64() got %d want %d", g64, e64)
+		}
+		g32 = uint32(slr.ReadULEB128())
+		if g32 != e32 {
+			t.Fatalf("slr.ReadULEB128() got %d want %d", g32, e32)
+		}
+		g64 = slr.ReadULEB128()
+		if g64 != e64 {
+			t.Fatalf("slr.ReadULEB128() got %d want %d", g64, e64)
+		}
+		gs1 := readStr(slr)
+		if gs1 != s1 {
+			t.Fatalf("readStr got %s want %s", gs1, s1)
+		}
+		gs2 := readStr(slr)
+		if gs2 != s2 {
+			t.Fatalf("readStr got %s want %s", gs2, s2)
+		}
+		if _, err := slr.Seek(4, io.SeekStart); err != nil {
+			t.Fatal(err)
+		}
+		off := slr.Offset()
+		if off != 4 {
+			t.Fatalf("Offset() returned %d wanted 4", off)
+		}
+		g64 = slr.ReadUint64()
+		if g64 != e64 {
+			t.Fatalf("post-seek slr.ReadUint64() got %d want %d", g64, e64)
+		}
+	}
+}
+
+func appendUleb128(b []byte, v uint) []byte {
+	for {
+		c := uint8(v & 0x7f)
+		v >>= 7
+		if v != 0 {
+			c |= 0x80
+		}
+		b = append(b, c)
+		if c&0x80 == 0 {
+			break
+		}
+	}
+	return b
+}