bytes: add Reader

copy of strings/reader{,_test}.go with
a few s/string/[]byte/

Fixes #2861

R=adg, rsc, r
CC=golang-dev
https://golang.org/cl/5666044

src/pkg/bytes/reader.go

+// 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 bytes
+
+import (
+	"errors"
+	"io"
+	"unicode/utf8"
+)
+
+// A Reader implements the io.Reader, io.Seeker, io.ByteScanner, and
+// io.RuneScanner interfaces by reading from a byte slice.
+// Unlike a Buffer, a Reader is read-only and supports seeking.
+type Reader struct {
+	s        []byte
+	i        int // current reading index
+	prevRune int // index of previous rune; or < 0
+}
+
+// Len returns the number of bytes of the unread portion of the
+// slice.
+func (r *Reader) Len() int {
+	if r.i >= len(r.s) {
+		return 0
+	}
+	return len(r.s) - r.i
+}
+
+func (r *Reader) Read(b []byte) (n int, err error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
+	if r.i >= len(r.s) {
+		return 0, io.EOF
+	}
+	n = copy(b, r.s[r.i:])
+	r.i += n
+	r.prevRune = -1
+	return
+}
+
+func (r *Reader) ReadByte() (b byte, err error) {
+	if r.i >= len(r.s) {
+		return 0, io.EOF
+	}
+	b = r.s[r.i]
+	r.i++
+	r.prevRune = -1
+	return
+}
+
+func (r *Reader) UnreadByte() error {
+	if r.i <= 0 {
+		return errors.New("bytes.Reader: at beginning of slice")
+	}
+	r.i--
+	r.prevRune = -1
+	return nil
+}
+
+func (r *Reader) ReadRune() (ch rune, size int, err error) {
+	if r.i >= len(r.s) {
+		return 0, 0, io.EOF
+	}
+	r.prevRune = r.i
+	if c := r.s[r.i]; c < utf8.RuneSelf {
+		r.i++
+		return rune(c), 1, nil
+	}
+	ch, size = utf8.DecodeRune(r.s[r.i:])
+	r.i += size
+	return
+}
+
+func (r *Reader) UnreadRune() error {
+	if r.prevRune < 0 {
+		return errors.New("bytes.Reader: previous operation was not ReadRune")
+	}
+	r.i = r.prevRune
+	r.prevRune = -1
+	return nil
+}
+
+// Seek implements the io.Seeker interface.
+func (r *Reader) Seek(offset int64, whence int) (int64, error) {
+	var abs int64
+	switch whence {
+	case 0:
+		abs = offset
+	case 1:
+		abs = int64(r.i) + offset
+	case 2:
+		abs = int64(len(r.s)) + offset
+	default:
+		return 0, errors.New("bytes: invalid whence")
+	}
+	if abs < 0 {
+		return 0, errors.New("bytes: negative position")
+	}
+	if abs >= 1<<31 {
+		return 0, errors.New("bytes: position out of range")
+	}
+	r.i = int(abs)
+	return abs, nil
+}
+
+// NewReader returns a new Reader reading from b.
+func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }

src/pkg/bytes/reader_test.go

+// 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 bytes_test
+
+import (
+	. "bytes"
+	"os"
+	"testing"
+)
+
+func TestReader(t *testing.T) {
+	r := NewReader([]byte("0123456789"))
+	tests := []struct {
+		off     int64
+		seek    int
+		n       int
+		want    string
+		wantpos int64
+		seekerr string
+	}{
+		{seek: os.SEEK_SET, off: 0, n: 20, want: "0123456789"},
+		{seek: os.SEEK_SET, off: 1, n: 1, want: "1"},
+		{seek: os.SEEK_CUR, off: 1, wantpos: 3, n: 2, want: "34"},
+		{seek: os.SEEK_SET, off: -1, seekerr: "bytes: negative position"},
+		{seek: os.SEEK_SET, off: 1<<31 - 1},
+		{seek: os.SEEK_CUR, off: 1, seekerr: "bytes: position out of range"},
+		{seek: os.SEEK_SET, n: 5, want: "01234"},
+		{seek: os.SEEK_CUR, n: 5, want: "56789"},
+		{seek: os.SEEK_END, 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 != nil {
+			t.Errorf("%d. read = %v", i, err)
+			continue
+		}
+		got := string(buf[:n])
+		if got != tt.want {
+			t.Errorf("%d. got %q; want %q", i, got, tt.want)
+		}
+	}
+}