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// 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.ReaderAt, io.WriterTo, 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.
// The zero value for Reader operates like a Reader of an empty slice.
type Reader struct {
s []byte
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
// slice.
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 byte slice.
// Size is the number of bytes available for reading via [Reader.ReadAt].
// The result is unaffected by any method calls except [Reader.Reset].
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("bytes.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 complements [Reader.ReadByte] in implementing the [io.ByteScanner] interface.
func (r *Reader) UnreadByte() error {
if r.i <= 0 {
return errors.New("bytes.Reader.UnreadByte: at beginning of slice")
}
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.DecodeRune(r.s[r.i:])
r.i += int64(size)
return
}
// UnreadRune complements [Reader.ReadRune] in implementing the [io.RuneScanner] interface.
func (r *Reader) UnreadRune() error {
if r.i <= 0 {
return errors.New("bytes.Reader.UnreadRune: at beginning of slice")
}
if r.prevRune < 0 {
return errors.New("bytes.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("bytes.Reader.Seek: invalid whence")
}
if abs < 0 {
return 0, errors.New("bytes.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
}
b := r.s[r.i:]
m, err := w.Write(b)
if m > len(b) {
panic("bytes.Reader.WriteTo: invalid Write count")
}
r.i += int64(m)
n = int64(m)
if m != len(b) && err == nil {
err = io.ErrShortWrite
}
return
}
// Reset resets the [Reader] to be reading from b.
func (r *Reader) Reset(b []byte) { *r = Reader{b, 0, -1} }
// NewReader returns a new [Reader] reading from b.
func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }
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