// 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.Reader] to be reading from b. func (r *Reader) Reset(b []byte) { *r = Reader{b, 0, -1} } // NewReader returns a new [Reader.Reader] reading from b. func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }