// 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} }