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Source file src/strings/reader.go

  // 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.Seeker, io.WriterTo,
  // io.ByteScanner, and io.RuneScanner interfaces by reading
  // from a 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 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)) }
  
  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
  }
  
  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
  }
  
  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
  }
  
  func (r *Reader) UnreadByte() error {
  	r.prevRune = -1
  	if r.i <= 0 {
  		return errors.New("strings.Reader.UnreadByte: at beginning of string")
  	}
  	r.i--
  	return nil
  }
  
  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
  }
  
  func (r *Reader) UnreadRune() error {
  	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 read-only.
  func NewReader(s string) *Reader { return &Reader{s, 0, -1} }
  

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