reader.go

     1  // Copyright 2012 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package bytes
     6  
     7  import (
     8  	"errors"
     9  	"io"
    10  	"unicode/utf8"
    11  )
    12  
    13  // A Reader implements the io.Reader, io.ReaderAt, io.WriterTo, io.Seeker,
    14  // io.ByteScanner, and io.RuneScanner interfaces by reading from
    15  // a byte slice.
    16  // Unlike a Buffer, a Reader is read-only and supports seeking.
    17  // The zero value for Reader operates like a Reader of an empty slice.
    18  type Reader struct {
    19  	s        []byte
    20  	i        int64 // current reading index
    21  	prevRune int   // index of previous rune; or < 0
    22  }
    23  
    24  // Len returns the number of bytes of the unread portion of the
    25  // slice.
    26  func (r *Reader) Len() int {
    27  	if r.i >= int64(len(r.s)) {
    28  		return 0
    29  	}
    30  	return int(int64(len(r.s)) - r.i)
    31  }
    32  
    33  // Size returns the original length of the underlying byte slice.
    34  // Size is the number of bytes available for reading via ReadAt.
    35  // The returned value is always the same and is not affected by calls
    36  // to any other method.
    37  func (r *Reader) Size() int64 { return int64(len(r.s)) }
    38  
    39  // Read implements the io.Reader interface.
    40  func (r *Reader) Read(b []byte) (n int, err error) {
    41  	if r.i >= int64(len(r.s)) {
    42  		return 0, io.EOF
    43  	}
    44  	r.prevRune = -1
    45  	n = copy(b, r.s[r.i:])
    46  	r.i += int64(n)
    47  	return
    48  }
    49  
    50  // ReadAt implements the io.ReaderAt interface.
    51  func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
    52  	// cannot modify state - see io.ReaderAt
    53  	if off < 0 {
    54  		return 0, errors.New("bytes.Reader.ReadAt: negative offset")
    55  	}
    56  	if off >= int64(len(r.s)) {
    57  		return 0, io.EOF
    58  	}
    59  	n = copy(b, r.s[off:])
    60  	if n < len(b) {
    61  		err = io.EOF
    62  	}
    63  	return
    64  }
    65  
    66  // ReadByte implements the io.ByteReader interface.
    67  func (r *Reader) ReadByte() (byte, error) {
    68  	r.prevRune = -1
    69  	if r.i >= int64(len(r.s)) {
    70  		return 0, io.EOF
    71  	}
    72  	b := r.s[r.i]
    73  	r.i++
    74  	return b, nil
    75  }
    76  
    77  // UnreadByte complements ReadByte in implementing the io.ByteScanner interface.
    78  func (r *Reader) UnreadByte() error {
    79  	if r.i <= 0 {
    80  		return errors.New("bytes.Reader.UnreadByte: at beginning of slice")
    81  	}
    82  	r.prevRune = -1
    83  	r.i--
    84  	return nil
    85  }
    86  
    87  // ReadRune implements the io.RuneReader interface.
    88  func (r *Reader) ReadRune() (ch rune, size int, err error) {
    89  	if r.i >= int64(len(r.s)) {
    90  		r.prevRune = -1
    91  		return 0, 0, io.EOF
    92  	}
    93  	r.prevRune = int(r.i)
    94  	if c := r.s[r.i]; c < utf8.RuneSelf {
    95  		r.i++
    96  		return rune(c), 1, nil
    97  	}
    98  	ch, size = utf8.DecodeRune(r.s[r.i:])
    99  	r.i += int64(size)
   100  	return
   101  }
   102  
   103  // UnreadRune complements ReadRune in implementing the io.RuneScanner interface.
   104  func (r *Reader) UnreadRune() error {
   105  	if r.i <= 0 {
   106  		return errors.New("bytes.Reader.UnreadRune: at beginning of slice")
   107  	}
   108  	if r.prevRune < 0 {
   109  		return errors.New("bytes.Reader.UnreadRune: previous operation was not ReadRune")
   110  	}
   111  	r.i = int64(r.prevRune)
   112  	r.prevRune = -1
   113  	return nil
   114  }
   115  
   116  // Seek implements the io.Seeker interface.
   117  func (r *Reader) Seek(offset int64, whence int) (int64, error) {
   118  	r.prevRune = -1
   119  	var abs int64
   120  	switch whence {
   121  	case io.SeekStart:
   122  		abs = offset
   123  	case io.SeekCurrent:
   124  		abs = r.i + offset
   125  	case io.SeekEnd:
   126  		abs = int64(len(r.s)) + offset
   127  	default:
   128  		return 0, errors.New("bytes.Reader.Seek: invalid whence")
   129  	}
   130  	if abs < 0 {
   131  		return 0, errors.New("bytes.Reader.Seek: negative position")
   132  	}
   133  	r.i = abs
   134  	return abs, nil
   135  }
   136  
   137  // WriteTo implements the io.WriterTo interface.
   138  func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
   139  	r.prevRune = -1
   140  	if r.i >= int64(len(r.s)) {
   141  		return 0, nil
   142  	}
   143  	b := r.s[r.i:]
   144  	m, err := w.Write(b)
   145  	if m > len(b) {
   146  		panic("bytes.Reader.WriteTo: invalid Write count")
   147  	}
   148  	r.i += int64(m)
   149  	n = int64(m)
   150  	if m != len(b) && err == nil {
   151  		err = io.ErrShortWrite
   152  	}
   153  	return
   154  }
   155  
   156  // Reset resets the Reader to be reading from b.
   157  func (r *Reader) Reset(b []byte) { *r = Reader{b, 0, -1} }
   158  
   159  // NewReader returns a new Reader reading from b.
   160  func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }
   161
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