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Source file src/bytes/reader.go

Documentation: bytes

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

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