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 [Reader.ReadAt].
    35  // The result is unaffected by any method calls except [Reader.Reset].
    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 [Reader.ReadByte] in implementing the [io.ByteScanner] interface.
    77  func (r *Reader) UnreadByte() error {
    78  	if r.i <= 0 {
    79  		return errors.New("bytes.Reader.UnreadByte: at beginning of slice")
    80  	}
    81  	r.prevRune = -1
    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 [Reader.ReadRune] in implementing the [io.RuneScanner] interface.
   103  func (r *Reader) UnreadRune() error {
   104  	if r.i <= 0 {
   105  		return errors.New("bytes.Reader.UnreadRune: at beginning of slice")
   106  	}
   107  	if r.prevRune < 0 {
   108  		return errors.New("bytes.Reader.UnreadRune: previous operation was not ReadRune")
   109  	}
   110  	r.i = int64(r.prevRune)
   111  	r.prevRune = -1
   112  	return nil
   113  }
   114  
   115  // Seek implements the [io.Seeker] interface.
   116  func (r *Reader) Seek(offset int64, whence int) (int64, error) {
   117  	r.prevRune = -1
   118  	var abs int64
   119  	switch whence {
   120  	case io.SeekStart:
   121  		abs = offset
   122  	case io.SeekCurrent:
   123  		abs = r.i + offset
   124  	case io.SeekEnd:
   125  		abs = int64(len(r.s)) + offset
   126  	default:
   127  		return 0, errors.New("bytes.Reader.Seek: invalid whence")
   128  	}
   129  	if abs < 0 {
   130  		return 0, errors.New("bytes.Reader.Seek: negative position")
   131  	}
   132  	r.i = abs
   133  	return abs, nil
   134  }
   135  
   136  // WriteTo implements the [io.WriterTo] interface.
   137  func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
   138  	r.prevRune = -1
   139  	if r.i >= int64(len(r.s)) {
   140  		return 0, nil
   141  	}
   142  	b := r.s[r.i:]
   143  	m, err := w.Write(b)
   144  	if m > len(b) {
   145  		panic("bytes.Reader.WriteTo: invalid Write count")
   146  	}
   147  	r.i += int64(m)
   148  	n = int64(m)
   149  	if m != len(b) && err == nil {
   150  		err = io.ErrShortWrite
   151  	}
   152  	return
   153  }
   154  
   155  // Reset resets the [Reader] to be reading from b.
   156  func (r *Reader) Reset(b []byte) { *r = Reader{b, 0, -1} }
   157  
   158  // NewReader returns a new [Reader] reading from b.
   159  func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }
   160
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