reader.mx raw
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