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
5package bytes
6
7import (
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.
18type 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.
26func (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.
37func (r *Reader) Size() int64 { return int64(len(r.s)) }
38
39// Read implements the io.Reader interface.
40func (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.
51func (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.
67func (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.
78func (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.
88func (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.
104func (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.
117func (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.
138func (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.
157func (r *Reader) Reset(b []byte) { *r = Reader{b, 0, -1} }
158
159// NewReader returns a new Reader reading from b.
160func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }
161