1// Copyright 2009 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 strings
6
7import (
8	"errors"
9	"io"
10	"unicode/utf8"
11)
12
13// A Reader implements the [io.Reader], [io.ReaderAt], [io.ByteReader], [io.ByteScanner],
14// [io.RuneReader], [io.RuneScanner], [io.Seeker], and [io.WriterTo] interfaces by reading
15// from a string.
16// The zero value for Reader operates like a Reader of an empty string.
17type Reader struct {
18	s        string
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// string.
25func (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 string.
33// Size is the number of bytes available for reading via [Reader.ReadAt].
34// The returned value is always the same and is not affected by calls
35// to any other method.
36func (r *Reader) Size() int64 { return int64(len(r.s)) }
37
38// Read implements the [io.Reader] interface.
39func (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.
50func (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("strings.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.
66func (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 implements the [io.ByteScanner] interface.
77func (r *Reader) UnreadByte() error {
78	if r.i <= 0 {
79		return errors.New("strings.Reader.UnreadByte: at beginning of string")
80	}
81	r.prevRune = -1
82	r.i--
83	return nil
84}
85
86// ReadRune implements the [io.RuneReader] interface.
87func (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.DecodeRuneInString(r.s[r.i:])
98	r.i += int64(size)
99	return
100}
101
102// UnreadRune implements the [io.RuneScanner] interface.
103func (r *Reader) UnreadRune() error {
104	if r.i <= 0 {
105		return errors.New("strings.Reader.UnreadRune: at beginning of string")
106	}
107	if r.prevRune < 0 {
108		return errors.New("strings.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.
116func (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("strings.Reader.Seek: invalid whence")
128	}
129	if abs < 0 {
130		return 0, errors.New("strings.Reader.Seek: negative position")
131	}
132	r.i = abs
133	return abs, nil
134}
135
136// WriteTo implements the [io.WriterTo] interface.
137func (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	s := r.s[r.i:]
143	m, err := io.WriteString(w, s)
144	if m > len(s) {
145		panic("strings.Reader.WriteTo: invalid WriteString count")
146	}
147	r.i += int64(m)
148	n = int64(m)
149	if m != len(s) && err == nil {
150		err = io.ErrShortWrite
151	}
152	return
153}
154
155// Reset resets the [Reader] to be reading from s.
156func (r *Reader) Reset(s string) { *r = Reader{s, 0, -1} }
157
158// NewReader returns a new [Reader] reading from s.
159// It is similar to [bytes.NewBufferString] but more efficient and non-writable.
160func NewReader(s string) *Reader { return &Reader{s, 0, -1} }
161