1// Copyright 2017 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	"unicode/utf8"
9	"unsafe"
10)
11
12// A Builder is used to efficiently build a string using Write methods.
13// It minimizes memory copying. The zero value is ready to use.
14// Do not copy a non-zero Builder.
15type Builder struct {
16	addr *Builder // of receiver, to detect copies by value
17	buf  []byte
18}
19
20// noescape hides a pointer from escape analysis.  noescape is
21// the identity function but escape analysis doesn't think the
22// output depends on the input. noescape is inlined and currently
23// compiles down to zero instructions.
24// USE CAREFULLY!
25// This was copied from the runtime; see issues 23382 and 7921.
26//go:nosplit
27//go:nocheckptr
28func noescape(p unsafe.Pointer) unsafe.Pointer {
29	x := uintptr(p)
30	return unsafe.Pointer(x ^ 0)
31}
32
33func (b *Builder) copyCheck() {
34	if b.addr == nil {
35		// This hack works around a failing of Go's escape analysis
36		// that was causing b to escape and be heap allocated.
37		// See issue 23382.
38		// TODO: once issue 7921 is fixed, this should be reverted to
39		// just "b.addr = b".
40		b.addr = (*Builder)(noescape(unsafe.Pointer(b)))
41	} else if b.addr != b {
42		panic("strings: illegal use of non-zero Builder copied by value")
43	}
44}
45
46// String returns the accumulated string.
47func (b *Builder) String() string {
48	return *(*string)(unsafe.Pointer(&b.buf))
49}
50
51// Len returns the number of accumulated bytes; b.Len() == len(b.String()).
52func (b *Builder) Len() int { return len(b.buf) }
53
54// Cap returns the capacity of the builder's underlying byte slice. It is the
55// total space allocated for the string being built and includes any bytes
56// already written.
57func (b *Builder) Cap() int { return cap(b.buf) }
58
59// Reset resets the Builder to be empty.
60func (b *Builder) Reset() {
61	b.addr = nil
62	b.buf = nil
63}
64
65// grow copies the buffer to a new, larger buffer so that there are at least n
66// bytes of capacity beyond len(b.buf).
67func (b *Builder) grow(n int) {
68	buf := make([]byte, len(b.buf), 2*cap(b.buf)+n)
69	copy(buf, b.buf)
70	b.buf = buf
71}
72
73// Grow grows b's capacity, if necessary, to guarantee space for
74// another n bytes. After Grow(n), at least n bytes can be written to b
75// without another allocation. If n is negative, Grow panics.
76func (b *Builder) Grow(n int) {
77	b.copyCheck()
78	if n < 0 {
79		panic("strings.Builder.Grow: negative count")
80	}
81	if cap(b.buf)-len(b.buf) < n {
82		b.grow(n)
83	}
84}
85
86// Write appends the contents of p to b's buffer.
87// Write always returns len(p), nil.
88func (b *Builder) Write(p []byte) (int, error) {
89	b.copyCheck()
90	b.buf = append(b.buf, p...)
91	return len(p), nil
92}
93
94// WriteByte appends the byte c to b's buffer.
95// The returned error is always nil.
96func (b *Builder) WriteByte(c byte) error {
97	b.copyCheck()
98	b.buf = append(b.buf, c)
99	return nil
100}
101
102// WriteRune appends the UTF-8 encoding of Unicode code point r to b's buffer.
103// It returns the length of r and a nil error.
104func (b *Builder) WriteRune(r rune) (int, error) {
105	b.copyCheck()
106	if r < utf8.RuneSelf {
107		b.buf = append(b.buf, byte(r))
108		return 1, nil
109	}
110	l := len(b.buf)
111	if cap(b.buf)-l < utf8.UTFMax {
112		b.grow(utf8.UTFMax)
113	}
114	n := utf8.EncodeRune(b.buf[l:l+utf8.UTFMax], r)
115	b.buf = b.buf[:l+n]
116	return n, nil
117}
118
119// WriteString appends the contents of s to b's buffer.
120// It returns the length of s and a nil error.
121func (b *Builder) WriteString(s string) (int, error) {
122	b.copyCheck()
123	b.buf = append(b.buf, s...)
124	return len(s), nil
125}
126