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 5 package strings 6 7 import ( 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. 15 type 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 func noescape(p unsafe.Pointer) unsafe.Pointer { 28 x := uintptr(p) 29 return unsafe.Pointer(x ^ 0) 30 } 31 32 func (b *Builder) copyCheck() { 33 if b.addr == nil { 34 // This hack works around a failing of Go's escape analysis 35 // that was causing b to escape and be heap allocated. 36 // See issue 23382. 37 // TODO: once issue 7921 is fixed, this should be reverted to 38 // just "b.addr = b". 39 b.addr = (*Builder)(noescape(unsafe.Pointer(b))) 40 } else if b.addr != b { 41 panic("strings: illegal use of non-zero Builder copied by value") 42 } 43 } 44 45 // String returns the accumulated string. 46 func (b *Builder) String() string { 47 return *(*string)(unsafe.Pointer(&b.buf)) 48 } 49 50 // Len returns the number of accumulated bytes; b.Len() == len(b.String()). 51 func (b *Builder) Len() int { return len(b.buf) } 52 53 // Cap returns the capacity of the builder's underlying byte slice. It is the 54 // total space allocated for the string being built and includes any bytes 55 // already written. 56 func (b *Builder) Cap() int { return cap(b.buf) } 57 58 // Reset resets the Builder to be empty. 59 func (b *Builder) Reset() { 60 b.addr = nil 61 b.buf = nil 62 } 63 64 // grow copies the buffer to a new, larger buffer so that there are at least n 65 // bytes of capacity beyond len(b.buf). 66 func (b *Builder) grow(n int) { 67 buf := make([]byte, len(b.buf), 2*cap(b.buf)+n) 68 copy(buf, b.buf) 69 b.buf = buf 70 } 71 72 // Grow grows b's capacity, if necessary, to guarantee space for 73 // another n bytes. After Grow(n), at least n bytes can be written to b 74 // without another allocation. If n is negative, Grow panics. 75 func (b *Builder) Grow(n int) { 76 b.copyCheck() 77 if n < 0 { 78 panic("strings.Builder.Grow: negative count") 79 } 80 if cap(b.buf)-len(b.buf) < n { 81 b.grow(n) 82 } 83 } 84 85 // Write appends the contents of p to b's buffer. 86 // Write always returns len(p), nil. 87 func (b *Builder) Write(p []byte) (int, error) { 88 b.copyCheck() 89 b.buf = append(b.buf, p...) 90 return len(p), nil 91 } 92 93 // WriteByte appends the byte c to b's buffer. 94 // The returned error is always nil. 95 func (b *Builder) WriteByte(c byte) error { 96 b.copyCheck() 97 b.buf = append(b.buf, c) 98 return nil 99 } 100 101 // WriteRune appends the UTF-8 encoding of Unicode code point r to b's buffer. 102 // It returns the length of r and a nil error. 103 func (b *Builder) WriteRune(r rune) (int, error) { 104 b.copyCheck() 105 if r < utf8.RuneSelf { 106 b.buf = append(b.buf, byte(r)) 107 return 1, nil 108 } 109 l := len(b.buf) 110 if cap(b.buf)-l < utf8.UTFMax { 111 b.grow(utf8.UTFMax) 112 } 113 n := utf8.EncodeRune(b.buf[l:l+utf8.UTFMax], r) 114 b.buf = b.buf[:l+n] 115 return n, nil 116 } 117 118 // WriteString appends the contents of s to b's buffer. 119 // It returns the length of s and a nil error. 120 func (b *Builder) WriteString(s string) (int, error) { 121 b.copyCheck() 122 b.buf = append(b.buf, s...) 123 return len(s), nil 124 } 125
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