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