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// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package strings
import (
// A Builder is used to efficiently build a string using Write methods.
// It minimizes memory copying. The zero value is ready to use.
// Do not copy a non-zero Builder.
type Builder struct {
addr *Builder // of receiver, to detect copies by value
buf []byte
// noescape hides a pointer from escape analysis. noescape is
// the identity function but escape analysis doesn't think the
// output depends on the input. noescape is inlined and currently
// compiles down to zero instructions.
// This was copied from the runtime; see issues 23382 and 7921.
func noescape(p unsafe.Pointer) unsafe.Pointer {
x := uintptr(p)
return unsafe.Pointer(x ^ 0)
func (b *Builder) copyCheck() {
if b.addr == nil {
// This hack works around a failing of Go's escape analysis
// that was causing b to escape and be heap allocated.
// See issue 23382.
// TODO: once issue 7921 is fixed, this should be reverted to
// just "b.addr = b".
b.addr = (*Builder)(noescape(unsafe.Pointer(b)))
} else if b.addr != b {
panic("strings: illegal use of non-zero Builder copied by value")
// String returns the accumulated string.
func (b *Builder) String() string {
return *(*string)(unsafe.Pointer(&b.buf))
// Len returns the number of accumulated bytes; b.Len() == len(b.String()).
func (b *Builder) Len() int { return len(b.buf) }
// Cap returns the capacity of the builder's underlying byte slice. It is the
// total space allocated for the string being built and includes any bytes
// already written.
func (b *Builder) Cap() int { return cap(b.buf) }
// Reset resets the Builder to be empty.
func (b *Builder) Reset() {
b.addr = nil
b.buf = nil
// grow copies the buffer to a new, larger buffer so that there are at least n
// bytes of capacity beyond len(b.buf).
func (b *Builder) grow(n int) {
buf := make([]byte, len(b.buf), 2*cap(b.buf)+n)
copy(buf, b.buf)
b.buf = buf
// Grow grows b's capacity, if necessary, to guarantee space for
// another n bytes. After Grow(n), at least n bytes can be written to b
// without another allocation. If n is negative, Grow panics.
func (b *Builder) Grow(n int) {
if n < 0 {
panic("strings.Builder.Grow: negative count")
if cap(b.buf)-len(b.buf) < n {
// Write appends the contents of p to b's buffer.
// Write always returns len(p), nil.
func (b *Builder) Write(p []byte) (int, error) {
b.buf = append(b.buf, p...)
return len(p), nil
// WriteByte appends the byte c to b's buffer.
// The returned error is always nil.
func (b *Builder) WriteByte(c byte) error {
b.buf = append(b.buf, c)
return nil
// WriteRune appends the UTF-8 encoding of Unicode code point r to b's buffer.
// It returns the length of r and a nil error.
func (b *Builder) WriteRune(r rune) (int, error) {
if r < utf8.RuneSelf {
b.buf = append(b.buf, byte(r))
return 1, nil
l := len(b.buf)
if cap(b.buf)-l < utf8.UTFMax {
n := utf8.EncodeRune(b.buf[l:l+utf8.UTFMax], r)
b.buf = b.buf[:l+n]
return n, nil
// WriteString appends the contents of s to b's buffer.
// It returns the length of s and a nil error.
func (b *Builder) WriteString(s string) (int, error) {
b.buf = append(b.buf, s...)
return len(s), nil