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