src/strings/builder.go

// 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 (
	"internal/abi"
	"internal/bytealg"
	"unicode/utf8"
	"unsafe"
)

// A Builder is used to efficiently build a string using [Builder.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

	// External users should never get direct access to this buffer, since
	// the slice at some point will be converted to a string using unsafe, also
	// data between len(buf) and cap(buf) might be uninitialized.
	buf []byte
}

// copyCheck implements a dynamic check to prevent modification after
// copying a non-zero Builder, which would be unsafe (see #25907, #47276).
//
// We cannot add a noCopy field to Builder, to cause vet's copylocks
// check to report copying, because copylocks cannot reliably
// discriminate the zero and nonzero cases.
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)(abi.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 unsafe.String(unsafe.SliceData(b.buf), len(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 := bytealg.MakeNoZero(2*cap(b.buf) + n)[:len(b.buf)]
	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) {
	b.copyCheck()
	if n < 0 {
		panic("strings.Builder.Grow: negative count")
	}
	if cap(b.buf)-len(b.buf) < n {
		b.grow(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.copyCheck()
	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.copyCheck()
	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) {
	b.copyCheck()
	n := len(b.buf)
	b.buf = utf8.AppendRune(b.buf, r)
	return len(b.buf) - 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.copyCheck()
	b.buf = append(b.buf, s...)
	return len(s), nil
}