Source: builder.go in package strings


package strings

import (
	"unicode/utf8"
	"unsafe"
)

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. USE CAREFULLY! This was copied from the runtime; see issues 23382 and 7921.go:nosplitgo:nocheckptr

func noescape(p unsafe.Pointer) unsafe.Pointer {
	x := uintptr(p)
	return unsafe.Pointer(x ^ 0)
}

func (b *Builder) copyCheck() {

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) {
	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()
	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 {
		b.grow(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.copyCheck()
	b.buf = append(b.buf, s...)
	return len(s), nil