proposal: strings: add Builder.Available and Builder.AvailableBuffer

[korzhao]

I propose adding functions Builder.Available and Builder.AvailableBuffer.

// AvailableBuffer returns an empty buffer with b.Available() capacity.
func (b *Builder) AvailableBuffer() []byte

// Available returns how many bytes are unused in the buffer.
func (b *Builder) Available() int

In recent years, we have added many append-like APIs. It would be great if strings.Builder could benefit from it.

One scenario I can think of is base64 encoding:

For example:

// EncodeToString returns the base64 encoding of src.
func (enc *Encoding) EncodeToString(src []byte) string {
	build := strings.Builder{}
	build.Grow(enc.EncodedLen(len(src)))

	buf := build.AvailableBuffer()
	buf = enc.AppendEncode(buf, src)

	build.Write(buf)
	return build.String()
}

Bench:

benchmark                      old ns/op     new ns/op     delta
BenchmarkEncodeToString-10     6115          4999          -18.25%

benchmark                      old MB/s     new MB/s     speedup
BenchmarkEncodeToString-10     1339.73      1638.63      1.22x

benchmark                      old allocs     new allocs     delta
BenchmarkEncodeToString-10     2              1              -50.00%

benchmark                      old bytes     new bytes     delta
BenchmarkEncodeToString-10     24576         12288         -50.00%

[mateusz834]

returning []byte from AvailableBuffer is not safe, this will allow modification of strings without unsafe package.

[korzhao]

returning []byte from AvailableBuffer is not safe, this will allow modification of strings without unsafe package.

It should be clarified that 'AvailableBuffer' returns the unused slice buf[len(buf):].

For the slice buf[0:len(buf)] being used, we cannot modify them using existing APIs.
@mateusz834

[mateusz834]

Yeah, but you can:

var s strings.Builder
s.Grow(16)
buf := s.AvailableBuffer()
s.Write([]byte("test"))
str := s.String()
copy(buf, []byte("oops"))
fmt.Println(str == "test") // false

[korzhao]

Thanks. I hadn't thought of that.
@mateusz834