go slices more tricks

slicestricks.go

// StringToImmutableBytes returns a slice of bytes from a string without allocating memory
// it is the caller's responsibility  not to mutate the bytes returned.
func StringToImmutableBytes(s string) []byte {
	if len(s) == 0 {
		return nil
	}

	// NB(xichen): We need to declare a real byte slice so internally the compiler
	// knows to use an unsafe.Pointer to keep track of the underlying memory so that
	// once the slice's array pointer is updated with the pointer to the string's
	// underlying bytes, the compiler won't prematurely GC the memory when the string
	// goes out of scope.
	var b []byte
	byteHeader := (*reflect.SliceHeader)(unsafe.Pointer(&b))

	// NB(xichen): This makes sure that even if GC relocates the string's underlying
	// memory after this assignment, the corresponding unsafe.Pointer in the internal
	// slice struct will be updated accordingly to reflect the memory relocation.
	byteHeader.Data = (*reflect.StringHeader)(unsafe.Pointer(&s)).Data

	// NB(xichen): It is important that we access s after we assign the Data
	// pointer of the string header to the Data pointer of the slice header to
	// make sure the string (and the underlying bytes backing the string) don't get
	// GC'ed before the assignment happens.
	l := len(s)
	byteHeader.Len = l
	byteHeader.Cap = l

	return b
}

func concatSlices(slices ...[]byte) []byte {
	var totalLen int
	for _, s := range slices {
		totalLen += len(s)
	}
	tmp := make([]byte, totalLen)
	var i int
	for _, s := range slices {
		i += copy(tmp[i:], s)
	}
	return tmp
}