Method parseUintHex

json/parse.go:211–256

parseUintHex parses a hexadecimanl representation of a uint64 from b. The function is equivalent to calling strconv.ParseUint(string(b), 16, 64) but it prevents Go from making a memory allocation for converting a byte slice to a string (escape analysis fails due to the error returned by strconv.Par

(b []byte)

// Because it only works with base 16 the function is also significantly faster
// than strconv.ParseUint.
func (d decoder) parseUintHex(b []byte) (uint64, []byte, error) {
	const max = math.MaxUint64
	const lim = max / 0x10

	var value uint64
	var count int

	if len(b) == 0 {
		return 0, b, syntaxError(b, "cannot decode hexadecimal value from an empty input")
	}

parseLoop:
	for i, c := range b {
		var x uint64

		switch {
		case c >= '0' && c <= '9':
			x = uint64(c - '0')

		case c >= 'A' && c <= 'F':
			x = uint64(c-'A') + 0xA

		case c >= 'a' && c <= 'f':
			x = uint64(c-'a') + 0xA

		default:
			if i == 0 {
				return 0, b, syntaxError(b, "expected hexadecimal digit but found '%c'", c)
			}
			break parseLoop
		}

		if value > lim {
			return 0, b, syntaxError(b, "hexadecimal value out of range")
		}

		if value *= 0x10; value > (max - x) {
			return 0, b, syntaxError(b, "hexadecimal value out of range")
		}

		value += x
		count++
	}

	return value, b[count:], nil
}

func (d decoder) parseNull(b []byte) ([]byte, []byte, Kind, error) {
	if hasNullPrefix(b) {