uuid.go
func Parse(s string) (UUID, error) {
	var uuid UUID
	switch len(s) {
	// xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	case 36:

	// urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	case 36 + 9:
		if strings.ToLower(s[:9]) != "urn:uuid:" {
			return uuid, fmt.Errorf("invalid urn prefix: %q", s[:9])
		}
		s = s[9:]

	// {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
	case 36 + 2:
		s = s[1:]

	// xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
	case 32:
		var ok bool
		for i := range uuid {
			uuid[i], ok = xtob(s[i*2], s[i*2+1])
			if !ok {
				return uuid, errors.New("invalid UUID format")
			}
		}
		return uuid, nil
	default:
		return uuid, fmt.Errorf("invalid UUID length: %d", len(s))
	}
	// s is now at least 36 bytes long
	// it must be of the form  xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	if s[8] != '-' || s[13] != '-' || s[18] != '-' || s[23] != '-' {
		return uuid, errors.New("invalid UUID format")
	}
	for i, x := range [16]int{
		0, 2, 4, 6,
		9, 11,
		14, 16,
		19, 21,
		24, 26, 28, 30, 32, 34} {
		v, ok := xtob(s[x], s[x+1])
		if !ok {
			return uuid, errors.New("invalid UUID format")
		}
		uuid[i] = v
	}
	return uuid, nil
}
func Parse(s string) (UUID, error) {
	var uuid UUID
	switch len(s) {
	// xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	case 36:

	// urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	case 36 + 9:
		if strings.ToLower(s[:9]) != "urn:uuid:" {
			return uuid, fmt.Errorf("invalid urn prefix: %q", s[:9])
		}
		s = s[9:]

	// {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}
	case 36 + 2:
		s = s[1:]

	// xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
	case 32:
		var ok bool
		for i := range uuid {
			uuid[i], ok = xtob(s[i*2], s[i*2+1])
			if !ok {
				return uuid, errors.New("invalid UUID format")
			}
		}
		return uuid, nil
	default:
		return uuid, fmt.Errorf("invalid UUID length: %d", len(s))
	}
	// s is now at least 36 bytes long
	// it must be of the form  xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
	if s[8] != '-' || s[13] != '-' || s[18] != '-' || s[23] != '-' {
		return uuid, errors.New("invalid UUID format")
	}
	for i, x := range [16]int{
		0, 2, 4, 6,
		9, 11,
		14, 16,
		19, 21,
		24, 26, 28, 30, 32, 34} {
		v, ok := xtob(s[x], s[x+1])
		if !ok {
			return uuid, errors.New("invalid UUID format")
		}
		uuid[i] = v
	}
	return uuid, nil
}
// Parse decodes s into a UUID or returns an error.  Both the standard UUID
// forms of xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx and
// urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx are decoded as well as the
// Microsoft encoding {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx} and the raw hex
// encoding: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.