internal/unicode/cesu8/cesu8.go
func RuneLen(r rune) int {
	switch {
	case r < 0:
		return -1
	case r <= rune1Max:
		return 1
	case r <= rune2Max:
		return 2
	case r <= rune3Max:
		return 3
	case r <= utf8.MaxRune:
		return CESUMax
	}
	return -1
}
func RuneLen(r rune) int {
	switch {
	case r < 0:
		return -1
	case r <= rune1Max:
		return 1
	case r <= rune2Max:
		return 2
	case r <= rune3Max:
		return 3
	case r <= utf8.MaxRune:
		return CESUMax
	}
	return -1
}
// RuneLen returns the number of bytes required to encode the rune.