func Index(s, sep []byte) int {
	n := len(sep)
	switch {
	case n == 0:
		return 0
	case n == 1:
		return IndexByte(s, sep[0])
	case n == len(s):
		if Equal(sep, s) {
			return 0
		}
		return -1
	case n > len(s):
		return -1
	case n <= bytealg.MaxLen:
		// Use brute force when s and sep both are small
		if len(s) <= bytealg.MaxBruteForce {