src/bytes/bytes_generic.go

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build !amd64,!s390x,!arm64

package bytes

// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
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
	}
	c := sep[0]
	i := 0
	fails := 0
	t := s[:len(s)-n+1]
	for i < len(t) {
		if t[i] != c {
			o := IndexByte(t[i:], c)
			if o < 0 {
				break
			}
			i += o
		}
		if Equal(s[i:i+n], sep) {
			return i
		}
		i++
		fails++
		if fails >= 4+i>>4 && i < len(t) {
			// Give up on IndexByte, it isn't skipping ahead
			// far enough to be better than Rabin-Karp.
			// Experiments (using IndexPeriodic) suggest
			// the cutover is about 16 byte skips.
			// TODO: if large prefixes of sep are matching
			// we should cutover at even larger average skips,
			// because Equal becomes that much more expensive.
			// This code does not take that effect into account.
			j := indexRabinKarp(s[i:], sep)
			if j < 0 {
				return -1
			}
			return i + j
		}
	}
	return -1
}

// Count counts the number of non-overlapping instances of sep in s.
// If sep is an empty slice, Count returns 1 + the number of UTF-8-encoded code points in s.
func Count(s, sep []byte) int {
	return countGeneric(s, sep)
}