bytes.go

				// IndexByte is faster than bytealg.Index, so use it as long as
				// we're not getting lots of false positives.
				o := IndexByte(s[i:t], c0)
				if o < 0 {
					return -1
				}
				i += o
			}
			if s[i+1] == c1 && Equal(s[i:i+n], sep) {
				return i
			}
			fails++
			i++
			// Switch to bytealg.Index when IndexByte produces too many false positives.
			if fails > bytealg.Cutover(i) {
				r := bytealg.Index(s[i:], sep)
				if r >= 0 {
					return r + i
				}
				return -1
			}
		}
		return -1
	}
	c0 := sep[0]
	c1 := sep[1]
	i := 0
	fails := 0
	t := len(s) - n + 1
	for i < t {
		if s[i] != c0 {
			o := IndexByte(s[i:t], c0)
			if o < 0 {
				break
			}
			i += o
		}
		if s[i+1] == c1 && Equal(s[i:i+n], sep) {
			return i
		}
		i++
		fails++
		if fails >= 4+i>>4 && i < 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
}

func indexRabinKarp(s, sep []byte) int {
	// Rabin-Karp search
	hashsep, pow := hashStr(sep)
	n := len(sep)
	var h uint32
	for i := 0; i < n; i++ {
		h = h*primeRK + uint32(s[i])
	}
	if h == hashsep && Equal(s[:n], sep) {
		return 0
	}
	for i := n; i < len(s); {
		h *= primeRK
		h += uint32(s[i])
		h -= pow * uint32(s[i-n])
		i++
		if h == hashsep && Equal(s[i-n:i], sep) {
			return i - n
		}
	}
	return -1
}

// primeRK is the prime base used in Rabin-Karp algorithm.
const primeRK = 16777619

// hashStr returns the hash and the appropriate multiplicative
// factor for use in Rabin-Karp algorithm.
func hashStr(sep []byte) (uint32, uint32) {
	hash := uint32(0)
	for i := 0; i < len(sep); i++ {
		hash = hash*primeRK + uint32(sep[i])
	}
	var pow, sq uint32 = 1, primeRK
	for i := len(sep); i > 0; i >>= 1 {
		if i&1 != 0 {
			pow *= sq
		}
		sq *= sq
	}
	return hash, pow
}

// hashStrRev returns the hash of the reverse of sep and the
// appropriate multiplicative factor for use in Rabin-Karp algorithm.
func hashStrRev(sep []byte) (uint32, uint32) {
	hash := uint32(0)
	for i := len(sep) - 1; i >= 0; i-- {
		hash = hash*primeRK + uint32(sep[i])
	}
	var pow, sq uint32 = 1, primeRK
	for i := len(sep); i > 0; i >>= 1 {
		if i&1 != 0 {
			pow *= sq
		}
		sq *= sq
	}
	return hash, pow
}