internal/bytealg: naive attempt to copy the main Go 1.15 implementatation

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/internal/bytealg/bytealg.go

@@ -124,3 +124,130 @@ func IndexString(str, sub string) int {
 	}
 	return -1
 }
+
+// Copyright 2020 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.
+
+// The following code has been copied from the Go 1.15 release tree.
+
+// PrimeRK is the prime base used in Rabin-Karp algorithm.
+const PrimeRK = 16777619
+
+// HashStrBytes returns the hash and the appropriate multiplicative
+// factor for use in Rabin-Karp algorithm.
+func HashStrBytes(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
+}
+
+// HashStr returns the hash and the appropriate multiplicative
+// factor for use in Rabin-Karp algorithm.
+func HashStr(sep string) (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
+}
+
+// HashStrRevBytes returns the hash of the reverse of sep and the
+// appropriate multiplicative factor for use in Rabin-Karp algorithm.
+func HashStrRevBytes(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
+}
+
+// HashStrRev returns the hash of the reverse of sep and the
+// appropriate multiplicative factor for use in Rabin-Karp algorithm.
+func HashStrRev(sep string) (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
+}
+
+// IndexRabinKarpBytes uses the Rabin-Karp search algorithm to return the index of the
+// first occurence of substr in s, or -1 if not present.
+func IndexRabinKarpBytes(s, sep []byte) int {
+	// Rabin-Karp search
+	hashsep, pow := HashStrBytes(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
+}
+
+// IndexRabinKarp uses the Rabin-Karp search algorithm to return the index of the
+// first occurence of substr in s, or -1 if not present.
+func IndexRabinKarp(s, substr string) int {
+	// Rabin-Karp search
+	hashss, pow := HashStr(substr)
+	n := len(substr)
+	var h uint32
+	for i := 0; i < n; i++ {
+		h = h*PrimeRK + uint32(s[i])
+	}
+	if h == hashss && s[:n] == substr {
+		return 0
+	}
+	for i := n; i < len(s); {
+		h *= PrimeRK
+		h += uint32(s[i])
+		h -= pow * uint32(s[i-n])
+		i++
+		if h == hashss && s[i-n:i] == substr {
+			return i - n
+		}
+	}
+	return -1
+}