bytes,internal/bytealg: add func bytealg.LastIndexRabinKarp

Also rename 'substr' to 'sep' in IndexRabinKarp for consistency.

Change-Id: Icc2ad1116aecaf002c8264daa2fa608306c9a88a
GitHub-Last-Rev: 1784b93f53d569991f86585f9011120ea26f193f
GitHub-Pull-Request: golang/go#63854
Reviewed-on: https://go-review.googlesource.com/c/go/+/538716
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

2 files changed, 32 insertions, 25 deletions

diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index c84accd8f5..0679b43a20 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -121,25 +121,7 @@ func LastIndex(s, sep []byte) int {
 	case n > len(s):
 		return -1
 	}
-	// Rabin-Karp search from the end of the string
-	hashss, pow := bytealg.HashStrRev(sep)
-	last := len(s) - n
-	var h uint32
-	for i := len(s) - 1; i >= last; i-- {
-		h = h*bytealg.PrimeRK + uint32(s[i])
-	}
-	if h == hashss && Equal(s[last:], sep) {
-		return last
-	}
-	for i := last - 1; i >= 0; i-- {
-		h *= bytealg.PrimeRK
-		h += uint32(s[i])
-		h -= pow * uint32(s[i+n])
-		if h == hashss && Equal(s[i:i+n], sep) {
-			return i
-		}
-	}
-	return -1
+	return bytealg.LastIndexRabinKarp(s, sep)
 }
 
 // LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
diff --git a/src/internal/bytealg/bytealg.go b/src/internal/bytealg/bytealg.go
index 92be8ea79b..1103891eee 100644
--- a/src/internal/bytealg/bytealg.go
+++ b/src/internal/bytealg/bytealg.go
@@ -62,16 +62,16 @@ func HashStrRev[T string | []byte](sep T) (uint32, uint32) {
 }
 
 // IndexRabinKarp uses the Rabin-Karp search algorithm to return the index of the
-// first occurrence of substr in s, or -1 if not present.
-func IndexRabinKarp[T string | []byte](s, substr T) int {
+// first occurrence of sep in s, or -1 if not present.
+func IndexRabinKarp[T string | []byte](s, sep T) int {
 	// Rabin-Karp search
-	hashss, pow := HashStr(substr)
-	n := len(substr)
+	hashss, pow := HashStr(sep)
+	n := len(sep)
 	var h uint32
 	for i := 0; i < n; i++ {
 		h = h*PrimeRK + uint32(s[i])
 	}
-	if h == hashss && string(s[:n]) == string(substr) {
+	if h == hashss && string(s[:n]) == string(sep) {
 		return 0
 	}
 	for i := n; i < len(s); {
@@ -79,13 +79,38 @@ func IndexRabinKarp[T string | []byte](s, substr T) int {
 		h += uint32(s[i])
 		h -= pow * uint32(s[i-n])
 		i++
-		if h == hashss && string(s[i-n:i]) == string(substr) {
+		if h == hashss && string(s[i-n:i]) == string(sep) {
 			return i - n
 		}
 	}
 	return -1
 }
 
+// LastIndexRabinKarp uses the Rabin-Karp search algorithm to return the last index of the
+// occurrence of sep in s, or -1 if not present.
+func LastIndexRabinKarp[T string | []byte](s, sep T) int {
+	// Rabin-Karp search from the end of the string
+	hashss, pow := HashStrRev(sep)
+	n := len(sep)
+	last := len(s) - n
+	var h uint32
+	for i := len(s) - 1; i >= last; i-- {
+		h = h*PrimeRK + uint32(s[i])
+	}
+	if h == hashss && string(s[last:]) == string(sep) {
+		return last
+	}
+	for i := last - 1; i >= 0; i-- {
+		h *= PrimeRK
+		h += uint32(s[i])
+		h -= pow * uint32(s[i+n])
+		if h == hashss && string(s[i:i+n]) == string(sep) {
+			return i
+		}
+	}
+	return -1
+}
+
 // MakeNoZero makes a slice of length and capacity n without zeroing the bytes.
 // It is the caller's responsibility to ensure uninitialized bytes
 // do not leak to the end user.