bytes,internal/bytealg: eliminate HashStrBytes,HashStrRevBytes using …

…generics

The logic of HashStrBytes, HashStrRevBytes and HashStr, HashStrRev,
are exactly the same, except that the types are different.

Since the bootstrap toolchain is bumped to 1.20, we can eliminate them
by using generics.

Change-Id: I4336b1cab494ba963f09646c169b45f6b1ee62e3
GitHub-Last-Rev: b11a2bf9476d54bed4bd18a3f9269b5c95a66d67
GitHub-Pull-Request: golang/go#63766
Reviewed-on: https://go-review.googlesource.com/c/go/+/538175


Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
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>

src/bytes/bytes.go

@@ -122,7 +122,7 @@ func LastIndex(s, sep []byte) int {
 		return -1
 	}
 	// Rabin-Karp search from the end of the string
-	hashss, pow := bytealg.HashStrRevBytes(sep)
+	hashss, pow := bytealg.HashStrRev(sep)
 	last := len(s) - n
 	var h uint32
 	for i := len(s) - 1; i >= last; i-- {

src/internal/bytealg/bytealg.go

@@ -24,33 +24,16 @@ const (
 // If MaxLen is not 0, make sure MaxLen >= 4.
 var MaxLen int
 
-// FIXME: the logic of HashStrBytes, HashStrRevBytes, IndexRabinKarpBytes and HashStr, HashStrRev,
-// IndexRabinKarp are exactly the same, except that the types are different. Can we eliminate
-// three of them without causing allocation?
+// FIXME: the logic of IndexRabinKarpBytes and IndexRabinKarp are exactly the same,
+// except that the types are different.
+// Can we eliminate one of them without causing allocation?
 
 // 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) {
+func HashStr[T string | []byte](sep T) (uint32, uint32) {
 	hash := uint32(0)
 	for i := 0; i < len(sep); i++ {
 		hash = hash*PrimeRK + uint32(sep[i])
@@ -65,26 +48,9 @@ func HashStr(sep string) (uint32, uint32) {
 	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) {
+func HashStrRev[T string | []byte](sep T) (uint32, uint32) {
 	hash := uint32(0)
 	for i := len(sep) - 1; i >= 0; i-- {
 		hash = hash*PrimeRK + uint32(sep[i])
@@ -103,7 +69,7 @@ func HashStrRev(sep string) (uint32, uint32) {
 // first occurrence of substr in s, or -1 if not present.
 func IndexRabinKarpBytes(s, sep []byte) int {
 	// Rabin-Karp search
-	hashsep, pow := HashStrBytes(sep)
+	hashsep, pow := HashStr(sep)
 	n := len(sep)
 	var h uint32
 	for i := 0; i < n; i++ {