path: use bytealg.LastIndexByteString

While strings.LastIndex{,Byte} cannot be used in package path, the
respective internal/bytealg function can be used.

Change-Id: If0ecc36484308221f50875c8609913f6f2887fba
Reviewed-on: https://go-review.googlesource.com/c/go/+/527855
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/path/path.go

@@ -11,6 +11,8 @@
 // operating system paths, use the [path/filepath] package.
 package path
 
+import "internal/bytealg"
+
 // A lazybuf is a lazily constructed path buffer.
 // It supports append, reading previously appended bytes,
 // and retrieving the final string. It does not allocate a buffer
@@ -135,22 +137,13 @@ func Clean(path string) string {
 	return out.string()
 }
 
-// lastSlash(s) is strings.LastIndex(s, "/") but we can't import strings.
-func lastSlash(s string) int {
-	i := len(s) - 1
-	for i >= 0 && s[i] != '/' {
-		i--
-	}
-	return i
-}
-
 // Split splits path immediately following the final slash,
 // separating it into a directory and file name component.
 // If there is no slash in path, Split returns an empty dir and
 // file set to path.
 // The returned values have the property that path = dir+file.
 func Split(path string) (dir, file string) {
-	i := lastSlash(path)
+	i := bytealg.LastIndexByteString(path, '/')
 	return path[:i+1], path[i+1:]
 }
 
@@ -205,7 +198,7 @@ func Base(path string) string {
 		path = path[0 : len(path)-1]
 	}
 	// Find the last element
-	if i := lastSlash(path); i >= 0 {
+	if i := bytealg.LastIndexByteString(path, '/'); i >= 0 {
 		path = path[i+1:]
 	}
 	// If empty now, it had only slashes.