predicates/forwarded: use strings.Index to parse header

Use strings.Cut instead of strings.Split to reduce memory allocations.

See #3403 for details.

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

predicates/forwarded/forwarded.go

@@ -133,8 +133,18 @@ func parseForwarded(fh string) *forwarded {
 
 	f := &forwarded{}
 
-	for _, forwardedFull := range strings.Split(fh, ",") {
-		for _, forwardedPair := range strings.Split(strings.TrimSpace(forwardedFull), ";") {
+	// TODO: use range over func added in go1.23 like:
+	// for forwardedFull := range split(fh, ",") {
+	// 	for forwardedPair := range split(strings.TrimSpace(forwardedFull), ";") {
+	// 		// ...
+	// 	}
+	// }
+	// once go1.24 is released
+	//
+	// Also use strings.SplitSeq added in go1.24 once go1.25 is released.
+
+	split(fh, ",")(func(forwardedFull string) bool {
+		split(strings.TrimSpace(forwardedFull), ";")(func(forwardedPair string) bool {
 			token, value, found := strings.Cut(forwardedPair, "=")
 			value = strings.Trim(value, `"`)
 			if found && value != "" {
@@ -145,8 +155,27 @@ func parseForwarded(fh string) *forwarded {
 					f.host = value
 				}
 			}
-		}
-	}
+			return true
+		})
+		return true
+	})
 
 	return f
 }
+
+func split(s string, sep string) func(yield func(string) bool) {
+	return func(yield func(string) bool) {
+		for {
+			i := strings.Index(s, sep)
+			if i < 0 {
+				break
+			}
+			frag := s[:i]
+			if !yield(frag) {
+				return
+			}
+			s = s[i+len(sep):]
+		}
+		yield(s)
+	}
+}