transform/urlreplacers: Cache the next position of urlreplacer.prefix

Improved performance due to `bytes.Index` repeated calls

Fixes #5942

transform/urlreplacers/absurlreplacer.go

@@ -41,6 +41,28 @@ type prefix struct {
 	disabled bool
 	b        []byte
 	f        func(l *absurllexer)
+
+	nextPos int
+}
+
+func (p *prefix) find(bs []byte, start int) bool {
+	if p.disabled {
+		return false
+	}
+
+	if p.nextPos == -1 {
+		idx := bytes.Index(bs[start:], p.b)
+
+		if idx == -1 {
+			p.disabled = true
+			// Find the closest match
+			return false
+		}
+
+		p.nextPos = start + idx + len(p.b)
+	}
+
+	return true
 }
 
 func newPrefixState() []*prefix {
@@ -179,35 +201,28 @@ func (l *absurllexer) replace() {
 			break
 		}
 
-		nextPos := -1
-
 		var match *prefix
 
 		for _, p := range prefixes {
-			if p.disabled {
+			if !p.find(l.content, l.pos) {
 				continue
 			}
-			idx := bytes.Index(l.content[l.pos:], p.b)
 
-			if idx == -1 {
-				p.disabled = true
-				// Find the closest match
-			} else if nextPos == -1 || idx < nextPos {
-				nextPos = idx
+			if match == nil || p.nextPos < match.nextPos {
 				match = p
 			}
 		}
 
-		if nextPos == -1 {
+		if match == nil {
 			// Done!
 			l.pos = contentLength
 			break
 		} else {
-			l.pos += nextPos + len(match.b)
+			l.pos = match.nextPos
+			match.nextPos = -1
 			match.f(l)
 		}
 	}
-
 	// Done!
 	if l.pos > l.start {
 		l.emit()