Optimize ackReply parsing

Signed-off-by: Derek Collison <derek@nats.io>

server/consumer.go

@@ -751,9 +751,9 @@ func (o *Consumer) sendAckReply(subj string) {
 
 // Process a message for the ack reply subject delivered with a message.
 func (o *Consumer) processAck(_ *subscription, _ *client, subject, reply string, msg []byte) {
-	sseq, dseq, dcount, _, _ := o.ReplyInfo(subject)
+	sseq, dseq, dcount := ackReplyInfo(subject)
 
-	var skipAckReply bool
+	skipAckReply := sseq == 0
 
 	switch {
 	case len(msg) == 0, bytes.Equal(msg, AckAck), bytes.Equal(msg, AckOK):
@@ -1830,23 +1830,73 @@ func (o *Consumer) checkPending() {
 
 // SeqFromReply will extract a sequence number from a reply subject.
 func (o *Consumer) SeqFromReply(reply string) uint64 {
-	_, seq, _, _, _ := o.ReplyInfo(reply)
-	return seq
+	_, dseq, _ := ackReplyInfo(reply)
+	return dseq
 }
 
 // StreamSeqFromReply will extract the stream sequence from the reply subject.
 func (o *Consumer) StreamSeqFromReply(reply string) uint64 {
-	seq, _, _, _, _ := o.ReplyInfo(reply)
-	return seq
+	sseq, _, _ := ackReplyInfo(reply)
+	return sseq
 }
 
+// Quick parser for positive numbers in ack reply encoding.
+func parseAckReplyNum(d string) (n int64) {
+	if len(d) == 0 {
+		return -1
+	}
+	for _, dec := range d {
+		if dec < asciiZero || dec > asciiNine {
+			return -1
+		}
+		n = n*10 + (int64(dec) - asciiZero)
+	}
+	return n
+}
+
+const expectedNumReplyTokens = 9
+
 // Grab encoded information in the reply subject for a delivered message.
-func (o *Consumer) ReplyInfo(reply string) (sseq, dseq, dcount uint64, ts int64, pending uint64) {
-	n, err := fmt.Sscanf(reply, o.ackReplyT, &dcount, &sseq, &dseq, &ts, &pending)
-	if err != nil || n != 5 {
+func (o *Consumer) ReplyInfo(subject string) (sseq, dseq, dcount uint64, ts int64, pending uint64) {
+	tsa := [expectedNumReplyTokens]string{}
+	start, tokens := 0, tsa[:0]
+	for i := 0; i < len(subject); i++ {
+		if subject[i] == btsep {
+			tokens = append(tokens, subject[start:i])
+			start = i + 1
+		}
+	}
+	tokens = append(tokens, subject[start:])
+	if len(tokens) != expectedNumReplyTokens || tokens[0] != "$JS" || tokens[1] != "ACK" {
 		return 0, 0, 0, 0, 0
 	}
-	return
+	// TODO(dlc) - Should we error if we do not match consumer name?
+	// stream is tokens[2], consumer is 3.
+	dcount = uint64(parseAckReplyNum(tokens[4]))
+	sseq, dseq = uint64(parseAckReplyNum(tokens[5])), uint64(parseAckReplyNum(tokens[6]))
+	ts = parseAckReplyNum(tokens[7])
+	pending = uint64(parseAckReplyNum(tokens[8]))
+
+	return sseq, dseq, dcount, ts, pending
+}
+
+func ackReplyInfo(subject string) (sseq, dseq, dcount uint64) {
+	tsa := [expectedNumReplyTokens]string{}
+	start, tokens := 0, tsa[:0]
+	for i := 0; i < len(subject); i++ {
+		if subject[i] == btsep {
+			tokens = append(tokens, subject[start:i])
+			start = i + 1
+		}
+	}
+	tokens = append(tokens, subject[start:])
+	if len(tokens) != expectedNumReplyTokens || tokens[0] != "$JS" || tokens[1] != "ACK" {
+		return 0, 0, 0
+	}
+	dcount = uint64(parseAckReplyNum(tokens[4]))
+	sseq, dseq = uint64(parseAckReplyNum(tokens[5])), uint64(parseAckReplyNum(tokens[6]))
+
+	return sseq, dseq, dcount
 }
 
 // NextSeq returns the next delivered sequence number for this observable.