[1975] Small optimisation

Don't compute the number of matched labels at the first match, there's
nothing to compare against anyway. We hope that there'll be only one one
partial match in practice.

src/lib/datasrc/list.cc

@@ -73,11 +73,13 @@ ConfigurableClientList::find(const dns::Name& name, bool want_exact_match,
     struct MutableResult {
         MutableResult() :
             datasrc_client(NULL),
-            matched_labels(0)
+            matched_labels(0),
+            matched(false)
         { }
         DataSourceClient* datasrc_client;
         ZoneFinderPtr finder;
         uint8_t matched_labels;
+        bool matched;
         operator FindResult() const {
             // Conversion to the right result. If we return this, there was
             // a partial match at best.
@@ -108,13 +110,25 @@ ConfigurableClientList::find(const dns::Name& name, bool want_exact_match,
                 if (!want_exact_match) {
                     // In case we have a partial match, check if it is better
                     // than what we have. If so, replace it.
-                    const uint8_t labels(
-                        result.zone_finder->getOrigin().getLabelCount());
-                    if (labels > candidate.matched_labels) {
+                    //
+                    // We don't need the labels at the first partial match,
+                    // we have nothing to compare with. So we don't get it
+                    // (as a performance) and hope we will not need it at all.
+                    const uint8_t labels(candidate.matched ?
+                        result.zone_finder->getOrigin().getLabelCount() : 0);
+                    if (candidate.matched && candidate.matched_labels == 0) {
+                        // But if the hope turns out to be false, we need to
+                        // compute it for the first match anyway.
+                        candidate.matched_labels = candidate.finder->
+                            getOrigin().getLabelCount();
+                    }
+                    if (labels > candidate.matched_labels ||
+                        !candidate.matched) {
                         // This one is strictly better. Replace it.
                         candidate.datasrc_client = info.data_src_client_;
                         candidate.finder = result.zone_finder;
                         candidate.matched_labels = labels;
+                        candidate.matched = true;
                     }
                 }
                 break;