File search: cache getMatchCount() result to avoid UI hangs

If the search view has matches for 100.000 files, it would hang forever
on updating the view via F5 because it asks getMatchCount() for every
element and that traverses entire result again and again.

This change remembers last computed result and returns that value if
there were no further changes on the search result itself.

See #2279

bundles/org.eclipse.search/newsearch/org/eclipse/search/ui/text/AbstractTextSearchResult.java

@@ -44,6 +44,7 @@ public abstract class AbstractTextSearchResult implements ISearchResult {
 	private final ConcurrentMap<Object, Set<Match>> fElementsToMatches;
 	private final List<ISearchResultListener> fListeners;
 	private final MatchEvent fMatchEvent;
+	private volatile int matchCount;
 
 	private MatchFilter[] fMatchFilters;
 
@@ -157,6 +158,7 @@ private MatchEvent getSearchResultEvent(Collection<Match> matches, int eventKind
 
 	private boolean didAddMatch(Match match) {
 		updateFilterState(match);
+		matchCount = -1;
 		return fElementsToMatches.computeIfAbsent(match.getElement(), k -> ConcurrentHashMap.newKeySet()).add(match);
 	}
 
@@ -178,6 +180,7 @@ public void removeAll() {
 		fireChange(new RemoveAllEvent(this));
 	}
 	private void doRemoveAll() {
+		matchCount = 0;
 		fElementsToMatches.clear();
 	}
 
@@ -216,6 +219,7 @@ public void removeMatches(Match[] matches) {
 
 	private boolean didRemoveMatch(Match match) {
 		boolean[] existed = new boolean[1];
+		matchCount = -1;
 		fElementsToMatches.computeIfPresent(match.getElement(), (f, matches) -> {
 			existed[0] = matches.remove(match);
 			if (matches.isEmpty()) {
@@ -302,10 +306,14 @@ private boolean updateFilterState(Match match) {
 	 * @return total number of matches
 	 */
 	public int getMatchCount() {
-		int count = 0;
+		int count = matchCount;
+		if (count != -1) {
+			return count;
+		}
 		for (Set<Match> element : fElementsToMatches.values()) {
 			count += element.size();
 		}
+		matchCount = count;
 		return count;
 	}