Build a Recommender for Reviewer Suggestion

Until now, reviewer suggestion was purely based on a search. I've built
a small recommender to improve the suggestions based on past
contributions by the individual reviewers and added an extension point
so that people can customize this feature.

The built-in recommender makes a default suggestion of reviewers before
the user types a query. These are based on people that have reviewed the
last contributions that a user made.

If the user starts typing in the box, we generate a list of candidates
using the account index and feed it into a small recommender. The
recommender ranks the list by looking at recent contributions of the
candidates made in the same project. Contributions include reviews,
owned-changes and comments at different weights.

Change-Id: I5aca23ddd2442146fd26bdc12e7c18da85de7ac1

gerrit-gwtui-common/src/main/java/com/google/gerrit/client/ui/RemoteSuggestOracle.java

@@ -14,6 +14,7 @@
 
 package com.google.gerrit.client.ui;
 
+import com.google.gwt.user.client.Timer;
 import com.google.gwt.user.client.ui.SuggestOracle;
 
 /**
@@ -31,6 +32,10 @@ public class RemoteSuggestOracle extends SuggestOracle {
   private final SuggestOracle oracle;
   private Query query;
   private String last;
+  private Timer requestRetentionTimer;
+  private boolean cancelOutstandingRequest;
+
+  private boolean serveSuggestions;
 
   public RemoteSuggestOracle(SuggestOracle src) {
     oracle = src;
@@ -42,13 +47,33 @@ public class RemoteSuggestOracle extends SuggestOracle {
 
   @Override
   public void requestSuggestions(Request req, Callback cb) {
-    Query q = new Query(req, cb);
-    if (query == null) {
-      query = q;
-      q.start();
-    } else {
-      query = q;
+    if (!serveSuggestions){
+      return;
     }
+
+    // Use a timer for key stroke retention, such that we don't query the
+    // backend for each and every keystroke we receive.
+    if (requestRetentionTimer != null) {
+      requestRetentionTimer.cancel();
+    }
+    requestRetentionTimer = new Timer() {
+      @Override
+      public void run() {
+        Query q = new Query(req, cb);
+        if (query == null) {
+          query = q;
+          q.start();
+        } else {
+          query = q;
+        }
+      }
+    };
+    requestRetentionTimer.schedule(200);
+  }
+
+  @Override
+  public void requestDefaultSuggestions(Request req, Callback cb) {
+    requestSuggestions(req, cb);
   }
 
   @Override
@@ -56,6 +81,19 @@ public class RemoteSuggestOracle extends SuggestOracle {
     return oracle.isDisplayStringHTML();
   }
 
+  public void cancelOutstandingRequest() {
+    if (requestRetentionTimer != null) {
+      requestRetentionTimer.cancel();
+    }
+    if (query != null) {
+      cancelOutstandingRequest = true;
+    }
+  }
+
+  public void setServeSuggestions(boolean serveSuggestions) {
+    this.serveSuggestions = serveSuggestions;
+  }
+
   private class Query implements Callback {
     final Request request;
     final Callback callback;
@@ -71,7 +109,11 @@ public class RemoteSuggestOracle extends SuggestOracle {
 
     @Override
     public void onSuggestionsReady(Request req, Response res) {
-      if (query == this) {
+      if (cancelOutstandingRequest || !serveSuggestions) {
+        // If cancelOutstandingRequest() was called, we ignore this response
+        cancelOutstandingRequest = false;
+        query = null;
+      } else if (query == this) {
         // No new request was started while this query was running.
         // Propose this request's response as the suggestions.
         query = null;