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

Documentation/config-gerrit.txt

@@ -67,6 +67,15 @@ be added at once by adding a group as reviewer.
 +
 Default is 20.
 
+[[addReviewer.baseWeight]]addReviewer.baseWeight::
++
+The weight that will be applied in the default reviewer ranking algorithm.
+This can be increased or decreased to give more or less influence to plugins.
+If set to zero, the base ranking will not have any effect. Reviewers will then
+be ordered as ranked by the plugins (if there are any).
++
+By default 1.
+
 [[auth]]
 === Section auth
 
@@ -3907,11 +3916,11 @@ By default 10.
 [[suggest.from]]suggest.from::
 +
 The number of characters that a user must have typed before suggestions
-are provided. If set to 0, suggestions are always provided.
+are provided. If set to 0, suggestions are always provided. This is only
+used for suggesting accounts when adding members to a group.
 +
 By default 0.
 
-
 [[theme]]
 === Section theme
 

Documentation/dev-plugins.txt

@@ -2399,6 +2399,41 @@ new RestApi("accounts").id("self").view("password.http")
 ----
 
 
+[[reviewer-suggestion]]
+== Reviewer Suggestion Plugins
+
+Gerrit provides an extension point that enables Plugins to rank
+the list of reviewer suggestion a user receives upon clicking "Add Reviewer" on
+the change screen.
+Gerrit supports both a default suggestion that appears when the user has not yet
+typed anything and a filtered suggestion that is shown as the user starts
+typing.
+Plugins receive a candidate list and can return a Set of suggested reviewers
+containing the Account.Id and a score for each reviewer.
+The candidate list is non-binding and plugins can choose to return reviewers not
+initially contained in the candidate list.
+Server administrators can configure the overall weight of each plugin using the
+weight config parameter on [addreviewer "<pluginName-exportName>"].
+
+[source, java]
+----
+import com.google.gerrit.server.change.ReviewerSuggestion;
+import com.google.gerrit.reviewdb.client.Account;
+import com.google.gerrit.reviewdb.client.Change;
+
+import java.util.Set;
+
+public class MyPlugin implements ReviewerSuggestion {
+  public Set<SuggestedReviewer> suggestReviewers(
+      Change.Id changeId, String query, Set<Account.Id> candidates){
+      Set<SuggestedReviewer> suggestions = new HashSet<>();
+      // Implement your ranking logic here
+      return suggestions;
+  }
+}
+----
+
+
 == SEE ALSO
 
 * link:js-api.html[JavaScript API]

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/change/SuggestReviewersIT.java

@@ -20,15 +20,20 @@ import com.google.common.collect.Iterables;
 import com.google.gerrit.acceptance.AbstractDaemonTest;
 import com.google.gerrit.acceptance.GerritConfig;
 import com.google.gerrit.acceptance.GerritConfigs;
+import com.google.gerrit.acceptance.Sandboxed;
 import com.google.gerrit.acceptance.TestAccount;
 import com.google.gerrit.common.data.GlobalCapability;
 import com.google.gerrit.common.data.GroupDescription;
 import com.google.gerrit.common.data.GroupDescriptions;
+import com.google.gerrit.extensions.api.changes.ReviewInput;
+import com.google.gerrit.extensions.common.ChangeInput;
 import com.google.gerrit.extensions.common.GroupInfo;
 import com.google.gerrit.extensions.common.SuggestedReviewerInfo;
+import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.extensions.restapi.TopLevelResource;
 import com.google.gerrit.extensions.restapi.Url;
 import com.google.gerrit.reviewdb.client.AccountGroup;
+import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.server.group.CreateGroup;
 import com.google.gerrit.server.group.GroupsCollection;
 import com.google.inject.Inject;
@@ -38,7 +43,9 @@ import org.junit.Test;
 
 import java.util.Arrays;
 import java.util.List;
+import java.util.stream.Collectors;
 
+@Sandboxed
 public class SuggestReviewersIT extends AbstractDaemonTest {
   @Inject
   private CreateGroup.Factory createGroupFactory;
@@ -88,15 +95,6 @@ public class SuggestReviewersIT extends AbstractDaemonTest {
     assertThat(reviewers).isEmpty();
   }
 
-  @Test
-  @GerritConfig(name = "suggest.from", value = "2")
-  public void suggestReviewersNoResult3() throws Exception {
-    String changeId = createChange().getChangeId();
-    List<SuggestedReviewerInfo> reviewers =
-        suggestReviewers(changeId, name("").substring(0, 1), 6);
-    assertThat(reviewers).isEmpty();
-  }
-
   @Test
   public void suggestReviewersChange() throws Exception {
     String changeId = createChange().getChangeId();
@@ -204,7 +202,7 @@ public class SuggestReviewersIT extends AbstractDaemonTest {
     assertThat(reviewers).hasSize(1);
 
     reviewers = suggestReviewers(changeId, "example.com", 7);
-    assertThat(reviewers).hasSize(6);
+    assertThat(reviewers).hasSize(5);
 
     reviewers = suggestReviewers(changeId, user1.email, 2);
     assertThat(reviewers).hasSize(1);
@@ -267,6 +265,145 @@ public class SuggestReviewersIT extends AbstractDaemonTest {
     assertThat(reviewer.confirm).isTrue();
   }
 
+  @Test
+  public void defaultReviewerSuggestion() throws Exception{
+    TestAccount user1 = user("customuser1", "User1");
+    TestAccount reviewer1 = user("customuser2", "User2");
+    TestAccount reviewer2 = user("customuser3", "User3");
+
+    setApiUser(user1);
+    String changeId1 = createChangeFromApi();
+
+    setApiUser(reviewer1);
+    reviewChange(changeId1);
+
+    setApiUser(user1);
+    String changeId2 = createChangeFromApi();
+
+    setApiUser(reviewer1);
+    reviewChange(changeId2);
+
+    setApiUser(reviewer2);
+    reviewChange(changeId2);
+
+    setApiUser(user1);
+    List<SuggestedReviewerInfo>  reviewers =
+        suggestReviewers(createChangeFromApi(), null, 4);
+    assertThat(
+        reviewers.stream()
+            .map(r -> r.account._accountId)
+            .collect(Collectors.toList()))
+        .containsExactly(
+            reviewer1.id.get(),
+            reviewer2.id.get())
+        .inOrder();
+  }
+
+  @Test
+  public void defaultReviewerSuggestionOnFirstChange() throws Exception{
+    TestAccount user1 = user("customuser1", "User1");
+    setApiUser(user1);
+    List<SuggestedReviewerInfo>  reviewers =
+        suggestReviewers(createChange().getChangeId(), "", 4);
+    assertThat(reviewers).isEmpty();
+  }
+
+  @Test
+  @GerritConfig(name = "suggest.maxSuggestedReviewers", value = "10")
+  public void reviewerRanking() throws Exception{
+    // Assert that user are ranked by the number of times they have applied a
+    // a label to a change (highest), added comments (medium) or owned a
+    // change (low).
+    String fullName = "Primum Finalis";
+    TestAccount userWhoOwns = user("customuser1", fullName);
+    TestAccount reviewer1 = user("customuser2", fullName);
+    TestAccount reviewer2 = user("customuser3", fullName);
+    TestAccount userWhoComments = user("customuser4", fullName);
+    TestAccount userWhoLooksForSuggestions = user("customuser5", fullName);
+
+    // Create a change as userWhoOwns and add some reviews
+    setApiUser(userWhoOwns);
+    String changeId1 = createChangeFromApi();
+
+    setApiUser(reviewer1);
+    reviewChange(changeId1);
+
+    setApiUser(user1);
+    String changeId2 = createChangeFromApi();
+
+    setApiUser(reviewer1);
+    reviewChange(changeId2);
+
+    setApiUser(reviewer2);
+    reviewChange(changeId2);
+
+    // Create a comment as a different user
+    setApiUser(userWhoComments);
+    ReviewInput ri = new ReviewInput();
+    ri.message = "Test";
+    gApi.changes().id(changeId1).revision(1).review(ri);
+
+    // Create a change as a new user to assert that we receive the correct
+    // ranking
+
+    setApiUser(userWhoLooksForSuggestions);
+    List<SuggestedReviewerInfo>  reviewers =
+        suggestReviewers(createChangeFromApi(), "Pri", 4);
+    assertThat(
+        reviewers.stream()
+            .map(r -> r.account._accountId)
+            .collect(Collectors.toList()))
+        .containsExactly(
+            reviewer1.id.get(),
+            reviewer2.id.get(),
+            userWhoOwns.id.get(),
+            userWhoComments.id.get())
+        .inOrder();
+  }
+
+  @Test
+  public void reviewerRankingProjectIsolation() throws Exception{
+    // Create new project
+    Project.NameKey newProject = createProject("test");
+
+    // Create users who review changes in both the default and the new project
+    String fullName = "Primum Finalis";
+    TestAccount userWhoOwns = user("customuser1", fullName);
+    TestAccount reviewer1 = user("customuser2", fullName);
+    TestAccount reviewer2 = user("customuser3", fullName);
+
+    setApiUser(userWhoOwns);
+    String changeId1 = createChangeFromApi();
+
+    setApiUser(reviewer1);
+    reviewChange(changeId1);
+
+    setApiUser(userWhoOwns);
+    String changeId2 = createChangeFromApi(newProject);
+
+    setApiUser(reviewer2);
+    reviewChange(changeId2);
+
+    setApiUser(userWhoOwns);
+    String changeId3 = createChangeFromApi(newProject);
+
+    setApiUser(reviewer2);
+    reviewChange(changeId3);
+
+    setApiUser(userWhoOwns);
+    List<SuggestedReviewerInfo> reviewers =
+        suggestReviewers(createChangeFromApi(), "Prim", 4);
+
+    // Assert that reviewer1 is on top, even though reviewer2 has more reviews
+    // in other projects
+    assertThat(
+        reviewers.stream()
+            .map(r -> r.account._accountId)
+            .collect(Collectors.toList()))
+        .containsExactly(reviewer1.id.get(), reviewer2.id.get())
+        .inOrder();
+  }
+
   private List<SuggestedReviewerInfo> suggestReviewers(String changeId,
       String query, int n) throws Exception {
     return gApi.changes()
@@ -296,4 +433,23 @@ public class SuggestReviewersIT extends AbstractDaemonTest {
       throws Exception {
     return user(name, fullName, name, groups);
   }
+
+  private void reviewChange(String changeId) throws RestApiException {
+    ReviewInput ri = new ReviewInput();
+    ri.label("Code-Review", 1);
+    gApi.changes().id(changeId).current().review(ri);
+  }
+
+  private String createChangeFromApi() throws RestApiException{
+    return createChangeFromApi(project);
+  }
+
+  private String createChangeFromApi(Project.NameKey project)
+      throws RestApiException{
+    ChangeInput ci = new ChangeInput();
+    ci.project = project.get();
+    ci.subject = "Test change at" + System.nanoTime();
+    ci.branch = "master";
+    return gApi.changes().create(ci).get().changeId;
+  }
 }

gerrit-gwtexpui/src/main/java/com/google/gwtexpui/safehtml/client/HighlightSuggestOracle.java

@@ -43,7 +43,7 @@ public abstract class HighlightSuggestOracle extends SuggestOracle {
   }
 
   @Override
-  public final void requestSuggestions(final Request request, final Callback cb) {
+  public final void requestSuggestions(Request request, Callback cb) {
     onRequestSuggestions(request, new Callback() {
       @Override
       public void onSuggestionsReady(final Request request,
@@ -88,27 +88,28 @@ public abstract class HighlightSuggestOracle extends SuggestOracle {
         ds = escape(ds);
       }
 
-      StringBuilder pattern = new StringBuilder();
+      if (qstr != null && !qstr.isEmpty()) {
-      for (String qterm : splitQuery(qstr)) {
+        StringBuilder pattern = new StringBuilder();
-        qterm = escape(qterm);
+        for (String qterm : splitQuery(qstr)) {
-        // We now surround qstr by <strong>. But the chosen approach is not too
+          qterm = escape(qterm);
-        // smooth, if qstr is small (e.g.: "t") and this small qstr may occur in
+          // We now surround qstr by <strong>. But the chosen approach is not too
-        // escapes (e.g.: "Tim &lt;email@example.org&gt;"). Those escapes will
+          // smooth, if qstr is small (e.g.: "t") and this small qstr may occur in
-        // get <strong>-ed as well (e.g.: "&lt;" -> "&<strong>l</strong>t;"). But
+          // escapes (e.g.: "Tim &lt;email@example.org&gt;"). Those escapes will
-        // as repairing those mangled escapes is easier than not mangling them in
+          // get <strong>-ed as well (e.g.: "&lt;" -> "&<strong>l</strong>t;"). But
-        // the first place, we repair them afterwards.
+          // as repairing those mangled escapes is easier than not mangling them in
-
+          // the first place, we repair them afterwards.
-        if (pattern.length() > 0) {
+          if (pattern.length() > 0) {
-          pattern.append("|");
+            pattern.append("|");
+          }
+          pattern.append(qterm);
         }
-        pattern.append(qterm);
+
+        ds = sgi(ds, "(" + pattern.toString() + ")", "<strong>$1</strong>");
+
+        // Repairing <strong>-ed escapes.
+        ds = sgi(ds, "(&[a-z]*)<strong>([a-z]*)</strong>([a-z]*;)", "$1$2$3");
       }
 
-      ds = sgi(ds, "(" + pattern.toString() + ")", "<strong>$1</strong>");
-
-      // Repairing <strong>-ed escapes.
-      ds = sgi(ds, "(&[a-z]*)<strong>([a-z]*)</strong>([a-z]*;)", "$1$2$3");
-
       displayString = ds;
     }
 

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 (!serveSuggestions){
-    if (query == null) {
+      return;
-      query = q;
-      q.start();
-    } else {
-      query = q;
     }
+
+    // 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;

gerrit-gwtui/src/main/java/com/google/gerrit/client/change/ReviewerSuggestOracle.java

@@ -21,21 +21,21 @@ import com.google.gerrit.client.info.GroupBaseInfo;
 import com.google.gerrit.client.rpc.GerritCallback;
 import com.google.gerrit.client.rpc.Natives;
 import com.google.gerrit.client.ui.AccountSuggestOracle;
-import com.google.gerrit.client.ui.SuggestAfterTypingNCharsOracle;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gwt.core.client.JavaScriptObject;
 import com.google.gwt.core.client.JsArray;
+import com.google.gwtexpui.safehtml.client.HighlightSuggestOracle;
 
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 
 /** REST API based suggestion Oracle for reviewers. */
-public class ReviewerSuggestOracle extends SuggestAfterTypingNCharsOracle {
+public class ReviewerSuggestOracle extends HighlightSuggestOracle {
   private Change.Id changeId;
 
   @Override
-  protected void _onRequestSuggestions(final Request req, final Callback cb) {
+  protected void onRequestSuggestions(final Request req, final Callback cb) {
     ChangeApi
         .suggestReviewers(changeId.get(), req.getQuery(), req.getLimit(), false)
         .get(new GerritCallback<JsArray<SuggestReviewerInfo>>() {
@@ -56,6 +56,11 @@ public class ReviewerSuggestOracle extends SuggestAfterTypingNCharsOracle {
         });
   }
 
+  @Override
+  public void requestDefaultSuggestions(final Request req, final Callback cb) {
+    requestSuggestions(req, cb);
+  }
+
   public void setChange(Change.Id changeId) {
     this.changeId = changeId;
   }

gerrit-gwtui/src/main/java/com/google/gerrit/client/change/Reviewers.java

@@ -81,6 +81,7 @@ public class Reviewers extends Composite {
   Reviewers() {
     reviewerSuggestOracle = new ReviewerSuggestOracle();
     suggestBox = new RemoteSuggestBox(reviewerSuggestOracle);
+    suggestBox.enableDefaultSuggestions();
     suggestBox.setVisibleLength(55);
     suggestBox.setHintText(Util.C.approvalTableAddReviewerHint());
     suggestBox.addCloseHandler(new CloseHandler<RemoteSuggestBox>() {
@@ -123,6 +124,7 @@ public class Reviewers extends Composite {
     UIObject.setVisible(form, true);
     UIObject.setVisible(error, false);
     addReviewerIcon.setVisible(false);
+    suggestBox.setServeSuggestionsOnOracle(true);
     suggestBox.setFocus(true);
   }
 
@@ -143,6 +145,7 @@ public class Reviewers extends Composite {
     UIObject.setVisible(form, false);
     suggestBox.setFocus(false);
     suggestBox.setText("");
+    suggestBox.setServeSuggestionsOnOracle(false);
   }
 
   private void addReviewer(final String reviewer, boolean confirmed) {

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeApi.java

@@ -171,10 +171,13 @@ public class ChangeApi {
   }
 
   public static RestApi suggestReviewers(int id, String q, int n, boolean e) {
-    return change(id).view("suggest_reviewers")
+    RestApi api = change(id).view("suggest_reviewers")
-        .addParameter("q", q)
         .addParameter("n", n)
         .addParameter("e", e);
+    if (q != null) {
+      api.addParameter("q", q);
+    }
+    return api;
   }
 
   public static RestApi vote(int id, int reviewer, String vote) {

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

@@ -61,7 +61,8 @@ public class AccountSuggestOracle extends SuggestAfterTypingNCharsOracle {
 
     public static String format(AccountInfo info, String query) {
       String s = FormatUtil.nameEmail(info);
-      if (!containsQuery(s, query) && info.secondaryEmails() != null) {
+      if (query != null && !containsQuery(s, query) &&
+          info.secondaryEmails() != null) {
         for (String email : Natives.asList(info.secondaryEmails())) {
           AccountInfo info2 = AccountInfo.create(info._accountId(), info.name(),
               email, info.username());

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

@@ -13,6 +13,8 @@
 // limitations under the License.
 package com.google.gerrit.client.ui;
 
+import com.google.gwt.event.dom.client.FocusEvent;
+import com.google.gwt.event.dom.client.FocusHandler;
 import com.google.gwt.event.dom.client.KeyCodes;
 import com.google.gwt.event.dom.client.KeyDownEvent;
 import com.google.gwt.event.dom.client.KeyDownHandler;
@@ -42,6 +44,7 @@ public class RemoteSuggestBox extends Composite implements Focusable, HasText,
 
   public RemoteSuggestBox(SuggestOracle oracle) {
     remoteSuggestOracle = new RemoteSuggestOracle(oracle);
+    remoteSuggestOracle.setServeSuggestions(true);
     display = new DefaultSuggestionDisplay();
 
     textBox = new HintTextBox();
@@ -49,7 +52,6 @@ public class RemoteSuggestBox extends Composite implements Focusable, HasText,
       @Override
       public void onKeyDown(KeyDownEvent e) {
         submitOnSelection = false;
-
         if (e.getNativeKeyCode() == KeyCodes.KEY_ESCAPE) {
           CloseEvent.fire(RemoteSuggestBox.this, RemoteSuggestBox.this);
         } else if (e.getNativeKeyCode() == KeyCodes.KEY_ENTER) {
@@ -70,10 +72,11 @@ public class RemoteSuggestBox extends Composite implements Focusable, HasText,
     suggestBox.addSelectionHandler(new SelectionHandler<Suggestion>() {
       @Override
       public void onSelection(SelectionEvent<Suggestion> event) {
-        textBox.setFocus(true);
         if (submitOnSelection) {
           SelectionEvent.fire(RemoteSuggestBox.this, getText());
         }
+        remoteSuggestOracle.cancelOutstandingRequest();
+        display.hideSuggestions();
       }
     });
     initWidget(suggestBox);
@@ -138,4 +141,19 @@ public class RemoteSuggestBox extends Composite implements Focusable, HasText,
   public void selectAll() {
     suggestBox.getValueBox().selectAll();
   }
+
+  public void enableDefaultSuggestions() {
+    textBox.addFocusHandler(new FocusHandler() {
+      @Override
+      public void onFocus(FocusEvent focusEvent) {
+        if (textBox.getText().equals("")) {
+          suggestBox.showSuggestionList();
+        }
+      }
+    });
+  }
+
+  public void setServeSuggestionsOnOracle(boolean serveSuggestions) {
+    remoteSuggestOracle.setServeSuggestions(serveSuggestions);
+  }
 }

gerrit-server/src/main/java/com/google/gerrit/server/ReviewerRecommender.java

@@ -0,0 +1,267 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server;
+
+import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
+import com.google.gerrit.common.data.LabelType;
+import com.google.gerrit.extensions.registration.DynamicMap;
+import com.google.gerrit.reviewdb.client.Account;
+import com.google.gerrit.reviewdb.client.PatchSetApproval;
+import com.google.gerrit.server.account.AccountDirectory.FillOptions;
+import com.google.gerrit.server.account.AccountLoader;
+import com.google.gerrit.server.change.ReviewerSuggestion;
+import com.google.gerrit.server.change.SuggestReviewers;
+import com.google.gerrit.server.change.SuggestedReviewer;
+import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.gerrit.server.git.WorkQueue;
+import com.google.gerrit.server.index.change.ChangeField;
+import com.google.gerrit.server.notedb.ChangeNotes;
+import com.google.gerrit.server.project.ProjectControl;
+import com.google.gerrit.server.query.Predicate;
+import com.google.gerrit.server.query.QueryParseException;
+import com.google.gerrit.server.query.change.ChangeData;
+import com.google.gerrit.server.query.change.ChangeQueryBuilder;
+import com.google.gerrit.server.query.change.InternalChangeQuery;
+import com.google.gwtorm.server.OrmException;
+import com.google.inject.Inject;
+
+import org.apache.commons.lang.mutable.MutableDouble;
+import org.eclipse.jgit.lib.Config;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.EnumSet;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+public class ReviewerRecommender {
+  private static final Logger log =
+      LoggerFactory.getLogger(ReviewersUtil.class);
+  private static final double BASE_REVIEWER_WEIGHT = 10;
+  private static final double BASE_OWNER_WEIGHT = 1;
+  private static final double BASE_COMMENT_WEIGHT = 0.5;
+  private static final long PLUGIN_QUERY_TIMEOUT = 500; //ms
+
+  private final ChangeQueryBuilder changeQueryBuilder;
+  private final Config config;
+  private final DynamicMap<ReviewerSuggestion> reviewerSuggestionPluginMap;
+  private final InternalChangeQuery internalChangeQuery;
+  private final WorkQueue workQueue;
+
+  @Inject
+  ReviewerRecommender(ChangeQueryBuilder changeQueryBuilder,
+      DynamicMap<ReviewerSuggestion> reviewerSuggestionPluginMap,
+      InternalChangeQuery internalChangeQuery,
+      WorkQueue workQueue,
+      @GerritServerConfig Config config) {
+    Set<FillOptions> fillOptions = EnumSet.of(FillOptions.SECONDARY_EMAILS);
+    fillOptions.addAll(AccountLoader.DETAILED_OPTIONS);
+    this.changeQueryBuilder = changeQueryBuilder;
+    this.config = config;
+    this.internalChangeQuery = internalChangeQuery;
+    this.reviewerSuggestionPluginMap = reviewerSuggestionPluginMap;
+    this.workQueue = workQueue;
+  }
+
+  public List<Account.Id> suggestReviewers(
+      ChangeNotes changeNotes,
+      SuggestReviewers suggestReviewers, ProjectControl projectControl,
+      List<Account.Id> candidateList)
+      throws OrmException {
+    String query = suggestReviewers.getQuery();
+    double baseWeight = config.getInt("addReviewer", "baseWeight", 1);
+
+    Map<Account.Id, MutableDouble> reviewerScores;
+    if (Strings.isNullOrEmpty(query)) {
+      reviewerScores = baseRankingForEmptyQuery(baseWeight);
+    } else {
+      reviewerScores = baseRankingForCandidateList(
+          candidateList, projectControl, baseWeight);
+    }
+
+    // Send the query along with a candidate list to all plugins and merge the
+    // results. Plugins don't necessarily need to use the candidates list, they
+    // can also return non-candidate account ids.
+    List<Callable<Set<SuggestedReviewer>>> tasks =
+        new ArrayList<>(reviewerSuggestionPluginMap.plugins().size());
+    List<Double> weights =
+        new ArrayList<>(reviewerSuggestionPluginMap.plugins().size());
+
+    for (DynamicMap.Entry<ReviewerSuggestion> plugin :
+        reviewerSuggestionPluginMap) {
+      tasks.add(() -> plugin.getProvider().get().suggestReviewers(
+          changeNotes.getChangeId(), query, reviewerScores.keySet()));
+      String pluginWeight = config.getString("addReviewer",
+          plugin.getPluginName() + "-" + plugin.getExportName(), "weight");
+      if (Strings.isNullOrEmpty(pluginWeight)) {
+        pluginWeight = "1";
+      }
+      try {
+        weights.add(Double.parseDouble(pluginWeight));
+      } catch (NumberFormatException e) {
+        log.error("Exception while parsing weight for " +
+            plugin.getPluginName() + "-" + plugin.getExportName(), e);
+        weights.add(1d);
+      }
+    }
+
+    try {
+      List<Future<Set<SuggestedReviewer>>> futures = workQueue
+          .getDefaultQueue()
+          .invokeAll(tasks, PLUGIN_QUERY_TIMEOUT, TimeUnit.MILLISECONDS);
+      Iterator<Double> weightIterator = weights.iterator();
+      for (Future<Set<SuggestedReviewer>> f : futures) {
+        double weight = weightIterator.next();
+        for (SuggestedReviewer s : f.get()) {
+          if (reviewerScores.containsKey(s.account)) {
+            reviewerScores.get(s.account).add(s.score * weight);
+          } else {
+            reviewerScores.put(s.account, new MutableDouble(s.score * weight));
+          }
+        }
+      }
+    } catch (ExecutionException | InterruptedException e) {
+      log.error("Exception while suggesting reviewers", e);
+      return ImmutableList.of();
+    }
+
+    // Remove change owner
+    reviewerScores.remove(changeNotes.getChange().getOwner());
+
+    // Sort results
+    Stream<Entry<Account.Id, MutableDouble>> sorted =
+        reviewerScores.entrySet().stream()
+            .sorted(Collections.reverseOrder(Map.Entry.comparingByValue()));
+    List<Account.Id> sortedSuggestions = sorted
+        .map(Map.Entry::getKey)
+        .collect(Collectors.toList());
+    return sortedSuggestions;
+  }
+
+  private Map<Account.Id, MutableDouble> baseRankingForEmptyQuery(
+      double baseWeight) throws OrmException{
+    // Get the user's last 50 changes, check approvals
+    try {
+      List<ChangeData> result = internalChangeQuery
+          .setLimit(50)
+          .setRequestedFields(ImmutableSet.of(ChangeField.REVIEWER.getName()))
+          .query(changeQueryBuilder.owner("self"));
+      Map<Account.Id, MutableDouble> suggestions = new HashMap<>();
+      for (ChangeData cd : result) {
+        for (PatchSetApproval approval : cd.currentApprovals()) {
+          Account.Id id = approval.getAccountId();
+          if (suggestions.containsKey(id)) {
+            suggestions.get(id).add(baseWeight);
+          } else {
+            suggestions.put(id, new MutableDouble(baseWeight));
+          }
+        }
+      }
+      return suggestions;
+    } catch (QueryParseException e) {
+      // Unhandled, because owner:self will never provoke a QueryParseException
+      log.error("Exception while suggesting reviewers", e);
+      return ImmutableMap.of();
+    }
+  }
+
+  private Map<Account.Id, MutableDouble> baseRankingForCandidateList(
+      List<Account.Id> candidates,
+      ProjectControl projectControl,
+      double baseWeight) throws OrmException {
+    // Get each reviewer's activity based on number of applied labels
+    // (weighted 10d), number of comments (weighted 0.5d) and number of owned
+    // changes (weighted 1d).
+    Map<Account.Id, MutableDouble> reviewers = new LinkedHashMap<>();
+    if (candidates.size() == 0) {
+      return reviewers;
+    }
+    List<Predicate<ChangeData>> predicates = new ArrayList<>();
+    for (Account.Id id : candidates) {
+      try {
+        Predicate<ChangeData> projectQuery =
+            changeQueryBuilder.project(projectControl.getProject().getName());
+
+        // Get all labels for this project and create a compound OR query to
+        // fetch all changes where users have applied one of these labels
+        List<LabelType> labelTypes =
+            projectControl.getLabelTypes().getLabelTypes();
+        List<Predicate<ChangeData>> labelPredicates =
+            new ArrayList<>(labelTypes.size());
+        for (LabelType type : labelTypes) {
+          labelPredicates
+              .add(changeQueryBuilder.label(type.getName() + ",user=" + id));
+        }
+        Predicate<ChangeData> reviewerQuery =
+            Predicate.and(projectQuery, Predicate.or(labelPredicates));
+
+        Predicate<ChangeData> ownerQuery = Predicate.and(projectQuery,
+            changeQueryBuilder.owner(id.toString()));
+        Predicate<ChangeData> commentedByQuery = Predicate.and(projectQuery,
+            changeQueryBuilder.commentby(id.toString()));
+
+        predicates.add(reviewerQuery);
+        predicates.add(ownerQuery);
+        predicates.add(commentedByQuery);
+        reviewers.put(id, new MutableDouble());
+      } catch (QueryParseException e) {
+        // Unhandled: If an exception is thrown, we won't increase the
+        // candidates's score
+        log.error("Exception while suggesting reviewers", e);
+      }
+    }
+
+    List<List<ChangeData>> result = internalChangeQuery
+        .setLimit(100 * predicates.size())
+        .setRequestedFields(ImmutableSet.of())
+        .query(predicates);
+
+    Iterator<List<ChangeData>> queryResultIterator = result.iterator();
+    Iterator<Account.Id> reviewersIterator = reviewers.keySet().iterator();
+
+    double[] weights = new double[]{
+        BASE_REVIEWER_WEIGHT, BASE_OWNER_WEIGHT, BASE_COMMENT_WEIGHT};
+
+    int i = 0;
+    Account.Id currentId = null;
+    while (queryResultIterator.hasNext()) {
+      List<ChangeData> currentResult = queryResultIterator.next();
+      if (i % weights.length == 0) {
+        currentId = reviewersIterator.next();
+      }
+
+      reviewers.get(currentId).add(weights[i % weights.length] *
+          baseWeight * currentResult.size());
+      i++;
+    }
+    return reviewers;
+  }
+}

gerrit-server/src/main/java/com/google/gerrit/server/ReviewersUtil.java

@@ -14,20 +14,15 @@
 
 package com.google.gerrit.server;
 
-import static java.util.Comparator.comparing;
-
-import com.google.common.base.MoreObjects;
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Lists;
-import com.google.common.collect.Ordering;
 import com.google.gerrit.common.data.GroupReference;
 import com.google.gerrit.common.errors.NoSuchGroupException;
 import com.google.gerrit.extensions.common.AccountInfo;
 import com.google.gerrit.extensions.common.GroupBaseInfo;
 import com.google.gerrit.extensions.common.SuggestedReviewerInfo;
-import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.Url;
 import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.AccountExternalId;
@@ -44,6 +39,7 @@ import com.google.gerrit.server.change.PostReviewers;
 import com.google.gerrit.server.change.SuggestReviewers;
 import com.google.gerrit.server.index.account.AccountIndex;
 import com.google.gerrit.server.index.account.AccountIndexCollection;
+import com.google.gerrit.server.notedb.ChangeNotes;
 import com.google.gerrit.server.project.NoSuchProjectException;
 import com.google.gerrit.server.project.ProjectControl;
 import com.google.gerrit.server.query.QueryParseException;
@@ -56,98 +52,95 @@ import com.google.inject.Provider;
 
 import java.io.IOException;
 import java.util.ArrayList;
-import java.util.Collection;
 import java.util.Collections;
 import java.util.EnumSet;
-import java.util.HashMap;
+import java.util.HashSet;
-import java.util.LinkedHashMap;
 import java.util.List;
-import java.util.Map;
 import java.util.Set;
 
 public class ReviewersUtil {
   private static final String MAX_SUFFIX = "\u9fa5";
-  private static final Ordering<SuggestedReviewerInfo> ORDERING =
+  // Generate a candidate list at 3x the size of what the user wants to see to
-      Ordering.<SuggestedReviewerInfo> from(comparing(
+  // give the ranking algorithm a good set of candidates it can work with
-          suggestedReviewerInfo -> {
+  private static final int CANDIDATE_LIST_MULTIPLIER = 3;
-            if (suggestedReviewerInfo == null) {
-              return null;
-            }
-            return suggestedReviewerInfo.account != null
-                ? MoreObjects.firstNonNull(suggestedReviewerInfo.account.email,
-                Strings.nullToEmpty(suggestedReviewerInfo.account.name))
-                : Strings.nullToEmpty(suggestedReviewerInfo.group.name);
-          }));
 
-  private final AccountLoader accountLoader;
   private final AccountCache accountCache;
-  private final AccountIndexCollection indexes;
-  private final AccountQueryBuilder queryBuilder;
-  private final AccountQueryProcessor queryProcessor;
   private final AccountControl accountControl;
-  private final Provider<ReviewDb> dbProvider;
+  private final AccountIndexCollection accountIndexes;
+  private final AccountLoader accountLoader;
+  private final AccountQueryBuilder accountQueryBuilder;
+  private final AccountQueryProcessor accountQueryProcessor;
   private final GroupBackend groupBackend;
   private final GroupMembers.Factory groupMembersFactory;
   private final Provider<CurrentUser> currentUser;
+  private final Provider<ReviewDb> dbProvider;
+  private final ReviewerRecommender reviewerRecommender;
 
   @Inject
-  ReviewersUtil(AccountLoader.Factory accountLoaderFactory,
+  ReviewersUtil(AccountCache accountCache,
-      AccountCache accountCache,
-      AccountIndexCollection indexes,
-      AccountQueryBuilder queryBuilder,
-      AccountQueryProcessor queryProcessor,
       AccountControl.Factory accountControlFactory,
-      Provider<ReviewDb> dbProvider,
+      AccountIndexCollection accountIndexes,
+      AccountLoader.Factory accountLoaderFactory,
+      AccountQueryBuilder accountQueryBuilder,
+      AccountQueryProcessor accountQueryProcessor,
       GroupBackend groupBackend,
       GroupMembers.Factory groupMembersFactory,
-      Provider<CurrentUser> currentUser) {
+      Provider<CurrentUser> currentUser,
+      Provider<ReviewDb> dbProvider,
+      ReviewerRecommender reviewerRecommender) {
     Set<FillOptions> fillOptions = EnumSet.of(FillOptions.SECONDARY_EMAILS);
     fillOptions.addAll(AccountLoader.DETAILED_OPTIONS);
-    this.accountLoader = accountLoaderFactory.create(fillOptions);
     this.accountCache = accountCache;
-    this.indexes = indexes;
-    this.queryBuilder = queryBuilder;
-    this.queryProcessor = queryProcessor;
     this.accountControl = accountControlFactory.get();
+    this.accountIndexes = accountIndexes;
+    this.accountLoader = accountLoaderFactory.create(fillOptions);
+    this.accountQueryBuilder = accountQueryBuilder;
+    this.accountQueryProcessor = accountQueryProcessor;
+    this.currentUser = currentUser;
     this.dbProvider = dbProvider;
     this.groupBackend = groupBackend;
     this.groupMembersFactory = groupMembersFactory;
-    this.currentUser = currentUser;
+    this.reviewerRecommender = reviewerRecommender;
   }
 
   public interface VisibilityControl {
     boolean isVisibleTo(Account.Id account) throws OrmException;
   }
 
-  public List<SuggestedReviewerInfo> suggestReviewers(
+  public List<SuggestedReviewerInfo> suggestReviewers(ChangeNotes changeNotes,
       SuggestReviewers suggestReviewers, ProjectControl projectControl,
       VisibilityControl visibilityControl, boolean excludeGroups)
-      throws IOException, OrmException, BadRequestException {
+      throws IOException, OrmException {
     String query = suggestReviewers.getQuery();
-    boolean suggestAccounts = suggestReviewers.getSuggestAccounts();
-    int suggestFrom = suggestReviewers.getSuggestFrom();
     int limit = suggestReviewers.getLimit();
 
-    if (Strings.isNullOrEmpty(query)) {
+    if (!suggestReviewers.getSuggestAccounts()) {
-      throw new BadRequestException("missing query field");
-    }
-
-    if (!suggestAccounts || query.length() < suggestFrom) {
       return Collections.emptyList();
     }
 
-    Collection<AccountInfo> suggestedAccounts =
+    List<Account.Id> candidateList = new ArrayList<>();
-        suggestAccounts(suggestReviewers, visibilityControl);
+    if (!Strings.isNullOrEmpty(query)) {
-
+      candidateList = suggestAccounts(suggestReviewers, visibilityControl);
-    List<SuggestedReviewerInfo> reviewer = new ArrayList<>();
-    for (AccountInfo a : suggestedAccounts) {
-      SuggestedReviewerInfo info = new SuggestedReviewerInfo();
-      info.account = a;
-      info.count = 1;
-      reviewer.add(info);
     }
 
-    if (!excludeGroups) {
+    List<Account.Id> sortedRecommendations = reviewerRecommender
+        .suggestReviewers(changeNotes, suggestReviewers, projectControl,
+            candidateList);
+
+    // Populate AccountInfo
+    List<SuggestedReviewerInfo> reviewer = new ArrayList<>();
+    for (Account.Id id : sortedRecommendations) {
+      AccountInfo account = accountLoader.get(id);
+      if (account != null) {
+        SuggestedReviewerInfo info = new SuggestedReviewerInfo();
+        info.account = account;
+        info.count = 1;
+        reviewer.add(info);
+      }
+    }
+    accountLoader.fill();
+
+    if (!excludeGroups && !Strings.isNullOrEmpty(query)) {
       for (GroupReference g : suggestAccountGroup(suggestReviewers, projectControl)) {
         GroupAsReviewer result = suggestGroupAsReviewer(
             suggestReviewers, projectControl.getProject(), g, visibilityControl);
@@ -161,59 +154,56 @@ public class ReviewersUtil {
           if (result.allowedWithConfirmation) {
             suggestedReviewerInfo.confirm = true;
           }
+          // Always add groups at the end as individual accounts are usually
+          // more important
           reviewer.add(suggestedReviewerInfo);
         }
       }
     }
 
-    reviewer = ORDERING.immutableSortedCopy(reviewer);
     if (reviewer.size() <= limit) {
       return reviewer;
     }
     return reviewer.subList(0, limit);
   }
 
-  private Collection<AccountInfo> suggestAccounts(SuggestReviewers suggestReviewers,
+  private List<Account.Id> suggestAccounts(SuggestReviewers suggestReviewers,
       VisibilityControl visibilityControl)
       throws OrmException {
-    AccountIndex searchIndex = indexes.getSearchIndex();
+    AccountIndex searchIndex = accountIndexes.getSearchIndex();
     if (searchIndex != null) {
       return suggestAccountsFromIndex(suggestReviewers);
     }
     return suggestAccountsFromDb(suggestReviewers, visibilityControl);
   }
 
-  private Collection<AccountInfo> suggestAccountsFromIndex(
+  private List<Account.Id> suggestAccountsFromIndex(
       SuggestReviewers suggestReviewers) throws OrmException {
     try {
-      Map<Account.Id, AccountInfo> matches = new LinkedHashMap<>();
+      Set<Account.Id> matches = new HashSet<>();
-      QueryResult<AccountState> result = queryProcessor
+      QueryResult<AccountState> result = accountQueryProcessor
-          .setLimit(suggestReviewers.getLimit())
+          .setLimit(suggestReviewers.getLimit() * CANDIDATE_LIST_MULTIPLIER)
-          .query(queryBuilder.defaultQuery(suggestReviewers.getQuery()));
+          .query(accountQueryBuilder.defaultQuery(suggestReviewers.getQuery()));
       for (AccountState accountState : result.entities()) {
         Account.Id id = accountState.getAccount().getId();
-        matches.put(id, accountLoader.get(id));
+        matches.add(id);
       }
-
+      return new ArrayList<>(matches);
-      accountLoader.fill();
-
-      return matches.values();
     } catch (QueryParseException e) {
       return ImmutableList.of();
     }
   }
 
-  private Collection<AccountInfo> suggestAccountsFromDb(
+  private List<Account.Id> suggestAccountsFromDb(
       SuggestReviewers suggestReviewers, VisibilityControl visibilityControl)
           throws OrmException {
     String query = suggestReviewers.getQuery();
-    int limit = suggestReviewers.getLimit();
+    int limit = suggestReviewers.getLimit() * CANDIDATE_LIST_MULTIPLIER;
 
     String a = query;
     String b = a + MAX_SUFFIX;
 
-    Map<Account.Id, AccountInfo> r = new LinkedHashMap<>();
+    Set<Account.Id> r = new HashSet<>();
-    Map<Account.Id, String> queryEmail = new HashMap<>();
 
     for (Account p : dbProvider.get().accounts()
         .suggestByFullName(a, b, limit)) {
@@ -234,36 +224,26 @@ public class ReviewersUtil {
     if (r.size() < limit) {
       for (AccountExternalId e : dbProvider.get().accountExternalIds()
           .suggestByEmailAddress(a, b, limit - r.size())) {
-        if (!r.containsKey(e.getAccountId())) {
+        if (!r.contains(e.getAccountId())) {
           Account p = accountCache.get(e.getAccountId()).getAccount();
           if (p.isActive()) {
-            if (addSuggestion(r, p.getId(), visibilityControl)) {
+            addSuggestion(r, p.getId(), visibilityControl);
-              queryEmail.put(e.getAccountId(), e.getEmailAddress());
-            }
           }
         }
       }
     }
-
+    return new ArrayList<>(r);
-    accountLoader.fill();
-    for (Map.Entry<Account.Id, String> p : queryEmail.entrySet()) {
-      AccountInfo info = r.get(p.getKey());
-      if (info != null) {
-        info.email = p.getValue();
-      }
-    }
-    return new ArrayList<>(r.values());
   }
 
-  private boolean addSuggestion(Map<Account.Id, AccountInfo> map,
+  private boolean addSuggestion(Set<Account.Id> map,
       Account.Id account, VisibilityControl visibilityControl)
       throws OrmException {
-    if (!map.containsKey(account)
+    if (!map.contains(account)
         // Can the suggestion see the change?
         && visibilityControl.isVisibleTo(account)
         // Can the current user see the account?
         && accountControl.canSee(account)) {
-      map.put(account, accountLoader.get(account));
+      map.add(account);
       return true;
     }
     return false;
@@ -282,7 +262,8 @@ public class ReviewersUtil {
     int size;
   }
 
-  private GroupAsReviewer suggestGroupAsReviewer(SuggestReviewers suggestReviewers,
+  private GroupAsReviewer suggestGroupAsReviewer(
+      SuggestReviewers suggestReviewers,
       Project project, GroupReference group,
       VisibilityControl visibilityControl) throws OrmException, IOException {
     GroupAsReviewer result = new GroupAsReviewer();

gerrit-server/src/main/java/com/google/gerrit/server/change/ReviewerSuggestion.java

@@ -0,0 +1,41 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.change;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+import com.google.gerrit.reviewdb.client.Account;
+import com.google.gerrit.reviewdb.client.Change;
+
+import java.util.Set;
+
+/**
+ * Listener to provide reviewer suggestions.
+ * <p>
+ * Invoked by Gerrit a user who is searching for a reviewer to add to a change.
+ */
+@ExtensionPoint
+public interface ReviewerSuggestion {
+  /**
+   * Reviewer suggestion.
+   *
+   * @param changeId The changeId that the suggestion is for.
+   * @param query The query as typed by the user. Can be an empty string.
+   * @param candidates A set of candidates for the ranking.
+   * @return Set of suggested reviewers as a tuple of account id and score.
+   *         The account ids listed here don't have to be a part of candidates.
+   */
+  Set<SuggestedReviewer> suggestReviewers(
+      Change.Id changeId, String query, Set<Account.Id> candidates);
+}

gerrit-server/src/main/java/com/google/gerrit/server/change/SuggestChangeReviewers.java

@@ -54,7 +54,7 @@ public class SuggestChangeReviewers extends SuggestReviewers
   @Override
   public List<SuggestedReviewerInfo> apply(ChangeResource rsrc)
       throws BadRequestException, OrmException, IOException {
-    return reviewersUtil.suggestReviewers(this,
+    return reviewersUtil.suggestReviewers(rsrc.getNotes(), this,
         rsrc.getControl().getProjectControl(), getVisibility(rsrc), excludeGroups);
   }
 

gerrit-server/src/main/java/com/google/gerrit/server/change/SuggestReviewers.java

@@ -33,7 +33,6 @@ public class SuggestReviewers {
   protected final ReviewersUtil reviewersUtil;
 
   private final boolean suggestAccounts;
-  private final int suggestFrom;
   private final int maxAllowed;
   private final int maxAllowedWithoutConfirmation;
   protected int limit;
@@ -62,10 +61,6 @@ public class SuggestReviewers {
     return suggestAccounts;
   }
 
-  public int getSuggestFrom() {
-    return suggestFrom;
-  }
-
   public int getLimit() {
     return limit;
   }
@@ -98,7 +93,6 @@ public class SuggestReviewers {
       this.suggestAccounts = (av != AccountVisibility.NONE);
     }
 
-    this.suggestFrom = cfg.getInt("suggest", null, "from", 0);
     this.maxAllowed = cfg.getInt("addreviewer", "maxAllowed",
         PostReviewers.DEFAULT_MAX_REVIEWERS);
     this.maxAllowedWithoutConfirmation = cfg.getInt(

gerrit-server/src/main/java/com/google/gerrit/server/change/SuggestedReviewer.java

@@ -0,0 +1,22 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.gerrit.server.change;
+
+import com.google.gerrit.reviewdb.client.Account;
+
+public class SuggestedReviewer {
+
+  public Account.Id account;
+  public double score;
+}

gerrit-server/src/main/java/com/google/gerrit/server/config/GerritGlobalModule.java

@@ -100,6 +100,7 @@ import com.google.gerrit.server.change.AccountPatchReviewStore;
 import com.google.gerrit.server.change.ChangeJson;
 import com.google.gerrit.server.change.ChangeKindCacheImpl;
 import com.google.gerrit.server.change.MergeabilityCacheImpl;
+import com.google.gerrit.server.change.ReviewerSuggestion;
 import com.google.gerrit.server.events.EventFactory;
 import com.google.gerrit.server.events.EventsMetrics;
 import com.google.gerrit.server.extensions.events.GitReferenceUpdated;
@@ -352,6 +353,7 @@ public class GerritGlobalModule extends FactoryModule {
     DynamicMap.mapOf(binder(), DownloadScheme.class);
     DynamicMap.mapOf(binder(), DownloadCommand.class);
     DynamicMap.mapOf(binder(), CloneCommand.class);
+    DynamicMap.mapOf(binder(), ReviewerSuggestion.class);
     DynamicSet.setOf(binder(), ExternalIncludedIn.class);
     DynamicMap.mapOf(binder(), ProjectConfigEntry.class);
     DynamicSet.setOf(binder(), PatchSetWebLink.class);