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/*
 * Copyright 2011 The GWT Project Authors
 *
 * 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 org.gwtproject.event.shared;

import org.gwtproject.event.shared.Event.Type;

import java.util.*;

/**
 * Basic implementation of {@link EventBus}.
 */
public class SimpleEventBus
    extends EventBus {

  /**
   * Map of event type to map of event source to list of their handlers.
   */
  private final Map, Map>> map = new HashMap<>();

  private int firingDepth = 0;

  /**
   * Add and remove operations received during dispatch.
   */
  private List deferredDeltas;

  @Override
  public  HandlerRegistration addHandler(Type type,
                                            H handler) {
    return doAdd(type,
                 null,
                 handler);
  }

  @Override
  public  HandlerRegistration addHandlerToSource(final Event.Type type,
                                                    final Object source,
                                                    final H handler) {
    if (source == null) {
      throw new NullPointerException("Cannot add a handler with a null source");
    }

    return doAdd(type,
                 source,
                 handler);
  }

  @Override
  public void fireEvent(Event event) {
    doFire(event,
           null);
  }

  @Override
  public void fireEventFromSource(Event event,
                                  Object source) {
    if (source == null) {
      throw new NullPointerException("Cannot fire from a null source");
    }
    doFire(event,
           source);
  }

  private  void doRemove(Event.Type type,
                            Object source,
                            H handler) {
    if (firingDepth > 0) {
      enqueueRemove(type,
                    source,
                    handler);
    } else {
      doRemoveNow(type,
                  source,
                  handler);
    }
  }

  private void defer(Command command) {
    if (deferredDeltas == null) {
      deferredDeltas = new ArrayList<>();
    }
    deferredDeltas.add(command);
  }

  private  HandlerRegistration doAdd(final Event.Type type,
                                        final Object source,
                                        final H handler) {
    if (type == null) {
      throw new NullPointerException("Cannot add a handler with a null type");
    }
    if (handler == null) {
      throw new NullPointerException("Cannot add a null handler");
    }

    if (firingDepth > 0) {
      enqueueAdd(type,
                 source,
                 handler);
    } else {
      doAddNow(type,
               source,
               handler);
    }

    return () -> doRemove(type,
                          source,
                          handler);
  }

  private  void doAddNow(Event.Type type,
                            Object source,
                            H handler) {
    List l = ensureHandlerList(type,
                                  source);
    l.add(handler);
  }

  private  void doFire(Event event,
                          Object source) {
    if (event == null) {
      throw new NullPointerException("Cannot fire null event");
    }
    try {
      firingDepth++;

      if (source != null) {
        setSourceOfEvent(event,
                         source);
      }

      List handlers = getDispatchList(event.getAssociatedType(),
                                         source);
      Set causes = null;

      for (H handler : handlers) {
        try {
          dispatchEvent(event,
                        handler);
        } catch (Throwable e) {
          if (causes == null) {
            causes = new HashSet<>();
          }
          causes.add(e);
        }
      }

      if (causes != null) {
        throw new UmbrellaException(causes);
      }
    } finally {
      firingDepth--;
      if (firingDepth == 0) {
        handleQueuedAddsAndRemoves();
      }
    }
  }

  private  void doRemoveNow(Event.Type type,
                               Object source,
                               H handler) {
    List l = getHandlerList(type,
                               source);

    boolean removed = l.remove(handler);

    if (removed && l.isEmpty()) {
      prune(type,
            source);
    }
  }

  private  void enqueueAdd(final Event.Type type,
                              final Object source,
                              final H handler) {
    defer(() -> doAddNow(type,
                         source,
                         handler));
  }

  private  void enqueueRemove(final Event.Type type,
                                 final Object source,
                                 final H handler) {
    defer(() -> doRemoveNow(type,
                            source,
                            handler));
  }

  private  List ensureHandlerList(Event.Type type,
                                        Object source) {
    Map> sourceMap = map.computeIfAbsent(type,
                                                         k -> new HashMap<>());

    // safe, we control the puts.
    @SuppressWarnings("unchecked") List handlers = (List) sourceMap.get(source);
    if (handlers == null) {
      handlers = new ArrayList<>();
      sourceMap.put(source,
                    handlers);
    }

    return handlers;
  }

  private  List getDispatchList(Event.Type type,
                                      Object source) {
    List directHandlers = getHandlerList(type,
                                            source);
    if (source == null) {
      return directHandlers;
    }

    List globalHandlers = getHandlerList(type,
                                            null);

    List rtn = new ArrayList<>(directHandlers);
    rtn.addAll(globalHandlers);
    return rtn;
  }

  private  List getHandlerList(Event.Type type,
                                     Object source) {
    Map> sourceMap = map.get(type);
    if (sourceMap == null) {
      return Collections.emptyList();
    }

    // safe, we control the puts.
    @SuppressWarnings("unchecked") List handlers = (List) sourceMap.get(source);
    if (handlers == null) {
      return Collections.emptyList();
    }

    return handlers;
  }

  private void handleQueuedAddsAndRemoves() {
    if (deferredDeltas != null) {
      try {
        for (Command c : deferredDeltas) {
          c.execute();
        }
      } finally {
        deferredDeltas = null;
      }
    }
  }

  private void prune(Event.Type type,
                     Object source) {
    Map> sourceMap = map.get(type);

    List pruned = sourceMap.remove(source);

    assert pruned != null : "Can't prune what wasn't there";
    assert pruned.isEmpty() : "Pruned unempty list!";

    if (sourceMap.isEmpty()) {
      map.remove(type);
    }
  }

  private interface Command {

    void execute();

  }

}