/**
 * Copyright 2010 Google Inc.
 *
 * 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.waveprotocol.wave.client.concurrencycontrol;

import com.google.common.base.Preconditions;

import org.waveprotocol.wave.model.operation.Operation;
import org.waveprotocol.wave.model.operation.SilentOperationSink;
import org.waveprotocol.wave.model.util.CollectionUtils;

import java.util.Queue;

/**
 * A sink that proxies for another sink. This sink is usable immediately after
 * construction. Operations that are consumed before the target sink is set are
 * simply queued. Setting the target sink flushes any operations that have been
 * queued. The target can be set only once, and after it is set, this sink
 * routes all operations to it.
 *
 */
public final class ProxyOperationSink<O extends Operation<?>> implements SilentOperationSink<O> {

  private Queue<O> queue;
  private SilentOperationSink<O> target;

  private ProxyOperationSink() {
  }

  /**
   * Creates a proxy sink.
   */
  public static <O extends Operation<?>> ProxyOperationSink<O> create() {
    return new ProxyOperationSink<O>();
  }

  /**
   * Sets this proxy's target. Any operations previously consumed by this proxy
   * will be forwarded to the target synchronously within this method. Future
   * operations consumed by this proxy will be sent directly to the target. This
   * target can be set at most once.
   *
   * @param target target to consume future operations
   */
  public void setTarget(SilentOperationSink<O> target) {
    Preconditions.checkState(this.target == null);
    this.target = target;

    if (queue != null) {
      while (!queue.isEmpty()) {
        target.consume(queue.poll());
      }
      queue = null;
    }
  }

  @Override
  public void consume(O op) {
    if (target != null) {
      target.consume(op);
    } else {
      if (queue == null) {
        queue = CollectionUtils.createQueue();
      }
      queue.add(op);
    }
  }
}