/*

  * Licensed to the Apache Software Foundation (ASF) under one

  * or more contributor license agreements.  See the NOTICE file

  * distributed with this work for additional information

  * regarding copyright ownership.  The ASF licenses this file

  * to you 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.apache.giraph.block_app.reducers;

 import org.apache.giraph.reducers.impl.KryoWrappedReduceOperation;

 import java.io.DataInput;

 import java.io.DataOutput;

 import java.io.IOException;

 import java.util.PriorityQueue;

 /**

  * Extracts top N largest elements

  *

  * @param <S> Single value type, objects passed on workers

  */

 public class TopNReduce<S extends Comparable<S>>

   extends KryoWrappedReduceOperation<S, PriorityQueue<S>> {

   private int capacity;

   public TopNReduce(int capacity) {

     this.capacity = capacity;

   }

   public TopNReduce() { }

   @Override

   public PriorityQueue<S> createValue() {

     return new PriorityQueue<S>();

   }

   @Override

   public void reduce(PriorityQueue<S> heap, S value) {

     if (capacity == 0) {

       return;

     }

     if (heap.size() < capacity) {

       heap.add(value);

     } else {

       S head = heap.peek();

       if (head.compareTo(value) < 0) {

         heap.poll();

         heap.add(value);

       }

     }

   }

   @Override

   public void reduceMerge(

     PriorityQueue<S> reduceInto,

     PriorityQueue<S> toReduce

   ) {

     for (S element : toReduce) {

       reduce(reduceInto, element);

     }

   }

   @Override

   public void write(DataOutput out) throws IOException {

     super.write(out);

     out.writeInt(capacity);

   }

   @Override

   public void readFields(DataInput in) throws IOException {

     super.readFields(in);

     capacity = in.readInt();

   }

 }