/*

  * 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.utils;

 import com.google.common.collect.Lists;

 import java.util.List;

 /**

  * Collection to keep pairs in, without creating a wrapper object around

  * each pair of objects.

  *

  * @param <U> Type of the first element in a pair

  * @param <V> Type of the second element in a pair

  */

 public class PairList<U, V> {

   /** List to keep first elements of pairs in */

   protected List<U> firstList;

   /** List to keep second elements of pairs in */

   protected List<V> secondList;

   /**

    * Constructor

    */

   public PairList() {

   }

   /**

    * Initialize the inner state. Must be called before {@code add()} is

    * called.

    */

   public void initialize() {

     firstList = Lists.newArrayList();

     secondList = Lists.newArrayList();

   }

   /**

    * Initialize the inner state, with a known size. Must be called before

    * {@code add()} is called.

    *

    * @param size Number of pairs which will be added to the list

    */

   public void initialize(int size) {

     firstList = Lists.newArrayListWithCapacity(size);

     secondList = Lists.newArrayListWithCapacity(size);

   }

   /**

    * Add a pair to the collection.

    *

    * @param first First element of the pair

    * @param second Second element of the pair

    */

   public void add(U first, V second) {

     firstList.add(first);

     secondList.add(second);

   }

   /**

    * Get number of pairs in this list.

    *

    * @return Number of pairs in the list

    */

   public int getSize() {

     return firstList.size();

   }

   /**

    * Check if the list is empty.

    *

    * @return True iff there are no pairs in the list

    */

   public boolean isEmpty() {

     return getSize() == 0;

   }

   /**

    * Get iterator through elements of this object.

    *

    * @return {@link Iterator} iterator

    */

   public Iterator getIterator() {

     return new Iterator();

   }

   /**

    * Special iterator class which we'll use to iterate through elements of

    * {@link PairList}, without having to create new object as wrapper for

    * each pair.

    *

    * Protocol is somewhat similar to the protocol of {@link java.util.Iterator}

    * only here next() doesn't return the next object, it just moves along in

    * the collection. Values related to current pair can be retrieved by calling

    * getCurrentFirst() and getCurrentSecond() methods.

    *

    * Not thread-safe.

    */

   public class Iterator {

     /** Current position of the iterator */

     private int position = -1;

     /**

      * Returns true if the iteration has more elements.

      *

      * @return True if the iteration has more elements.

      */

     public boolean hasNext() {

       return position < getSize() - 1;

     }

     /**

      * Moves to the next element in the iteration.

      */

     public void next() {

       position++;

     }

     /**

      * Get first element of the current pair of the iteration.

      *

      * @return First element of the current pair of the iteration

      */

     public U getCurrentFirst() {

       return firstList.get(position);

     }

     /**

      * Get second element of the current pair of the iteration.

      *

      * @return Second element of the current pair of the iteration

      */

     public V getCurrentSecond() {

       return secondList.get(position);

     }

   }

 }