/*
    * Created on Apr 2, 2006
    *
    * Copyright (c) 2006, the JUNG Project and the Regents of the University 
    * of California
    * All rights reserved.
    *
    * This software is open-source under the BSD license; see either
    * "license.txt" or
   * http://jung.sourceforge.net/license.txt for a description.
   */
  package edu.uci.ics.jung.graph.util;
  
  import java.io.Serializable;
  import java.util.Collection;
  import java.util.Iterator;
  
  
  /**
   * An implementation of <code>Collection</code> that stores exactly
   * 2 non-null objects and is not mutable.  They respect <code>equals</code>
   * and may be used as indices or map keys.<p>
   * Note that they do not protect from malevolent behavior: if one or another
   * object in the tuple is mutable, then it can be changed with the usual bad
   * effects.
   */
  @SuppressWarnings("serial")
  public final class Pair<T> implements Collection<T>, Serializable
  {
      private T first;
      private T second;
  
      /**
       * Creates a <code>Pair</code> from the specified elements.
       * @param value1 the first value in the new <code>Pair</code>
       * @param value2 the second value in the new <code>Pair</code>
       * @throws IllegalArgumentException if either argument is null
       */
      public Pair(T value1, T value2) 
      {
      	if(value1 == null || value2 == null) 
      		throw new IllegalArgumentException("Pair cannot contain null values");
          first = value1;
          second = value2;
      }
      
      /**
       * Creates a Pair from the passed Collection.
       * The size of the Collection must be 2.
       * @param values the elements of the new <code>Pair</code>
       * @throws IllegalArgumentException if the input collection is null,
       * contains null values, or has != 2 elements.
       */
      public Pair(Collection<? extends T> values) 
      {
          if (values == null)
              throw new IllegalArgumentException("Input collection cannot be null");
      	if (values.size() == 2)
          {
              if(values.contains(null)) 
                  throw new IllegalArgumentException("Pair cannot contain null values");
              Iterator<? extends T> iter = values.iterator();
              first = iter.next();
              second = iter.next();
         }
          else
              throw new IllegalArgumentException("Pair may only be created from a Collection of exactly 2 elements");
          
      }
      
      /**
       * Creates a <code>Pair</code> from the passed array.
       * The size of the array must be 2.
       * @throws IllegalArgumentException if the input array is null,
       * contains null values, or has != 2 elements.
       */
      public Pair(T[] values)
      {
          if (values == null)
              throw new IllegalArgumentException("Input array cannot be null");
          if (values.length == 2)
          {
              if(values[0] == null || values[1] == null) 
                  throw new IllegalArgumentException("Pair cannot contain null values");
              first = values[0];
              second = values[1];
          }
          else
              throw new IllegalArgumentException("Pair may only be created from an " +
              		"array of 2 elements");
      }
  
      /**
       * Returns the first element.
       */
      public T getFirst() 
      {
          return first;
      }
     
     /**
      * Returns the second element.
      */
     public T getSecond() 
     {
         return second;
     }
     
     @SuppressWarnings("unchecked")
     @Override
     public boolean equals( Object o ) {
         if (o == this)
             return true;
 
         if (o instanceof Pair) {
             Pair otherPair = (Pair) o;
             Object otherFirst = otherPair.getFirst();
             Object otherSecond = otherPair.getSecond();
             return 
             	(this.first  == otherFirst  || 
             			(this.first != null  && this.first.equals(otherFirst)))   
             			&&
                 (this.second == otherSecond || 
                 		(this.second != null && this.second.equals(otherSecond)));
         } else {
             return false;
         }
     }
     
     @Override
     public int hashCode() 
     {
     	int hashCode = 1;
 	    hashCode = 31*hashCode + (first==null ? 0 : first.hashCode());
 	    hashCode = 31*hashCode + (second==null ? 0 : second.hashCode());
     	return hashCode;
     }
     
     @Override
     public String toString()
     {
         return "<" + first.toString() + ", " + second.toString() + ">";
     }
 
     public boolean add(T o) {
         throw new UnsupportedOperationException("Pairs cannot be mutated");
     }
 
     public boolean addAll(Collection<? extends T> c) {
         throw new UnsupportedOperationException("Pairs cannot be mutated");
     }
 
     public void clear() {
         throw new UnsupportedOperationException("Pairs cannot be mutated");
     }
 
     public boolean contains(Object o) {
         return (first == o || first.equals(o) || second == o || second.equals(o));
     }
 
     public boolean containsAll(Collection<?> c) {
         if (c.size() > 2)
             return false;
         Iterator<?> iter = c.iterator();
         Object c_first = iter.next();
         Object c_second = iter.next();
         return this.contains(c_first) && this.contains(c_second);
     }
 
     public boolean isEmpty() {
         return false;
     }
 
     public Iterator<T> iterator() {
         return new PairIterator();
     }
 
     public boolean remove(Object o) {
         throw new UnsupportedOperationException("Pairs cannot be mutated");
     }
 
     public boolean removeAll(Collection<?> c) {
         throw new UnsupportedOperationException("Pairs cannot be mutated");
     }
 
     public boolean retainAll(Collection<?> c) {
         throw new UnsupportedOperationException("Pairs cannot be mutated");
     }
 
     public int size() {
         return 2;
     }
 
     public Object[] toArray() {
         Object[] to_return = new Object[2];
         to_return[0] = first;
         to_return[1] = second;
         return to_return;
     }
 
     @SuppressWarnings("unchecked")
     public <S> S[] toArray(S[] a) {
         S[] to_return = a;
         Class<?> type = a.getClass().getComponentType();
         if (a.length < 2)
             to_return = (S[])java.lang.reflect.Array.newInstance(type, 2);
         to_return[0] = (S)first;
         to_return[1] = (S)second;
         
         if (to_return.length > 2)
             to_return[2] = null;
         return to_return;
     }
     
     private class PairIterator implements Iterator<T>
     {
         int position;
         
         private PairIterator()
         {
             position = 0;
         }
 
         public boolean hasNext()
         {
             return position < 2;
         }
 
         public T next()
         {
             position++;
             if (position == 1)
                 return first;
             else if (position == 2)
                 return second;
             else
                 return null;
         }
 
         public void remove()
         {
             throw new UnsupportedOperationException("Pairs cannot be mutated");
         }
     }
 }