/*
 *  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.commons.collections.iterators;

import java.util.ArrayList;
import java.util.BitSet;
import java.util.Collection;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

import org.apache.commons.collections.list.UnmodifiableList;

/**
 * Provides an ordered iteration over the elements contained in
 * a collection of ordered Iterators.
 * <p>
 * Given two ordered {@link Iterator} instances <code>A</code> and <code>B</code>,
 * the {@link #next} method on this iterator will return the lesser of 
 * <code>A.next()</code> and <code>B.next()</code>.
 *
 * @since Commons Collections 2.1
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Rodney Waldhoff
 * @author Stephen Colebourne
 */
public class CollatingIterator implements Iterator {

    /** The {@link Comparator} used to evaluate order. */
    private Comparator comparator = null;

    /** The list of {@link Iterator}s to evaluate. */
    private ArrayList iterators = null;
   
    /** {@link Iterator#next Next} objects peeked from each iterator. */
    private ArrayList values = null;
    
    /** Whether or not each {@link #values} element has been set. */
    private BitSet valueSet = null;

    /** Index of the {@link #iterators iterator} from whom the last returned value was obtained. */
    private int lastReturned = -1;

    // Constructors
    // ----------------------------------------------------------------------
    /**
     * Constructs a new <code>CollatingIterator</code>.  Natural sort order
     * will be used, and child iterators will have to be manually added 
     * using the {@link #addIterator(Iterator)} method.
     */
    public CollatingIterator() {
        this(null,2);
    }
    
    /**
     * Constructs a new <code>CollatingIterator</code> that will used the
     * specified comparator for ordering.  Child iterators will have to be 
     * manually added using the {@link #addIterator(Iterator)} method.
     *
     * @param comp  the comparator to use to sort, or null to use natural sort order
     */
    public CollatingIterator(final Comparator comp) {
        this(comp,2);
    }
    
    /**
     * Constructs a new <code>CollatingIterator</code> that will used the
     * specified comparator for ordering and have the specified initial
     * capacity.  Child iterators will have to be 
     * manually added using the {@link #addIterator(Iterator)} method.
     *
     * @param comp  the comparator to use to sort, or null to use natural sort order
     * @param initIterCapacity  the initial capacity for the internal list
     *    of child iterators
     */
    public CollatingIterator(final Comparator comp, final int initIterCapacity) {
        iterators = new ArrayList(initIterCapacity);
        setComparator(comp);
    }

    /**
     * Constructs a new <code>CollatingIterator</code> that will use the
     * specified comparator to provide ordered iteration over the two
     * given iterators.
     *
     * @param comp  the comparator to use to sort, or null to use natural sort order
     * @param a  the first child ordered iterator
     * @param b  the second child ordered iterator
     * @throws NullPointerException if either iterator is null
     */
    public CollatingIterator(final Comparator comp, final Iterator a, final Iterator b) {
        this(comp,2);
        addIterator(a);
        addIterator(b);
    }

    /**
     * Constructs a new <code>CollatingIterator</code> that will use the
     * specified comparator to provide ordered iteration over the array
     * of iterators.
     *
     * @param comp  the comparator to use to sort, or null to use natural sort order
     * @param iterators  the array of iterators
     * @throws NullPointerException if iterators array is or contains null
     */
    public CollatingIterator(final Comparator comp, final Iterator[] iterators) {
        this(comp, iterators.length);
        for (int i = 0; i < iterators.length; i++) {
            addIterator(iterators[i]);
        }
    }

    /**
     * Constructs a new <code>CollatingIterator</code> that will use the
     * specified comparator to provide ordered iteration over the collection
     * of iterators.
     *
     * @param comp  the comparator to use to sort, or null to use natural sort order
     * @param iterators  the collection of iterators
     * @throws NullPointerException if the iterators collection is or contains null
     * @throws ClassCastException if the iterators collection contains an
     *         element that's not an {@link Iterator}
     */
    public CollatingIterator(final Comparator comp, final Collection iterators) {
        this(comp, iterators.size());
        for (Iterator it = iterators.iterator(); it.hasNext();) {
            Iterator item = (Iterator) it.next();
            addIterator(item);
        }
    }

    // Public Methods
    // ----------------------------------------------------------------------
    /**
     * Adds the given {@link Iterator} to the iterators being collated.
     * 
     * @param iterator  the iterator to add to the collation, must not be null
     * @throws IllegalStateException if iteration has started
     * @throws NullPointerException if the iterator is null
     */
    public void addIterator(final Iterator iterator) {
        checkNotStarted();
        if (iterator == null) {
            throw new NullPointerException("Iterator must not be null");
        }
        iterators.add(iterator);
    }

    /**
     * Sets the iterator at the given index.
     * 
     * @param index  index of the Iterator to replace
     * @param iterator  Iterator to place at the given index
     * @throws IndexOutOfBoundsException if index &lt; 0 or index &gt; size()
     * @throws IllegalStateException if iteration has started
     * @throws NullPointerException if the iterator is null
     */
    public void setIterator(final int index, final Iterator iterator) {
        checkNotStarted();
        if (iterator == null) {
            throw new NullPointerException("Iterator must not be null");
        }
        iterators.set(index, iterator);
    }

    /**
     * Gets the list of Iterators (unmodifiable).
     * 
     * @return the unmodifiable list of iterators added
     */
    public List getIterators() {
        return UnmodifiableList.decorate(iterators);
    }

    /**
     * Gets the {@link Comparator} by which collatation occurs.
     */
    public Comparator getComparator() {
        return comparator;
    }

    /**
     * Sets the {@link Comparator} by which collation occurs.
     * 
     * @throws IllegalStateException if iteration has started
     */
    public void setComparator(final Comparator comp) {
        checkNotStarted();
        comparator = comp;
    }

    // Iterator Methods
    // -------------------------------------------------------------------
    /**
     * Returns <code>true</code> if any child iterator has remaining elements.
     *
     * @return true if this iterator has remaining elements
     */
    public boolean hasNext() {
        start();
        return anyValueSet(valueSet) || anyHasNext(iterators);
    }

    /**
     * Returns the next ordered element from a child iterator.
     *
     * @return the next ordered element
     * @throws NoSuchElementException if no child iterator has any more elements
     */
    public Object next() throws NoSuchElementException {
        if (hasNext() == false) {
            throw new NoSuchElementException();
        }
        int leastIndex = least();
        if (leastIndex == -1) {
            throw new NoSuchElementException();
        } else {
            Object val = values.get(leastIndex);
            clear(leastIndex);
            lastReturned = leastIndex;
            return val;
        }
    }

    /**
     * Removes the last returned element from the child iterator that 
     * produced it.
     *
     * @throws IllegalStateException if there is no last returned element,
     *  or if the last returned element has already been removed
     */
    public void remove() {
        if (lastReturned == -1) {
            throw new IllegalStateException("No value can be removed at present");
        }
        Iterator it = (Iterator) (iterators.get(lastReturned));
        it.remove();
    }

    // Private Methods
    // -------------------------------------------------------------------
    /** 
     * Initializes the collating state if it hasn't been already.
     */
    private void start() {
        if (values == null) {
            values = new ArrayList(iterators.size());
            valueSet = new BitSet(iterators.size());
            for (int i = 0; i < iterators.size(); i++) {
                values.add(null);
                valueSet.clear(i);
            }
        }
    }

    /** 
     * Sets the {@link #values} and {@link #valueSet} attributes 
     * at position <i>i</i> to the next value of the 
     * {@link #iterators iterator} at position <i>i</i>, or 
     * clear them if the <i>i</i><sup>th</sup> iterator
     * has no next value.
     *
     * @return <tt>false</tt> iff there was no value to set
     */
    private boolean set(int i) {
        Iterator it = (Iterator)(iterators.get(i));
        if (it.hasNext()) {
            values.set(i, it.next());
            valueSet.set(i);
            return true;
        } else {
            values.set(i,null);
            valueSet.clear(i);
            return false;
        }
    }

    /** 
     * Clears the {@link #values} and {@link #valueSet} attributes 
     * at position <i>i</i>.
     */
    private void clear(int i) {
        values.set(i,null);
        valueSet.clear(i);
    }

    /** 
     * Throws {@link IllegalStateException} if iteration has started 
     * via {@link #start}.
     * 
     * @throws IllegalStateException if iteration started
     */
    private void checkNotStarted() throws IllegalStateException {
        if (values != null) {
            throw new IllegalStateException("Can't do that after next or hasNext has been called.");
        }
    }

    /** 
     * Returns the index of the least element in {@link #values},
     * {@link #set(int) setting} any uninitialized values.
     * 
     * @throws IllegalStateException
     */
    private int least() {
        int leastIndex = -1;
        Object leastObject = null;                
        for (int i = 0; i < values.size(); i++) {
            if (valueSet.get(i) == false) {
                set(i);
            }
            if (valueSet.get(i)) {
                if (leastIndex == -1) {
                    leastIndex = i;
                    leastObject = values.get(i);
                } else {
                    Object curObject = values.get(i);
                    if (comparator.compare(curObject,leastObject) < 0) {
                        leastObject = curObject;
                        leastIndex = i;
                    }
                }
            }
        }
        return leastIndex;
    }

    /**
     * Returns <code>true</code> iff any bit in the given set is 
     * <code>true</code>.
     */
    private boolean anyValueSet(BitSet set) {
        for (int i = 0; i < set.size(); i++) {
            if (set.get(i)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Returns <code>true</code> iff any {@link Iterator} 
     * in the given list has a next value.
     */
    private boolean anyHasNext(ArrayList iters) {
        for (int i = 0; i < iters.size(); i++) {
            Iterator it = (Iterator) iters.get(i);
            if (it.hasNext()) {
                return true;
            }
        }
        return false;
    }

}
