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/*
 * Copyright 2011,2012 Metamarkets Group 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 com.metamx.common.guava;

import com.google.common.collect.Iterators;
import com.google.common.collect.PeekingIterator;

import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;

/**
*/
public class MergeIterator implements Iterator
{
  private final PriorityQueue> pQueue;

  public MergeIterator(
      final Comparator comparator,
      List> iterators
  )
  {
    pQueue = new PriorityQueue<>(
        16,
        new Comparator>()
        {
          @Override
          public int compare(PeekingIterator lhs, PeekingIterator rhs)
          {
            return comparator.compare(lhs.peek(), rhs.peek());
          }
        }
    );

    for (Iterator iterator : iterators) {
      final PeekingIterator iter = Iterators.peekingIterator(iterator);

      if (iter != null && iter.hasNext()) {
        pQueue.add(iter);
      }
    }

  }

  @Override
  public boolean hasNext()
  {
    return ! pQueue.isEmpty();
  }

  @Override
  public T next()
  {
    if (! hasNext()) {
      throw new NoSuchElementException();
    }

    PeekingIterator retIt = pQueue.remove();
    T retVal = retIt.next();

    if (retIt.hasNext()) {
      pQueue.add(retIt);
    }

    return retVal;
  }

  @Override
  public void remove()
  {
    throw new UnsupportedOperationException();
  }
}