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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreemnets.  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 opennlp.tools.util.eval;

import java.io.IOException;
import java.util.Collection;
import java.util.NoSuchElementException;

import opennlp.tools.util.CollectionObjectStream;
import opennlp.tools.util.ObjectStream;

/**
 * Provides access to training and test partitions for n-fold cross validation.
 * 
 * Cross validation is used to evaluate the performance of a classifier when only
 * training data is available. The training set is split into n parts
 * and the training / evaluation is performed n times on these parts. 
 * The training partition always consists of n -1 parts and one part is used for testing.
 * 
 * To use the CrossValidationPartioner a client iterates over the n
 * TrainingSampleStreams. Each TrainingSampleStream represents
 * one partition and is used first for training and afterwards for testing.
 * The TestSampleStream can be obtained from the TrainingSampleStream
 * with the getTestSampleStream method.
 */
public class CrossValidationPartitioner {

  /**
   * The TestSampleStream iterates over all test elements.
   *
   * @param 
   */
  private static class TestSampleStream implements ObjectStream {
    
    private ObjectStream sampleStream;
    
    private final int numberOfPartitions;
    
    private final int testIndex;
    
    private int index;
    
    private boolean isPoisened;
    
    private TestSampleStream(ObjectStream sampleStream, int numberOfPartitions, int testIndex) {
      this.numberOfPartitions = numberOfPartitions;
      this.sampleStream = sampleStream;
      this.testIndex = testIndex;
    }
    
    public E read() throws IOException {
      if (isPoisened) {
        throw new IllegalStateException();
      }
      
      // skip training samples
      while (index % numberOfPartitions != testIndex) {
        sampleStream.read();
        index++;
      }
      
      index++;
      
      return sampleStream.read();
    }
    
    /**
     * Throws UnsupportedOperationException
     */
    public void reset() {
      throw new UnsupportedOperationException();
    }
    
    public void close() throws IOException {
      sampleStream.close();
      isPoisened = true;
    }
    
    void poison() {
      isPoisened = true;
    }
  }
  
  /**
   * The TrainingSampleStream which iterates over
   * all training elements.
   * 
   * Note:
   * After the TestSampleStream was obtained
   * the TrainingSampleStream must not be used
   * anymore, otherwise a {@link IllegalStateException}
   * is thrown.
   * 
   * The ObjectStream>s must not be used anymore after the
   * CrossValidationPartitioner was moved
   * to one of next partitions. If they are called anyway
   * a {@link IllegalStateException} is thrown.
   * 
   * @param 
   */
  public static class TrainingSampleStream implements ObjectStream {

    private ObjectStream sampleStream;
    
    private final int numberOfPartitions;
    
    private final int testIndex;
    
    private int index;
    
    private boolean isPoisened;
    
    private TestSampleStream testSampleStream;
    
    TrainingSampleStream(ObjectStream sampleStream, int numberOfPartitions, int testIndex) {
      this.numberOfPartitions = numberOfPartitions;
      this.sampleStream = sampleStream;
      this.testIndex = testIndex;
    }

    public E read() throws IOException {
      
      if (testSampleStream != null || isPoisened) {
        throw new IllegalStateException();
      }
      
      // If the test element is reached skip over it to not include it in
      // the training data
      if (index % numberOfPartitions == testIndex) {
        sampleStream.read();
        index++;
      }
      
      index++;
      
      return sampleStream.read();
    }
    
    /**
     * Throws UnsupportedOperationException
     */
    public void reset() {
      throw new UnsupportedOperationException();
    }
    
    public void close() throws IOException {
      sampleStream.close();
      poison();
    }
    
    void poison() {
      isPoisened = true;
      if (testSampleStream != null)
        testSampleStream.poison();
    }
    
    /**
     * Retrieves the ObjectStream over the test/evaluations
     * elements and poisons this TrainingSampleStream.
     * From now on calls to the hasNext and next methods are forbidden
     * and will raise anIllegalArgumentException.
     *  
     * @return
     */
    public ObjectStream getTestSampleStream() throws IOException {
      
      if (isPoisened) {
        throw new IllegalStateException();
      }
      
      if (testSampleStream == null) {
      
        sampleStream.reset();
        testSampleStream =  new TestSampleStream(sampleStream, numberOfPartitions, testIndex);
      }
      
      return testSampleStream;
    }
  }
  
  /**
   * An ObjectStream over the whole set of data samples which
   * are used for the cross validation.
   */
  private ObjectStream sampleStream;
  
  /**
   * The number of parts the data is divided into.
   */
  private final int numberOfPartitions;
  
  /**
   * The index of test part.
   */
  private int testIndex;

  /**
   * The last handed out TrainingIterator. The reference
   * is needed to poison the instance to fail fast if it is used
   * despite the fact that it is forbidden!.
   */
  private TrainingSampleStream lastTrainingSampleStream;
  
  /**
   * Initializes the current instance.
   * 
   * @param inElements
   * @param numberOfPartitions
   */
  public CrossValidationPartitioner(ObjectStream inElements, int numberOfPartitions) {
    this.sampleStream = inElements;
    this.numberOfPartitions = numberOfPartitions;
  }
  
  /**
   * Initializes the current instance.
   * 
   * @param elements
   * @param numberOfPartitions
   */
  public CrossValidationPartitioner(Collection elements, int numberOfPartitions) {
    this(new CollectionObjectStream(elements), numberOfPartitions);
  }

  /**
   * Checks if there are more partitions available.
   */
  public boolean hasNext() {
    return testIndex < numberOfPartitions;
  }

  /**
   * Retrieves the next training and test partitions.
   */
  public TrainingSampleStream next() throws IOException {
    if (hasNext()) {
      if (lastTrainingSampleStream != null)
        lastTrainingSampleStream.poison();
      
      sampleStream.reset();
      
      TrainingSampleStream trainingSampleStream = new TrainingSampleStream(sampleStream,
          numberOfPartitions, testIndex);
      
      testIndex++;
      
      lastTrainingSampleStream = trainingSampleStream;
      
      return trainingSampleStream;
    }
    else {
      throw new NoSuchElementException();
    }
  }
  
  @Override
  public String toString() {
    return "At partition" + Integer.toString(testIndex + 1) +
        " of " + Integer.toString(numberOfPartitions);
  }
}