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package edu.isi.nlp.collections;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Random;

/**
 * This is an {@link Iterable} which provides its data in a random order every time an iterator is
 * requested. Note this must load its entire underlying {@link Iterable} into memory.
 *
 * @author Ryan Gabbard, Constantine Lignos
 */
public final class ShufflingIterable implements Iterable {
  private final ImmutableList data;
  private final Random rng;

  private ShufflingIterable(final Iterable iterable, final Random rng) {
    this.data = ImmutableList.copyOf(iterable);
    this.rng = checkNotNull(rng);
  }

  /**
   * Returns a new {@link ShufflingIterable} for the specified iterable and random number generator.
   *
   * @param iterable the source iterable
   * @param rng the random number generator to use in shuffling the iterable
   * @return a new shuffling iterable for contents of the original iterable
   */
  public static  ShufflingIterable from(final Iterable iterable, final Random rng) {
    return new ShufflingIterable<>(iterable, rng);
  }

  /** Returns a new iterator that iterates over a new random ordering of the data. */
  @Override
  public Iterator iterator() {
    final List shuffledList = Lists.newArrayList(data);
    Collections.shuffle(shuffledList, rng);
    return Collections.unmodifiableList(shuffledList).iterator();
  }
}