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package edu.isi.nlp.math;

import com.google.common.base.Function;
import com.google.common.base.Optional;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Ordering;

/**
 * General mathematical utilities.
 *
 * @author Ryan Gabbard, Jay DeYoung
 */
public final class MathUtils {

  private MathUtils() {
    throw new UnsupportedOperationException();
  }

  /** Returns the sum of all elements in the given non-null array {@code arr}. */
  public static int sum(int[] arr) {
    int ret = 0;
    for (final int x : arr) {
      ret += x;
    }
    return ret;
  }

  public static Optional medianOfIntegers(Iterable sizes) {
    final ImmutableList sorted = Ordering.natural().immutableSortedCopy(sizes);
    if (sorted.size() == 0) {
      return Optional.absent();
    }
    if (sorted.size() % 2 == 0) {
      return Optional.of(0.5 * (sorted.get(sorted.size() / 2) + sorted.get(sorted.size() / 2 - 1)));
    } else {
      return Optional.of((double) sorted.get(sorted.size() / 2));
    }
  }

  public static Optional medianOfDoubles(Iterable sizes) {
    final ImmutableList sorted = Ordering.natural().immutableSortedCopy(sizes);
    if (sorted.size() == 0) {
      return Optional.absent();
    }
    if (sorted.size() % 2 == 0) {
      return Optional.of(0.5 * (sorted.get(sorted.size() / 2) + sorted.get(sorted.size() / 2 - 1)));
    } else {
      return Optional.of(sorted.get(sorted.size() / 2));
    }
  }

  public static Function, Optional> medianOfDoublesFunction() {
    return new Function, Optional>() {
      @Override
      public Optional apply(final Iterable input) {
        return medianOfDoubles(input);
      }
    };
  }

  /** {@code x * log(x)} when x is non-zero, zero otherwise. */
  public static double xLogX(double d) {
    if (d == 0.0) {
      return 0.0;
    } else {
      return d * Math.log(d);
    }
  }
}