001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.math3.ml.distance;
018
019import org.apache.commons.math3.util.FastMath;
020
021/**
022 * Calculates the Earh Mover's distance (also known as Wasserstein metric) between two distributions.
023 *
024 * @see <a href="http://en.wikipedia.org/wiki/Earth_mover's_distance">Earth Mover's distance (Wikipedia)</a>
025 *
026 * @version $Id: EarthMoversDistance.html 885258 2013-11-03 02:46:49Z tn $
027 * @since 3.3
028 */
029public class EarthMoversDistance implements DistanceMeasure {
030
031    /** Serializable version identifier. */
032    private static final long serialVersionUID = -5406732779747414922L;
033
034    /** {@inheritDoc} */
035    public double compute(double[] a, double[] b) {
036        double lastDistance = 0;
037        double totalDistance = 0;
038        for (int i = 0; i < a.length; i++) {
039            final double currentDistance = (a[i] + lastDistance) - b[i];
040            totalDistance += FastMath.abs(currentDistance);
041            lastDistance = currentDistance;
042        }
043        return totalDistance;
044    }
045}