1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one or more 3 * contributor license agreements. See the NOTICE file distributed with 4 * this work for additional information regarding copyright ownership. 5 * The ASF licenses this file to You under the Apache License, Version 2.0 6 * (the "License"); you may not use this file except in compliance with 7 * the License. You may obtain a copy of the License at 8 * 9 * http://www.apache.org/licenses/LICENSE-2.0 10 * 11 * Unless required by applicable law or agreed to in writing, software 12 * distributed under the License is distributed on an "AS IS" BASIS, 13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 14 * See the License for the specific language governing permissions and 15 * limitations under the License. 16 */ 17 package org.apache.commons.math3.ml.distance; 18 19 import org.apache.commons.math3.exception.DimensionMismatchException; 20 import org.apache.commons.math3.util.FastMath; 21 import org.apache.commons.math3.util.MathArrays; 22 23 /** 24 * Calculates the Earh Mover's distance (also known as Wasserstein metric) between two distributions. 25 * 26 * @see <a href="http://en.wikipedia.org/wiki/Earth_mover's_distance">Earth Mover's distance (Wikipedia)</a> 27 * 28 * @since 3.3 29 */ 30 public class EarthMoversDistance implements DistanceMeasure { 31 32 /** Serializable version identifier. */ 33 private static final long serialVersionUID = -5406732779747414922L; 34 35 /** {@inheritDoc} */ compute(double[] a, double[] b)36 public double compute(double[] a, double[] b) 37 throws DimensionMismatchException { 38 MathArrays.checkEqualLength(a, b); 39 double lastDistance = 0; 40 double totalDistance = 0; 41 for (int i = 0; i < a.length; i++) { 42 final double currentDistance = (a[i] + lastDistance) - b[i]; 43 totalDistance += FastMath.abs(currentDistance); 44 lastDistance = currentDistance; 45 } 46 return totalDistance; 47 } 48 } 49