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 * @since 3.3 027 */ 028public class EarthMoversDistance implements DistanceMeasure { 029 030 /** Serializable version identifier. */ 031 private static final long serialVersionUID = -5406732779747414922L; 032 033 /** {@inheritDoc} */ 034 public double compute(double[] a, double[] b) { 035 double lastDistance = 0; 036 double totalDistance = 0; 037 for (int i = 0; i < a.length; i++) { 038 final double currentDistance = (a[i] + lastDistance) - b[i]; 039 totalDistance += FastMath.abs(currentDistance); 040 lastDistance = currentDistance; 041 } 042 return totalDistance; 043 } 044}