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.math4.legacy.ml.distance;
18
19 import org.apache.commons.math4.core.jdkmath.JdkMath;
20 import org.apache.commons.math4.legacy.core.MathArrays;
21
22 /**
23 * Calculates the Earh Mover's distance (also known as Wasserstein metric) between two distributions.
24 *
25 * @see <a href="http://en.wikipedia.org/wiki/Earth_mover's_distance">Earth Mover's distance (Wikipedia)</a>
26 *
27 * @since 3.3
28 */
29 public class EarthMoversDistance implements DistanceMeasure {
30 /** {@inheritDoc} */
31 @Override
32 public double compute(double[] a, double[] b) {
33 MathArrays.checkEqualLength(a, b);
34 double lastDistance = 0;
35 double totalDistance = 0;
36 for (int i = 0; i < a.length; i++) {
37 final double currentDistance = (a[i] + lastDistance) - b[i];
38 totalDistance += JdkMath.abs(currentDistance);
39 lastDistance = currentDistance;
40 }
41 return totalDistance;
42 }
43 }