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 * https://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
18 /*
19 * This is not the original file distributed by the Apache Software Foundation
20 * It has been modified by the Hipparchus project
21 */
22 package org.hipparchus.clustering.distance;
23
24 import org.hipparchus.exception.MathIllegalArgumentException;
25 import org.hipparchus.util.FastMath;
26 import org.hipparchus.util.MathArrays;
27
28 /**
29 * Calculates the Earh Mover's distance (also known as Wasserstein metric) between two distributions.
30 *
31 * @see <a href="http://en.wikipedia.org/wiki/Earth_mover's_distance">Earth Mover's distance (Wikipedia)</a>
32 *
33 */
34 public class EarthMoversDistance implements DistanceMeasure {
35
36 /** Serializable version identifier. */
37 private static final long serialVersionUID = -5406732779747414922L;
38
39 /** Empty constructor.
40 * <p>
41 * This constructor is not strictly necessary, but it prevents spurious
42 * javadoc warnings with JDK 18 and later.
43 * </p>
44 * @since 3.0
45 */
46 public EarthMoversDistance() { // NOPMD - unnecessary constructor added intentionally to make javadoc happy
47 // nothing to do
48 }
49
50 /** {@inheritDoc} */
51 @Override
52 public double compute(double[] a, double[] b)
53 throws MathIllegalArgumentException {
54 MathArrays.checkEqualLength(a, b);
55 double lastDistance = 0;
56 double totalDistance = 0;
57 for (int i = 0; i < a.length; i++) {
58 final double currentDistance = (a[i] + lastDistance) - b[i];
59 totalDistance += FastMath.abs(currentDistance);
60 lastDistance = currentDistance;
61 }
62 return totalDistance;
63 }
64
65 }