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
18 package org.apache.commons.math4.legacy.ml.clustering;
19
20 import java.util.List;
21
22 /**
23 * Defines a measure of the quality of clusters.
24 */
25 public interface ClusterEvaluator {
26 /**
27 * @param cList List of clusters.
28 * @return the score attributed by the evaluator.
29 */
30 double score(List<? extends Cluster<? extends Clusterable>> cList);
31
32 /**
33 * Provides a means to interpret the {@link #score(List) score value}.
34 *
35 * @param a Score computed by this evaluator.
36 * @param b Score computed by this evaluator.
37 * @return {@code true} if the evaluator considers that score
38 * {@code a} is better than score {@code b}.
39 */
40 boolean isBetterScore(double a, double b);
41
42 /**
43 * Converts to a {@link ClusterRanking ranking function}
44 * (as required by clustering implementations).
45 *
46 * @param <T> the type of points that can be clustered
47 * @param eval Evaluator function.
48 * @return a ranking function.
49 */
50 static <T extends Clusterable> ClusterRanking ranking(ClusterEvaluator eval) {
51 return eval.isBetterScore(1, 2) ?
52 clusters -> 1 / eval.score(clusters) :
53 clusters -> eval.score(clusters);
54 }
55 }