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A suite of classification clustering algorithm implementations for Java. A number of partitional, hierarchical and density-based algorithms including DBSCAN, k-Means, k-Medoids, MeanShift, Affinity Propagation, HDBSCAN and more.

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/*******************************************************************************
 *    Copyright 2015, 2016 Taylor G Smith
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 *******************************************************************************/
package com.clust4j.kernel;

import org.apache.commons.math3.util.FastMath;

/**
 * The Power kernel is also known as the (unrectified) triangular kernel. 
 * It is an example of scale-invariant kernel 
 * (Sahbi and Fleuret, 2004) and is also only conditionally positive definite.
 * 
 * @see Souza, Cesar R. -- Kernel Functions for Machine Learning Applications.
 * @author Taylor G Smith
 */
public class PowerKernel extends Kernel {
	/**
	 * 
	 */
	private static final long serialVersionUID = -861680950436032350L;
	public static final double DEFAULT_DEGREE = 1;
	private final double degree;
	
	public PowerKernel() {
		this(DEFAULT_DEGREE);
	}
	
	public PowerKernel(final double degree) {
		this.degree = degree;
	}

	@Override
	public double getSimilarity(double[] a, double[] b) {
		return -(FastMath.pow(toHilbertPSpace(a, b), getDegree()));
	}

	@Override
	public String getName() {
		return "PowerKernel";
	}

	public double getDegree() {
		return degree;
	}
}