001    /*
002     * JNI_SVM-light - A Java Native Interface for SVM-light
003     * 
004     * Copyright (C) 2005 
005     * Tom Crecelius & Martin Theobald 
006     * Max-Planck Institute for Computer Science
007     * 
008     * This program is free software; you can redistribute it and/or modify it under
009     * the terms of the GNU General Public License as published by the Free Software
010     * Foundation.
011     * 
012     * This program is distributed in the hope that it will be useful, but WITHOUT
013     * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
014     * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
015     * details.
016     * 
017     * You should have received a copy of the GNU General Public License along with
018     * this program; if not, write to the Free Software Foundation, Inc., 51
019     * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
020     */
021    
022    package jnisvmlight;
023    
024    /**
025     * A kernel with sigmoid smoothing.
026     * 
027     * @author Tom Crecelius & Martin Theobald
028     */
029    public class SigmoidKernel extends ExtendedKernel {
030    
031      protected SigmoidKernel() {
032        super(new LinearKernel(), 1.0, 0.0);
033      }
034    
035      public SigmoidKernel(Kernel nestedKernel, double a, double c) {
036        super(nestedKernel, a, c);
037      }
038    
039      public double evaluate(FeatureVector v1, FeatureVector v2) {
040        return tanh(m_a * m_kernel.evaluate(v1, v2) + m_c);
041      }
042    
043      private double tanh(double a) {
044        double x = Math.exp(a);
045        double y = Math.exp(-a);
046        return (x - y) / (x + y);
047      }
048    
049      public String toString() {
050        return "Sigmoid kernel K(x, k) = tanh(" + m_a + ".k(x) + " + m_c + ")"
051            + ". k = " + m_kernel.toString();
052      }
053    }