samer@0: /*
samer@0:  *	Copyright (c) 2000, Samer Abdallah, King's College London.
samer@0:  *	All rights reserved.
samer@0:  *
samer@0:  *	This software is provided AS iS and WITHOUT ANY WARRANTY; 
samer@0:  *	without even the implied warranty of MERCHANTABILITY or 
samer@0:  *	FITNESS FOR A PARTICULAR PURPOSE.
samer@0:  */
samer@0: 
samer@0: package samer.functions;
samer@0: import  samer.maths.*;
samer@0: 
samer@0: /**
samer@0: Sparsified Laplacian, v1
samer@0: 	Log prior for an laplacian distribution with
samer@0: 	an extra mass at zero, eg a mixture of a laplacian
samer@0: 	with another much narrower laplacian. The log prior is
samer@0: 	basically an abs function with with a extra-deep pit at
samer@0: 	zero.
samer@0: 	Paramters control the width and depth of the pit.
samer@0:   */
samer@0: 
samer@0: public class BiLaplacian extends Function
samer@0: {
samer@0: 	double width=1, A=1, slope=2;
samer@0: 
samer@0: 	public void setWidth(double b) { width=b; slope=(width+A)/width; }
samer@0: 	public void setStrength(double a) { A=a; slope=(width+A)/width; }
samer@0: 	public double getJump() { return slope; }
samer@0: 
samer@0: 	public String format(String t) { return "BiLaplace("+t+")"; }
samer@0: 
samer@0: 	public final double apply(double t) { 
samer@0: 		t = Math.abs(t);
samer@0: 		if  (t<width) return -A+slope*t;
samer@0: 		else return t;
samer@0: 	}
samer@0: 
samer@0: 	public Function derivative() { 
samer@0: 		return new Function() {
samer@0: 			public String format(String t) { return "dBiLaplace("+t+")"; }
samer@0: 			public final double apply(double t) { 
samer@0: 				if (t>0) {
samer@0: 					if  (t<width) return slope;
samer@0: 					else return 1;
samer@0: 				} else if (t<0) {
samer@0: 					if  (t>-width) return -slope;
samer@0: 					else return -1;
samer@0: 				} else return 0;
samer@0: 			}
samer@0: 		};
samer@0: 	}
samer@0: }
samer@0: