/*
Copyright (C) 2001 Kyle Siegrist, Dawn Duehring
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your option)
any later version.
This program is distributed in the hope that it will be useful, but without
any warranty; without even the implied warranty of merchantability or
fitness for a particular purpose. See the GNU General Public License for
more details. You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
package distributions;
/**The Poisson distribution with a specified rate parameter*/
public class PoissonDistribution extends Distribution{
//Variables
double parameter;
/**Default constructor: creates a new Poisson distribution with a given
parameter value*/
public PoissonDistribution(double r){
setParameter(r);
}
/**Default constructor: creates a new Poisson distribtiton with parameter 1*/
public PoissonDistribution(){
this(1);
}
/**Sets the parameter*/
public void setParameter(double r){
//Correct for invalid parameter:
if(r < 0) r = 1;
parameter = r;
//Sets the truncated set of values
double a = Math.ceil(getMean() - 4 * getSD()), b = Math.ceil(getMean() + 4 * getSD());
if (a < 0) a = 0;
super.setParameters(a, b, 1, DISCRETE);
}
/**Parameter*/
public double getParameter(){
return parameter;
}
/**Density function*/
public double getDensity(double x){
int k = (int)Math.rint(x);
if(k < 0) return 0;
else return Math.exp(-parameter) * (Math.pow(parameter, k) / factorial(k));
}
/**Maximum value of the getDensity function*/
public double getMaxDensity(){
double mode = Math.floor(parameter);
return getDensity(mode);
}
/**Cumulative distribution function*/
public double getCDF(double x){
return 1 - gammaCDF(parameter, x + 1);
}
/**Mean*/
public double getMean(){
return parameter;
}
/**Variance*/
public double getVariance(){
return parameter;
}
/**Simulate a value*/
public double simulate(){
int arrivals = 0;
double sum = -Math.log(1 - Math.random());
while (sum <= parameter){
arrivals++;
sum = sum - Math.log(1 - Math.random());
}
return arrivals;
}
}