/*
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 distribution of the number of matches in a random permutation*/
public class MatchDistribution extends Distribution{
int parameter;
int[] b;
/**This general constructor creates a new matching distribution with a
specified parameter*/
public MatchDistribution(int n){
setParameter(n);
}
/**this default constructor creates a new mathcing distribuiton with
parameter 5*/
public MatchDistribution(){
this(5);
}
/**This method sets the parameter of the distribution (the size of the
random permutation*/
public void setParameter(int n){
if (n < 1) n = 1;
parameter = n;
super.setParameters(0, parameter, 1, DISCRETE);
b = new int[n];
}
/**This method computes the getDensity function*/
public double getDensity(double x){
int k = (int)Math.rint(x);
double sum = 0;
int sign = -1;
for (int j = 0; j <= parameter - k; j++){
sign = -sign;
sum = sum + sign / factorial(j);
}
return sum / factorial(k);
}
/**This method gives the maximum value of the getDensity function*/
public double getMaxDensity(){
if (parameter == 2) return getDensity(0);
else return getDensity(1);
}
/**This method returns the mean*/
public double getMean(){
return 1;
}
/**This method returns the variance*/
public double getVariance(){
return 1;
}
/**This method gets the parameter*/
public int getParameter(){
return parameter;
}
/**This method simulates a value from the distribution, by generating
a random permutation and computing the number of matches*/
public double simulate(){
int j, k, u;
double matches = 0;
for (int i = 0; i < parameter; i++) b[i] = i + 1;
for (int i = 0; i < parameter; i++){
j = parameter - i;
u = (int)(j * Math.random());
if (b[u] == i + 1) matches = matches + 1;
k = b[j - 1];
b[j - 1] = b[u];
b[u] = k;
}
return matches;
}
}