// ============================================================================
// Copyright 2006-2012 Daniel W. Dyer
//
// 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 org.uncommons.maths.demo;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import org.uncommons.maths.Maths;
import org.uncommons.maths.random.BinomialGenerator;
/**
* @author Daniel Dyer
*/
class BinomialDistribution extends ProbabilityDistribution
{
private final int n;
private final double p;
public BinomialDistribution(int n, double p)
{
this.n = n;
this.p = p;
}
public Map<Double, Double> getExpectedValues()
{
Map<Double, Double> values = new HashMap<Double, Double>();
for (int i = 0; i <= n; i++)
{
values.put((double) i, getExpectedProbability(i));
}
return values;
}
/**
* This is the probability mass function
* (http://en.wikipedia.org/wiki/Probability_mass_function) of
* the Binomial distribution represented by this number generator.
* @param successes The number of successful trials to determine
* the probability for.
* @return The probability of obtaining the specified number of
* successful trials given the current values of n and p.
*/
private double getExpectedProbability(int successes)
{
double prob = Math.pow(p, successes) * Math.pow(1 - p, n - successes);
BigDecimal coefficient = new BigDecimal(binomialCoefficient(n, successes));
return coefficient.multiply(new BigDecimal(prob)).doubleValue();
}
private BigInteger binomialCoefficient(int n, int k)
{
BigInteger nFactorial = Maths.bigFactorial(n);
BigInteger kFactorial = Maths.bigFactorial(k);
BigInteger nMinusKFactorial = Maths.bigFactorial(n - k);
BigInteger divisor = kFactorial.multiply(nMinusKFactorial);
return nFactorial.divide(divisor);
}
protected BinomialGenerator createValueGenerator(Random rng)
{
return new BinomialGenerator(n, p, rng);
}
public double getExpectedMean()
{
return n * p;
}
public double getExpectedStandardDeviation()
{
return Math.sqrt(n * p * (1 - p));
}
public String getDescription()
{
return "Binomial Distribution (n = " + n + ", p = " + p + ")";
}
public boolean isDiscrete()
{
return true;
}
}