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Apache commons math 2.0 » org » apache » commons » math » distribution » PoissonDistributionTest.java
/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.commons.math.distribution;

import org.apache.commons.math.MathException;

/**
 * <code>PoissonDistributionTest</code>
 * 
 * @version $Revision: 762087 $ $Date: 2009-04-05 10:20:18 -0400 (Sun, 05 Apr 2009) $
 */
public class PoissonDistributionTest extends IntegerDistributionAbstractTest {

    /**
     * Poisson parameter value for the test distribution.
     */
    private static final double DEFAULT_TEST_POISSON_PARAMETER = 4.0;

    /**
     * Constructor.
     * @param name
     */
    public PoissonDistributionTest(String name) {
        super(name);
        setTolerance(1e-12);
    }

    /** 
     * Creates the default discrete distribution instance to use in tests. 
     */
    @Override
    public IntegerDistribution makeDistribution() {
        return new PoissonDistributionImpl(DEFAULT_TEST_POISSON_PARAMETER);  
    }

    /** 
     * Creates the default probability density test input values.
     */
    @Override
    public int[] makeDensityTestPoints() {
        return new int[] { -1, 0, 1, 2, 3, 4, 5, 10, 20};
    }

    /**
     * Creates the default probability density test expected values.
     * These and all other test values are generated by R, version 1.8.1
     */
    @Override
    public double[] makeDensityTestValues() {
        return new double[] { 0d, 0.0183156388887d,  0.073262555555d,
                0.14652511111d, 0.195366814813d, 0.195366814813, 
                0.156293451851d, 0.00529247667642d, 8.27746364655e-09};
    }

    /**
     * Creates the default cumulative probability density test input values.
     */
    @Override
    public int[] makeCumulativeTestPoints() {
        return new int[] { -1, 0, 1, 2, 3, 4, 5, 10, 20 };
    }

    /**
     * Creates the default cumulative probability density test expected values.
     */
    @Override
    public double[] makeCumulativeTestValues() {
        return new double[] { 0d,  0.0183156388887d, 0.0915781944437d, 
                0.238103305554d, 0.433470120367d, 0.62883693518,
                0.78513038703d,  0.99716023388d, 0.999999998077 };
    }

    /** 
     * Creates the default inverse cumulative probability test input values.
     * Increased 3rd and 7th values slightly as computed cumulative
     * probabilities for corresponding values exceeds the target value (still 
     * within tolerance).
     */
    @Override
    public double[] makeInverseCumulativeTestPoints() {
        return new double[] { 0d,  0.018315638889d, 0.0915781944437d, 
                0.238103305554d, 0.433470120367d, 0.62883693518,
                0.78513038704d,  0.99716023388d, 0.999999998077 };
    }

    /**
     * Creates the default inverse cumulative probability density test expected values.
     */
    @Override
    public int[] makeInverseCumulativeTestValues() {
        return new int[] { -1, 0, 1, 2, 3, 4, 5, 10, 20};
    }

    /**
     * Test the normal approximation of the Poisson distribution by
     * calculating P(90 &le; X &le; 110) for X = Po(100) and
     * P(9900 &le; X &le; 10200) for X  = Po(10000)
     */
    public void testNormalApproximateProbability() throws Exception {
        PoissonDistribution dist = new PoissonDistributionImpl(100);
        double result = dist.normalApproximateProbability(110)
                - dist.normalApproximateProbability(89);
        assertEquals(0.706281887248, result, 1E-10);
        dist.setMean(10000);
        result = dist.normalApproximateProbability(10200)
        - dist.normalApproximateProbability(9899);
        assertEquals(0.820070051552, result, 1E-10);
    }

    /**
     * Test the degenerate cases of a 0.0 and 1.0 inverse cumulative probability.
     * @throws Exception
     */
    public void testDegenerateInverseCumulativeProbability() throws Exception {
        PoissonDistribution dist = new PoissonDistributionImpl(DEFAULT_TEST_POISSON_PARAMETER);
        assertEquals(Integer.MAX_VALUE, dist.inverseCumulativeProbability(1.0d));
        assertEquals(-1, dist.inverseCumulativeProbability(0d));
    }
    
    public void testMean() {
        PoissonDistribution dist = new PoissonDistributionImpl(DEFAULT_TEST_POISSON_PARAMETER);
        try {
            dist.setMean(-1);
            fail("negative mean.  IllegalArgumentException expected");
        } catch(IllegalArgumentException ex) {
        }
        
        dist.setMean(10.0);
        assertEquals(10.0, dist.getMean(), 0.0);
    }
    
    public void testLargeMeanCumulativeProbability() {
        PoissonDistribution dist = new PoissonDistributionImpl(1.0);
        double mean = 1.0;
        while (mean <= 10000000.0) {
            dist.setMean(mean);
            
            double x = mean * 2.0;
            double dx = x / 10.0;
            while (x >= 0) {
                try {
                    dist.cumulativeProbability(x);
                } catch (MathException ex) {
                    fail("mean of " + mean + " and x of " + x + " caused " + ex.getMessage());
                }
                x -= dx;
            }
            
            mean *= 10.0;
        }
    }
    
    public void testLargeMeanInverseCumulativeProbability() {
        PoissonDistribution dist = new PoissonDistributionImpl(1.0);
        double mean = 1.0;
        while (mean <= 10000000.0) {
            dist.setMean(mean);
            
            double p = 0.1;
            double dp = p;
            while (p < 1.0) {
                try {
                    dist.inverseCumulativeProbability(p);
                } catch (MathException ex) {
                    fail("mean of " + mean + " and p of " + p + " caused " + ex.getMessage());
                }
                p += dp;
            }
            
            mean *= 10.0;
        }
    }
}
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