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Serializable pseudo-random number generators and distributions.
There is a newer version: 0.6.1
/*
 * Copyright (c) 2023 See AUTHORS file.
 *
 * 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 com.github.tommyettinger.random.distribution;

import com.github.tommyettinger.digital.MathTools;
import com.github.tommyettinger.random.EnhancedRandom;
import com.github.tommyettinger.random.AceRandom;

/**
 * A one-parameter discrete distribution with integer range from 0 inclusive to positive infinity.
 * @see Wikipedia's page on this distribution.
 */
public class PoissonDistribution extends Distribution {
    public String getTag() {
        return "Poisson";
    }

    @Override
    public PoissonDistribution copy() {
        return new PoissonDistribution(generator.copy(), lambda);
    }

    private double lambda;

    public double getLambda() {
        return lambda;
    }

    @Override
    public double getParameterA() {
        return lambda;
    }

    /**
     * Uses an {@link AceRandom}, lambda = 1.0 .
     */
    public PoissonDistribution() {
        this(new AceRandom(), 1.0);
    }

    /**
     * Uses an {@link AceRandom} and the given lambda.
     */
    public PoissonDistribution(double lambda) {
        this(new AceRandom(), lambda);
    }

    /**
     * Uses the given EnhancedRandom directly. Uses the given lambda.
     */
    public PoissonDistribution(EnhancedRandom generator, double lambda)
    {
        this.generator = generator;
        if(!setParameters(lambda, 0.0, 0.0))
            throw new IllegalArgumentException("Given lambda is invalid.");
    }

    @Override
    public double getMaximum() {
        return Double.POSITIVE_INFINITY;
    }

    @Override
    public double getMean() {
        return lambda;
    }

    @Override
    public double getMedian() {
        throw new UnsupportedOperationException("Median is undefined.");
    }

    @Override
    public double getMinimum() {
        return 0.0;
    }

    @Override
    public double[] getMode() {
        return MathTools.isEqual(lambda, Math.floor(lambda), 0x1p-24)
                ? new double[] { lambda - 1.0, lambda }
                : new double[] { MathTools.floor(lambda) };
    }

    @Override
    public double getVariance() {
        return lambda;
    }

    /**
     * Sets all parameters and returns true if they are valid, otherwise leaves parameters unchanged and returns false.
     * @param a lambda; should be greater than 0.0
     * @param b ignored
     * @param c ignored
     * @return true if the parameters given are valid and will be used
     */
    @Override
    public boolean setParameters(double a, double b, double c) {
        if(a > 0.0) {
            lambda = a;
            return true;
        }
        return false;
    }

    @Override
    public double nextDouble() {
        return sample(generator, lambda);
    }

    public static double sample(EnhancedRandom generator, double lambda) {
        double x = 0.0;
        double p = Math.exp(-lambda), s = p, u = generator.nextDouble();
        while (u > s)
        {
            ++x;
            p *= lambda / x;
            s += p;
        }
        return x;
    }
}