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Serializable pseudo-random number generators and distributions.
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/*
 * 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.random.EnhancedRandom;
import com.github.tommyettinger.random.AceRandom;

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

    @Override
    public BinomialDistribution copy() {
        return new BinomialDistribution(generator.copy(), alpha, beta);
    }

    private double alpha;
    private int beta;

    public double getAlpha() {
        return alpha;
    }

    public int getBeta() {
        return beta;
    }

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

    @Override
    public double getParameterB() {
        return beta;
    }

    /**
     * Uses an {@link AceRandom}, alpha = 0.5, beta = 1 .
     */
    public BinomialDistribution() {
        this(new AceRandom(), 0.5, 1);
    }

    /**
     * Uses an {@link AceRandom} and the given beta and alpha.
     */
    public BinomialDistribution(double alpha, int beta) {
        this(new AceRandom(), alpha, beta);
    }

    /**
     * Uses the given EnhancedRandom directly. Uses the given beta and alpha.
     */
    public BinomialDistribution(EnhancedRandom generator, double alpha, int beta)
    {
        this.generator = generator;
        if(!setParameters(alpha, beta, 0.0))
            throw new IllegalArgumentException("Given alpha and/or beta are invalid.");
    }

    @Override
    public double getMaximum() {
        return beta;
    }

    @Override
    public double getMean() {
        return beta * alpha;
    }

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

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

    @Override
    public double[] getMode() {
        return new double[] { Math.floor(alpha * (beta + 1.0)) };
    }

    @Override
    public double getVariance() {
        return alpha * (1.0 - alpha) * beta;
    }

    /**
     * Sets all parameters and returns true if they are valid, otherwise leaves parameters unchanged and returns false.
     * @param a alpha; should be greater than or equal to 0.0 and less than or equal to 1.0
     * @param b beta; will be cast to an int, and should be greater or equal to 0
     * @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 && a <= 1.0 && (int)b >= 0.0){
            alpha = a;
            beta = (int)b;
            return true;
        }
        return false;
    }

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

    public static double sample(EnhancedRandom generator, double alpha, int beta) {
        double successes = 0;
        for (int i = 0; i < beta; i++)
        {
            if (generator.nextExclusiveDouble() < alpha)
            {
                successes++;
            }
        }
        return successes;

    }
}