org.rhwlab.BHC.GammaRatio.java Source code

Introduction

Here is the source code for org.rhwlab.BHC.GammaRatio.java

Source

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package org.rhwlab.BHC;

import java.util.HashMap;
import org.apache.commons.math3.special.Gamma;

/**
 *
 * @author gevirl
 */
public class GammaRatio {
    public GammaRatio(int d, int nu) {
        this.d = d;
        this.nu = nu;
        ratios = new HashMap<>();
    }

    public double getRatio(int n) {

        Double ret = ratios.get(n);
        if (ret != null) {
            return ret;
        }

        double logr = 0.0;
        double r;
        if (n % 2 == 0) {

            for (int j = 1; j <= n / 2; ++j) {
                for (int i = 1; i <= d; ++i) {
                    logr = logr + Math.log(((nu - 1.0 - i) / 2.0) + j);
                }
            }
            r = Math.exp(logr);
            ratios.put(n, r);
        } else {
            for (int j = 1; j <= (n - 1) / 2; ++j) {
                for (int i = 1; i <= d; ++i) {
                    logr = logr + Math.log(((nu - i) / 2.0) + j);
                }
            }
            for (int i = 1; i <= d; ++i) {
                logr = logr + Gamma.logGamma((nu + 2.0 - i) / 2.0) - Gamma.logGamma((nu + 1.0 - i) / 2.0);
            }
            r = Math.exp(logr);
        }

        if (!Double.isFinite(r)) {
            System.out.println("Gamma ratio error");
            System.exit(n);
        }
        return r;
    }

    static public void main(String[] args) {
        int d = 3;
        int nu = 4;
        GammaRatio ratio = new GammaRatio(d, nu);
        int n = 1;
        while (true) {
            double nuP = nu + n;
            double r = 1.0;
            for (int i = 1; i <= d; ++i) {
                r = r * Gamma.gamma((nuP + 1.0 - i) / 2.0) / Gamma.gamma((nu + 1.0 - i) / 2.0);
            }
            System.out.printf("%d  Ratio = %e   byGamma = %e\n", n, ratio.getRatio(n), r);
            ++n;
        }
    }

    int d;
    int nu;
    HashMap<Integer, Double> ratios;
}