The R Project SVN R

/*
 *  R : A Computer Langage for Statistical Data Analysis
 *  Copyright (C) 1995, 1996  Robert Gentleman and Ross Ihaka
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */

#include "Mathlib.h"

double qhyper(double x, double NR, double NB, double n)
{
    double N, xstart, xend, xr, xb, sum, term;

    NR = floor(NR + 0.5);
    NB = floor(NB + 0.5);
    N = NR + NB;
    n = floor(n + 0.5);
    if (x < 0.0 || x > 1.0 || NR <= 0 || NR <= 0 || n <= 0 || n > N)
        DOMAIN_ERROR;
    xstart = fmax2(0, n - NB);
    xend = fmin2(n, NR);
    if(x <= 0.0) return xstart;
    if(x >= 1.0) return xend;
    xr = xstart;
    xb = n - xr;
    term = exp(lfastchoose(NR, xr) + lfastchoose(NB, xb)
            - lfastchoose(N, n));
    NR = NR - xr;
    NB = NB - xb;
    sum = term;
    while(sum < x && xr < xend) {
        xr++;
        NB++;
        term *= (NR / xr) * (xb / NB);
        sum += term;
        xb--;
        NR--;
    }
    return xr;
}