The R Project SVN R

/*
 *  Mathlib : A C Library of Special Functions
 *  Copyright (C) 2000-2025 The R Core Team
 *  Copyright (C) 2005-2020 The R Foundation
 *  Copyright (C) 1998 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, a copy is available at
 *  https://www.R-project.org/Licenses/
 *
 *  SYNOPSIS
 *
 *    #include <Rmath.h>
 *    double rhyper(double NR, double NB, double n);
 *
 *  DESCRIPTION
 *
 *    Random variates from the hypergeometric distribution.
 *    Returns the number of white balls drawn when kk balls
 *    are drawn at random from an urn containing nn1 white
 *    and nn2 black balls.
 *
 *  REFERENCE
 *
 *    V. Kachitvichyanukul and B. Schmeiser (1985).
 *    ``Computer generation of hypergeometric random variates,''
 *    Journal of Statistical Computation and Simulation 22, 127-145.
 *
 *    The original algorithm had a bug -- R bug report PR#7314 --
 *    giving numbers slightly too small in case III h2pe
 *    where (m < 100 || ix <= 50) , see below.
 */

#include "nmath.h"
#include "dpq.h"
#include <limits.h>

// afc(i) :=  ln( i! )  [logarithm of the factorial i] = {R:} lgamma(i + 1) = {C:} lgammafn(i + 1)
static double afc(int i)
{
    // If (i > 7), use Stirling's approximation, otherwise use table lookup.
    const static double al[8] =
    {
    0.0,/*ln(0!)=ln(1)*/
    0.0,/*ln(1!)=ln(1)*/
    0.69314718055994530941723212145817,/*ln(2) */
    1.79175946922805500081247735838070,/*ln(6) */
    3.17805383034794561964694160129705,/*ln(24)*/
    4.78749174278204599424770093452324,
    6.57925121201010099506017829290394,
    8.52516136106541430016553103634712
    /* 10.60460290274525022841722740072165, approx. value below =
       10.6046028788027; rel.error = 2.26 10^{-9}

      FIXME: Use constants and if(n > ..) decisions from ./stirlerr.c
      -----  will be even *faster* for n > 500 (or so)
    */
    };

    if (i < 0) {
    MATHLIB_WARNING(("rhyper.c: afc(i), i=%d < 0 -- SHOULD NOT HAPPEN!\n"), i);
    return -1; // unreached
    }
    if (i <= 7)
    return al[i];
    // else i >= 8 :
    double di = i, i2 = di*di;
    return (di + 0.5) * log(di) - di + M_LN_SQRT_2PI +
    (0.0833333333333333 - 0.00277777777777778 / i2) / di;
}

//     rhyper(NR, NB, n) -- NR 'red', NB 'blue', n drawn, how many are 'red'
double rhyper(double nn1in, double nn2in, double kkin)
{
    /* extern double afc(int); */

    /* check parameter validity */

    if(!R_FINITE(nn1in) || !R_FINITE(nn2in) || !R_FINITE(kkin))
    ML_WARN_return_NAN;

    nn1in = R_forceint(nn1in);
    nn2in = R_forceint(nn2in);
    kkin  = R_forceint(kkin);

    if (nn1in < 0 || nn2in < 0 || kkin < 0 || kkin > nn1in + nn2in)
    ML_WARN_return_NAN;
    if (nn1in >= INT_MAX || nn2in >= INT_MAX || kkin >= INT_MAX) {
    /* large n -- evade integer overflow (and inappropriate algorithms)
       -------- */
#ifdef DEBUG_rhyper
    REprintf("rhyper(nn1=%.0f, nn2=%.0f, kk=%.0f): 'large n' case\n", nn1in, nn2in, kkin);
#endif
        // FIXME: Much faster to give rbinom() approx when appropriate; -> see Kuensch(1989)
    // Johnson, Kotz,.. p.258 (top) mention the *four* different binomial approximations
    if(kkin == 1.) { // Bernoulli
        return rbinom(kkin, nn1in / (nn1in + nn2in));
    }
    // Slow, but safe: return  F^{-1}(U)  where F(.) = phyper(.) and  U ~ U[0,1]
    return qhyper(unif_rand(), nn1in, nn2in, kkin,
              /*lower_tail =*/ false, /*log_p = */ false);
    // lower_tail=false: a thinko, is still "correct" as equiv. to  U <--> 1-U
    }

    int
    nn1 = (int)nn1in,
    nn2 = (int)nn2in,
    kk  = (int)kkin;

    /* These should become 'thread_local globals' : */
    static int ks = -1, n1s = -1, n2s = -1;
    static int m, minjx, maxjx;
    static int k, n1, n2; // <- not allowing larger integer par
    static double N;

    bool setup1, setup2;
    /* if new parameter values, initialize */
    if (nn1 != n1s || nn2 != n2s) { // n1 | n2 is changed: setup all
    setup1 = true;  setup2 = true;
    } else if (kk != ks) { // n1 & n2 are unchanged: setup 'k' only
    setup1 = false; setup2 = true;
    } else { // all three unchanged ==> no setup
    setup1 = false; setup2 = false;
    }
    if (setup1) { // n1 & n2
    n1s = nn1; n2s = nn2; // save
    N = nn1 + (double)nn2; // avoid int overflow
    if (nn1 <= nn2) {
        n1 = nn1; n2 = nn2;
    } else { // nn2 < nn1
        n1 = nn2; n2 = nn1;
    }
    // now have n1 <= n2
    }
    if (setup2) { // k
    ks = kk; // save
    if ((double)kk + kk >= N) { // this could overflow
        k = (int)(N - kk);
    } else {
        k = kk;
    }
    // now have  k < N/2 = (n1+n2)/2
    }
    if (setup1 || setup2) {
    m = (int) ((k + 1.) * (n1 + 1.) / (N + 2.)); // m := floor(adjusted mean E[.])
    minjx = imax2(0, k - n2);
    maxjx = imin2(n1, k);
#ifdef DEBUG_rhyper
    REprintf("rhyper(n1=%d, n2=%d, k=%d), setup: floor(a.mean)=: m = %d, [min,maxjx]= [%d,%d]\n",
         nn1, nn2, kk, m, minjx, maxjx);
#endif
    }
#ifdef DEBUG_rhyper
    else
    REprintf("rhyper(n1=%d, n2=%d, k=%d); setup UNchanged\n", nn1, nn2, kk);
#endif

    // generate random variate --- Three basic cases ---------------------------

    int ix; // return value (coerced to double at the very end) .. FIXME?? what if overflows

    if (minjx == maxjx) { /* I: degenerate distribution ---------------- */
#ifdef DEBUG_rhyper
    REprintf("rhyper(), branch I (degenerate): ix := maxjx = %d\n", maxjx);
#endif
    ix = maxjx;
    goto L_finis; // return appropriate variate

    } else if (m - minjx < 10) { // II: (Scaled) algorithm HIN (inverse transformation) ----
    const static double scale = 1e25; // scaling factor against (early) underflow
    const static double con = 57.5646273248511421;
              // 25*log(10) = log(scale) { <==> exp(con) == scale }
    static double w,
        lw; // = log(w);  w = exp(lw) * scale = exp(lw + log(scale)) = exp(lw + con)
    if (setup1 || setup2) {
        // NB:  n1 <= n2  here
        if (k < n2) {
        lw = afc(n2) + afc(n1 + n2 - k) - afc(n2 - k) - afc(n1 + n2);
        } else {
        lw = afc(n1) + afc(     k     ) - afc(k - n2) - afc(n1 + n2);
        }
        w = exp(lw + con);
    }

#ifdef DEBUG_rhyper
    REprintf("  branch II; w = %g %s\n", w,
         (w > 0) ? "> 0" : "= 0 <==> Underflow __NOT GOOD__");
#endif
    if(w <= 0.)
        MATHLIB_ERROR(_("w = %g <= 0: Underflow in rhyper()  SHOULD NOT HAPPEN!"), w);
    // FIXME: MM has code "rhyper.c+M3" for switching to log space if w == 0

    double p, u;
      L10:
    p = w;
    ix = minjx;
    u = unif_rand() * scale;
#ifdef DEBUG_rhyper
    REprintf("  _new_ u = %g\n", u);
#endif
    while (u > p) {
        u -= p;
        p *= ((double) n1 - ix) * (k - ix);
        ix++;
        p = p / ix / (n2 - k + ix);
#ifdef DEBUG_rhyper
        REprintf("       ix=%3d, u=%11g, p=%20.14g (u-p=%g)\n", ix, u, p, u-p);
#endif
        if (ix > maxjx)
        goto L10;
        // FIXME  if(p == 0.)  we also "have lost"  => goto L10
    }

    } else { /* III : H2PE Algorithm --------------------------------------- */

    static double a, xl, xr, lamdl, lamdr, p1, p2, p3;
    if (setup1 || setup2) {
        double
        s = sqrt((N - k) * k * n1 * n2 / (N - 1) / N / N),

        /* remark: d is defined in reference without int. */
        /* the truncation centers the cell boundaries at 0.5 */
        d = (int) (1.5 * s) + .5;
        xl = m - d + .5;
        xr = m + d + .5;
        a = afc(m) + afc(n1 - m) + afc(k - m) + afc(n2 - k + m);
        double
        kl = exp(a - afc((int) (xl)) - afc((int) (n1 - xl))
             - afc((int) (k - xl))
             - afc((int) (n2 - k + xl))),
        kr = exp(a - afc((int) (xr - 1))
             - afc((int) (n1 - xr + 1))
             - afc((int) (k - xr + 1))
             - afc((int) (n2 - k + xr - 1)));
        lamdl = -log(xl * (n2 - k + xl) / (n1 - xl + 1) / (k - xl + 1));
        lamdr = -log((n1 - xr + 1) * (k - xr + 1) / xr / (n2 - k + xr));
        p1 = d + d;
        p2 = p1 + kl / lamdl;
        p3 = p2 + kr / lamdr;
#ifdef DEBUG_rhyper
        REprintf("rhyper(), branch III {accept/reject}: (xl,xr)= (%g,%g); lamdl,r = (%g,%g)\n",
             xl, xr, lamdl, lamdr);
        REprintf(" --------> (p1,p2,p3) = (%g,%g,%g)\n", p1,p2,p3);
    }
    else
        REprintf("rhyper(), branch III {accept/reject}: NO setup needed; lamdl,r = (%g,%g)\n",
             lamdl, lamdr);
#else
        }
#endif

    /* acceptance/rejection test */
    bool reject = true;
        int n_uv = 0;
    do { // L30: -------------- acceptance / rejection : loop while reject=true -------------
    double
        u = unif_rand() * p3,
        v = unif_rand();
    n_uv++;
    if(n_uv >= 10000) {
        REprintf("rhyper(*, n1=%d, n2=%d, k=%d): branch III: giving up after %d rejections\n",
             nn1, nn2, kk, n_uv);
        ML_WARN_return_NAN;
        }
#ifdef DEBUG_rhyper
    REprintf(" ... L30 [%d]: new (u=%g, v ~ U[0,1]=%g): ", n_uv, u,v);
#endif

    if (u < p1) {       /* rectangular region */
        ix = (int) (xl + u);
#ifdef DEBUG_rhyper
        REprintf(" rectangular: ix=%d\n", ix);
#endif
    } else if (u <= p2) {   /* left tail */
        ix = (int) (xl + log(v) / lamdl);
#ifdef DEBUG_rhyper
        REprintf(" left tail: ix=%d %s", ix, (ix < minjx)? "< minjx (=> L30)\n" : ">= minjx;");
#endif
        if (ix < minjx)
        continue; // goto L30
        v = v * (u - p1) * lamdl;
    } else {        /* right tail */
        ix = (int) (xr - log(v) / lamdr);
#ifdef DEBUG_rhyper
        REprintf(" right tail: ix=%d %s", ix, (ix > maxjx)? "> maxjx (=> L30)\n" : "< maxjx;");
#endif
        if (ix > maxjx)
        continue; // goto L30
        v = v * (u - p2) * lamdr;
    }
#ifdef DEBUG_rhyper
    if(u >= p1) REprintf(" new v = %g\n", v);
#endif

    if (m < 100 || ix <= 50) {
        /* explicit evaluation */
        /* The original algorithm (and TOMS 668) have
           f = f * i * (n2 - k + i) / (n1 - i) / (k - i);
           in the (m > ix) case, but the definition of the
           recurrence relation on p134 shows that the +1 is
           needed. */
        double f = 1.0;
        if (m < ix) {
        for (int i = m + 1; i <= ix; i++)
            f = f * (n1 - i + 1) * (k - i + 1) / (n2 - k + i) / i;
        } else if (m > ix) {
        for (int i = ix + 1; i <= m; i++)
            f = f * i * (n2 - k + i) / (n1 - i + 1) / (k - i + 1);
        }
#ifdef DEBUG_rhyper
        REprintf(" small m or ix: f = %g\n", f);
#endif
        if (v <= f) {
        reject = false;
        }
    } else {

        const static double deltal = 0.0078;
        const static double deltau = 0.0034;

#ifdef DEBUG_rhyper
        REprintf(" ... accept/reject 'large' case v=%g\n", v);
#endif
        /* squeeze using upper and lower bounds */
        double
        y = ix,
        y1 = y + 1.0,
        ym = y - m,
        yn = n1 - y + 1.0,
        yk = k  - y + 1.0,
        nk = n2 - k + y1,
        r = -ym / y1,
        s =  ym / yn,
        t =  ym / yk,
        e = -ym / nk,
        g = yn * yk / (y1 * nk) - 1.0,
        dg = (g < 0.0) ? 1. + g : 1.,
        gu = g * (1.0 + g * (-0.5 + g / 3.0)),
        gl = gu - .25 * (g * g * g * g) / dg,
        xm = m + 0.5,
        xn = n1 - m + 0.5,
        xk = k - m + 0.5,
        nm = n2 - k + xm,
        ub = y * gu - m * gl + deltau
        + xm * r * (1. + r * (-0.5 + r / 3.0))
        + xn * s * (1. + s * (-0.5 + s / 3.0))
        + xk * t * (1. + t * (-0.5 + t / 3.0))
        + nm * e * (1. + e * (-0.5 + e / 3.0));
        /* test against upper bound */
        double alv = log(v);
        if (alv > ub) {
        reject = true;
        } else {
        /* test against lower bound */
        double dr = xm * (r * r * r * r);
        if (r < 0.)
            dr /= (1.0 + r);
        double ds = xn * (s * s * s * s);
        if (s < 0.0)
            ds /= (1.0 + s);
        double dt = xk * (t * t * t * t);
        if (t < 0.0)
            dt /= (1.0 + t);
        double de = nm * (e * e * e * e);
        if (e < 0.0)
            de /= (1.0 + e);
        if (alv < ub - 0.25 * (dr + ds + dt + de)
            + (y + m) * (gl - gu) - deltal) {
            reject = false;
        }
        else {
            /* * Stirling's formula to machine accuracy
             */
            if (alv <= (a - afc(ix) - afc(n1 - ix)
                - afc(k - ix) - afc(n2 - k + ix))) {
            reject = false;
            } else {
            reject = true;
            }
        }
        }
    } // end{ III "large" }

    } while (reject);

    } // end{branch III}


L_finis:  /* return appropriate variate */

#ifdef DEBUG_rhyper
    REprintf(" L_finis: ix = %d, then", ix);
#endif
    if ((double)kk + kk >= N) {
    if (nn1 > nn2) {
        ix = kk - nn2 + ix;
    } else {
        ix = nn1 - ix;
    }
    } else if (nn1 > nn2) {
    ix = kk - ix;
    }
#ifdef DEBUG_rhyper
    REprintf(" %d\n", ix);
#endif
    return ix;
}