The R Project SVN R

/*
 *  Mathlib : A C Library of Special Functions
 *  Copyright (C) 1998 Ross Ihaka
 *  Copyright (C) 2000 The R Development Core Team
 *
 *  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., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA.
 *
 *  SYNOPSIS
 *
 * #include "Rmath.h"
 *
 * double pbeta_raw(double x, double pin, double qin, int lower_tail)
 * double pbeta    (double x, double pin, double qin, int lower_tail, int log_p)
 *
 *  DESCRIPTION
 *
 *  Returns distribution function of the beta distribution.
 *  ( = The incomplete beta ratio I_x(p,q) ).
 *
 *  NOTES
 *
 *  This routine is a translation into C of a Fortran subroutine
 *  by W. Fullerton of Los Alamos Scientific Laboratory.
 *
 *  REFERENCE
 *
 *  Bosten and Battiste (1974).
 *  Remark on Algorithm 179, CACM 17, p153, (1974).
 */

#include "nmath.h"
#include "dpq.h"

/* This is called from  qbeta(.) in a root-finding loop --- be FAST! */

double pbeta_raw(double x, double pin, double qin, int lower_tail)
{
    double ans, c, finsum, p, ps, p1, q, term, xb, xi, y;
    int n, i, ib, swap_tail;

    const double eps = .5*DBL_EPSILON;
    const double sml = DBL_MIN;
    const double lneps = log(eps);
    const double lnsml = log(sml);

    /* swap tails if x is greater than the mean */
    if (pin / (pin + qin) < x) {
    swap_tail = 1;
    y = 1 - x;
    p = qin;
    q = pin;
    }
    else {
    swap_tail = 0;
    y = x;
    p = pin;
    q = qin;
    }

    if ((p + q) * y / (p + 1) < eps) {

    /* tail approximation */

    ans = 0;
    xb = p * log(fmax2(y, sml)) - log(p) - lbeta(p, q);
    if (xb > lnsml && y != 0)
        ans = exp(xb);
    if (swap_tail == lower_tail)
        ans = 1 - ans;
    }
    else {
    /*___ FIXME ___:  This takes forever (or ends wrongly)
      when (one or) both p & q  are huge
    */

    /* evaluate the infinite sum first.  term will equal */
    /* y^p / beta(ps, p) * (1 - ps)-sub-i * y^i / fac(i) */

    ps = q - floor(q);
    if (ps == 0)
        ps = 1;
    xb = p * log(y) - lbeta(ps, p) - log(p);
    ans = 0;
    if (xb >= lnsml) {
        ans = exp(xb);
        term = ans * p;
        if (ps != 1) {
        n = fmax2(lneps/log(y), 4.0);
        for(i=1 ; i <= n ; i++) {
            xi = i;
            term *= (xi - ps) * y / xi;
            ans += term / (p + xi);
        }
        }
    }

    /* now evaluate the finite sum, maybe. */

    if (q > 1) {
        xb = p * log(y) + q * log(1 - y) - lbeta(p, q) - log(q);
        ib = fmax2(xb / lnsml, 0.0);
        term = exp(xb - ib * lnsml);
        c = 1 / (1 - y);
        p1 = q * c / (p + q - 1);

        finsum = 0;
        n = q;
        if (q == n)
        n--;
        for(i=1 ; i<=n ; i++) {
        if (p1 <= 1 && term / eps <= finsum)
            break;
        xi = i;
        term = (q - xi + 1) * c * term / (p + q - xi);
        if (term > 1) {
            ib--;
            term *= sml;
        }
        if (ib == 0)
            finsum += term;
        }
        ans += finsum;
    }
    if (swap_tail == lower_tail)
        ans = 1 - ans;
    ans = fmax2(fmin2(ans, 1.), 0.);
    }
    return ans;
} /* pbeta_raw() */

double pbeta(double x, double pin, double qin, int lower_tail, int log_p)
{
#ifdef IEEE_754
    if (ISNAN(x) || ISNAN(pin) || ISNAN(qin))
    return x + pin + qin;
#endif

    if (pin <= 0 || qin <= 0) ML_ERR_return_NAN;

    if (x <= 0)
    return R_DT_0;
    if (x >= 1)
    return R_DT_1;
    return R_D_val(pbeta_raw(x, pin, qin, lower_tail));
}