// -*- mode: C ; delete-old-versions: never -*-

/* Based on C translation of ACM TOMS 708
   Please do not change this, e.g. to use R's versions of the
   ancillary routines, without investigating the error analysis as we
   do need very high relative accuracy.  This version has about
   14 digits accuracy.
*/

#undef min
#define min(a,b) ((a < b)?a:b)
#undef max
#define max(a,b) ((a > b)?a:b)

#include "nmath.h" /* includes config.h, math.h */
#include "dpq.h"
/* after config.h to avoid warning on Solaris */
#include <limits.h>

/**----------- DEBUGGING -------------
 *
 *	make CFLAGS='-DDEBUG_bratio  ...'
 *MM (w/ Debug, w/o Optimization):
 (cd `R-devel-pbeta-dbg RHOME`/src/nmath ; gcc -I. -I../../src/include -I../../../R/src/include  -DHAVE_CONFIG_H -fopenmp -g -pedantic -Wall --std=gnu99 -DDEBUG_q -DDEBUG_bratio -Wcast-align -Wclobbered  -c ../../../R/src/nmath/toms708.c -o toms708.o; cd ../..; make R)
*/
#ifdef DEBUG_bratio
# define R_ifDEBUG_printf(...) REprintf(__VA_ARGS__)
#else
# define R_ifDEBUG_printf(...)
#endif

/* MM added R_D_LExp, so redefine here in terms of rexpm1 */
#undef R_Log1_Exp
#define R_Log1_Exp(x)   ((x) > -M_LN2 ? log(-rexpm1(x)) : log1p(-exp(x)))


static double bfrac(double, double, double, double, double, double, int log_p);
static void bgrat(double, double, double, double, double *, double, int *, Rboolean log_w);
static double grat_r(double a, double x, double r, double eps);
static double apser(double, double, double, double);
static double bpser(double, double, double, double, int log_p);
static double basym(double, double, double, double, int log_p);
static double fpser(double, double, double, double, int log_p);
static double bup(double, double, double, double, int, double, int give_log);
static double exparg(int);
static double psi(double);
static double gam1(double);
static double gamln1(double);
static double betaln(double, double);
static double algdiv(double, double);
static double brcmp1(int, double, double, double, double, int give_log);
static double brcomp(double, double, double, double, int log_p);
static double rlog1(double);
static double bcorr(double, double);
static double gamln(double);
static double alnrel(double);
static double esum(int, double, int give_log);
static double erf__(double);
static double rexpm1(double);
static double erfc1(int, double);
static double gsumln(double, double);

/*      ALGORITHM 708, COLLECTED ALGORITHMS FROM ACM.
 *      This work published in  Transactions On Mathematical Software,
 *      vol. 18, no. 3, September 1992, pp. 360-373.
 */

/* Changes by R Core Team :
 * add log_p  and work towards gaining precision in that case
 */

void attribute_hidden
bratio(double a, double b, double x, double y, double *w, double *w1,
       int *ierr, int log_p)
{
/* -----------------------------------------------------------------------

 *	      Evaluation of the Incomplete Beta function I_x(a,b)

 *		       --------------------

 *     It is assumed that a and b are nonnegative, and that x <= 1
 *     and y = 1 - x.  Bratio assigns w and w1 the values

 *			w  = I_x(a,b)
 *			w1 = 1 - I_x(a,b)

 *     ierr is a variable that reports the status of the results.
 *     If no input errors are detected then ierr is set to 0 and
 *     w and w1 are computed. otherwise, if an error is detected,
 *     then w and w1 are assigned the value 0 and ierr is set to
 *     one of the following values ...

 *	  ierr = 1  if a or b is negative
 *	  ierr = 2  if a = b = 0
 *	  ierr = 3  if x < 0 or x > 1
 *	  ierr = 4  if y < 0 or y > 1
 *	  ierr = 5  if x + y != 1
 *	  ierr = 6  if x = a = 0
 *	  ierr = 7  if y = b = 0
 *	  ierr = 8	(not used currently)
 *	  ierr = 9  NaN in a, b, x, or y
 *	  ierr = 10     (not used currently)
 *	  ierr = 11  bgrat() error code 1 [+ warning in bgrat()]
 *	  ierr = 12  bgrat() error code 2   (no warning here)
 *	  ierr = 13  bgrat() error code 3   (no warning here)
 *	  ierr = 14  bgrat() error code 4 [+ WARNING in bgrat()]


 * --------------------
 *     Written by Alfred H. Morris, Jr.
 *	  Naval Surface Warfare Center
 *	  Dahlgren, Virginia
 *     Revised ... Nov 1991
* ----------------------------------------------------------------------- */

    Rboolean do_swap;
    int n, ierr1 = 0;
    double z, a0, b0, x0, y0, lambda;

/*  eps is a machine dependent constant: the smallest
 *      floating point number for which   1. + eps > 1.
 * NOTE: for almost all purposes it is replaced by 1e-15 (~= 4.5 times larger) below */
    double eps = 2. * Rf_d1mach(3); /* == DBL_EPSILON (in R, Rmath) */

/* ----------------------------------------------------------------------- */
    *w  = R_D__0;
    *w1 = R_D__0;

#ifdef IEEE_754
    // safeguard, preventing infinite loops further down
    if (ISNAN(x) || ISNAN(y) ||
	ISNAN(a) || ISNAN(b)) { *ierr = 9; return; }
#endif
    if (a < 0. || b < 0.)   { *ierr = 1; return; }
    if (a == 0. && b == 0.) { *ierr = 2; return; }
    if (x < 0. || x > 1.)   { *ierr = 3; return; }
    if (y < 0. || y > 1.)   { *ierr = 4; return; }

    /* check that  'y == 1 - x' : */
    z = x + y - 0.5 - 0.5;

    if (fabs(z) > eps * 3.) { *ierr = 5; return; }

    R_ifDEBUG_printf("bratio(a=%g, b=%g, x=%9g, y=%9g, .., log_p=%d): ",
		     a,b,x,y, log_p);
    *ierr = 0;
    if (x == 0.) goto L200;
    if (y == 0.) goto L210;

    if (a == 0.) goto L211;
    if (b == 0.) goto L201;

    eps = max(eps, 1e-15);
    Rboolean a_lt_b = (a < b);
    if (/* max(a,b) */ (a_lt_b ? b : a) < eps * .001) { /* procedure for a and b < 0.001 * eps */
	// L230:  -- result *independent* of x (!)
	// *w  = a/(a+b)  and  w1 = b/(a+b) :
	if(log_p) {
	    if(a_lt_b) {
		*w  = log1p(-a/(a+b)); // notably if a << b
		*w1 = log  ( a/(a+b));
	    } else { // b <= a
		*w  = log  ( b/(a+b));
		*w1 = log1p(-b/(a+b));
	    }
	} else {
	    *w	= b / (a + b);
	    *w1 = a / (a + b);
	}

	R_ifDEBUG_printf("a & b very small -> simple ratios (%g,%g)\n", *w,*w1);
	return;
    }

#define SET_0_noswap \
    a0 = a;  x0 = x; \
    b0 = b;  y0 = y;

#define SET_0_swap   \
    a0 = b;  x0 = y; \
    b0 = a;  y0 = x;

    if (min(a,b) <= 1.) { /*------------------------ a <= 1  or  b <= 1 ---- */

	do_swap = (x > 0.5);
	if (do_swap) {
	    SET_0_swap;
	} else {
	    SET_0_noswap;
	}
	/* now have  x0 <= 1/2 <= y0  (still  x0+y0 == 1) */

	R_ifDEBUG_printf(" min(a,b) <= 1, do_swap=%d;", do_swap);

	if (b0 < min(eps, eps * a0)) { /* L80: */
	    *w = fpser(a0, b0, x0, eps, log_p);
	    *w1 = log_p ? R_Log1_Exp(*w) : 0.5 - *w + 0.5;
	    R_ifDEBUG_printf("  b0 small -> w := fpser(*) = %.15g\n", *w);
	    goto L_end;
	}

	if (a0 < min(eps, eps * b0) && b0 * x0 <= 1.) { /* L90: */
	    *w1 = apser(a0, b0, x0, eps);
	    R_ifDEBUG_printf("  a0 small -> w1 := apser(*) = %.15g\n", *w1);
	    goto L_end_from_w1;
	}

	Rboolean did_bup = FALSE;
	if (max(a0,b0) > 1.) { /* L20:  min(a,b) <= 1 < max(a,b)  */
	    R_ifDEBUG_printf("\n L20:  min(a,b) <= 1 < max(a,b); ");
	    if (b0 <= 1.) goto L_w_bpser;

	    if (x0 >= 0.29) /* was 0.3, PR#13786 */	goto L_w1_bpser;

	    if (x0 < 0.1 && pow(x0*b0, a0) <= 0.7)	goto L_w_bpser;

	    if (b0 > 15.) {
		*w1 = 0.;
		goto L131;
	    }
	} else { /*  a, b <= 1 */
	    R_ifDEBUG_printf("\n      both a,b <= 1; ");
	    if (a0 >= min(0.2, b0))	goto L_w_bpser;

	    if (pow(x0, a0) <= 0.9) 	goto L_w_bpser;

	    if (x0 >= 0.3)		goto L_w1_bpser;
	}
	n = 20; /* goto L130; */
	*w1 = bup(b0, a0, y0, x0, n, eps, FALSE); did_bup = TRUE;
	R_ifDEBUG_printf("  ... n=20 and *w1 := bup(*) = %.15g; ", *w1);
	b0 += n;
    L131:
	R_ifDEBUG_printf(" L131: bgrat(*, w1=%.15g) ", *w1);
	bgrat(b0, a0, y0, x0, w1, 15*eps, &ierr1, FALSE);
#ifdef DEBUG_bratio
	REprintf(" ==> new w1=%.15g", *w1);
	if(ierr1) REprintf(" ERROR(code=%d)\n", ierr1) ; else REprintf("\n");
#endif
	if(*w1 == 0 || (0 < *w1 && *w1 < DBL_MIN)) { // w1=0 or very close:
	    // "almost surely" from underflow, try more: [2013-03-04]
// FIXME: it is even better to do this in bgrat *directly* at least for the case
//  !did_bup, i.e., where *w1 = (0 or -Inf) on entry
	    R_ifDEBUG_printf(" denormalized or underflow (?) -> retrying: ");
	    if(did_bup) { // re-do that part on log scale:
		*w1 = bup(b0-n, a0, y0, x0, n, eps, TRUE);
	    }
	    else *w1 = ML_NEGINF; // = 0 on log-scale
	    bgrat(b0, a0, y0, x0, w1, 15*eps, &ierr1, TRUE);
	    if(ierr1) *ierr = 10 + ierr1;
#ifdef DEBUG_bratio
	    REprintf(" ==> new log(w1)=%.15g", *w1);
	    if(ierr1) REprintf(" Error(code=%d)\n", ierr1) ; else REprintf("\n");
#endif
	    goto L_end_from_w1_log;
	}
	// else
	if(ierr1) *ierr = 10 + ierr1;
	if(*w1 < 0)
	    MATHLIB_WARNING4("bratio(a=%g, b=%g, x=%g): bgrat() -> w1 = %g",
			     a,b,x, *w1);
	goto L_end_from_w1;
    }
    else { /* L30: -------------------- both  a, b > 1  {a0 > 1  &  b0 > 1} ---*/

	/* lambda := a y - b x  =  (a + b)y  =  a - (a+b)x    {using x + y == 1},
	 * ------ using the numerically best version : */
	lambda = R_FINITE(a+b)
	    ? ((a > b) ? (a + b) * y - b : a - (a + b) * x)
	    : a*y - b*x;
	do_swap = (lambda < 0.);
	if (do_swap) {
	    lambda = -lambda;
	    SET_0_swap;
	} else {
	    SET_0_noswap;
	}

	R_ifDEBUG_printf("  L30:  both  a, b > 1; |lambda| = %#g, do_swap = %d\n",
			 lambda, do_swap);

	if (b0 < 40.) {
	    R_ifDEBUG_printf("  b0 < 40;");
	    if (b0 * x0 <= 0.7
		|| (log_p && lambda > 650.)) // << added 2010-03; svn r51327
		goto L_w_bpser;
	    else
		goto L140;
	}
	else if (a0 > b0) { /* ----  a0 > b0 >= 40  ---- */
	    R_ifDEBUG_printf("  a0 > b0 >= 40;");
	    if (b0 <= 100. || lambda > b0 * 0.03)
		goto L_bfrac;

	} else if (a0 <= 100.) {
	    R_ifDEBUG_printf("  a0 <= 100; a0 <= b0 >= 40;");
	    goto L_bfrac;
	}
	else if (lambda > a0 * 0.03) {
	    R_ifDEBUG_printf("  b0 >= a0 > 100; lambda > a0 * 0.03 ");
	    goto L_bfrac;
	}

	/* else if none of the above    L180: */
	*w = basym(a0, b0, lambda, eps * 100., log_p);
	*w1 = log_p ? R_Log1_Exp(*w) : 0.5 - *w + 0.5;
	R_ifDEBUG_printf("  b0 >= a0 > 100; lambda <= a0 * 0.03: *w:= basym(*) =%.15g\n",
			 *w);
	goto L_end;

    } /* else: a, b > 1 */

/*            EVALUATION OF THE APPROPRIATE ALGORITHM */

L_w_bpser: // was L100
    *w = bpser(a0, b0, x0, eps, log_p);
    *w1 = log_p ? R_Log1_Exp(*w) : 0.5 - *w + 0.5;
    R_ifDEBUG_printf(" L_w_bpser: *w := bpser(*) = %.15g\n", *w);
    goto L_end;

L_w1_bpser:  // was L110
    *w1 = bpser(b0, a0, y0, eps, log_p);
    *w  = log_p ? R_Log1_Exp(*w1) : 0.5 - *w1 + 0.5;
    R_ifDEBUG_printf(" L_w1_bpser: *w1 := bpser(*) = %.15g\n", *w1);
    goto L_end;

L_bfrac:
    *w = bfrac(a0, b0, x0, y0, lambda, eps * 15., log_p);
    *w1 = log_p ? R_Log1_Exp(*w) : 0.5 - *w + 0.5;
    R_ifDEBUG_printf(" L_bfrac: *w := bfrac(*) = %g\n", *w);
    goto L_end;

L140:
    /* b0 := fractional_part( b0 )  in (0, 1]  */
    n = (int) b0;
    b0 -= n;
    if (b0 == 0.) {
	--n; b0 = 1.;
    }

    *w = bup(b0, a0, y0, x0, n, eps, FALSE);

    if(*w < DBL_MIN && log_p) { /* do not believe it; try bpser() : */
	R_ifDEBUG_printf(" L140: bup(b0=%g,..)=%.15g < DBL_MIN - not used; ", b0, *w);
	/*revert: */ b0 += n;
	/* which is only valid if b0 <= 1 || b0*x0 <= 0.7 */
	goto L_w_bpser;
    }
    R_ifDEBUG_printf(" L140: *w := bup(b0=%g,..) = %.15g; ", b0, *w);
    if (x0 <= 0.7) {
	/* log_p :  TODO:  w = bup(.) + bpser(.)  -- not so easy to use log-scale */
	*w += bpser(a0, b0, x0, eps, /* log_p = */ FALSE);
	R_ifDEBUG_printf(" x0 <= 0.7: *w := *w + bpser(*) = %.15g\n", *w);
	goto L_end_from_w;
    }
    /* L150: */
    if (a0 <= 15.) {
	n = 20;
	*w += bup(a0, b0, x0, y0, n, eps, FALSE);
	R_ifDEBUG_printf("\n a0 <= 15: *w := *w + bup(*) = %.15g;", *w);
	a0 += n;
    }
    R_ifDEBUG_printf(" bgrat(*, w=%.15g) ", *w);
    bgrat(a0, b0, x0, y0, w, 15*eps, &ierr1, FALSE);
    if(ierr1) *ierr = 10 + ierr1;
#ifdef DEBUG_bratio
    REprintf("==> new w=%.15g", *w);
    if(ierr1) REprintf(" Error(code=%d)\n", ierr1) ; else REprintf("\n");
#endif
    goto L_end_from_w;


/* TERMINATION OF THE PROCEDURE */

L200:
    if (a == 0.) { *ierr = 6;    return; }
    // else:
L201: *w  = R_D__0; *w1 = R_D__1; return;

L210:
    if (b == 0.) { *ierr = 7;    return; }
    // else:
L211: *w  = R_D__1; *w1 = R_D__0; return;

L_end_from_w:
    if(log_p) {
	*w1 = log1p(-*w);
	*w  = log(*w);
    } else {
	*w1 = 0.5 - *w + 0.5;
    }
    goto L_end;

L_end_from_w1:
    if(log_p) {
	*w  = log1p(-*w1);
	*w1 = log(*w1);
    } else {
	*w = 0.5 - *w1 + 0.5;
    }
    goto L_end;

L_end_from_w1_log:
    // *w1 = log(w1) already; w = 1 - w1  ==> log(w) = log(1 - w1) = log(1 - exp(*w1))
    if(log_p) {
	*w = R_Log1_Exp(*w1);
    } else {
	*w  = /* 1 - exp(*w1) */ -expm1(*w1);
	*w1 = exp(*w1);
    }
    goto L_end;


L_end:
    if (do_swap) { /* swap */
	double t = *w; *w = *w1; *w1 = t;
    }
    return;

} /* bratio */

#undef SET_0_noswap
#undef SET_0_swap

double fpser(double a, double b, double x, double eps, int log_p)
{
/* ----------------------------------------------------------------------- *

 *                 EVALUATION OF I (A,B)
 *                                X

 *          FOR B < MIN(EPS, EPS*A) AND X <= 0.5

 * ----------------------------------------------------------------------- */

    double ans, c, s, t, an, tol;

    /* SET  ans := x^a : */
    if (log_p) {
	ans = a * log(x);
    } else if (a > eps * 0.001) {
	t = a * log(x);
	if (t < exparg(1)) { /* exp(t) would underflow */
	    return 0.;
	}
	ans = exp(t);
    } else
	ans = 1.;

/*                NOTE THAT 1/B(A,B) = B */

    if (log_p)
	ans += log(b) - log(a);
    else
	ans *= b / a;

    tol = eps / a;
    an = a + 1.;
    t = x;
    s = t / an;
    do {
	an += 1.;
	t = x * t;
	c = t / an;
	s += c;
    } while (fabs(c) > tol);

    if (log_p)
	ans += log1p(a * s);
    else
	ans *= a * s + 1.;
    return ans;
} /* fpser */

static double apser(double a, double b, double x, double eps)
{
/* -----------------------------------------------------------------------
 *     apser() yields the incomplete beta ratio  I_{1-x}(b,a)  for
 *     a <= min(eps,eps*b), b*x <= 1, and x <= 0.5,  i.e., a is very small.
 *     Use only if above inequalities are satisfied.
 * ----------------------------------------------------------------------- */

    static double const g = .577215664901533;

    double tol, c, j, s, t, aj;
    double bx = b * x;

    t = x - bx;
    if (b * eps <= 0.02)
	c = log(x) + psi(b) + g + t;
    else // b > 2e13 : psi(b) ~= log(b)
	c = log(bx) + g + t;

    tol = eps * 5. * fabs(c);
    j = 1.;
    s = 0.;
    do {
	j += 1.;
	t *= x - bx / j;
	aj = t / j;
	s += aj;
    } while (fabs(aj) > tol);

    return -a * (c + s);
} /* apser */

static double bpser(double a, double b, double x, double eps, int log_p)
{
/* -----------------------------------------------------------------------
 * Power SERies expansion for evaluating I_x(a,b) when
 *	       b <= 1 or b*x <= 0.7.   eps is the tolerance used.
 * NB: if log_p is TRUE, also use it if   (b < 40  & lambda > 650)
 * ----------------------------------------------------------------------- */

    int i, m;
    double ans, c, t, u, z, a0, b0, apb;

    if (x == 0.) {
	return R_D__0;
    }
/* ----------------------------------------------------------------------- */
/*	      compute the factor  x^a/(a*Beta(a,b)) */
/* ----------------------------------------------------------------------- */
    a0 = min(a,b);
    if (a0 >= 1.) { /*		 ------	 1 <= a0 <= b0  ------ */
	z = a * log(x) - betaln(a, b);
	ans = log_p ? z - log(a) : exp(z) / a;
    }
    else {
	b0 = max(a,b);

	if (b0 < 8.) {

	    if (b0 <= 1.) { /*	 ------	 a0 < 1	 and  b0 <= 1  ------ */

		if(log_p) {
		    ans = a * log(x);
		} else {
		    ans = pow(x, a);
		    if (ans == 0.) /* once underflow, always underflow .. */
			return ans;
		}
		apb = a + b;
		if (apb > 1.) {
		    u = a + b - 1.;
		    z = (gam1(u) + 1.) / apb;
		} else {
		    z = gam1(apb) + 1.;
		}
		c = (gam1(a) + 1.) * (gam1(b) + 1.) / z;

		if(log_p) /* FIXME ? -- improve quite a bit for c ~= 1 */
		    ans += log(c * (b / apb));
		else
		    ans *=  c * (b / apb);

	    } else { /* 	------	a0 < 1 < b0 < 8	 ------ */

		u = gamln1(a0);
		m = (int)(b0 - 1.);
		if (m >= 1) {
		    c = 1.;
		    for (i = 1; i <= m; ++i) {
			b0 += -1.;
			c *= b0 / (a0 + b0);
		    }
		    u += log(c);
		}

		z = a * log(x) - u;
		b0 += -1.; // => b0 in (0, 7)
		apb = a0 + b0;
		if (apb > 1.) {
		    u = a0 + b0 - 1.;
		    t = (gam1(u) + 1.) / apb;
		} else {
		    t = gam1(apb) + 1.;
		}

		if(log_p) /* FIXME? potential for improving log(t) */
		    ans = z + log(a0 / a) + log1p(gam1(b0)) - log(t);
		else
		    ans = exp(z) * (a0 / a) * (gam1(b0) + 1.) / t;
	    }

	} else { /* 		------  a0 < 1 < 8 <= b0  ------ */

	    u = gamln1(a0) + algdiv(a0, b0);
	    z = a * log(x) - u;

	    if(log_p)
		ans = z + log(a0 / a);
	    else
		ans = a0 / a * exp(z);
	}
    }
    R_ifDEBUG_printf(" bpser(a=%g, b=%g, x=%g, log=%d): prelim.ans = %.14g;\n",
		     a,b,x, log_p, ans);
    if (ans == R_D__0 || (!log_p && a <= eps * 0.1)) {
	return ans;
    }

/* ----------------------------------------------------------------------- */
/*		       COMPUTE THE SERIES */
/* ----------------------------------------------------------------------- */
    double tol = eps / a,
	n = 0.,
	sum = 0., w;
    c = 1.;
    do { // sum is alternating as long as n < b (<==> 1 - b/n < 0)
	n += 1.;
	c *= (0.5 - b / n + 0.5) * x;
	w = c / (a + n);
	sum += w;
    } while (n < 1e7 && fabs(w) > tol);
    if(fabs(w) > tol) { // the series did not converge (in time)
	// warn only when the result seems to matter:
	if(( log_p && !(a*sum > -1. && fabs(log1p(a * sum)) < eps*fabs(ans))) ||
	   (!log_p && fabs(a*sum + 1.) != 1.))
	    MATHLIB_WARNING5(
		" bpser(a=%g, b=%g, x=%g,...) did not converge (n=1e7, |w|/tol=%g > 1; A=%g)",
		a,b,x, fabs(w)/tol, ans);
    }
    R_ifDEBUG_printf("  -> n=%.0f iterations, |w|=%g %s %g=tol:=eps/a ==> a*sum=%g\n",
		     n, fabs(w), (fabs(w) > tol) ? ">!!>" : "<=",
		     tol, a*sum);
    if(log_p) {
	if (a*sum > -1.) ans += log1p(a * sum);
	else {
	    if(ans > ML_NEGINF)
		MATHLIB_WARNING3(
		    "pbeta(*, log.p=TRUE) -> bpser(a=%g, b=%g, x=%g,...) underflow to -Inf",
		    a,b,x);
	    ans = ML_NEGINF;
	}
    } else if (a*sum > -1.)
	ans *= (a * sum + 1.);
    else // underflow to
	ans = 0.;
    return ans;
} /* bpser */

static double bup(double a, double b, double x, double y, int n, double eps,
		  int give_log)
{
/* ----------------------------------------------------------------------- */
/*     EVALUATION OF I_x(A,B) - I_x(A+N,B) WHERE N IS A POSITIVE INT. */
/*     EPS IS THE TOLERANCE USED. */
/* ----------------------------------------------------------------------- */

    double ret_val;
    int i, k, mu;
    double d, l;

// Obtain the scaling factor exp(-mu) and exp(mu)*(x^a * y^b / beta(a,b))/a

    double apb = a + b,
	ap1 = a + 1.;
    if (n > 1 && a >= 1. && apb >= ap1 * 1.1) {
	mu = (int)fabs(exparg(1));
	k = (int) exparg(0);
	if (mu > k)
	    mu = k;
 	d = exp(-(double) mu);
    }
    else {
	mu = 0;
	d = 1.;
    }

    /* L10: */
    ret_val = give_log
	? brcmp1(mu, a, b, x, y, TRUE) - log(a)
	: brcmp1(mu, a, b, x, y, FALSE)  / a;
    if (n == 1 ||
	(give_log && ret_val == ML_NEGINF) || (!give_log && ret_val == 0.))
	return ret_val;

    int nm1 = n - 1;
    double w = d;

/*          LET K BE THE INDEX OF THE MAXIMUM TERM */

    k = 0;
    if (b > 1.) {
	if (y > 1e-4) {
	    double r = (b - 1.) * x / y - a;
	    if (r >= 1.)
		k = (r < nm1) ? (int) r : nm1;
	} else
	    k = nm1;

//          ADD THE INCREASING TERMS OF THE SERIES - if k > 0
/* L30: */
	for (i = 0; i < k; ++i) {
	    l = (double) i;
	    d *= (apb + l) / (ap1 + l) * x;
	    w += d;
	}
    }

// L40:     ADD THE REMAINING TERMS OF THE SERIES

    for (i = k; i < nm1; ++i) {
	l = (double) i;
	d *= (apb + l) / (ap1 + l) * x;
	w += d;
	if (d <= eps * w) /* relativ convergence (eps) */
	    break;
    }

    // L50: TERMINATE THE PROCEDURE
    if(give_log) {
	ret_val += log(w);
    } else
	ret_val *= w;

    return ret_val;
} /* bup */

static double bfrac(double a, double b, double x, double y, double lambda,
		    double eps, int log_p)
{
/* -----------------------------------------------------------------------
       Continued fraction expansion for I_x(a,b) when a, b > 1.
       It is assumed that  lambda = (a + b)*y - b.
   -----------------------------------------------------------------------*/

    double c, e, n, p, r, s, t, w, c0, c1, r0, an, bn, yp1, anp1, bnp1,
	beta, alpha, brc;

    if(!R_FINITE(lambda)) return ML_NAN;// TODO: can return 0 or 1 (?)
    R_ifDEBUG_printf(" bfrac(a=%g, b=%g, x=%g, y=%g, lambda=%g, eps=%g, log_p=%d):",
		     a,b,x,y, lambda, eps, log_p);
    brc = brcomp(a, b, x, y, log_p);
    if(ISNAN(brc)) { // e.g. from   L <- 1e308; pnbinom(L, L, mu = 5)
	R_ifDEBUG_printf(" --> brcomp(a,b,x,y) = NaN\n");
	ML_WARN_return_NAN; // TODO: could we know better?
    }
    if (!log_p && brc == 0.) {
	R_ifDEBUG_printf(" --> brcomp(a,b,x,y) underflowed to 0.\n");
	return 0.;
    }
#ifdef DEBUG_bratio
    else
	REprintf("\n");
#endif

    c = lambda + 1.;
    c0 = b / a;
    c1 = 1. / a + 1.;
    yp1 = y + 1.;

    n = 0.;
    p = 1.;
    s = a + 1.;
    an = 0.;
    bn = 1.;
    anp1 = 1.;
    bnp1 = c / c1;
    r = c1 / c;

/*        CONTINUED FRACTION CALCULATION */

    do {
	n += 1.;
	t = n / a;
	w = n * (b - n) * x;
	e = a / s;
	alpha = p * (p + c0) * e * e * (w * x);
	e = (t + 1.) / (c1 + t + t);
	beta = n + w / s + e * (c + n * yp1);
	p = t + 1.;
	s += 2.;

	/* update an, bn, anp1, and bnp1 */

	t = alpha * an + beta * anp1;	an = anp1;	anp1 = t;
	t = alpha * bn + beta * bnp1;	bn = bnp1;	bnp1 = t;

	r0 = r;
	r = anp1 / bnp1;
#ifdef _not_normally_DEBUG_bfrac
	R_ifDEBUG_printf(" n=%5.0f, a_{n,n+1}= (%12g,%12g),  b_{n,n+1} = (%12g,%12g) => r0,r = (%14g,%14g)\n",
			 n, an,anp1, bn,bnp1, r0, r);
#endif
	if (fabs(r - r0) <= eps * r)
	    break;

	/* rescale an, bn, anp1, and bnp1 */

	an /= bnp1;
	bn /= bnp1;
	anp1 = r;
	bnp1 = 1.;
    } while (n < 10000);// arbitrary; had '1' --> infinite loop for  lambda = Inf
    R_ifDEBUG_printf("  in bfrac(): n=%.0f terms cont.frac.; brc=%g, r=%g\n",
		     n, brc, r);
    if(n >= 10000 && fabs(r - r0) > eps * r)
	MATHLIB_WARNING5(
	    " bfrac(a=%g, b=%g, x=%g, y=%g, lambda=%g) did *not* converge (in 10000 steps)\n",
	    a,b,x,y, lambda);
    return (log_p ? brc + log(r) : brc * r);
} /* bfrac */

static double brcomp(double a, double b, double x, double y, int log_p)
{
/* -----------------------------------------------------------------------
 *		 Evaluation of x^a * y^b / Beta(a,b)
 * ----------------------------------------------------------------------- */

    static double const__ = .398942280401433; /* == 1/sqrt(2*pi); */
    /* R has  M_1_SQRT_2PI , and M_LN_SQRT_2PI = ln(sqrt(2*pi)) = 0.918938.. */
    int i, n;
    double c, e, u, v, z, a0, b0, apb;

    if (x == 0. || y == 0.) {
	return R_D__0;
    }
    a0 = min(a, b);
    if (a0 < 8.) {
	double lnx, lny;
	if (x <= .375) {
	    lnx = log(x);
	    lny = alnrel(-x);
	}
	else {
	    if (y > .375) {
		lnx = log(x);
		lny = log(y);
	    } else {
		lnx = alnrel(-y);
		lny = log(y);
	    }
	}

	z = a * lnx + b * lny;
	if (a0 >= 1.) {
	    z -= betaln(a, b);
	    return R_D_exp(z);
	}

/* ----------------------------------------------------------------------- */
/*		PROCEDURE FOR a < 1 OR b < 1 */
/* ----------------------------------------------------------------------- */

	b0 = max(a, b);
	if (b0 >= 8.) { /* L80: */
	    u = gamln1(a0) + algdiv(a0, b0);

	    return (log_p ? log(a0) + (z - u)  : a0 * exp(z - u));
	}
	/* else : */

	if (b0 <= 1.) { /*		algorithm for max(a,b) = b0 <= 1 */

	    double e_z = R_D_exp(z);

	    if (!log_p && e_z == 0.) /* exp() underflow */
		return 0.;

	    apb = a + b;
	    if (apb > 1.) {
		u = a + b - 1.;
		z = (gam1(u) + 1.) / apb;
	    } else {
		z = gam1(apb) + 1.;
	    }

	    c = (gam1(a) + 1.) * (gam1(b) + 1.) / z;
	    /* FIXME? log(a0*c)= log(a0)+ log(c) and that is improvable */
	    return (log_p
		    ? e_z + log(a0 * c) - log1p(a0/b0)
		    : e_z * (a0 * c) / (a0 / b0 + 1.));
	}

	/* else : 		  ALGORITHM FOR 1 < b0 < 8 */

	u = gamln1(a0);
	n = (int)(b0 - 1.);
	if (n >= 1) {
	    c = 1.;
	    for (i = 1; i <= n; ++i) {
		b0 += -1.;
		c *= b0 / (a0 + b0);
	    }
	    u = log(c) + u;
	}
	z -= u;
	b0 += -1.;
	apb = a0 + b0;
	double t;
	if (apb > 1.) {
	    u = a0 + b0 - 1.;
	    t = (gam1(u) + 1.) / apb;
	} else {
	    t = gam1(apb) + 1.;
	}

	return (log_p
		? log(a0) + z + log1p(gam1(b0))  - log(t)
		: a0 * exp(z) * (gam1(b0) + 1.) / t);

    } else {
/* ----------------------------------------------------------------------- */
/*		PROCEDURE FOR A >= 8 AND B >= 8 */
/* ----------------------------------------------------------------------- */
	double h, x0, y0, lambda;
	if (a <= b) {
	    h = a / b;
	    x0 = h / (h + 1.);
	    y0 = 1. / (h + 1.);
	    lambda = a - (a + b) * x;
	} else {
	    h = b / a;
	    x0 = 1. / (h + 1.);
	    y0 = h / (h + 1.);
	    lambda = (a + b) * y - b;
	}

	e = -lambda / a;
	if (fabs(e) > .6)
	    u = e - log(x / x0);
	else
	    u = rlog1(e);

	e = lambda / b;
	if (fabs(e) <= .6)
	    v = rlog1(e);
	else
	    v = e - log(y / y0);

	z = log_p ? -(a * u + b * v) : exp(-(a * u + b * v));

	return(log_p
	       ? -M_LN_SQRT_2PI + .5*log(b * x0) + z - bcorr(a,b)
	       : const__ * sqrt(b * x0) * z * exp(-bcorr(a, b)));
    }
} /* brcomp */

// called only once from  bup(),  as   r = brcmp1(mu, a, b, x, y, FALSE) / a;
//                        -----
static double brcmp1(int mu, double a, double b, double x, double y, int give_log)
{
/* -----------------------------------------------------------------------
 *          Evaluation of    exp(mu) * x^a * y^b / beta(a,b)
 * ----------------------------------------------------------------------- */

    static double const__ = .398942280401433; /* == 1/sqrt(2*pi); */
    /* R has  M_1_SQRT_2PI */

    /* Local variables */
    double c, t, u, v, z, a0, b0, apb;

    a0 = min(a,b);
    if (a0 < 8.) {
	double lnx, lny;
	if (x <= .375) {
	    lnx = log(x);
	    lny = alnrel(-x);
	} else if (y > .375) {
	    // L11:
	    lnx = log(x);
	    lny = log(y);
	} else {
	    lnx = alnrel(-y);
	    lny = log(y);
	}

	// L20:
	z = a * lnx + b * lny;
	if (a0 >= 1.) {
	    z -= betaln(a, b);
	    return esum(mu, z, give_log);
	}
	// else :
	/* ----------------------------------------------------------------------- */
	/*              PROCEDURE FOR A < 1 OR B < 1 */
	/* ----------------------------------------------------------------------- */
	// L30:
	b0 = max(a,b);
	if (b0 >= 8.) {
	/* L80:                  ALGORITHM FOR b0 >= 8 */
	    u = gamln1(a0) + algdiv(a0, b0);
	    R_ifDEBUG_printf(" brcmp1(mu,a,b,*): a0 < 1, b0 >= 8;  z=%.15g\n", z);
	    return give_log
		? log(a0) + esum(mu, z - u, TRUE)
		:     a0  * esum(mu, z - u, FALSE);

	} else if (b0 <= 1.) {
	    //                   a0 < 1, b0 <= 1
	    double ans = esum(mu, z, give_log);
	    if (ans == (give_log ? ML_NEGINF : 0.))
		return ans;

	    apb = a + b;
	    if (apb > 1.) {
		// L40:
		u = a + b - 1.;
		z = (gam1(u) + 1.) / apb;
	    } else {
		z = gam1(apb) + 1.;
	    }
	    // L50:
	    c = give_log
		? log1p(gam1(a)) + log1p(gam1(b)) - log(z)
		: (gam1(a) + 1.) * (gam1(b) + 1.) / z;
	    R_ifDEBUG_printf(" brcmp1(mu,a,b,*): a0 < 1, b0 <= 1;  c=%.15g\n", c);
	    return give_log
		? ans + log(a0) + c - log1p(a0 / b0)
		: ans * (a0 * c) / (a0 / b0 + 1.);
	}
	// else:               algorithm for	a0 < 1 < b0 < 8
	// L60:
	u = gamln1(a0);
	int n = (int)(b0 - 1.);
	if (n >= 1) {
	    c = 1.;
	    for (int i = 1; i <= n; ++i) {
		b0 += -1.;
		c *= b0 / (a0 + b0);
		/* L61: */
	    }
	    u += log(c); // TODO?: log(c) = log( prod(...) ) =  sum( log(...) )
	}
	// L70:
	z -= u;
	b0 += -1.;
	apb = a0 + b0;
	if (apb > 1.) {
	    // L71:
	    t = (gam1(apb - 1.) + 1.) / apb;
	} else {
	    t = gam1(apb) + 1.;
	}
	R_ifDEBUG_printf(" brcmp1(mu,a,b,*): a0 < 1 < b0 < 8;  t=%.15g\n", t);
	// L72:
	return give_log
	    ? log(a0)+ esum(mu, z, TRUE) + log1p(gam1(b0)) - log(t) // TODO? log(t) = log1p(..)
	    :     a0 * esum(mu, z, FALSE) * (gam1(b0) + 1.) / t;

    } else {

/* ----------------------------------------------------------------------- */
/*              PROCEDURE FOR A >= 8 AND B >= 8 */
/* ----------------------------------------------------------------------- */
	// L100:
	double h, x0, y0, lambda;
	if (a > b) {
	    // L101:
	    h = b / a;
	    x0 = 1. / (h + 1.);// => lx0 := log(x0) = 0 - log1p(h)
	    y0 = h / (h + 1.);
	    lambda = (a + b) * y - b;
	} else {
	    h = a / b;
	    x0 = h / (h + 1.);  // => lx0 := log(x0) = - log1p(1/h)
	    y0 = 1. / (h + 1.);
	    lambda = a - (a + b) * x;
	}
	double lx0 = -log1p(b/a); // in both cases

	R_ifDEBUG_printf(" brcmp1(mu,a,b,*): a,b >= 8;	x0=%.15g, lx0=log(x0)=%.15g\n",
			 x0, lx0);
	// L110:
	double e = -lambda / a;
	if (fabs(e) > 0.6) {
	    // L111:
	    u = e - log(x / x0);
	} else {
	    u = rlog1(e);
	}

	// L120:
	e = lambda / b;
	if (fabs(e) > 0.6) {
	    // L121:
	    v = e - log(y / y0);
	} else {
	    v = rlog1(e);
	}

	// L130:
	z = esum(mu, -(a * u + b * v), give_log);
	return give_log
	    ? log(const__)+ (log(b) + lx0)/2. + z      - bcorr(a, b)
	    :     const__ * sqrt(b * x0)      * z * exp(-bcorr(a, b));
    }

} /* brcmp1 */

static void bgrat(double a, double b, double x, double y, double *w,
		  double eps, int *ierr, Rboolean log_w)
{
/* -----------------------------------------------------------------------
*     Asymptotic Expansion for I_x(a,b)  when a is larger than b.
*     Compute   w := w + I_x(a,b)
*     It is assumed a >= 15 and b <= 1.
*     eps is the tolerance used.
*     ierr is a variable that reports the status of the results.
*
* if(log_w),  *w  itself must be in log-space;
*     compute   w := w + I_x(a,b)  but return *w = log(w):
*          *w := log(exp(*w) + I_x(a,b)) = logspace_add(*w, log( I_x(a,b) ))
* ----------------------------------------------------------------------- */

#define n_terms_bgrat 30
    double c[n_terms_bgrat], d[n_terms_bgrat];
    double bm1 = b - 0.5 - 0.5,
	nu = a + bm1 * 0.5, /* nu = a + (b-1)/2 =: T, in (9.1) of
			     * Didonato & Morris(1992), p.362 */
	lnx = (y > 0.375) ? log(x) : alnrel(-y),
	z = -nu * lnx; // z =: u in (9.1) of D.&M.(1992)

    if (b * z == 0.) { // should not happen, but does, e.g.,
	// for  pbeta(1e-320, 1e-5, 0.5)  i.e., _subnormal_ x,
	// Warning ... bgrat(a=20.5, b=1e-05, x=1, y=9.99989e-321): ..
	MATHLIB_WARNING5(
	    "bgrat(a=%g, b=%g, x=%g, y=%g): z=%g, b*z == 0 underflow, hence inaccurate pbeta()",
	    a,b,x,y, z);
	/* L_Error:    THE EXPANSION CANNOT BE COMPUTED */
	 *ierr = 1; return;
    }

/*                 COMPUTATION OF THE EXPANSION */
    double
	/* r1 = b * (gam1(b) + 1.) * exp(b * log(z)),// = b/gamma(b+1) z^b = z^b / gamma(b)
	 * set r := exp(-z) * z^b / gamma(b) ;
	 *          gam1(b) = 1/gamma(b+1) - 1 , b in [-1/2, 3/2] */
	// exp(a*lnx) underflows for large (a * lnx); e.g. large a ==> using log_r := log(r):
	// r = r1 * exp(a * lnx) * exp(bm1 * 0.5 * lnx);
	// log(r)=log(b) + log1p(gam1(b)) + b * log(z) + (a * lnx) + (bm1 * 0.5 * lnx),
	log_r = log(b) + log1p(gam1(b)) + b * log(z) + nu * lnx,
	// FIXME work with  log_u = log(u)  also when log_p=FALSE  (??)
	// u is 'factored out' from the expansion {and multiplied back, at the end}:
	log_u = log_r - (algdiv(b, a) + b * log(nu)),// algdiv(b,a) = log(gamma(a)/gamma(a+b))
	/* u = (log_p) ? log_r - u : exp(log_r-u); // =: M  in (9.2) of {reference above} */
	/* u = algdiv(b, a) + b * log(nu);// algdiv(b,a) = log(gamma(a)/gamma(a+b)) */
	// u = (log_p) ? log_u : exp(log_u); // =: M  in (9.2) of {reference above}
	u = exp(log_u);

    if (log_u == ML_NEGINF) {
	R_ifDEBUG_printf(" bgrat(*): underflow log_u = -Inf  = log_r -u', log_r = %g ",
			 log_r);
	/* L_Error:    THE EXPANSION CANNOT BE COMPUTED */ *ierr = 2; return;
    }

    Rboolean u_0 = (u == 0.); // underflow --> do work with log(u) == log_u !
    double l = // := *w/u .. but with care: such that it also works when u underflows to 0:
	log_w
	? ((*w == ML_NEGINF) ? 0. : exp(  *w    - log_u))
	: ((*w == 0.)        ? 0. : exp(log(*w) - log_u));

    R_ifDEBUG_printf(" bgrat(a=%g, b=%g, x=%g, *)\n -> u=%g, l='w/u'=%g, ",
		     a,b,x, u, l);
    double
	q_r = grat_r(b, z, log_r, eps), // = q/r of former grat1(b,z, r, &p, &q)
	v = 0.25 / (nu * nu),
	t2 = lnx * 0.25 * lnx,
	j = q_r,
	sum = j,
	t = 1., cn = 1., n2 = 0.;
    for (int n = 1; n <= n_terms_bgrat; ++n) {
	double bp2n = b + n2;
	j = (bp2n * (bp2n + 1.) * j + (z + bp2n + 1.) * t) * v;
	n2 += 2.;
	t *= t2;
	cn /= n2 * (n2 + 1.);
	int nm1 = n - 1;
	c[nm1] = cn;
	double s = 0.;
	if (n > 1) {
	    double coef = b - n;
	    for (int i = 1; i <= nm1; ++i) {
		s += coef * c[i - 1] * d[nm1 - i];
		coef += b;
	    }
	}
	d[nm1] = bm1 * cn + s / n;
	double dj = d[nm1] * j;
	sum += dj;
	if (sum <= 0.) {
	    R_ifDEBUG_printf(" bgrat(*): sum_n(..) <= 0; should not happen (n=%d)\n", n);
	    /* L_Error:    THE EXPANSION CANNOT BE COMPUTED */ *ierr = 3; return;
	}
	if (fabs(dj) <= eps * (sum + l)) {
	    *ierr = 0;
	    break;
	} else if(n == n_terms_bgrat) { // never? ; please notify R-core if seen:
	    *ierr = 4;
	    MATHLIB_WARNING5(
	"bgrat(a=%g, b=%g, x=%g) *no* convergence: NOTIFY R-core!\n dj=%g, rel.err=%g\n",
		a,b,x, dj, fabs(dj) /(sum + l));
	}
    } // for(n .. n_terms..)

/*                    ADD THE RESULTS TO W */

    if(log_w) // *w is in log space already:
	*w = logspace_add(*w, log_u + log(sum));
    else
	*w += (u_0 ? exp(log_u + log(sum)) : u * sum);
    return;
} /* bgrat */


// called only from bgrat() , as   q_r = grat_r(b, z, log_r, eps)  :
static double grat_r(double a, double x, double log_r, double eps)
{
/* -----------------------------------------------------------------------
 *        Scaled complement of incomplete gamma ratio function
 *                   grat_r(a,x,r) :=  Q(a,x) / r
 * where
 *               Q(a,x) = pgamma(x,a, lower.tail=FALSE)
 *     and            r = e^(-x)* x^a / Gamma(a) ==  exp(log_r)
 *
 *     It is assumed that a <= 1.  eps is the tolerance to be used.
 * ----------------------------------------------------------------------- */

    if (a * x == 0.) { /* L130: */
	if (x <= a) {
	    /* L100: */ return exp(-log_r);
	} else {
	    /* L110:*/  return 0.;
	}
    }
    else if (a == 0.5) { // e.g. when called from pt()
	/* L120: */
	if (x < 0.25) {
	    double p = erf__(sqrt(x));
	    R_ifDEBUG_printf(" grat_r(a=%g, x=%g ..)): a=1/2 --> p=erf__(.)= %g\n",
			     a, x, p);
	    return (0.5 - p + 0.5)*exp(-log_r);

        } else { // 2013-02-27: improvement for "large" x: direct computation of q/r:
	    double sx = sqrt(x),
		q_r = erfc1(1, sx)/sx * M_SQRT_PI;
	    R_ifDEBUG_printf(" grat_r(a=%g, x=%g ..)): a=1/2 --> q_r=erfc1(..)/r= %g\n",
			     a,x, q_r);
	    return q_r;
	}

    } else if (x < 1.1) { /* L10:  Taylor series for  P(a,x)/x^a */

	double an = 3.,
	    c = x,
	    sum = x / (a + 3.),
	    tol = eps * 0.1 / (a + 1.), t;
	do {
	    an += 1.;
	    c *= -(x / an);
	    t = c / (a + an);
	    sum += t;
	} while (fabs(t) > tol);

	R_ifDEBUG_printf(" grat_r(a=%g, x=%g, log_r=%g): sum=%g; Taylor w/ %.0f terms",
			 a,x,log_r, sum, an-3.);
	double j = a * x * ((sum/6. - 0.5/(a + 2.)) * x + 1./(a + 1.)),
	    z = a * log(x),
	    h = gam1(a),
	    g = h + 1.;

	if ((x >= 0.25 && (a < x / 2.59)) || (z > -0.13394)) {
	    // L40:
	    double l = rexpm1(z),
		q = ((l + 0.5 + 0.5) * j - l) * g - h;
	    if (q <= 0.) {
		R_ifDEBUG_printf(" => q_r= 0.\n");
		/* L110:*/ return 0.;
	    } else {
		R_ifDEBUG_printf(" => q_r=%.15g\n", q * exp(-log_r));
		return q * exp(-log_r);
	    }

	} else {
	    double p = exp(z) * g * (0.5 - j + 0.5);
	    R_ifDEBUG_printf(" => q_r=%.15g\n", (0.5 - p + 0.5) * exp(-log_r));
	    return /* q/r = */ (0.5 - p + 0.5) * exp(-log_r);
	}

    } else {
	/* L50: ----  (x >= 1.1)  ---- Continued Fraction Expansion */

	double a2n_1 = 1.,
	    a2n = 1.,
	    b2n_1 = x,
	    b2n = x + (1. - a),
	    c = 1., am0, an0;

	do {
	    a2n_1 = x * a2n + c * a2n_1;
	    b2n_1 = x * b2n + c * b2n_1;
	    am0 = a2n_1 / b2n_1;
	    c += 1.;
	    double c_a = c - a;
	    a2n = a2n_1 + c_a * a2n;
	    b2n = b2n_1 + c_a * b2n;
	    an0 = a2n / b2n;
	} while (fabs(an0 - am0) >= eps * an0);

	R_ifDEBUG_printf(" grat_r(a=%g, x=%g, log_r=%g): Cont.frac. %.0f terms => q_r=%.15g\n",
			 a,x, log_r, c-1., an0);
	return /* q/r = (r * an0)/r = */ an0;
    }
} /* grat_r */


static double basym(double a, double b, double lambda, double eps, int log_p)
{
/* ----------------------------------------------------------------------- */
/*     ASYMPTOTIC EXPANSION FOR I_x(A,B) FOR LARGE A AND B. */
/*     LAMBDA = (A + B)*Y - B  AND EPS IS THE TOLERANCE USED. */
/*     IT IS ASSUMED THAT LAMBDA IS NONNEGATIVE AND THAT */
/*     A AND B ARE GREATER THAN OR EQUAL TO 15. */
/* ----------------------------------------------------------------------- */


/* ------------------------ */
/*     ****** NUM IS THE MAXIMUM VALUE THAT N CAN TAKE IN THE DO LOOP */
/*            ENDING AT STATEMENT 50. IT IS REQUIRED THAT NUM BE EVEN. */
#define num_IT 20
/*            THE ARRAYS A0, B0, C, D HAVE DIMENSION NUM + 1. */

    static double const e0 = 1.12837916709551;/* e0 == 2/sqrt(pi) */
    static double const e1 = .353553390593274;/* e1 == 2^(-3/2)   */
    static double const ln_e0 = 0.120782237635245; /* == ln(e0) */

    double a0[num_IT + 1], b0[num_IT + 1], c[num_IT + 1], d[num_IT + 1];

    double f = a * rlog1(-lambda/a) + b * rlog1(lambda/b), t;
    if(log_p)
	t = -f;
    else {
	t = exp(-f);
	if (t == 0.) {
	    return 0; /* once underflow, always underflow .. */
	}
    }
    double z0 = sqrt(f),
	z = z0 / e1 * 0.5,
	z2 = f + f,
	h, r0, r1, w0;

    if (a < b) {
	h = a / b;
	r0 = 1. / (h + 1.);
	r1 = (b - a) / b;
	w0 = 1. / sqrt(a * (h + 1.));
    } else {
	h = b / a;
	r0 = 1. / (h + 1.);
	r1 = (b - a) / a;
	w0 = 1. / sqrt(b * (h + 1.));
    }

    a0[0] = r1 * .66666666666666663;
    c[0] = a0[0] * -0.5;
    d[0] = -c[0];
    double j0 = 0.5 / e0 * erfc1(1, z0),
	j1 = e1,
	sum = j0 + d[0] * w0 * j1;

    double s = 1.,
	h2 = h * h,
	hn = 1.,
	w = w0,
	znm1 = z,
	zn = z2;
    for (int n = 2; n <= num_IT; n += 2) {
	hn *= h2;
	a0[n - 1] = r0 * 2. * (h * hn + 1.) / (n + 2.);
	int np1 = n + 1;
	s += hn;
	a0[np1 - 1] = r1 * 2. * s / (n + 3.);

	for (int i = n; i <= np1; ++i) {
	    double r = (i + 1.) * -0.5;
	    b0[0] = r * a0[0];
	    for (int m = 2; m <= i; ++m) {
		double bsum = 0.;
		for (int j = 1; j <= m-1; ++j) {
		    int mmj = m - j;
		    bsum += (j * r - mmj) * a0[j - 1] * b0[mmj - 1];
		}
		b0[m - 1] = r * a0[m - 1] + bsum / m;
	    }
	    c[i - 1] = b0[i - 1] / (i + 1.);

	    double dsum = 0.;
	    for (int j = 1; j <= i-1; ++j) {
		dsum += d[i - j - 1] * c[j - 1];
	    }
	    d[i - 1] = -(dsum + c[i - 1]);
	}

	j0 = e1 * znm1 + (n - 1.) * j0;
	j1 = e1 * zn + n * j1;
	znm1 = z2 * znm1;
	zn = z2 * zn;
	w *= w0;
	double t0 = d[n - 1] * w * j0;
	w *= w0;
	double t1 = d[np1 - 1] * w * j1;
	sum += t0 + t1;
	if (fabs(t0) + fabs(t1) <= eps * sum) {
	    break;
	}
    }

    if(log_p)
	return ln_e0 + t - bcorr(a, b) + log(sum);
    else {
	double u = exp(-bcorr(a, b));
	return e0 * t * u * sum;
    }

} /* basym_ */


static double exparg(int l)
{
/* --------------------------------------------------------------------
 *     If l = 0 then  exparg(l) = The largest positive W for which
 *     exp(W) can be computed. With 0.99999 fuzz  ==> exparg(0) =   709.7756  nowadays

 *     if l = 1 (nonzero) then  exparg(l) = the largest negative W for
 *     which the computed value of exp(W) is nonzero.
 *     With 0.99999 fuzz			  ==> exparg(1) =  -709.0825  nowadays

 *     Note... only an approximate value for exparg(L) is needed.
 * -------------------------------------------------------------------- */

    static double const lnb = .69314718055995;
    int m = (l == 0) ? Rf_i1mach(16) : Rf_i1mach(15) - 1;

    return m * lnb * .99999;
} /* exparg */

static double esum(int mu, double x, int give_log)
{
/* ----------------------------------------------------------------------- */
/*                    EVALUATION OF EXP(MU + X) */
/* ----------------------------------------------------------------------- */

    if(give_log)
	return x + (double) mu;

    // else :
    double w;
    if (x > 0.) { /* L10: */
	if (mu > 0)  return exp((double) mu) * exp(x);
	w = mu + x;
	if (w < 0.) return exp((double) mu) * exp(x);
    }
    else { /* x <= 0 */
	if (mu < 0)  return exp((double) mu) * exp(x);
	w = mu + x;
	if (w > 0.) return exp((double) mu) * exp(x);
    }
    return exp(w);

} /* esum */

double rexpm1(double x)
{
/* ----------------------------------------------------------------------- */
/*            EVALUATION OF THE FUNCTION EXP(X) - 1 */
/* ----------------------------------------------------------------------- */

    static double p1 = 9.14041914819518e-10;
    static double p2 = .0238082361044469;
    static double q1 = -.499999999085958;
    static double q2 = .107141568980644;
    static double q3 = -.0119041179760821;
    static double q4 = 5.95130811860248e-4;

    if (fabs(x) <= 0.15) {
	return x * (((p2 * x + p1) * x + 1.) /
		    ((((q4 * x + q3) * x + q2) * x + q1) * x + 1.));
    }
    else { /* |x| > 0.15 : */
	double w = exp(x);
	if (x > 0.)
	    return w * (0.5 - 1. / w + 0.5);
	else
	    return w - 0.5 - 0.5;
    }

} /* rexpm1 */

static double alnrel(double a)
{
/* -----------------------------------------------------------------------
 *            Evaluation of the function ln(1 + a)
 * ----------------------------------------------------------------------- */

    if (fabs(a) > 0.375)
	return log(1. + a);
    // else : |a| <= 0.375
    static double
	p1 = -1.29418923021993,
	p2 = .405303492862024,
	p3 = -.0178874546012214,
	q1 = -1.62752256355323,
	q2 = .747811014037616,
	q3 = -.0845104217945565;
    double
	t = a / (a + 2.),
	t2 = t * t,
	w = (((p3 * t2 + p2) * t2 + p1) * t2 + 1.) /
	(((q3 * t2 + q2) * t2 + q1) * t2 + 1.);
    return t * 2. * w;

} /* alnrel */

static double rlog1(double x)
{
/* -----------------------------------------------------------------------
 *             Evaluation of the function  x - ln(1 + x)
 * ----------------------------------------------------------------------- */

    static double a = .0566749439387324;
    static double b = .0456512608815524;
    static double p0 = .333333333333333;
    static double p1 = -.224696413112536;
    static double p2 = .00620886815375787;
    static double q1 = -1.27408923933623;
    static double q2 = .354508718369557;

    double h, r, t, w, w1;
    if (x < -0.39 || x > 0.57) { /* direct evaluation */
	w = x + 0.5 + 0.5;
	return x - log(w);
    }
    /* else */
    if (x < -0.18) { /* L10: */
	h = x + .3;
	h /= .7;
	w1 = a - h * .3;
    }
    else if (x > 0.18) { /* L20: */
	h = x * .75 - .25;
	w1 = b + h / 3.;
    }
    else { /*		Argument Reduction */
	h = x;
	w1 = 0.;
    }

/* L30:              	Series Expansion */

    r = h / (h + 2.);
    t = r * r;
    w = ((p2 * t + p1) * t + p0) / ((q2 * t + q1) * t + 1.);
    return t * 2. * (1. / (1. - r) - r * w) + w1;

} /* rlog1 */

static double erf__(double x)
{
/* -----------------------------------------------------------------------
 *             EVALUATION OF THE REAL ERROR FUNCTION
 * ----------------------------------------------------------------------- */

    /* Initialized data */

    static double c = .564189583547756;
    static double a[5] = { 7.7105849500132e-5,-.00133733772997339,
	    .0323076579225834,.0479137145607681,.128379167095513 };
    static double b[3] = { .00301048631703895,.0538971687740286,
	    .375795757275549 };
    static double p[8] = { -1.36864857382717e-7,.564195517478974,
	    7.21175825088309,43.1622272220567,152.98928504694,
	    339.320816734344,451.918953711873,300.459261020162 };
    static double q[8] = { 1.,12.7827273196294,77.0001529352295,
	    277.585444743988,638.980264465631,931.35409485061,
	    790.950925327898,300.459260956983 };
    static double r[5] = { 2.10144126479064,26.2370141675169,
	    21.3688200555087,4.6580782871847,.282094791773523 };
    static double s[4] = { 94.153775055546,187.11481179959,
	    99.0191814623914,18.0124575948747 };

    /* Local variables */
    double t, x2, ax, bot, top;

    ax = fabs(x);
    if (ax <= 0.5) {
	t = x * x;
	top = (((a[0] * t + a[1]) * t + a[2]) * t + a[3]) * t + a[4] + 1.;
	bot = ((b[0] * t + b[1]) * t + b[2]) * t + 1.;

	return x * (top / bot);
    }

    // else:  |x| > 0.5

    if (ax <= 4.) { //  |x| in (0.5, 4]
	top = ((((((p[0] * ax + p[1]) * ax + p[2]) * ax + p[3]) * ax + p[4]) * ax
		+ p[5]) * ax + p[6]) * ax + p[7];
	bot = ((((((q[0] * ax + q[1]) * ax + q[2]) * ax + q[3]) * ax + q[4]) * ax
		+ q[5]) * ax + q[6]) * ax + q[7];
	double R = 0.5 - exp(-x * x) * top / bot + 0.5;
	return (x < 0) ? -R : R;
    }

    // else:  |x| > 4

    if (ax >= 5.8) {
	return x > 0 ? 1 : -1;
    }

    // else:  4 < |x| < 5.8
    x2 = x * x;
    t = 1. / x2;
    top = (((r[0] * t + r[1]) * t + r[2]) * t + r[3]) * t + r[4];
    bot = (((s[0] * t + s[1]) * t + s[2]) * t + s[3]) * t + 1.;
    t = (c - top / (x2 * bot)) / ax;
    double R = 0.5 - exp(-x2) * t + 0.5;
    return (x < 0) ? -R : R;
} /* erf */

static double erfc1(int ind, double x)
{
/* ----------------------------------------------------------------------- */
/*         EVALUATION OF THE COMPLEMENTARY ERROR FUNCTION */

/*          ERFC1(IND,X) = ERFC(X)            IF IND = 0 */
/*          ERFC1(IND,X) = EXP(X*X)*ERFC(X)   OTHERWISE */
/* ----------------------------------------------------------------------- */

    /* Initialized data */

    static double c = .564189583547756;
    static double a[5] = { 7.7105849500132e-5,-.00133733772997339,
	    .0323076579225834,.0479137145607681,.128379167095513 };
    static double b[3] = { .00301048631703895,.0538971687740286,
	    .375795757275549 };
    static double p[8] = { -1.36864857382717e-7,.564195517478974,
	    7.21175825088309,43.1622272220567,152.98928504694,
	    339.320816734344,451.918953711873,300.459261020162 };
    static double q[8] = { 1.,12.7827273196294,77.0001529352295,
	    277.585444743988,638.980264465631,931.35409485061,
	    790.950925327898,300.459260956983 };
    static double r[5] = { 2.10144126479064,26.2370141675169,
	    21.3688200555087,4.6580782871847,.282094791773523 };
    static double s[4] = { 94.153775055546,187.11481179959,
	    99.0191814623914,18.0124575948747 };

    double ret_val;
    double e, t, w, bot, top;

    double ax = fabs(x);
    //				|X| <= 0.5 */
    if (ax <= 0.5) {
	double t = x * x,
	    top = (((a[0] * t + a[1]) * t + a[2]) * t + a[3]) * t + a[4] + 1.,
	    bot = ((b[0] * t + b[1]) * t + b[2]) * t + 1.;
	ret_val = 0.5 - x * (top / bot) + 0.5;
	if (ind != 0) {
	    ret_val = exp(t) * ret_val;
	}
	return ret_val;
    }
    // else (L10:):		0.5 < |X| <= 4
    if (ax <= 4.) {
	top = ((((((p[0] * ax + p[1]) * ax + p[2]) * ax + p[3]) * ax + p[4]) * ax
		+ p[5]) * ax + p[6]) * ax + p[7];
	bot = ((((((q[0] * ax + q[1]) * ax + q[2]) * ax + q[3]) * ax + q[4]) * ax
		+ q[5]) * ax + q[6]) * ax + q[7];
	ret_val = top / bot;

    } else { //			|X| > 4
	// L20:
	if (x <= -5.6) {
	    // L50:            	LIMIT VALUE FOR "LARGE" NEGATIVE X
	    ret_val = 2.;
	    if (ind != 0) {
		ret_val = exp(x * x) * 2.;
	    }
	    return ret_val;
	}
	if (ind == 0 && (x > 100. || x * x > -exparg(1))) {
	    // LIMIT VALUE FOR LARGE POSITIVE X   WHEN IND = 0
	    // L60:
	    return 0.;
	}

	// L30:
	t = 1. / (x * x);
	top = (((r[0] * t + r[1]) * t + r[2]) * t + r[3]) * t + r[4];
	bot = (((s[0] * t + s[1]) * t + s[2]) * t + s[3]) * t + 1.;
	ret_val = (c - t * top / bot) / ax;
    }

    // L40:                 FINAL ASSEMBLY
    if (ind != 0) {
	if (x < 0.)
	    ret_val = exp(x * x) * 2. - ret_val;
    } else {
	// L41:  ind == 0 :
	w = x * x;
	t = w;
	e = w - t;
	ret_val = (0.5 - e + 0.5) * exp(-t) * ret_val;
	if (x < 0.)
	    ret_val = 2. - ret_val;
    }
    return ret_val;

} /* erfc1 */

static double gam1(double a)
{
/*     ------------------------------------------------------------------ */
/*     COMPUTATION OF 1/GAMMA(A+1) - 1  FOR -0.5 <= A <= 1.5 */
/*     ------------------------------------------------------------------ */

    double d, t, w, bot, top;

    t = a;
    d = a - 0.5;
    // t := if(a > 1/2)  a-1  else  a
    if (d > 0.)
	t = d - 0.5;
    if (t < 0.) { /* L30: */
	static double
	    r[9] = { -.422784335098468,-.771330383816272,
		     -.244757765222226,.118378989872749,9.30357293360349e-4,
		     -.0118290993445146,.00223047661158249,2.66505979058923e-4,
		     -1.32674909766242e-4 },
	    s1 = .273076135303957,
	    s2 = .0559398236957378;

	top = (((((((r[8] * t + r[7]) * t + r[6]) * t + r[5]) * t + r[4]
		     ) * t + r[3]) * t + r[2]) * t + r[1]) * t + r[0];
	bot = (s2 * t + s1) * t + 1.;
	w = top / bot;
	R_ifDEBUG_printf("  gam1(a = %.15g): t < 0: w=%.15g\n", a, w);
	if (d > 0.)
	    return t * w / a;
	else
	    return a * (w + 0.5 + 0.5);

    } else if (t == 0) { // L10: a in {0, 1}
	return 0.;

    } else { /* t > 0;  L20: */
	static double
	    p[7] = { .577215664901533,-.409078193005776,
		     -.230975380857675,.0597275330452234,.0076696818164949,
		     -.00514889771323592,5.89597428611429e-4 },
	    q[5] = { 1.,.427569613095214,.158451672430138,
		     .0261132021441447,.00423244297896961 };

	top = (((((p[6] * t + p[5]) * t + p[4]) * t + p[3]) * t + p[2]
		   ) * t + p[1]) * t + p[0];
	bot = (((q[4] * t + q[3]) * t + q[2]) * t + q[1]) * t + 1.;
	w = top / bot;
	R_ifDEBUG_printf("  gam1(a = %.15g): t > 0: (is a < 1.5 ?)  w=%.15g\n",
			 a, w);
	if (d > 0.) /* L21: */
	    return t / a * (w - 0.5 - 0.5);
	else
	    return a * w;
    }
} /* gam1 */

static double gamln1(double a)
{
/* ----------------------------------------------------------------------- */
/*     EVALUATION OF LN(GAMMA(1 + A)) FOR -0.2 <= A <= 1.25 */
/* ----------------------------------------------------------------------- */

    double w;
    if (a < 0.6) {
	static double p0 = .577215664901533;
	static double p1 = .844203922187225;
	static double p2 = -.168860593646662;
	static double p3 = -.780427615533591;
	static double p4 = -.402055799310489;
	static double p5 = -.0673562214325671;
	static double p6 = -.00271935708322958;
	static double q1 = 2.88743195473681;
	static double q2 = 3.12755088914843;
	static double q3 = 1.56875193295039;
	static double q4 = .361951990101499;
	static double q5 = .0325038868253937;
	static double q6 = 6.67465618796164e-4;
	w = ((((((p6 * a + p5)* a + p4)* a + p3)* a + p2)* a + p1)* a + p0) /
	    ((((((q6 * a + q5)* a + q4)* a + q3)* a + q2)* a + q1)* a + 1.);
	return -(a) * w;
    }
    else { /* 0.6 <= a <= 1.25 */
	static double r0 = .422784335098467;
	static double r1 = .848044614534529;
	static double r2 = .565221050691933;
	static double r3 = .156513060486551;
	static double r4 = .017050248402265;
	static double r5 = 4.97958207639485e-4;
	static double s1 = 1.24313399877507;
	static double s2 = .548042109832463;
	static double s3 = .10155218743983;
	static double s4 = .00713309612391;
	static double s5 = 1.16165475989616e-4;
	double x = a - 0.5 - 0.5;
	w = (((((r5 * x + r4) * x + r3) * x + r2) * x + r1) * x + r0) /
	    (((((s5 * x + s4) * x + s3) * x + s2) * x + s1) * x + 1.);
	return x * w;
    }
} /* gamln1 */

static double psi(double x)
{
/* ---------------------------------------------------------------------

 *                 Evaluation of the Digamma function psi(x)

 *                           -----------

 *     Psi(xx) is assigned the value 0 when the digamma function cannot
 *     be computed.

 *     The main computation involves evaluation of rational Chebyshev
 *     approximations published in Math. Comp. 27, 123-127(1973) by
 *     Cody, Strecok and Thacher. */

/* --------------------------------------------------------------------- */
/*     Psi was written at Argonne National Laboratory for the FUNPACK */
/*     package of special function subroutines. Psi was modified by */
/*     A.H. Morris (NSWC). */
/* --------------------------------------------------------------------- */

    static double piov4 = .785398163397448; /* == pi / 4 */
/*     dx0 = zero of psi() to extended precision : */
    static double dx0 = 1.461632144968362341262659542325721325;

/* --------------------------------------------------------------------- */
/*     COEFFICIENTS FOR RATIONAL APPROXIMATION OF */
/*     PSI(X) / (X - X0),  0.5 <= X <= 3. */
    static double p1[7] = { .0089538502298197,4.77762828042627,
	    142.441585084029,1186.45200713425,3633.51846806499,
	    4138.10161269013,1305.60269827897 };
    static double q1[6] = { 44.8452573429826,520.752771467162,
	    2210.0079924783,3641.27349079381,1908.310765963,
	    6.91091682714533e-6 };
/* --------------------------------------------------------------------- */


/* --------------------------------------------------------------------- */
/*     COEFFICIENTS FOR RATIONAL APPROXIMATION OF */
/*     PSI(X) - LN(X) + 1 / (2*X),  X > 3. */

    static double p2[4] = { -2.12940445131011,-7.01677227766759,
	    -4.48616543918019,-.648157123766197 };
    static double q2[4] = { 32.2703493791143,89.2920700481861,
	    54.6117738103215,7.77788548522962 };
/* --------------------------------------------------------------------- */

    int i, m, n, nq;
    double d2;
    double w, z;
    double den, aug, sgn, xmx0, xmax1, upper, xsmall;

/* --------------------------------------------------------------------- */


/*     MACHINE DEPENDENT CONSTANTS ... */

/* --------------------------------------------------------------------- */
/*	  XMAX1	 = THE SMALLEST POSITIVE FLOATING POINT CONSTANT
		   WITH ENTIRELY INT REPRESENTATION.  ALSO USED
		   AS NEGATIVE OF LOWER BOUND ON ACCEPTABLE NEGATIVE
		   ARGUMENTS AND AS THE POSITIVE ARGUMENT BEYOND WHICH
		   PSI MAY BE REPRESENTED AS LOG(X).
 * Originally:  xmax1 = amin1(ipmpar(3), 1./spmpar(1))  */
    xmax1 = (double) INT_MAX;
    d2 = 0.5 / Rf_d1mach(3); /*= 0.5 / (0.5 * DBL_EPS) = 1/DBL_EPSILON = 2^52 */
    if(xmax1 > d2) xmax1 = d2;

/* --------------------------------------------------------------------- */
/*        XSMALL = ABSOLUTE ARGUMENT BELOW WHICH PI*COTAN(PI*X) */
/*                 MAY BE REPRESENTED BY 1/X. */
    xsmall = 1e-9;
/* --------------------------------------------------------------------- */
    aug = 0.;
    if (x < 0.5) {
/* --------------------------------------------------------------------- */
/*     X < 0.5,  USE REFLECTION FORMULA */
/*     PSI(1-X) = PSI(X) + PI * COTAN(PI*X) */
/* --------------------------------------------------------------------- */
	if (fabs(x) <= xsmall) {

	    if (x == 0.) {
		goto L_err;
	    }
/* --------------------------------------------------------------------- */
/*     0 < |X| <= XSMALL.  USE 1/X AS A SUBSTITUTE */
/*     FOR  PI*COTAN(PI*X) */
/* --------------------------------------------------------------------- */
	    aug = -1. / x;
	} else { /* |x| > xsmall */
/* --------------------------------------------------------------------- */
/*     REDUCTION OF ARGUMENT FOR COTAN */
/* --------------------------------------------------------------------- */
	    /* L100: */
	    w = -x;
	    sgn = piov4;
	    if (w <= 0.) {
		w = -w;
		sgn = -sgn;
	    }
/* --------------------------------------------------------------------- */
/*     MAKE AN ERROR EXIT IF |X| >= XMAX1 */
/* --------------------------------------------------------------------- */
	    if (w >= xmax1) {
		goto L_err;
	    }
	    nq = (int) w;
	    w -= (double) nq;
	    nq = (int) (w * 4.);
	    w = (w - (double) nq * 0.25) * 4.;
/* --------------------------------------------------------------------- */
/*     W IS NOW RELATED TO THE FRACTIONAL PART OF  4. * X. */
/*     ADJUST ARGUMENT TO CORRESPOND TO VALUES IN FIRST */
/*     QUADRANT AND DETERMINE SIGN */
/* --------------------------------------------------------------------- */
	    n = nq / 2;
	    if (n + n != nq) {
		w = 1. - w;
	    }
	    z = piov4 * w;
	    m = n / 2;
	    if (m + m != n) {
		sgn = -sgn;
	    }
/* --------------------------------------------------------------------- */
/*     DETERMINE FINAL VALUE FOR  -PI*COTAN(PI*X) */
/* --------------------------------------------------------------------- */
	    n = (nq + 1) / 2;
	    m = n / 2;
	    m += m;
	    if (m == n) {
/* --------------------------------------------------------------------- */
/*     CHECK FOR SINGULARITY */
/* --------------------------------------------------------------------- */
		if (z == 0.) {
		    goto L_err;
		}
/* --------------------------------------------------------------------- */
/*     USE COS/SIN AS A SUBSTITUTE FOR COTAN, AND */
/*     SIN/COS AS A SUBSTITUTE FOR TAN */
/* --------------------------------------------------------------------- */
		aug = sgn * (cos(z) / sin(z) * 4.);

	    } else { /* L140: */
		aug = sgn * (sin(z) / cos(z) * 4.);
	    }
	}

	x = 1. - x;

    }
    /* L200: */
    if (x <= 3.) {
/* --------------------------------------------------------------------- */
/*     0.5 <= X <= 3. */
/* --------------------------------------------------------------------- */
	den = x;
	upper = p1[0] * x;

	for (i = 1; i <= 5; ++i) {
	    den = (den + q1[i - 1]) * x;
	    upper = (upper + p1[i]) * x;
	}

	den = (upper + p1[6]) / (den + q1[5]);
	xmx0 = x - dx0;
	return den * xmx0 + aug;
    }

/* --------------------------------------------------------------------- */
/*     IF X >= XMAX1, PSI = LN(X) */
/* --------------------------------------------------------------------- */
    if (x < xmax1) {
/* --------------------------------------------------------------------- */
/*     3. < X < XMAX1 */
/* --------------------------------------------------------------------- */
	w = 1. / (x * x);
	den = w;
	upper = p2[0] * w;

	for (i = 1; i <= 3; ++i) {
	    den = (den + q2[i - 1]) * w;
	    upper = (upper + p2[i]) * w;
	}

	aug = upper / (den + q2[3]) - 0.5 / x + aug;
    }
    return aug + log(x);

/* --------------------------------------------------------------------- */
/*     ERROR RETURN */
/* --------------------------------------------------------------------- */
L_err:
    return 0.;
} /* psi */

static double betaln(double a0, double b0)
{
/* -----------------------------------------------------------------------
 *     Evaluation of the logarithm of the beta function  ln(beta(a0,b0))
 * ----------------------------------------------------------------------- */

    static double e = .918938533204673;/* e == 0.5*LN(2*PI) */

    double
	a = min(a0 ,b0),
	b = max(a0, b0);

    if (a < 8.) {
	if (a < 1.) {
/* ----------------------------------------------------------------------- */
//                    		A < 1
/* ----------------------------------------------------------------------- */
	    if (b < 8.)
		return gamln(a) + (gamln(b) - gamln(a+b));
	    else
		return gamln(a) + algdiv(a, b);
	}
	/* else */
/* ----------------------------------------------------------------------- */
//				1 <= A < 8
/* ----------------------------------------------------------------------- */
	double w;
	if (a < 2.) {
	    if (b <= 2.) {
		return gamln(a) + gamln(b) - gsumln(a, b);
	    }
	    /* else */

	    if (b < 8.) {
		w = 0.;
		goto L40;
	    }
	    return gamln(a) + algdiv(a, b);
	}
	// else L30:    REDUCTION OF A WHEN B <= 1000

	if (b <= 1e3) {
	    int n = (int)(a - 1.);
	    w = 1.;
	    for (int i = 1; i <= n; ++i) {
		a += -1.;
		double h = a / b;
		w *= h / (h + 1.);
	    }
	    w = log(w);

	    if (b >= 8.)
		return w + gamln(a) + algdiv(a, b);

	    // else
	L40:
	    // 	1 < A <= B < 8 :  reduction of B
	    n = (int)(b - 1.);
	    double z = 1.;
	    for (int i = 1; i <= n; ++i) {
		b += -1.;
		z *= b / (a + b);
	    }
	    return w + log(z) + (gamln(a) + (gamln(b) - gsumln(a, b)));
	}
	else { // L50:	reduction of A when  B > 1000
	    int n = (int)(a - 1.);
	    w = 1.;
	    for (int i = 1; i <= n; ++i) {
		a += -1.;
		w *= a / (a / b + 1.);
	    }
	    return log(w) - n * log(b) + (gamln(a) + algdiv(a, b));
	}

    } else {
/* ----------------------------------------------------------------------- */
	// L60:			A >= 8
/* ----------------------------------------------------------------------- */

	double
	    w = bcorr(a, b),
	    h = a / b,
	    u = -(a - 0.5) * log(h / (h + 1.)),
	    v = b * alnrel(h);
	if (u > v)
	    return log(b) * -0.5 + e + w - v - u;
	else
	    return log(b) * -0.5 + e + w - u - v;
    }

} /* betaln */

static double gsumln(double a, double b)
{
/* ----------------------------------------------------------------------- */
/*          EVALUATION OF THE FUNCTION LN(GAMMA(A + B)) */
/*          FOR 1 <= A <= 2  AND  1 <= B <= 2 */
/* ----------------------------------------------------------------------- */

    double x = a + b - 2.;/* in [0, 2] */

    if (x <= 0.25)
	return gamln1(x + 1.);

    /* else */
    if (x <= 1.25)
	return gamln1(x) + alnrel(x);
    /* else x > 1.25 : */
    return gamln1(x - 1.) + log(x * (x + 1.));

} /* gsumln */

static double bcorr(double a0, double b0)
{
/* ----------------------------------------------------------------------- */

/*     EVALUATION OF  DEL(A0) + DEL(B0) - DEL(A0 + B0)  WHERE */
/*     LN(GAMMA(A)) = (A - 0.5)*LN(A) - A + 0.5*LN(2*PI) + DEL(A). */
/*     IT IS ASSUMED THAT A0 >= 8 AND B0 >= 8. */

/* ----------------------------------------------------------------------- */
    /* Initialized data */

    static double c0 = .0833333333333333;
    static double c1 = -.00277777777760991;
    static double c2 = 7.9365066682539e-4;
    static double c3 = -5.9520293135187e-4;
    static double c4 = 8.37308034031215e-4;
    static double c5 = -.00165322962780713;

    /* System generated locals */
    double ret_val, r1;

    /* Local variables */
    double a, b, c, h, t, w, x, s3, s5, x2, s7, s9, s11;
/* ------------------------ */
    a = min(a0, b0);
    b = max(a0, b0);

    h = a / b;
    c = h / (h + 1.);
    x = 1. / (h + 1.);
    x2 = x * x;

/*                SET SN = (1 - X^N)/(1 - X) */

    s3 = x + x2 + 1.;
    s5 = x + x2 * s3 + 1.;
    s7 = x + x2 * s5 + 1.;
    s9 = x + x2 * s7 + 1.;
    s11 = x + x2 * s9 + 1.;

/*                SET W = DEL(B) - DEL(A + B) */

/* Computing 2nd power */
    r1 = 1. / b;
    t = r1 * r1;
    w = ((((c5 * s11 * t + c4 * s9) * t + c3 * s7) * t + c2 * s5) * t + c1 *
	    s3) * t + c0;
    w *= c / b;

/*                   COMPUTE  DEL(A) + W */

/* Computing 2nd power */
    r1 = 1. / a;
    t = r1 * r1;
    ret_val = (((((c5 * t + c4) * t + c3) * t + c2) * t + c1) * t + c0) / a +
	    w;
    return ret_val;
} /* bcorr */

static double algdiv(double a, double b)
{
/* ----------------------------------------------------------------------- */

/*     COMPUTATION OF LN(GAMMA(B)/GAMMA(A+B)) WHEN B >= 8 */

/*                         -------- */

/*     IN THIS ALGORITHM, DEL(X) IS THE FUNCTION DEFINED BY */
/*     LN(GAMMA(X)) = (X - 0.5)*LN(X) - X + 0.5*LN(2*PI) + DEL(X). */

/* ----------------------------------------------------------------------- */

    /* Initialized data */

    static double c0 = .0833333333333333;
    static double c1 = -.00277777777760991;
    static double c2 = 7.9365066682539e-4;
    static double c3 = -5.9520293135187e-4;
    static double c4 = 8.37308034031215e-4;
    static double c5 = -.00165322962780713;

    double c, d, h, t, u, v, w, x, s3, s5, x2, s7, s9, s11;

/* ------------------------ */
    if (a > b) {
	h = b / a;
	c = 1. / (h + 1.);
	x = h / (h + 1.);
	d = a + (b - 0.5);
    }
    else {
	h = a / b;
	c = h / (h + 1.);
	x = 1. / (h + 1.);
	d = b + (a - 0.5);
    }

/* Set s<n> = (1 - x^n)/(1 - x) : */

    x2 = x * x;
    s3 = x + x2 + 1.;
    s5 = x + x2 * s3 + 1.;
    s7 = x + x2 * s5 + 1.;
    s9 = x + x2 * s7 + 1.;
    s11 = x + x2 * s9 + 1.;

/* w := Del(b) - Del(a + b) */

    t = 1./ (b * b);
    w = ((((c5 * s11 * t + c4 * s9) * t + c3 * s7) * t + c2 * s5) * t + c1 *
	    s3) * t + c0;
    w *= c / b;

/*                    COMBINE THE RESULTS */

    u = d * alnrel(a / b);
    v = a * (log(b) - 1.);
    if (u > v)
	return w - v - u;
    else
	return w - u - v;
} /* algdiv */

static double gamln(double a)
{
/* -----------------------------------------------------------------------
 *            Evaluation of  ln(gamma(a))  for positive a
 * ----------------------------------------------------------------------- */
/*     Written by Alfred H. Morris */
/*          Naval Surface Warfare Center */
/*          Dahlgren, Virginia */
/* ----------------------------------------------------------------------- */

    static double d = .418938533204673;/* d == 0.5*(LN(2*PI) - 1) */

    static double c0 = .0833333333333333;
    static double c1 = -.00277777777760991;
    static double c2 = 7.9365066682539e-4;
    static double c3 = -5.9520293135187e-4;
    static double c4 = 8.37308034031215e-4;
    static double c5 = -.00165322962780713;

    if (a <= 0.8)
	return gamln1(a) - log(a); /* ln(G(a+1)) - ln(a) == ln(G(a+1)/a) = ln(G(a)) */
    else if (a <= 2.25)
	return gamln1(a - 0.5 - 0.5);

    else if (a < 10.) {
	int i, n = (int)(a - 1.25);
	double t = a;
	double w = 1.;
	for (i = 1; i <= n; ++i) {
	    t += -1.;
	    w *= t;
	}
	return gamln1(t - 1.) + log(w);
    }
    else { /* a >= 10 */
	double t = 1. / (a * a);
	double w = (((((c5 * t + c4) * t + c3) * t + c2) * t + c1) * t + c0) / a;
	return d + w + (a - 0.5) * (log(a) - 1.);
    }
} /* gamln */