* Copyright (c) 1992, 1993

/* $NetBSD: n_gamma.c,v 1.9 2013/11/09 21:41:03 christos Exp $ */
/*-
* Copyright (c) 1992, 1993
* The Regents of the University of California. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/

#ifndef lint
#if 0
static char sccsid[] = "@(#)gamma.c 8.1 (Berkeley) 6/4/93";
#endif
#endif /* not lint */

/*
* This code by P. McIlroy, Oct 1992;
*
* The financial support of UUNET Communications Services is gratefully
* acknowledged.
*/

#include <math.h>
#include "mathimpl.h"
#include <errno.h>

/* METHOD:
* x < 0: Use reflection formula, G(x) = pi/(sin(pi*x)*x*G(x))
* At negative integers, return +Inf, and set errno.
*
* x < 6.5:
* Use argument reduction G(x+1) = xG(x) to reach the
* range [1.066124,2.066124]. Use a rational
* approximation centered at the minimum (x0+1) to
* ensure monotonicity.
*
* x >= 6.5: Use the asymptotic approximation (Stirling's formula)
* adjusted for equal-ripples:
*
* log(G(x)) ~= (x-.5)*(log(x)-1) + .5(log(2*pi)-1) + 1/x*P(1/(x*x))
*
* Keep extra precision in multiplying (x-.5)(log(x)-1), to
* avoid premature round-off.
*
* Special values:
* non-positive integer: Set overflow trap; return +Inf;
* x > 171.63: Set overflow trap; return +Inf;
* NaN: Set invalid trap; return NaN
*
* Accuracy: Gamma(x) is accurate to within
* x > 0: error provably < 0.9ulp.
* Maximum observed in 1,000,000 trials was .87ulp.
* x < 0:
* Maximum observed error < 4ulp in 1,000,000 trials.
*/

static double neg_gam (double);
static double small_gam (double);
static double smaller_gam (double);
static struct Double large_gam (double);
static struct Double ratfun_gam (double, double);

/*
* Rational approximation, A0 + x*x*P(x)/Q(x), on the interval
* [1.066.., 2.066..] accurate to 4.25e-19.
*/
#define LEFT -.3955078125 /* left boundary for rat. approx */
#define x0 .461632144968362356785 /* xmin - 1 */

#define a0_hi 0.88560319441088874992
#define a0_lo -.00000000000000004996427036469019695
#define P0 6.21389571821820863029017800727e-01
#define P1 2.65757198651533466104979197553e-01
#define P2 5.53859446429917461063308081748e-03
#define P3 1.38456698304096573887145282811e-03
#define P4 2.40659950032711365819348969808e-03
#define Q0 1.45019531250000000000000000000e+00
#define Q1 1.06258521948016171343454061571e+00
#define Q2 -2.07474561943859936441469926649e-01
#define Q3 -1.46734131782005422506287573015e-01
#define Q4 3.07878176156175520361557573779e-02
#define Q5 5.12449347980666221336054633184e-03
#define Q6 -1.76012741431666995019222898833e-03
#define Q7 9.35021023573788935372153030556e-05
#define Q8 6.13275507472443958924745652239e-06
/*
* Constants for large x approximation (x in [6, Inf])
* (Accurate to 2.8*10^-19 absolute)
*/
#define lns2pi_hi 0.418945312500000
#define lns2pi_lo -.000006779295327258219670263595
#define Pa0 8.33333333333333148296162562474e-02
#define Pa1 -2.77777777774548123579378966497e-03
#define Pa2 7.93650778754435631476282786423e-04
#define Pa3 -5.95235082566672847950717262222e-04
#define Pa4 8.41428560346653702135821806252e-04
#define Pa5 -1.89773526463879200348872089421e-03
#define Pa6 5.69394463439411649408050664078e-03
#define Pa7 -1.44705562421428915453880392761e-02

static const double zero = 0., one = 1.0, tiny = _TINY;
/*
* TRUNC sets trailing bits in a floating-point number to zero.
* is a temporary variable.
*/
#if defined(__vax__) || defined(tahoe)
#define _IEEE 0
#define TRUNC(x) x = (double) (float) (x)
#else
static int endian;
#define _IEEE 1
#define TRUNC(x) *(((int *) &x) + endian) &= 0xf8000000
#define infnan(x) 0.0
#endif

double
gamma(double x)
{
double b;
struct Double u;
#if _IEEE
int endian = (*(int *) &one) ? 1 : 0;
#endif

if (x >= 6) {
if(x > 171.63)
return(one/zero);
u = large_gam(x);
return(__exp__D(u.a, u.b));
} else if (x >= 1.0 + LEFT + x0) {
return (small_gam(x));
} else if (x > 1.e-17) {
return (smaller_gam(x));
} else if (x > -1.e-17) {
if (x == 0.0) {
if (!_IEEE) return (infnan(ERANGE));
else return (one/x);
}
b =one+1e-20; /* Raise inexact flag. ??? -ragge */
__USE(b);
return (one/x);
} else if (!finite(x)) {
if (_IEEE) /* x = NaN, -Inf */
return (x*x);
else
return (infnan(EDOM));
} else
return (neg_gam(x));
}
/*
* Accurate to max(ulp(1/128) absolute, 2^-66 relative) error.
*/
static struct Double
large_gam(double x)
{
double z, p;
struct Double t, u, v;

z = one/(x*x);
p = Pa0+z*(Pa1+z*(Pa2+z*(Pa3+z*(Pa4+z*(Pa5+z*(Pa6+z*Pa7))))));
p = p/x;

u = __log__D(x);
u.a -= one;
v.a = (x -= .5);
TRUNC(v.a);
v.b = x - v.a;
t.a = v.a*u.a; /* t = (x-.5)*(log(x)-1) */
t.b = v.b*u.a + x*u.b;
/* return t.a + t.b + lns2pi_hi + lns2pi_lo + p */
t.b += lns2pi_lo; t.b += p;
u.a = lns2pi_hi + t.b; u.a += t.a;
u.b = t.a - u.a;
u.b += lns2pi_hi; u.b += t.b;
return (u);
}
/*
* Good to < 1 ulp. (provably .90 ulp; .87 ulp on 1,000,000 runs.)
* It also has correct monotonicity.
*/
static double
small_gam(double x)
{
double y, ym1, t;
struct Double yy, r;
y = x - one;
ym1 = y - one;
if (y <= 1.0 + (LEFT + x0)) {
yy = ratfun_gam(y - x0, 0);
return (yy.a + yy.b);
}
r.a = y;
TRUNC(r.a);
yy.a = r.a - one;
y = ym1;
yy.b = r.b = y - yy.a;
/* Argument reduction: G(x+1) = x*G(x) */
for (ym1 = y-one; ym1 > LEFT + x0; y = ym1--, yy.a--) {
t = r.a*yy.a;
r.b = r.a*yy.b + y*r.b;
r.a = t;
TRUNC(r.a);
r.b += (t - r.a);
}
/* Return r*gamma(y). */
yy = ratfun_gam(y - x0, 0);
y = r.b*(yy.a + yy.b) + r.a*yy.b;
y += yy.a*r.a;
return (y);
}
/*
* Good on (0, 1+x0+LEFT]. Accurate to 1ulp.
*/
static double
smaller_gam(double x)
{
double t, d;
struct Double r, xx;
if (x < x0 + LEFT) {
t = x, TRUNC(t);
d = (t+x)*(x-t);
t *= t;
xx.a = (t + x), TRUNC(xx.a);
xx.b = x - xx.a; xx.b += t; xx.b += d;
t = (one-x0); t += x;
d = (one-x0); d -= t; d += x;
x = xx.a + xx.b;
} else {
xx.a = x, TRUNC(xx.a);
xx.b = x - xx.a;
t = x - x0;
d = (-x0 -t); d += x;
}
r = ratfun_gam(t, d);
d = r.a/x, TRUNC(d);
r.a -= d*xx.a; r.a -= d*xx.b; r.a += r.b;
return (d + r.a/x);
}
/*
* returns (z+c)^2 * P(z)/Q(z) + a0
*/
static struct Double
ratfun_gam(double z, double c)
{
double p, q;
struct Double r, t;

q = Q0 +z*(Q1+z*(Q2+z*(Q3+z*(Q4+z*(Q5+z*(Q6+z*(Q7+z*Q8)))))));
p = P0 + z*(P1 + z*(P2 + z*(P3 + z*P4)));

/* return r.a + r.b = a0 + (z+c)^2*p/q, with r.a truncated to 26 bits. */
p = p/q;
t.a = z, TRUNC(t.a); /* t ~= z + c */
t.b = (z - t.a) + c;
t.b *= (t.a + z);
q = (t.a *= t.a); /* t = (z+c)^2 */
TRUNC(t.a);
t.b += (q - t.a);
r.a = p, TRUNC(r.a); /* r = P/Q */
r.b = p - r.a;
t.b = t.b*p + t.a*r.b + a0_lo;
t.a *= r.a; /* t = (z+c)^2*(P/Q) */
r.a = t.a + a0_hi, TRUNC(r.a);
r.b = ((a0_hi-r.a) + t.a) + t.b;
return (r); /* r = a0 + t */
}

static double
neg_gam(double x)
{
int sgn = 1;
struct Double lg, lsine;
double y, z;

y = floor(x + .5);
if (y == x) { /* Negative integer. */
if(!_IEEE)
return (infnan(ERANGE));
else
return (one/zero);
}
z = fabs(x - y);
y = .5*ceil(x);
if (y == ceil(y))
sgn = -1;
if (z < .25)
z = sin(M_PI*z);
else
z = cos(M_PI*(0.5-z));
/* Special case: G(1-x) = Inf; G(x) may be nonzero. */
if (x < -170) {
if (x < -190)
return ((double)sgn*tiny*tiny);
y = one - x; /* exact: 128 < |x| < 255 */
lg = large_gam(y);
lsine = __log__D(M_PI/z); /* = TRUNC(log(u)) + small */
lg.a -= lsine.a; /* exact (opposite signs) */
lg.b -= lsine.b;
y = -(lg.a + lg.b);
z = (y + lg.a) + lg.b;
y = __exp__D(y, z);
if (sgn < 0) y = -y;
return (y);
}
y = one-x;
if (one-y == x)
y = gamma(y);
else /* 1-x is inexact */
y = -x*gamma(-x);
if (sgn < 0) y = -y;
return (M_PI / (y*z));
}