diff options
Diffstat (limited to 'lib/libm/common_source/gamma.c')
| -rw-r--r-- | lib/libm/common_source/gamma.c | 336 |
1 files changed, 336 insertions, 0 deletions
diff --git a/lib/libm/common_source/gamma.c b/lib/libm/common_source/gamma.c new file mode 100644 index 0000000..5d270f0 --- /dev/null +++ b/lib/libm/common_source/gamma.c @@ -0,0 +1,336 @@ +/*- + * 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. All advertising materials mentioning features or use of this software + * must display the following acknowledgement: + * This product includes software developed by the University of + * California, Berkeley and its contributors. + * 4. 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 +static char sccsid[] = "@(#)gamma.c 8.1 (Berkeley) 6/4/93"; +#endif /* not lint */ + +/* + * This code by P. McIlroy, Oct 1992; + * + * The financial support of UUNET Communications Services is greatfully + * 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 __P((double)); +static double small_gam __P((double)); +static double smaller_gam __P((double)); +static struct Double large_gam __P((double)); +static struct Double ratfun_gam __P((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 = 1e-300; +static int endian; +/* + * 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 +#define _IEEE 1 +#define TRUNC(x) *(((int *) &x) + endian) &= 0xf8000000 +#define infnan(x) 0.0 +#endif + +double +gamma(x) + double x; +{ + struct Double u; + endian = (*(int *) &one) ? 1 : 0; + + 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); + one+1e-20; /* Raise inexact flag. */ + 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(x) + double x; +{ + double z, p; + int i; + 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(x) + double x; +{ + double y, ym1, t, x1; + 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(x) + 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(z, c) + double z, c; +{ + int i; + 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(x) + 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)); +} |
