2 * Copyright (c) 1992, 1993
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29 * @(#)gamma.c 8.1 (Berkeley) 6/4/93
30 * $FreeBSD: head/lib/msun/bsdsrc/b_tgamma.c 176449 2008-02-22 02:26:51Z das $
34 * This code by P. McIlroy, Oct 1992;
36 * The financial support of UUNET Communications Services is greatfully
44 * x < 0: Use reflection formula, G(x) = pi/(sin(pi*x)*x*G(x))
45 * At negative integers, return NaN and raise invalid.
48 * Use argument reduction G(x+1) = xG(x) to reach the
49 * range [1.066124,2.066124]. Use a rational
50 * approximation centered at the minimum (x0+1) to
51 * ensure monotonicity.
53 * x >= 6.5: Use the asymptotic approximation (Stirling's formula)
54 * adjusted for equal-ripples:
56 * log(G(x)) ~= (x-.5)*(log(x)-1) + .5(log(2*pi)-1) + 1/x*P(1/(x*x))
58 * Keep extra precision in multiplying (x-.5)(log(x)-1), to
59 * avoid premature round-off.
62 * -Inf: return NaN and raise invalid;
63 * negative integer: return NaN and raise invalid;
64 * other x ~< 177.79: return +-0 and raise underflow;
65 * +-0: return +-Inf and raise divide-by-zero;
66 * finite x ~> 171.63: return +Inf and raise overflow;
70 * Accuracy: tgamma(x) is accurate to within
71 * x > 0: error provably < 0.9ulp.
72 * Maximum observed in 1,000,000 trials was .87ulp.
74 * Maximum observed error < 4ulp in 1,000,000 trials.
77 static double neg_gam(double);
78 static double small_gam(double);
79 static double smaller_gam(double);
80 static struct Double large_gam(double);
81 static struct Double ratfun_gam(double, double);
84 * Rational approximation, A0 + x*x*P(x)/Q(x), on the interval
85 * [1.066.., 2.066..] accurate to 4.25e-19.
87 #define LEFT -.3955078125 /* left boundary for rat. approx */
88 #define x0 .461632144968362356785 /* xmin - 1 */
90 #define a0_hi 0.88560319441088874992
91 #define a0_lo -.00000000000000004996427036469019695
92 #define P0 6.21389571821820863029017800727e-01
93 #define P1 2.65757198651533466104979197553e-01
94 #define P2 5.53859446429917461063308081748e-03
95 #define P3 1.38456698304096573887145282811e-03
96 #define P4 2.40659950032711365819348969808e-03
97 #define Q0 1.45019531250000000000000000000e+00
98 #define Q1 1.06258521948016171343454061571e+00
99 #define Q2 -2.07474561943859936441469926649e-01
100 #define Q3 -1.46734131782005422506287573015e-01
101 #define Q4 3.07878176156175520361557573779e-02
102 #define Q5 5.12449347980666221336054633184e-03
103 #define Q6 -1.76012741431666995019222898833e-03
104 #define Q7 9.35021023573788935372153030556e-05
105 #define Q8 6.13275507472443958924745652239e-06
107 * Constants for large x approximation (x in [6, Inf])
108 * (Accurate to 2.8*10^-19 absolute)
110 #define lns2pi_hi 0.418945312500000
111 #define lns2pi_lo -.000006779295327258219670263595
112 #define Pa0 8.33333333333333148296162562474e-02
113 #define Pa1 -2.77777777774548123579378966497e-03
114 #define Pa2 7.93650778754435631476282786423e-04
115 #define Pa3 -5.95235082566672847950717262222e-04
116 #define Pa4 8.41428560346653702135821806252e-04
117 #define Pa5 -1.89773526463879200348872089421e-03
118 #define Pa6 5.69394463439411649408050664078e-03
119 #define Pa7 -1.44705562421428915453880392761e-02
121 static const double zero = 0., one = 1.0, tiny = 1e-300;
132 return(__exp__D(u.a, u.b));
133 } else if (x >= 1.0 + LEFT + x0)
134 return (small_gam(x));
136 return (smaller_gam(x));
137 else if (x > -1.e-17) {
139 u.a = one - tiny; /* raise inexact */
141 } else if (!finite(x))
142 return (x - x); /* x is NaN or -Inf */
147 * Accurate to max(ulp(1/128) absolute, 2^-66 relative) error.
153 struct Double t, u, v;
156 p = Pa0+z*(Pa1+z*(Pa2+z*(Pa3+z*(Pa4+z*(Pa5+z*(Pa6+z*Pa7))))));
164 t.a = v.a*u.a; /* t = (x-.5)*(log(x)-1) */
165 t.b = v.b*u.a + x*u.b;
166 /* return t.a + t.b + lns2pi_hi + lns2pi_lo + p */
167 t.b += lns2pi_lo; t.b += p;
168 u.a = lns2pi_hi + t.b; u.a += t.a;
170 u.b += lns2pi_hi; u.b += t.b;
174 * Good to < 1 ulp. (provably .90 ulp; .87 ulp on 1,000,000 runs.)
175 * It also has correct monotonicity.
184 if (y <= 1.0 + (LEFT + x0)) {
185 yy = ratfun_gam(y - x0, 0);
186 return (yy.a + yy.b);
192 yy.b = r.b = y - yy.a;
193 /* Argument reduction: G(x+1) = x*G(x) */
194 for (ym1 = y-one; ym1 > LEFT + x0; y = ym1--, yy.a--) {
196 r.b = r.a*yy.b + y*r.b;
201 /* Return r*tgamma(y). */
202 yy = ratfun_gam(y - x0, 0);
203 y = r.b*(yy.a + yy.b) + r.a*yy.b;
208 * Good on (0, 1+x0+LEFT]. Accurate to 1ulp.
211 smaller_gam(double x)
219 xx.a = (t + x), TRUNC(xx.a);
220 xx.b = x - xx.a; xx.b += t; xx.b += d;
221 t = (one-x0); t += x;
222 d = (one-x0); d -= t; d += x;
225 xx.a = x, TRUNC(xx.a);
228 d = (-x0 -t); d += x;
230 r = ratfun_gam(t, d);
232 r.a -= d*xx.a; r.a -= d*xx.b; r.a += r.b;
236 * returns (z+c)^2 * P(z)/Q(z) + a0
239 ratfun_gam(double z, double c)
244 q = Q0 +z*(Q1+z*(Q2+z*(Q3+z*(Q4+z*(Q5+z*(Q6+z*(Q7+z*Q8)))))));
245 p = P0 + z*(P1 + z*(P2 + z*(P3 + z*P4)));
247 /* return r.a + r.b = a0 + (z+c)^2*p/q, with r.a truncated to 26 bits. */
249 t.a = z, TRUNC(t.a); /* t ~= z + c */
252 q = (t.a *= t.a); /* t = (z+c)^2 */
255 r.a = p, TRUNC(r.a); /* r = P/Q */
257 t.b = t.b*p + t.a*r.b + a0_lo;
258 t.a *= r.a; /* t = (z+c)^2*(P/Q) */
259 r.a = t.a + a0_hi, TRUNC(r.a);
260 r.b = ((a0_hi-r.a) + t.a) + t.b;
261 return (r); /* r = a0 + t */
268 struct Double lg, lsine;
272 if (y == x) /* Negative integer. */
273 return ((x - x) / zero);
283 z = cos(M_PI*(0.5-z));
284 /* Special case: G(1-x) = Inf; G(x) may be nonzero. */
287 return ((double)sgn*tiny*tiny);
288 y = one - x; /* exact: 128 < |x| < 255 */
290 lsine = __log__D(M_PI/z); /* = TRUNC(log(u)) + small */
291 lg.a -= lsine.a; /* exact (opposite signs) */
294 z = (y + lg.a) + lg.b;
302 else /* 1-x is inexact */
305 return (M_PI / (y*z));