1 /* $NetBSD: n_lgamma.c,v 1.2 1997/10/20 14:12:59 ragge Exp $ */ 2 /*- 3 * Copyright (c) 1992, 1993 4 * The Regents of the University of California. All rights reserved. 5 * 6 * Redistribution and use in source and binary forms, with or without 7 * modification, are permitted provided that the following conditions 8 * are met: 9 * 1. Redistributions of source code must retain the above copyright 10 * notice, this list of conditions and the following disclaimer. 11 * 2. Redistributions in binary form must reproduce the above copyright 12 * notice, this list of conditions and the following disclaimer in the 13 * documentation and/or other materials provided with the distribution. 14 * 3. All advertising materials mentioning features or use of this software 15 * must display the following acknowledgement: 16 * This product includes software developed by the University of 17 * California, Berkeley and its contributors. 18 * 4. Neither the name of the University nor the names of its contributors 19 * may be used to endorse or promote products derived from this software 20 * without specific prior written permission. 21 * 22 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32 * SUCH DAMAGE. 33 */ 34 35 #ifndef lint 36 #if 0 37 static char sccsid[] = "@(#)lgamma.c 8.2 (Berkeley) 11/30/93"; 38 #endif 39 #endif /* not lint */ 40 41 /* 42 * Coded by Peter McIlroy, Nov 1992; 43 * 44 * The financial support of UUNET Communications Services is greatfully 45 * acknowledged. 46 */ 47 48 #include <math.h> 49 #include <errno.h> 50 51 #include "mathimpl.h" 52 53 /* Log gamma function. 54 * Error: x > 0 error < 1.3ulp. 55 * x > 4, error < 1ulp. 56 * x > 9, error < .6ulp. 57 * x < 0, all bets are off. (When G(x) ~ 1, log(G(x)) ~ 0) 58 * Method: 59 * x > 6: 60 * Use the asymptotic expansion (Stirling's Formula) 61 * 0 < x < 6: 62 * Use gamma(x+1) = x*gamma(x) for argument reduction. 63 * Use rational approximation in 64 * the range 1.2, 2.5 65 * Two approximations are used, one centered at the 66 * minimum to ensure monotonicity; one centered at 2 67 * to maintain small relative error. 68 * x < 0: 69 * Use the reflection formula, 70 * G(1-x)G(x) = PI/sin(PI*x) 71 * Special values: 72 * non-positive integer returns +Inf. 73 * NaN returns NaN 74 */ 75 static int endian; 76 #if defined(vax) || defined(tahoe) 77 #define _IEEE 0 78 /* double and float have same size exponent field */ 79 #define TRUNC(x) x = (double) (float) (x) 80 #else 81 #define _IEEE 1 82 #define TRUNC(x) *(((int *) &x) + endian) &= 0xf8000000 83 #define infnan(x) 0.0 84 #endif 85 86 static double small_lgam(double); 87 static double large_lgam(double); 88 static double neg_lgam(double); 89 static double one = 1.0; 90 int signgam; 91 92 #define UNDERFL (1e-1020 * 1e-1020) 93 94 #define LEFT (1.0 - (x0 + .25)) 95 #define RIGHT (x0 - .218) 96 /* 97 * Constants for approximation in [1.244,1.712] 98 */ 99 #define x0 0.461632144968362356785 100 #define x0_lo -.000000000000000015522348162858676890521 101 #define a0_hi -0.12148629128932952880859 102 #define a0_lo .0000000007534799204229502 103 #define r0 -2.771227512955130520e-002 104 #define r1 -2.980729795228150847e-001 105 #define r2 -3.257411333183093394e-001 106 #define r3 -1.126814387531706041e-001 107 #define r4 -1.129130057170225562e-002 108 #define r5 -2.259650588213369095e-005 109 #define s0 1.714457160001714442e+000 110 #define s1 2.786469504618194648e+000 111 #define s2 1.564546365519179805e+000 112 #define s3 3.485846389981109850e-001 113 #define s4 2.467759345363656348e-002 114 /* 115 * Constants for approximation in [1.71, 2.5] 116 */ 117 #define a1_hi 4.227843350984671344505727574870e-01 118 #define a1_lo 4.670126436531227189e-18 119 #define p0 3.224670334241133695662995251041e-01 120 #define p1 3.569659696950364669021382724168e-01 121 #define p2 1.342918716072560025853732668111e-01 122 #define p3 1.950702176409779831089963408886e-02 123 #define p4 8.546740251667538090796227834289e-04 124 #define q0 1.000000000000000444089209850062e+00 125 #define q1 1.315850076960161985084596381057e+00 126 #define q2 6.274644311862156431658377186977e-01 127 #define q3 1.304706631926259297049597307705e-01 128 #define q4 1.102815279606722369265536798366e-02 129 #define q5 2.512690594856678929537585620579e-04 130 #define q6 -1.003597548112371003358107325598e-06 131 /* 132 * Stirling's Formula, adjusted for equal-ripple. x in [6,Inf]. 133 */ 134 #define lns2pi .418938533204672741780329736405 135 #define pb0 8.33333333333333148296162562474e-02 136 #define pb1 -2.77777777774548123579378966497e-03 137 #define pb2 7.93650778754435631476282786423e-04 138 #define pb3 -5.95235082566672847950717262222e-04 139 #define pb4 8.41428560346653702135821806252e-04 140 #define pb5 -1.89773526463879200348872089421e-03 141 #define pb6 5.69394463439411649408050664078e-03 142 #define pb7 -1.44705562421428915453880392761e-02 143 144 __pure double 145 lgamma(double x) 146 { 147 double r; 148 149 signgam = 1; 150 endian = ((*(int *) &one)) ? 1 : 0; 151 152 if (!finite(x)) 153 if (_IEEE) 154 return (x+x); 155 else return (infnan(EDOM)); 156 157 if (x > 6 + RIGHT) { 158 r = large_lgam(x); 159 return (r); 160 } else if (x > 1e-16) 161 return (small_lgam(x)); 162 else if (x > -1e-16) { 163 if (x < 0) 164 signgam = -1, x = -x; 165 return (-log(x)); 166 } else 167 return (neg_lgam(x)); 168 } 169 170 static double 171 large_lgam(double x) 172 { 173 double z, p, x1; 174 struct Double t, u, v; 175 u = __log__D(x); 176 u.a -= 1.0; 177 if (x > 1e15) { 178 v.a = x - 0.5; 179 TRUNC(v.a); 180 v.b = (x - v.a) - 0.5; 181 t.a = u.a*v.a; 182 t.b = x*u.b + v.b*u.a; 183 if (_IEEE == 0 && !finite(t.a)) 184 return(infnan(ERANGE)); 185 return(t.a + t.b); 186 } 187 x1 = 1./x; 188 z = x1*x1; 189 p = pb0+z*(pb1+z*(pb2+z*(pb3+z*(pb4+z*(pb5+z*(pb6+z*pb7)))))); 190 /* error in approximation = 2.8e-19 */ 191 192 p = p*x1; /* error < 2.3e-18 absolute */ 193 /* 0 < p < 1/64 (at x = 5.5) */ 194 v.a = x = x - 0.5; 195 TRUNC(v.a); /* truncate v.a to 26 bits. */ 196 v.b = x - v.a; 197 t.a = v.a*u.a; /* t = (x-.5)*(log(x)-1) */ 198 t.b = v.b*u.a + x*u.b; 199 t.b += p; t.b += lns2pi; /* return t + lns2pi + p */ 200 return (t.a + t.b); 201 } 202 203 static double 204 small_lgam(double x) 205 { 206 int x_int; 207 double y, z, t, r = 0, p, q, hi, lo; 208 struct Double rr; 209 x_int = (x + .5); 210 y = x - x_int; 211 if (x_int <= 2 && y > RIGHT) { 212 t = y - x0; 213 y--; x_int++; 214 goto CONTINUE; 215 } else if (y < -LEFT) { 216 t = y +(1.0-x0); 217 CONTINUE: 218 z = t - x0_lo; 219 p = r0+z*(r1+z*(r2+z*(r3+z*(r4+z*r5)))); 220 q = s0+z*(s1+z*(s2+z*(s3+z*s4))); 221 r = t*(z*(p/q) - x0_lo); 222 t = .5*t*t; 223 z = 1.0; 224 switch (x_int) { 225 case 6: z = (y + 5); 226 case 5: z *= (y + 4); 227 case 4: z *= (y + 3); 228 case 3: z *= (y + 2); 229 rr = __log__D(z); 230 rr.b += a0_lo; rr.a += a0_hi; 231 return(((r+rr.b)+t+rr.a)); 232 case 2: return(((r+a0_lo)+t)+a0_hi); 233 case 0: r -= log1p(x); 234 default: rr = __log__D(x); 235 rr.a -= a0_hi; rr.b -= a0_lo; 236 return(((r - rr.b) + t) - rr.a); 237 } 238 } else { 239 p = p0+y*(p1+y*(p2+y*(p3+y*p4))); 240 q = q0+y*(q1+y*(q2+y*(q3+y*(q4+y*(q5+y*q6))))); 241 p = p*(y/q); 242 t = (double)(float) y; 243 z = y-t; 244 hi = (double)(float) (p+a1_hi); 245 lo = a1_hi - hi; lo += p; lo += a1_lo; 246 r = lo*y + z*hi; /* q + r = y*(a0+p/q) */ 247 q = hi*t; 248 z = 1.0; 249 switch (x_int) { 250 case 6: z = (y + 5); 251 case 5: z *= (y + 4); 252 case 4: z *= (y + 3); 253 case 3: z *= (y + 2); 254 rr = __log__D(z); 255 r += rr.b; r += q; 256 return(rr.a + r); 257 case 2: return (q+ r); 258 case 0: rr = __log__D(x); 259 r -= rr.b; r -= log1p(x); 260 r += q; r-= rr.a; 261 return(r); 262 default: rr = __log__D(x); 263 r -= rr.b; 264 q -= rr.a; 265 return (r+q); 266 } 267 } 268 } 269 270 static double 271 neg_lgam(double x) 272 { 273 int xi; 274 double y, z, one = 1.0, zero = 0.0; 275 276 /* avoid destructive cancellation as much as possible */ 277 if (x > -170) { 278 xi = x; 279 if (xi == x) 280 if (_IEEE) 281 return(one/zero); 282 else 283 return(infnan(ERANGE)); 284 y = gamma(x); 285 if (y < 0) 286 y = -y, signgam = -1; 287 return (log(y)); 288 } 289 z = floor(x + .5); 290 if (z == x) { /* convention: G(-(integer)) -> +Inf */ 291 if (_IEEE) 292 return (one/zero); 293 else 294 return (infnan(ERANGE)); 295 } 296 y = .5*ceil(x); 297 if (y == ceil(y)) 298 signgam = -1; 299 x = -x; 300 z = fabs(x + z); /* 0 < z <= .5 */ 301 if (z < .25) 302 z = sin(M_PI*z); 303 else 304 z = cos(M_PI*(0.5-z)); 305 z = log(M_PI/(z*x)); 306 y = large_lgam(x); 307 return (z - y); 308 } 309