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e_lgamma_r.c
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1 
2 /* @(#)e_lgamma_r.c 1.3 95/01/18 */
3 /*
4  * ====================================================
5  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
6  *
7  * Developed at SunSoft, a Sun Microsystems, Inc. business.
8  * Permission to use, copy, modify, and distribute this
9  * software is freely granted, provided that this notice
10  * is preserved.
11  * ====================================================
12  *
13  */
14 
15 /* __ieee754_lgamma_r(x, signgamp)
16  * Reentrant version of the logarithm of the Gamma function
17  * with user provide pointer for the sign of Gamma(x).
18  *
19  * Method:
20  * 1. Argument Reduction for 0 < x <= 8
21  * Since gamma(1+s)=s*gamma(s), for x in [0,8], we may
22  * reduce x to a number in [1.5,2.5] by
23  * lgamma(1+s) = log(s) + lgamma(s)
24  * for example,
25  * lgamma(7.3) = log(6.3) + lgamma(6.3)
26  * = log(6.3*5.3) + lgamma(5.3)
27  * = log(6.3*5.3*4.3*3.3*2.3) + lgamma(2.3)
28  * 2. Polynomial approximation of lgamma around its
29  * minimun ymin=1.461632144968362245 to maintain monotonicity.
30  * On [ymin-0.23, ymin+0.27] (i.e., [1.23164,1.73163]), use
31  * Let z = x-ymin;
32  * lgamma(x) = -1.214862905358496078218 + z^2*poly(z)
33  * where
34  * poly(z) is a 14 degree polynomial.
35  * 2. Rational approximation in the primary interval [2,3]
36  * We use the following approximation:
37  * s = x-2.0;
38  * lgamma(x) = 0.5*s + s*P(s)/Q(s)
39  * with accuracy
40  * |P/Q - (lgamma(x)-0.5s)| < 2**-61.71
41  * Our algorithms are based on the following observation
42  *
43  * zeta(2)-1 2 zeta(3)-1 3
44  * lgamma(2+s) = s*(1-Euler) + --------- * s - --------- * s + ...
45  * 2 3
46  *
47  * where Euler = 0.5771... is the Euler constant, which is very
48  * close to 0.5.
49  *
50  * 3. For x>=8, we have
51  * lgamma(x)~(x-0.5)log(x)-x+0.5*log(2pi)+1/(12x)-1/(360x**3)+....
52  * (better formula:
53  * lgamma(x)~(x-0.5)*(log(x)-1)-.5*(log(2pi)-1) + ...)
54  * Let z = 1/x, then we approximation
55  * f(z) = lgamma(x) - (x-0.5)(log(x)-1)
56  * by
57  * 3 5 11
58  * w = w0 + w1*z + w2*z + w3*z + ... + w6*z
59  * where
60  * |w - f(z)| < 2**-58.74
61  *
62  * 4. For negative x, since (G is gamma function)
63  * -x*G(-x)*G(x) = pi/sin(pi*x),
64  * we have
65  * G(x) = pi/(sin(pi*x)*(-x)*G(-x))
66  * since G(-x) is positive, sign(G(x)) = sign(sin(pi*x)) for x<0
67  * Hence, for x<0, signgam = sign(sin(pi*x)) and
68  * lgamma(x) = log(|Gamma(x)|)
69  * = log(pi/(|x*sin(pi*x)|)) - lgamma(-x);
70  * Note: one should avoid compute pi*(-x) directly in the
71  * computation of sin(pi*(-x)).
72  *
73  * 5. Special Cases
74  * lgamma(2+s) ~ s*(1-Euler) for tiny s
75  * lgamma(1)=lgamma(2)=0
76  * lgamma(x) ~ -log(x) for tiny x
77  * lgamma(0) = lgamma(inf) = inf
78  * lgamma(-integer) = +-inf
79  *
80  */
81 
82 #include "fdlibm.h"
83 
84 #ifdef __STDC__
85 static const double
86 #else
87 static double
88 #endif
89 two52= 4.50359962737049600000e+15, /* 0x43300000, 0x00000000 */
90 half= 5.00000000000000000000e-01, /* 0x3FE00000, 0x00000000 */
91 one = 1.00000000000000000000e+00, /* 0x3FF00000, 0x00000000 */
92 pi = 3.14159265358979311600e+00, /* 0x400921FB, 0x54442D18 */
93 a0 = 7.72156649015328655494e-02, /* 0x3FB3C467, 0xE37DB0C8 */
94 a1 = 3.22467033424113591611e-01, /* 0x3FD4A34C, 0xC4A60FAD */
95 a2 = 6.73523010531292681824e-02, /* 0x3FB13E00, 0x1A5562A7 */
96 a3 = 2.05808084325167332806e-02, /* 0x3F951322, 0xAC92547B */
97 a4 = 7.38555086081402883957e-03, /* 0x3F7E404F, 0xB68FEFE8 */
98 a5 = 2.89051383673415629091e-03, /* 0x3F67ADD8, 0xCCB7926B */
99 a6 = 1.19270763183362067845e-03, /* 0x3F538A94, 0x116F3F5D */
100 a7 = 5.10069792153511336608e-04, /* 0x3F40B6C6, 0x89B99C00 */
101 a8 = 2.20862790713908385557e-04, /* 0x3F2CF2EC, 0xED10E54D */
102 a9 = 1.08011567247583939954e-04, /* 0x3F1C5088, 0x987DFB07 */
103 a10 = 2.52144565451257326939e-05, /* 0x3EFA7074, 0x428CFA52 */
104 a11 = 4.48640949618915160150e-05, /* 0x3F07858E, 0x90A45837 */
105 tc = 1.46163214496836224576e+00, /* 0x3FF762D8, 0x6356BE3F */
106 tf = -1.21486290535849611461e-01, /* 0xBFBF19B9, 0xBCC38A42 */
107 /* tt = -(tail of tf) */
108 tt = -3.63867699703950536541e-18, /* 0xBC50C7CA, 0xA48A971F */
109 t0 = 4.83836122723810047042e-01, /* 0x3FDEF72B, 0xC8EE38A2 */
110 t1 = -1.47587722994593911752e-01, /* 0xBFC2E427, 0x8DC6C509 */
111 t2 = 6.46249402391333854778e-02, /* 0x3FB08B42, 0x94D5419B */
112 t3 = -3.27885410759859649565e-02, /* 0xBFA0C9A8, 0xDF35B713 */
113 t4 = 1.79706750811820387126e-02, /* 0x3F9266E7, 0x970AF9EC */
114 t5 = -1.03142241298341437450e-02, /* 0xBF851F9F, 0xBA91EC6A */
115 t6 = 6.10053870246291332635e-03, /* 0x3F78FCE0, 0xE370E344 */
116 t7 = -3.68452016781138256760e-03, /* 0xBF6E2EFF, 0xB3E914D7 */
117 t8 = 2.25964780900612472250e-03, /* 0x3F6282D3, 0x2E15C915 */
118 t9 = -1.40346469989232843813e-03, /* 0xBF56FE8E, 0xBF2D1AF1 */
119 t10 = 8.81081882437654011382e-04, /* 0x3F4CDF0C, 0xEF61A8E9 */
120 t11 = -5.38595305356740546715e-04, /* 0xBF41A610, 0x9C73E0EC */
121 t12 = 3.15632070903625950361e-04, /* 0x3F34AF6D, 0x6C0EBBF7 */
122 t13 = -3.12754168375120860518e-04, /* 0xBF347F24, 0xECC38C38 */
123 t14 = 3.35529192635519073543e-04, /* 0x3F35FD3E, 0xE8C2D3F4 */
124 u0 = -7.72156649015328655494e-02, /* 0xBFB3C467, 0xE37DB0C8 */
125 u1 = 6.32827064025093366517e-01, /* 0x3FE4401E, 0x8B005DFF */
126 u2 = 1.45492250137234768737e+00, /* 0x3FF7475C, 0xD119BD6F */
127 u3 = 9.77717527963372745603e-01, /* 0x3FEF4976, 0x44EA8450 */
128 u4 = 2.28963728064692451092e-01, /* 0x3FCD4EAE, 0xF6010924 */
129 u5 = 1.33810918536787660377e-02, /* 0x3F8B678B, 0xBF2BAB09 */
130 v1 = 2.45597793713041134822e+00, /* 0x4003A5D7, 0xC2BD619C */
131 v2 = 2.12848976379893395361e+00, /* 0x40010725, 0xA42B18F5 */
132 v3 = 7.69285150456672783825e-01, /* 0x3FE89DFB, 0xE45050AF */
133 v4 = 1.04222645593369134254e-01, /* 0x3FBAAE55, 0xD6537C88 */
134 v5 = 3.21709242282423911810e-03, /* 0x3F6A5ABB, 0x57D0CF61 */
135 s0 = -7.72156649015328655494e-02, /* 0xBFB3C467, 0xE37DB0C8 */
136 s1 = 2.14982415960608852501e-01, /* 0x3FCB848B, 0x36E20878 */
137 s2 = 3.25778796408930981787e-01, /* 0x3FD4D98F, 0x4F139F59 */
138 s3 = 1.46350472652464452805e-01, /* 0x3FC2BB9C, 0xBEE5F2F7 */
139 s4 = 2.66422703033638609560e-02, /* 0x3F9B481C, 0x7E939961 */
140 s5 = 1.84028451407337715652e-03, /* 0x3F5E26B6, 0x7368F239 */
141 s6 = 3.19475326584100867617e-05, /* 0x3F00BFEC, 0xDD17E945 */
142 r1 = 1.39200533467621045958e+00, /* 0x3FF645A7, 0x62C4AB74 */
143 r2 = 7.21935547567138069525e-01, /* 0x3FE71A18, 0x93D3DCDC */
144 r3 = 1.71933865632803078993e-01, /* 0x3FC601ED, 0xCCFBDF27 */
145 r4 = 1.86459191715652901344e-02, /* 0x3F9317EA, 0x742ED475 */
146 r5 = 7.77942496381893596434e-04, /* 0x3F497DDA, 0xCA41A95B */
147 r6 = 7.32668430744625636189e-06, /* 0x3EDEBAF7, 0xA5B38140 */
148 w0 = 4.18938533204672725052e-01, /* 0x3FDACFE3, 0x90C97D69 */
149 w1 = 8.33333333333329678849e-02, /* 0x3FB55555, 0x5555553B */
150 w2 = -2.77777777728775536470e-03, /* 0xBF66C16C, 0x16B02E5C */
151 w3 = 7.93650558643019558500e-04, /* 0x3F4A019F, 0x98CF38B6 */
152 w4 = -5.95187557450339963135e-04, /* 0xBF4380CB, 0x8C0FE741 */
153 w5 = 8.36339918996282139126e-04, /* 0x3F4B67BA, 0x4CDAD5D1 */
154 w6 = -1.63092934096575273989e-03; /* 0xBF5AB89D, 0x0B9E43E4 */
155 
156 static double zero= 0.00000000000000000000e+00;
157 
158 #ifdef __STDC__
159  static double sin_pi(double x)
160 #else
161  static double sin_pi(x)
162  double x;
163 #endif
164 {
165  double y,z;
166  int n,ix;
167 
168  ix = 0x7fffffff&__HI(x);
169 
170  if(ix<0x3fd00000) return __kernel_sin(pi*x,zero,0);
171  y = -x; /* x is assume negative */
172 
173  /*
174  * argument reduction, make sure inexact flag not raised if input
175  * is an integer
176  */
177  z = floor(y);
178  if(z!=y) { /* inexact anyway */
179  y *= 0.5;
180  y = 2.0*(y - floor(y)); /* y = |x| mod 2.0 */
181  n = (int) (y*4.0);
182  } else {
183  if(ix>=0x43400000) {
184  y = zero; n = 0; /* y must be even */
185  } else {
186  if(ix<0x43300000) z = y+two52; /* exact */
187  n = __LO(z)&1; /* lower word of z */
188  y = n;
189  n<<= 2;
190  }
191  }
192  switch (n) {
193  case 0: y = __kernel_sin(pi*y,zero,0); break;
194  case 1:
195  case 2: y = __kernel_cos(pi*(0.5-y),zero); break;
196  case 3:
197  case 4: y = __kernel_sin(pi*(one-y),zero,0); break;
198  case 5:
199  case 6: y = -__kernel_cos(pi*(y-1.5),zero); break;
200  default: y = __kernel_sin(pi*(y-2.0),zero,0); break;
201  }
202  return -y;
203 }
204 
205 
206 #ifdef __STDC__
207  double __ieee754_lgamma_r(double x, int *signgamp)
208 #else
209  double __ieee754_lgamma_r(x,signgamp)
210  double x; int *signgamp;
211 #endif
212 {
213  double t,y,z,nadj=0,p,p1,p2,p3,q,r,w;
214  int i,hx,lx,ix;
215 
216  hx = __HI(x);
217  lx = __LO(x);
218 
219  /* purge off +-inf, NaN, +-0, and negative arguments */
220  *signgamp = 1;
221  ix = hx&0x7fffffff;
222  if(ix>=0x7ff00000) return x*x;
223  if((ix|lx)==0) return one/zero;
224  if(ix<0x3b900000) { /* |x|<2**-70, return -log(|x|) */
225  if(hx<0) {
226  *signgamp = -1;
227  return -__ieee754_log(-x);
228  } else return -__ieee754_log(x);
229  }
230  if(hx<0) {
231  if(ix>=0x43300000) /* |x|>=2**52, must be -integer */
232  return one/zero;
233  t = sin_pi(x);
234  if(t==zero) return one/zero; /* -integer */
235  nadj = __ieee754_log(pi/fabs(t*x));
236  if(t<zero) *signgamp = -1;
237  x = -x;
238  }
239 
240  /* purge off 1 and 2 */
241  if((((ix-0x3ff00000)|lx)==0)||(((ix-0x40000000)|lx)==0)) r = 0;
242  /* for x < 2.0 */
243  else if(ix<0x40000000) {
244  if(ix<=0x3feccccc) { /* lgamma(x) = lgamma(x+1)-log(x) */
245  r = -__ieee754_log(x);
246  if(ix>=0x3FE76944) {y = one-x; i= 0;}
247  else if(ix>=0x3FCDA661) {y= x-(tc-one); i=1;}
248  else {y = x; i=2;}
249  } else {
250  r = zero;
251  if(ix>=0x3FFBB4C3) {y=2.0-x;i=0;} /* [1.7316,2] */
252  else if(ix>=0x3FF3B4C4) {y=x-tc;i=1;} /* [1.23,1.73] */
253  else {y=x-one;i=2;}
254  }
255  switch(i) {
256  case 0:
257  z = y*y;
258  p1 = a0+z*(a2+z*(a4+z*(a6+z*(a8+z*a10))));
259  p2 = z*(a1+z*(a3+z*(a5+z*(a7+z*(a9+z*a11)))));
260  p = y*p1+p2;
261  r += (p-0.5*y); break;
262  case 1:
263  z = y*y;
264  w = z*y;
265  p1 = t0+w*(t3+w*(t6+w*(t9 +w*t12))); /* parallel comp */
266  p2 = t1+w*(t4+w*(t7+w*(t10+w*t13)));
267  p3 = t2+w*(t5+w*(t8+w*(t11+w*t14)));
268  p = z*p1-(tt-w*(p2+y*p3));
269  r += (tf + p); break;
270  case 2:
271  p1 = y*(u0+y*(u1+y*(u2+y*(u3+y*(u4+y*u5)))));
272  p2 = one+y*(v1+y*(v2+y*(v3+y*(v4+y*v5))));
273  r += (-0.5*y + p1/p2);
274  }
275  }
276  else if(ix<0x40200000) { /* x < 8.0 */
277  i = (int)x;
278  t = zero;
279  y = x-(double)i;
280  p = y*(s0+y*(s1+y*(s2+y*(s3+y*(s4+y*(s5+y*s6))))));
281  q = one+y*(r1+y*(r2+y*(r3+y*(r4+y*(r5+y*r6)))));
282  r = half*y+p/q;
283  z = one; /* lgamma(1+s) = log(s) + lgamma(s) */
284  switch(i) {
285  case 7: z *= (y+6.0); /* FALLTHRU */
286  case 6: z *= (y+5.0); /* FALLTHRU */
287  case 5: z *= (y+4.0); /* FALLTHRU */
288  case 4: z *= (y+3.0); /* FALLTHRU */
289  case 3: z *= (y+2.0); /* FALLTHRU */
290  r += __ieee754_log(z); break;
291  }
292  /* 8.0 <= x < 2**58 */
293  } else if (ix < 0x43900000) {
294  t = __ieee754_log(x);
295  z = one/x;
296  y = z*z;
297  w = w0+z*(w1+y*(w2+y*(w3+y*(w4+y*(w5+y*w6)))));
298  r = (x-half)*(t-one)+w;
299  } else
300  /* 2**58 <= x <= inf */
301  r = x*(__ieee754_log(x)-one);
302  if(hx<0) r = nadj - r;
303  return r;
304 }