2 * Copyright (c) 1992, 1993
3 * The Regents of the University of California. All rights reserved.
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15 * This product includes software developed by the University of
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21 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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23 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
24 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
25 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
26 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
27 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
28 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
29 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
30 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
33 * @(#)lgamma.c 8.2 (Berkeley) 11/30/93
37 * Coded by Peter McIlroy, Nov 1992;
39 * The financial support of UUNET Communications Services is greatfully
48 /* Log gamma function.
49 * Error: x > 0 error < 1.3ulp.
50 * x > 4, error < 1ulp.
51 * x > 9, error < .6ulp.
52 * x < 0, all bets are off. (When G(x) ~ 1, log(G(x)) ~ 0)
55 * Use the asymptotic expansion (Stirling's Formula)
57 * Use gamma(x+1) = x*gamma(x) for argument reduction.
58 * Use rational approximation in
60 * Two approximations are used, one centered at the
61 * minimum to ensure monotonicity; one centered at 2
62 * to maintain small relative error.
64 * Use the reflection formula,
65 * G(1-x)G(x) = PI/sin(PI*x)
67 * non-positive integer returns +Inf.
71 #if defined(vax) || defined(tahoe)
73 /* double and float have same size exponent field */
74 #define TRUNC(x) x = (double) (float) (x)
77 #define TRUNC(x) *(((int *) &x) + endian) &= 0xf8000000
81 static double small_lgam(double);
82 static double large_lgam(double);
83 static double neg_lgam(double);
84 static double zero = 0.0, one = 1.0;
87 #define UNDERFL (1e-1020 * 1e-1020)
89 #define LEFT (1.0 - (x0 + .25))
90 #define RIGHT (x0 - .218)
92 * Constants for approximation in [1.244,1.712]
94 #define x0 0.461632144968362356785
95 #define x0_lo -.000000000000000015522348162858676890521
96 #define a0_hi -0.12148629128932952880859
97 #define a0_lo .0000000007534799204229502
98 #define r0 -2.771227512955130520e-002
99 #define r1 -2.980729795228150847e-001
100 #define r2 -3.257411333183093394e-001
101 #define r3 -1.126814387531706041e-001
102 #define r4 -1.129130057170225562e-002
103 #define r5 -2.259650588213369095e-005
104 #define s0 1.714457160001714442e+000
105 #define s1 2.786469504618194648e+000
106 #define s2 1.564546365519179805e+000
107 #define s3 3.485846389981109850e-001
108 #define s4 2.467759345363656348e-002
110 * Constants for approximation in [1.71, 2.5]
112 #define a1_hi 4.227843350984671344505727574870e-01
113 #define a1_lo 4.670126436531227189e-18
114 #define p0 3.224670334241133695662995251041e-01
115 #define p1 3.569659696950364669021382724168e-01
116 #define p2 1.342918716072560025853732668111e-01
117 #define p3 1.950702176409779831089963408886e-02
118 #define p4 8.546740251667538090796227834289e-04
119 #define q0 1.000000000000000444089209850062e+00
120 #define q1 1.315850076960161985084596381057e+00
121 #define q2 6.274644311862156431658377186977e-01
122 #define q3 1.304706631926259297049597307705e-01
123 #define q4 1.102815279606722369265536798366e-02
124 #define q5 2.512690594856678929537585620579e-04
125 #define q6 -1.003597548112371003358107325598e-06
127 * Stirling's Formula, adjusted for equal-ripple. x in [6,Inf].
129 #define lns2pi .418938533204672741780329736405
130 #define pb0 8.33333333333333148296162562474e-02
131 #define pb1 -2.77777777774548123579378966497e-03
132 #define pb2 7.93650778754435631476282786423e-04
133 #define pb3 -5.95235082566672847950717262222e-04
134 #define pb4 8.41428560346653702135821806252e-04
135 #define pb5 -1.89773526463879200348872089421e-03
136 #define pb6 5.69394463439411649408050664078e-03
137 #define pb7 -1.44705562421428915453880392761e-02
145 endian = ((*(int *) &one)) ? 1 : 0;
150 else return (infnan(EDOM));
155 } else if (x > 1e-16)
156 return (small_lgam(x));
157 else if (x > -1e-16) {
159 signgam = -1, x = -x;
162 return (neg_lgam(x));
170 struct Double t, u, v;
176 v.b = (x - v.a) - 0.5;
178 t.b = x*u.b + v.b*u.a;
179 if (_IEEE == 0 && !finite(t.a))
180 return(infnan(ERANGE));
185 p = pb0+z*(pb1+z*(pb2+z*(pb3+z*(pb4+z*(pb5+z*(pb6+z*pb7))))));
186 /* error in approximation = 2.8e-19 */
188 p = p*x1; /* error < 2.3e-18 absolute */
189 /* 0 < p < 1/64 (at x = 5.5) */
191 TRUNC(v.a); /* truncate v.a to 26 bits. */
193 t.a = v.a*u.a; /* t = (x-.5)*(log(x)-1) */
194 t.b = v.b*u.a + x*u.b;
195 t.b += p; t.b += lns2pi; /* return t + lns2pi + p */
203 double y, z, t, r = 0, p, q, hi, lo;
207 if (x_int <= 2 && y > RIGHT) {
211 } else if (y < -LEFT) {
215 p = r0+z*(r1+z*(r2+z*(r3+z*(r4+z*r5))));
216 q = s0+z*(s1+z*(s2+z*(s3+z*s4)));
217 r = t*(z*(p/q) - x0_lo);
222 case 5: z *= (y + 4);
223 case 4: z *= (y + 3);
224 case 3: z *= (y + 2);
226 rr.b += a0_lo; rr.a += a0_hi;
227 return(((r+rr.b)+t+rr.a));
228 case 2: return(((r+a0_lo)+t)+a0_hi);
229 case 0: r -= log1p(x);
230 default: rr = __log__D(x);
231 rr.a -= a0_hi; rr.b -= a0_lo;
232 return(((r - rr.b) + t) - rr.a);
235 p = p0+y*(p1+y*(p2+y*(p3+y*p4)));
236 q = q0+y*(q1+y*(q2+y*(q3+y*(q4+y*(q5+y*q6)))));
238 t = (double)(float) y;
240 hi = (double)(float) (p+a1_hi);
241 lo = a1_hi - hi; lo += p; lo += a1_lo;
242 r = lo*y + z*hi; /* q + r = y*(a0+p/q) */
247 case 5: z *= (y + 4);
248 case 4: z *= (y + 3);
249 case 3: z *= (y + 2);
253 case 2: return (q+ r);
254 case 0: rr = __log__D(x);
255 r -= rr.b; r -= log1p(x);
258 default: rr = __log__D(x);
270 double y, z, one = 1.0, zero = 0.0;
271 extern double gamma();
273 /* avoid destructive cancellation as much as possible */
280 return(infnan(ERANGE));
283 y = -y, signgam = -1;
287 if (z == x) { /* convention: G(-(integer)) -> +Inf */
291 return (infnan(ERANGE));
297 z = fabs(x + z); /* 0 < z <= .5 */
301 z = cos(M_PI*(0.5-z));