1 files changed, 82 insertions, 0 deletions
diff --git a/missing/tgamma.c b/missing/tgamma.c
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+/* tgamma.c  - public domain implementation of function tgamma(3m)
+
+reference - Haruhiko Okumura: C-gengo niyoru saishin algorithm jiten
+            (New Algorithm handbook in C language) (Gijyutsu hyouron
+            sha, Tokyo, 1991) [in Japanese]
+            http://oku.edu.mie-u.ac.jp/~okumura/algo/
+*/
+
+/***********************************************************
+    gamma.c -- Gamma function
+***********************************************************/
+#include "ruby/internal/config.h"
+#include "ruby/missing.h"
+#include <math.h>
+#include <errno.h>
+
+#ifndef HAVE_LGAMMA_R
+
+#include <errno.h>
+#define PI      3.14159265358979324  /* $\pi$ */
+#define LOG_2PI 1.83787706640934548  /* $\log 2\pi$ */
+#define N       8
+
+#define B0  1                 /* Bernoulli numbers */
+#define B1  (-1.0 / 2.0)
+#define B2  ( 1.0 / 6.0)
+#define B4  (-1.0 / 30.0)
+#define B6  ( 1.0 / 42.0)
+#define B8  (-1.0 / 30.0)
+#define B10 ( 5.0 / 66.0)
+#define B12 (-691.0 / 2730.0)
+#define B14 ( 7.0 / 6.0)
+#define B16 (-3617.0 / 510.0)
+
+static double
+loggamma(double x)  /* the natural logarithm of the Gamma function. */
+{
+    double v, w;
+
+    v = 1;
+    while (x < N) {  v *= x;  x++;  }
+    w = 1 / (x * x);
+    return ((((((((B16 / (16 * 15))  * w + (B14 / (14 * 13))) * w
+                + (B12 / (12 * 11))) * w + (B10 / (10 *  9))) * w
+                + (B8  / ( 8 *  7))) * w + (B6  / ( 6 *  5))) * w
+                + (B4  / ( 4 *  3))) * w + (B2  / ( 2 *  1))) / x
+                + 0.5 * LOG_2PI - log(v) - x + (x - 0.5) * log(x);
+}
+#endif
+
+double tgamma(double x)  /* Gamma function */
+{
+    int sign;
+    if (x == 0.0) { /* Pole Error */
+        errno = ERANGE;
+        return 1/x < 0 ? -HUGE_VAL : HUGE_VAL;
+    }
+    if (isinf(x)) {
+        if (x < 0) goto domain_error;
+        return x;
+    }
+    if (x < 0) {
+        static double zero = 0.0;
+        double i, f;
+        f = modf(-x, &i);
+        if (f == 0.0) { /* Domain Error */
+          domain_error:
+            errno = EDOM;
+            return zero/zero;
+        }
+#ifndef HAVE_LGAMMA_R
+        sign = (fmod(i, 2.0) != 0.0) ? 1 : -1;
+        return sign * PI / (sin(PI * f) * exp(loggamma(1 - x)));
+#endif
+    }
+#ifndef HAVE_LGAMMA_R
+    return exp(loggamma(x));
+#else
+    x = lgamma_r(x, &sign);
+    return sign * exp(x);
+#endif
+}