WSJT-X/lib/sfrsd2/sfrsd_paper/binomial_subs.c
Joe Taylor 036d408908 Make binomial_subs.c OK on 32-bit machines.
git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@6217 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
2015-12-02 16:13:44 +00:00

56 lines
1.4 KiB
C

#include <stdio.h>
#include <limits.h>
/* Original code copied from
http://rosettacode.org/wiki/Evaluate_binomial_coefficients
*/
/* We go to some effort to handle overflow situations */
static unsigned long long gcd_ui(unsigned long long x, unsigned long long y) {
unsigned long long t;
if (y < x) { t = x; x = y; y = t; }
while (y > 0) {
t = y; y = x % y; x = t; /* y1 <- x0 % y0 ; x1 <- y0 */
}
return x;
}
unsigned long long binomial(unsigned long long n, unsigned long long k) {
unsigned long long d, g, r = 1;
if (k == 0) return 1;
if (k == 1) return n;
if (k >= n) return (k == n);
if (k > n/2) k = n-k;
for (d = 1; d <= k; d++) {
if (r >= ULONG_LONG_MAX/n) { /* Possible overflow */
unsigned long long nr, dr; /* reduced numerator / denominator */
g = gcd_ui(n, d); nr = n/g; dr = d/g;
g = gcd_ui(r, dr); r = r/g; dr = dr/g;
if (r >= ULONG_LONG_MAX/nr) return 0; /* Unavoidable overflow */
r *= nr;
r /= dr;
n--;
} else {
r *= n--;
r /= d;
}
}
return r;
}
unsigned long long binomial_(int *n, int *k)
{
// printf("n=%d k=%d %lu\n",*n,*k,binomial(*n,*k));
return binomial(*n,*k);
}
double hypergeo_(int *x, int *NN, int *XX, int *s)
{
double a,b,c;
a=(double)binomial(*XX, *x);
b=(double)binomial(*NN-*XX, *s-*x);
c=(double)binomial(*NN, *s);
return a*b/c;
}