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