| /* randist/hyperg.c |
| * |
| * Copyright (C) 1996, 1997, 1998, 1999, 2000 James Theiler, Brian Gough |
| * |
| * This program is free software; you can redistribute it and/or modify |
| * it under the terms of the GNU General Public License as published by |
| * the Free Software Foundation; either version 2 of the License, or (at |
| * your option) any later version. |
| * |
| * This program is distributed in the hope that it will be useful, but |
| * WITHOUT ANY WARRANTY; without even the implied warranty of |
| * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| * General Public License for more details. |
| * |
| * You should have received a copy of the GNU General Public License |
| * along with this program; if not, write to the Free Software |
| * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. |
| */ |
| |
| #include <config.h> |
| #include <math.h> |
| #include <gsl/gsl_rng.h> |
| #include <gsl/gsl_randist.h> |
| #include <gsl/gsl_sf_gamma.h> |
| |
| /* The hypergeometric distribution has the form, |
| |
| prob(k) = choose(n1,t) choose(n2, t-k) / choose(n1+n2,t) |
| |
| where choose(a,b) = a!/(b!(a-b)!) |
| |
| n1 + n2 is the total population (tagged plus untagged) |
| n1 is the tagged population |
| t is the number of samples taken (without replacement) |
| k is the number of tagged samples found |
| |
| */ |
| |
| unsigned int |
| gsl_ran_hypergeometric (const gsl_rng * r, unsigned int n1, unsigned int n2, |
| unsigned int t) |
| { |
| const unsigned int n = n1 + n2; |
| |
| unsigned int i = 0; |
| unsigned int a = n1; |
| unsigned int b = n1 + n2; |
| unsigned int k = 0; |
| |
| if (t > n) |
| { |
| t = n ; |
| } |
| |
| if (t < n / 2) |
| { |
| for (i = 0 ; i < t ; i++) |
| { |
| double u = gsl_rng_uniform(r) ; |
| |
| if (b * u < a) |
| { |
| k++ ; |
| if (k == n1) |
| return k ; |
| a-- ; |
| } |
| b-- ; |
| } |
| return k; |
| } |
| else |
| { |
| for (i = 0 ; i < n - t ; i++) |
| { |
| double u = gsl_rng_uniform(r) ; |
| |
| if (b * u < a) |
| { |
| k++ ; |
| if (k == n1) |
| return n1 - k ; |
| a-- ; |
| } |
| b-- ; |
| } |
| return n1 - k; |
| } |
| |
| |
| } |
| |
| double |
| gsl_ran_hypergeometric_pdf (const unsigned int k, |
| const unsigned int n1, |
| const unsigned int n2, |
| unsigned int t) |
| { |
| if (t > n1 + n2) |
| { |
| t = n1 + n2 ; |
| } |
| |
| if (k > n1 || k > t) |
| { |
| return 0 ; |
| } |
| else if (t > n2 && k + n2 < t ) |
| { |
| return 0 ; |
| } |
| else |
| { |
| double p; |
| |
| double c1 = gsl_sf_lnchoose(n1,k); |
| double c2 = gsl_sf_lnchoose(n2,t-k); |
| double c3 = gsl_sf_lnchoose(n1+n2,t); |
| |
| p = exp(c1 + c2 - c3) ; |
| |
| return p; |
| } |
| } |