1 files changed, 146 insertions, 0 deletions
diff --git a/src/power_law/power_law.c b/src/power_law/power_law.c
new file mode 100644
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+++ b/src/power_law/power_law.c
@@ -0,0 +1,146 @@
+/**
+ *  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 3 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, see
+ *  <http://www.gnu.org/licenses/>.
+ *
+ *  (c) Vincenzo Nicosia 2009-2017 -- <v.nicosia@qmul.ac.uk>
+ * 
+ *  This file is part of NetBunch, a package for complex network
+ *  analysis and modelling. For more information please visit:
+ *
+ *             http://www.complex-networks.net/
+ *
+ *  If you use this software, please add a reference to 
+ *
+ *               V. Latora, V. Nicosia, G. Russo             
+ *   "Complex Networks: Principles, Methods and Applications"
+ *              Cambridge University Press (2017) 
+ *                ISBN: 9781107103184
+ *
+ ***********************************************************************
+ *
+ *  This program samples a degree sequence from a discrete power-law
+ *  distribution with a given exponent.
+ *
+ */
+
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+#include <time.h>
+#include <errno.h>
+
+
+void usage(char *argv[]){
+  printf("********************************************************************\n"
+         "**                                                                **\n"
+         "**                   -*-    power_law    -*-                      **\n"
+         "**                                                                **\n"
+         "**  Sample 'N' elements from a power law degree distribution      **\n"
+         "**                      P(k) ~= k^{gamma}                         **\n"
+         "**  with degrees in the range [k_min, k_max], and print them      **\n"
+         "**  on STDOUT.                                                    **\n"
+         "**                                                                **\n"
+         "**  If the obtained degree sequence is even, i.e., if the sum     **\n"
+         "**  of all the sampled degrees is even, the program returns 0     **\n"
+         "**  (zero), otherwise it returns 1.                               **\n"
+         "**                                                                **\n"
+         "********************************************************************\n"
+         " This is Free Software - You can use and distribute it under \n"
+         " the terms of the GNU General Public License, version 3 or later\n\n"
+         " (c) Vincenzo Nicosia 2010-2017 (v.nicosia@qmul.ac.uk)\n\n"
+         "********************************************************************\n\n"
+         );
+  printf("Usage: %s <gamma> <k_min> <k_max> <N>\n" , argv[0]);
+}
+
+
+
+void create_distr(double *v, double gamma, int k_min, int k_max){
+  int n, i;
+  double sum, new_sum;
+  
+  n = k_max-k_min + 1;
+  
+  sum = 0;
+  for(i=0; i<n; i++){
+    v[i] = pow((k_min + i), gamma);
+    sum += v[i];
+  }
+  new_sum = 0;
+  /* Now we normalize the array*/
+  for(i=0; i<n; i++){
+    v[i]/= sum;
+    new_sum += v[i];
+  }
+  /* Now we compute the cumulative distribution*/
+  for(i=1; i<n; i++){
+    v[i] += v[i-1];
+  }
+}
+
+
+int find_degree(double *v, int dim, double xi){
+  int i;
+  
+  i=0;
+  while(xi > v[i])
+    i++;
+  return i;
+  
+}
+
+
+int main(int argc, char *argv[]){
+  
+  double gamma,  xi, val, q;
+  unsigned int N, i, distr_num, k, k_min, k_max, K;
+  double *distr;
+    
+  if (argc < 5){
+    usage(argv);
+    exit(1);
+  }
+  
+  srand(time(NULL));
+
+  gamma = atof(argv[1]);
+  k_min = atoi(argv[2]);
+  k_max = atoi(argv[3]);
+  N = atoi(argv[4]);
+
+  K = 0;
+  
+  distr_num = k_max - k_min + 1;
+  distr = malloc(distr_num * sizeof(double));
+
+  create_distr(distr, gamma, k_min, k_max);
+  
+  for(i=0; i<N;){
+    xi = rand()* 1.0 / RAND_MAX;
+    k = find_degree(distr, distr_num, xi);
+    val = rand()*1.0/RAND_MAX;
+    q = k_min + xi * distr_num;
+    q = q / (floor(q) + 1);
+    q = pow(q, gamma);
+    if (val <= q){
+      printf("%d\n", k+k_min);
+      K += k+k_min;
+      i++;
+    }
+  }
+  free(distr);
+  /* Return 0 if the degree sequence is even, or 1 otherwise */
+  return K%2;
+}