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/**
* 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;
}
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