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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 simple graph (i.e., without self-loops
+ * and/or multiple edges) from the configuration model, using the
+ * degree sequence provided as input.
+ *
+ */
+
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include "utils.h"
+#include "iltree_double.h"
+
+
+#define MAXREJECTS 1000000
+
+
+/* Usage */
+void usage(char *argv[]){
+ printf("********************************************************************\n"
+ "** **\n"
+ "** -*- conf_model_deg -*- **\n"
+ "** **\n"
+ "** Sample a simple graph from the configuration model associated **\n"
+ "** to the degree sequence provided as input. **\n"
+ "** **\n"
+ "** The degree sequence is a file with format: **\n"
+ "** **\n"
+ "** k_1 **\n"
+ "** k_2 **\n"
+ "** k_3 **\n"
+ "** .... **\n"
+ "** k_N **\n"
+ "** **\n"
+ "** where 'k_i' is the degree of the i-th node. The program **\n"
+ "** prints on output the edge-list of the resulting graph. **\n"
+ "** **\n"
+ "** The second (optional) parameter 'threshold' indicates the **\n"
+ "** number of edge stubs that are allowed to remain unmatched **\n"
+ "** in the final graph. This parameter might be used to **\n"
+ "** obtain a graph from a too-constrained degree sequence, e.g., **\n"
+ "** a power-law sequence whose maximum degree is larger than **\n"
+ "** the structural cut-off. **\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"
+ " Please visit http://www.complex-networks.net for more information\n\n"
+ " (c) Vincenzo Nicosia 2010-2017 (v.nicosia@qmul.ac.uk)\n"
+ "********************************************************************\n\n"
+ );
+ printf("Usage: %s <degs> [<threshold>]\n\n" , argv[0]);
+}
+
+
+unsigned int count_stubs(unsigned int *nodes, int N){
+
+ int i;
+ unsigned int cnt;
+
+ cnt = 0;
+ for (i=0; i<N; i ++){
+ cnt += nodes[i];
+ }
+ return cnt;
+}
+
+
+void sample_graph_conf(unsigned int *nodes, int N, iltree_t *t,
+ unsigned int thresh){
+
+ unsigned int S;
+ unsigned int i, j;
+ unsigned int n_stubs;
+ unsigned int *stubs;
+ unsigned int countreject;
+ unsigned int L1, L2, I1, I2, tmp;
+ long double k;
+ FILE *tmp_file;
+
+ n_stubs = count_stubs(nodes, N);
+
+ if (n_stubs %2){
+ fprintf(stderr, "The sequence is not even!\n");
+ exit(3);
+ }
+ /* ind = malloc(n_stubs * sizeof(unsigned int)); */
+ /* stub2node = malloc(n_stubs * sizeof(unsigned int)); */
+
+ stubs = malloc(n_stubs * sizeof(unsigned int));
+
+
+ do{
+ countreject = 0;
+ S = 0;
+ /*
+ * We first initialize the stubs
+ *
+ * This is needed only before the first iteration, indeed....
+ *
+ */
+ for(i=0; i<N; i ++){
+ for(j=0; j<nodes[i]; j ++){
+ stubs[S] = i;
+ S += 1;
+ }
+ }
+ while (S > 0 && countreject < MAXREJECTS){
+ /* sample the first stub */
+ L1 = rand() % S;
+ I1 = stubs[L1];
+ S -= 1;
+
+ /* sample the second stub */
+ L2 = rand() % S;
+ I2 = stubs[L2];
+ S -= 1;
+
+ if (L1 == L2){/* If L1 and L2 are the same element, discard the
+ sample */
+ /* reset S to the value it had before */
+ S += 2;
+ continue;
+ }
+
+ if (I1 == I2){
+ /* reject the pair: it's the same node! */
+ countreject += 1;
+ /* reset S to the value it had before */
+ S += 2;
+ continue;
+ }
+
+
+ if (I1 > I2){ /* we require that I1 < I2 */
+ tmp = I2;
+ I2 = I1;
+ I1 = tmp;
+ }
+ k = 1.0 * (I2)* (I2-1) / 2 + I1 + 1; /* This number represents the
+ couple I1, I2 (I1 < I2)*/
+ if (!iltree_lookup(*t, &k)){ /* The element "k" is not yet present in the BST*/
+ /* We can accept the new pair... */
+ /* ...so we insert it into the tree */
+ iltree_insert(*t, &k);
+ /* remove the two stubs */
+ tmp = stubs[L1];
+ stubs[L1] = stubs[S+1];
+ stubs[S+1] = tmp;
+
+ tmp = stubs[L2];
+ stubs[L2] = stubs[S];
+ stubs[S] = tmp;
+ }
+ else{/* reject the pair */
+ /* reset S to the value it had before */
+ countreject += 1;
+ S += 2;
+ }
+ }
+
+ /* Let's check if we got a graph....*/
+ if (countreject >= MAXREJECTS){
+ if (S <= thresh){
+ fprintf(stderr, "Found a graph (unmatched stubs: %d <= threshold: %d)\n",
+ S, thresh);
+ break;
+ }
+ else{
+ fprintf(stderr, "Graph not found (unmatched stubs: %d > threshold: %d)\n",
+ S, thresh);
+ S = 0;
+ countreject = 0;
+ tmp_file = iltree_get_fileout(*t);
+ iltree_destroy(*t);
+ *t = NULL;
+ *t = iltree_double_init(*t, tmp_file);
+ }
+ }
+ else{
+ fprintf(stderr, "Found a graph!\n");
+ break;
+ }
+ } while(1);
+ free(stubs);
+}
+
+
+
+int main(int argc, char *argv[]){
+
+ unsigned int *nodes= NULL;
+ int N;
+ iltree_t t = NULL;
+ FILE *filein;
+ unsigned int thresh;
+
+ if (argc < 2){
+ usage(argv);
+ exit(1);
+ }
+
+ srand(time(NULL));
+
+ if (!strcmp(argv[1], "-")){
+ /* take the input from STDIN */
+ filein = stdin;
+ }
+ else {
+ filein = openfile_or_exit(argv[1], "r", 2);
+ }
+
+ if (argc > 2) /* Set the threshold */
+ thresh = atoi(argv[2]);
+ else
+ thresh = 0;
+
+
+ t = iltree_double_init(t, stdout);
+
+ N = read_deg_seq(filein, &nodes);
+ sample_graph_conf(nodes, N, &t, thresh);
+ iltree_double_dump_edges(t);
+ fclose(filein);
+ free(nodes);
+ iltree_destroy(t);
+}
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