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diff --git a/src/knn/knn.c b/src/knn/knn.c
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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
+ *
+ ***********************************************************************
+ *
+ * Compute knn(k) for a given unweighted graph. If requested, the
+ * average is computed for ranges of degrees falling into a number of
+ * equal bins or into bins or exponentially increasing width.
+ *
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+#include <string.h>
+
+#include "utils.h"
+#include "knntree.h"
+
+#define NO_BIN 0x01
+#define BIN_LIN 0x02
+#define BIN_EXP 0x04
+
+void usage(char *argv[]){
+
+ printf("********************************************************************\n"
+ "** **\n"
+ "** -*- knn -*- **\n"
+ "** **\n"
+ "** Compute the average nearest-neighbour degree function knn(k) **\n"
+ "** of the unweighted graph provided as input. **\n"
+ "** **\n"
+ "** The input file 'graph_in' is an edge-list: **\n"
+ "** **\n"
+ "** I_1 J_1 **\n"
+ "** I_2 J_2 **\n"
+ "** I_3 J_3 **\n"
+ "** ... ... **\n"
+ "** I_K J_K **\n"
+ "** **\n"
+ "** If 'graph_in' is equal to '-' (dash), read the file from **\n"
+ "** the standard input (STDIN). **\n"
+ "** **\n"
+ "** If no second parameter is given (or the second parameter **\n"
+ "** is equal to 'NO'), the program prints on output the value **\n"
+ "** of knn(k) for each degree in the graph, in the format: **\n"
+ "** **\n"
+ "** k1 knn(k1) **\n"
+ "** k2 knn(k2) **\n"
+ "** ..... **\n"
+ "** **\n"
+ "** If the second parameter is 'LIN', the values of knn(k) **\n"
+ "** will be grouped into 'bin_param' equal bins. **\n"
+ "** **\n"
+ "** If the second parameter is 'EXP;, the values of knn(k) **\n"
+ "** will be grouped into bins whose width increases **\n"
+ "** exponentially with exponent 'bin_param' (also known as **\n"
+ "** 'logarithmic binning'). **\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 <graph_in> [<NO|LIN|EXP> <bin_param>]\n", argv[0]);
+}
+
+
+
+typedef struct{
+ unsigned int num_bins;
+ unsigned int *degs;
+ unsigned int *num;
+ double *values;
+} knn_hist_t;
+
+
+/* compute the sum of the degrees of the neighbours of the node with
+ label 'idx' */
+double neigh_degs_sum(unsigned int *J_slap, unsigned int *r_slap, unsigned int idx){
+
+ double sum = 0;
+ unsigned j, k;
+
+ for(j=r_slap[idx]; j < r_slap[idx+1]; j++){
+ k = J_slap[j];
+ sum += r_slap[k+1] - r_slap[k];
+ }
+ return sum;
+}
+
+/* compute knn(k) for each degree k */
+void compute_knn(unsigned int *J_slap, unsigned int *r_slap, unsigned int N,
+ knntree_t t){
+
+ knnsum_t elem, *ptr;
+ unsigned int i;
+
+ for(i=0; i<N; i++){
+ elem.k = r_slap[i+1] - r_slap[i];
+ elem.knnsum = neigh_degs_sum(J_slap, r_slap, i);
+ ptr = iltree_lookup(t, &elem);
+ if (!ptr){
+ elem.Nk = 1;
+ iltree_insert(t, &elem);
+ }
+ else{
+ ptr->Nk +=1;
+ ptr->knnsum += elem.knnsum;
+ }
+ }
+}
+
+/* aggregate all the values of knn(k) for the degrees falling in a
+ certain bin */
+void aggregate_knn(void *elem, void *h){
+
+ knn_hist_t *hist;
+ knnsum_t *s;
+ unsigned int cur, low, high;
+
+ hist = (knn_hist_t*)h;
+ s = (knnsum_t*)elem;
+
+
+ cur = hist->num_bins / 2;
+
+ low = 0;
+ high = hist->num_bins - 1;
+
+ while (low < high){
+ if (hist->degs[cur] < s->k){
+ low = cur+1;
+ }
+ else if(hist->degs[cur] >= s->k){
+ high = cur;
+ }
+ cur = (high + low) / 2;
+ }
+
+ hist->num[cur] += s->Nk * s->k;
+ hist->values[cur] += s->knnsum;
+}
+
+
+/* print knn(k) for each value of k in the graph (i.e., without any
+ binning) */
+void print_knn_nobin(knntree_t t){
+
+ iltree_view_pre(t);
+}
+
+
+
+/* print the values of knn(k) averaging over num_bins of equal size */
+void print_knn_lin(knntree_t t, unsigned int num_bins){
+
+ unsigned int kmin, kmax, step, i;
+ knn_hist_t hist;
+
+ if (!t){
+ return;
+ }
+
+
+ kmin = ((knnsum_t*)iltree_getmin(t))->k;
+ kmax = ((knnsum_t*)iltree_getmax(t))->k;
+
+ hist.num_bins = num_bins;
+
+ step = (kmax - kmin)/num_bins + 1;
+
+ hist.degs = malloc(num_bins * sizeof(unsigned int));
+ hist.num = malloc(num_bins * sizeof(unsigned int));
+ hist.values = malloc(num_bins * sizeof(double));
+
+
+ hist.degs[0] = kmin + step;
+ hist.values[0] = 0;
+ hist.num[0] = 0;
+
+ for(i=1; i<num_bins; i++){
+ hist.degs[i] = hist.degs[i-1] + step;
+ hist.values[i] = 0;
+ hist.num[i] = 0;
+ }
+
+ iltree_map_args(t, aggregate_knn, &hist);
+
+ for(i=0; i<hist.num_bins; i++){
+ if (hist.num[i])
+ printf("%d %2.8g\n", hist.degs[i], hist.values[i] / hist.num[i] );
+ }
+
+ free(hist.degs);
+ free(hist.num);
+ free(hist.values);
+
+}
+
+
+/* print knn(k) using exponential (a.k.a. logarithmic) binning */
+
+void print_knn_exp(knntree_t t, double alpha){
+
+ unsigned int kmin, kmax, i;
+ double width;
+ knn_hist_t hist;
+
+ if (!t){
+ return;
+ }
+
+
+ kmin = ((knnsum_t*)iltree_getmin(t))->k;
+ kmax = ((knnsum_t*)iltree_getmax(t))->k;
+
+
+ hist.num_bins = (int)ceil(log(kmax)/log(alpha)) + 1;
+
+
+ hist.degs = malloc(hist.num_bins * sizeof(unsigned int));
+ hist.num = malloc(hist.num_bins * sizeof(unsigned int));
+ hist.values = malloc(hist.num_bins * sizeof(double));
+
+ width = 2;
+
+ hist.degs[0] = kmin + width;
+ hist.values[0] = 0;
+ hist.num[0] = 0;
+
+ for(i=1; i<hist.num_bins; i++){
+ hist.values[i] = 0;
+ hist.num[i] = 0;
+ width *= alpha;
+ hist.degs[i] = (int)ceil(hist.degs[i-1] + width);
+ }
+
+ iltree_map_args(t, aggregate_knn, &hist);
+
+ for(i=0; i<hist.num_bins; i++){
+ if(hist.num[i])
+ printf("%d %2.8g\n", hist.degs[i], hist.values[i] / hist.num[i] );
+ }
+
+ free(hist.degs);
+ free(hist.num);
+ free(hist.values);
+
+}
+
+
+
+int main(int argc, char *argv[]){
+
+ unsigned int N, K;
+ unsigned int *J_slap=NULL, *r_slap=NULL;
+ FILE *filein;
+ char bin_type;
+ unsigned int num_bins;
+ double alpha;
+ knntree_t t = NULL;
+
+
+
+ if(argc < 2){
+ usage(argv);
+ exit(1);
+ }
+
+ if(!strcmp(argv[1], "-")){
+ filein = stdin;
+ }
+ else{
+ filein = openfile_or_exit(argv[1], "r", 2);
+ }
+
+ t = iltree_create(t);
+
+ bin_type = NO_BIN;
+ if (argc > 2){ /* the user has specified a binning type */
+ if (!my_strcasecmp(argv[2], "lin")){
+ bin_type = BIN_LIN;
+ if(argc < 4){
+ fprintf(stderr, "you must provide a number of bins for linear binning\n");
+ exit(3);
+ }
+ num_bins = atoi(argv[3]);
+ }
+ else if (!my_strcasecmp(argv[2], "exp")){
+ bin_type = BIN_EXP;
+ if(argc < 4){
+ fprintf(stderr, "you must provide an exponent for exponential binning\n");
+ exit(4);
+ }
+ alpha = atof(argv[3]);
+ }
+ }
+
+ read_slap(filein, &K, &N, &J_slap, &r_slap);
+ fclose(filein);
+
+ t = knntree_init(t, stdout);
+ compute_knn(J_slap, r_slap, N, t);
+
+
+ switch(bin_type){
+ case BIN_LIN:
+ print_knn_lin(t, num_bins);
+ break;
+ case BIN_EXP:
+ print_knn_exp(t, alpha);
+ break;
+ default:
+ print_knn_nobin(t);
+ break;
+ }
+ iltree_destroy(t);
+ free(J_slap);
+ free(r_slap);
+}
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