1 /* -*- mode: C -*- */
2 /* vim:set ts=4 sw=4 sts=4 et: */
3 /*
4 IGraph library.
5 Copyright (C) 2003-2021 The igraph development team
6
7 This program is free software; you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation; either version 2 of the License, or
10 (at your option) any later version.
11
12 This program is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
16
17 You should have received a copy of the GNU General Public License
18 along with this program; if not, write to the Free Software
19 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
20 02110-1301 USA
21
22 */
23
24 #include "igraph_games.h"
25
26 #include "igraph_interface.h"
27
28 /**
29 * \ingroup generators
30 * \function igraph_k_regular_game
31 * \brief Generates a random graph where each vertex has the same degree.
32 *
33 * This game generates a directed or undirected random graph where the
34 * degrees of vertices are equal to a predefined constant k. For undirected
35 * graphs, at least one of k and the number of vertices must be even.
36 *
37 * </para><para>
38 * Currently, this game simply uses \ref igraph_degree_sequence_game with
39 * the \c SIMPLE_NO_MULTIPLE method and appropriately constructed degree sequences.
40 * Thefore, it does not sample uniformly: while it can generate all k-regular graphs
41 * with the given number of vertices, it does not generate each one with the same
42 * probability.
43 *
44 * \param graph Pointer to an uninitialized graph object.
45 * \param no_of_nodes The number of nodes in the generated graph.
46 * \param k The degree of each vertex in an undirected graph, or
47 * the out-degree and in-degree of each vertex in a
48 * directed graph.
49 * \param directed Whether the generated graph will be directed.
50 * \param multiple Whether to allow multiple edges in the generated graph.
51 *
52 * \return Error code:
53 * \c IGRAPH_EINVAL: invalid parameter; e.g., negative number of nodes,
54 * or odd number of nodes and odd k for undirected
55 * graphs.
56 * \c IGRAPH_ENOMEM: there is not enough memory for the operation.
57 *
58 * Time complexity: O(|V|+|E|) if \c multiple is true, otherwise not known.
59 */
igraph_k_regular_game(igraph_t * graph,igraph_integer_t no_of_nodes,igraph_integer_t k,igraph_bool_t directed,igraph_bool_t multiple)60 int igraph_k_regular_game(igraph_t *graph,
61 igraph_integer_t no_of_nodes, igraph_integer_t k,
62 igraph_bool_t directed, igraph_bool_t multiple) {
63 igraph_vector_t degseq;
64 igraph_degseq_t mode = multiple ? IGRAPH_DEGSEQ_SIMPLE : IGRAPH_DEGSEQ_SIMPLE_NO_MULTIPLE;
65
66 /* Note to self: we are not using IGRAPH_DEGSEQ_VL when multiple = false
67 * because the VL method is not really good at generating k-regular graphs.
68 * Actually, that's why we have added SIMPLE_NO_MULTIPLE. */
69
70 if (no_of_nodes < 0) {
71 IGRAPH_ERROR("number of nodes must be non-negative", IGRAPH_EINVAL);
72 }
73 if (k < 0) {
74 IGRAPH_ERROR("degree must be non-negative", IGRAPH_EINVAL);
75 }
76
77 IGRAPH_VECTOR_INIT_FINALLY(°seq, no_of_nodes);
78 igraph_vector_fill(°seq, k);
79 IGRAPH_CHECK(igraph_degree_sequence_game(graph, °seq, directed ? °seq : 0, mode));
80
81 igraph_vector_destroy(°seq);
82 IGRAPH_FINALLY_CLEAN(1);
83
84 return IGRAPH_SUCCESS;
85 }
86