/* -*- mode: C -*-  */
/* 
   IGraph library.
   Copyright (C) 2005  Gabor Csardi <csardi@rmki.kfki.hu>
   MTA RMKI, Konkoly-Thege Miklos st. 29-33, Budapest 1121, Hungary
   
   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 2 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, write to the Free Software
   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 
   02110-1301 USA

*/

#include "igraph.h"
#include "memory.h"
#include "random.h"


void igraph_i_cliques_free_res(igraph_vector_ptr_t *res) {
  long i, n;
  
  n = igraph_vector_ptr_size(res);
  for (i=0; i<n; i++) {
    if (VECTOR(*res)[i] != 0) {
      igraph_vector_destroy(VECTOR(*res)[i]);
      igraph_free(VECTOR(*res)[i]);
    }
  }
  igraph_vector_ptr_clear(res);
}

int igraph_i_find_k_cliques(const igraph_t *graph,
			    long int size,
			    const igraph_real_t *member_storage,
			    igraph_real_t **new_member_storage,
			    long int old_clique_count,
			    long int *clique_count,
			    igraph_vector_t *neis,
			    igraph_bool_t independent_vertices) {

  long int j, k, l, m, n;
  const igraph_real_t *c1, *c2;
  igraph_real_t v1, v2;
  igraph_bool_t ok;
  
  /* Allocate the storage */
  *new_member_storage=Realloc(*new_member_storage, 
			      size*old_clique_count*(old_clique_count-1)/2,
			      igraph_real_t);
  if (*new_member_storage == 0) {
    IGRAPH_ERROR("cliques failed", IGRAPH_ENOMEM);
  }
  IGRAPH_FINALLY(igraph_free, *new_member_storage);

  m=n=0;
  
  /* Now consider all pairs of i-1-cliques and see if they can be merged */
  for (j=0; j<old_clique_count; j++) {
    for (k=j+1; k<old_clique_count; k++) {
      IGRAPH_ALLOW_INTERRUPTION();
        
      /* Since cliques are represented by their vertex indices in increasing
       * order, two cliques can be merged iff they have exactly the same
       * indices excluding one AND there is an edge between the two different
       * vertices */
      c1 = member_storage+j*(size-1);
      c2 = member_storage+k*(size-1);
      /* Find the longest prefixes of c1 and c2 that are equal */
      for (l=0; l<size-1 && c1[l] == c2[l]; l++)
	(*new_member_storage)[m++]=c1[l];
      /* Now, if l == size-1, the two vectors are totally equal.
	 This is a bug */
      if (l == size-1) {
	IGRAPH_WARNING("possible bug in igraph_cliques");
	m=n;
      } else {
	/* Assuming that j<k, c1[l] is always less than c2[l], since cliques
	 * are ordered alphabetically. Now add c1[l] and store c2[l] in a
	 * dummy variable */
	(*new_member_storage)[m++]=c1[l];
	v1=c1[l];
	v2=c2[l];
	l++;
	/* Copy the remaining part of the two vectors. Every member pair
	 * found in the remaining parts satisfies the following:
	 * 1. If they are equal, they should be added.
	 * 2. If they are not equal, the smaller must be equal to the
	 *    one stored in the dummy variable. If not, the two vectors
	 *    differ in more than one place. The larger will be stored in
	 *    the dummy variable again.
	 */
	ok=1;
	for (; l<size-1; l++) {
	  if (c1[l] == c2[l]) {
	    (*new_member_storage)[m++]=c1[l];
	    ok=0;
	  } else if (ok) {
	    if (c1[l] < c2[l]) {
	      if (c1[l] == v1) {
		(*new_member_storage)[m++]=c1[l];
		v2 = c2[l];
	      } else break;
	    } else {
	      if (ok && c2[l] == v1) {
		(*new_member_storage)[m++]=c2[l];
		v2 = c1[l];
	      } else break;
	    }
	  } else break;
	}
	/* Now, if l != size-1, the two vectors had a difference in more than
	 * one place, so the whole clique is invalid. */
	if (l != size-1) {
	  /* Step back in new_member_storage */
	  m=n;
	} else {
	  /* v1 and v2 are the two different vertices. Check for an edge
	   * if we are looking for cliques and check for the absence of an
	   * edge if we are looking for independent vertex sets */
	  IGRAPH_CHECK(igraph_neighbors(graph, neis, v1, IGRAPH_ALL));
	  l=igraph_vector_search(neis, 0, v2, 0);
	  if ((l && !independent_vertices) || (!l && independent_vertices)) {
	    /* Found a new clique, step forward in new_member_storage */
	    if (m==n || v2>(*new_member_storage)[m-1]) {
	      (*new_member_storage)[m++]=v2;
	      n=m;
	    } else {
	      m=n;
	    }
	  } else {
	    m=n;
	  }
	}
      }
    }
  }

  /* Calculate how many cliques have we found */
  *clique_count = n/size;
  
  IGRAPH_FINALLY_CLEAN(1);
  return 0;
}  

/* Internal function for calculating cliques or independent vertex sets.
 * They are practically the same except that the complementer of the graph
 * should be used in the latter case.
 */
int igraph_i_cliques(const igraph_t *graph, igraph_vector_ptr_t *res,
		     igraph_integer_t min_size, igraph_integer_t max_size,
		     igraph_bool_t independent_vertices) {

  igraph_integer_t no_of_nodes;
  igraph_vector_t neis;
  igraph_real_t *member_storage=0, *new_member_storage, *c1;
  long int i, j, k, clique_count, old_clique_count;
  
  no_of_nodes = igraph_vcount(graph);
  
  if (min_size < 0) { min_size = 0; }
  if (max_size > no_of_nodes || max_size <= 0) { max_size = no_of_nodes; }

  igraph_vector_ptr_clear(res);
  
  IGRAPH_VECTOR_INIT_FINALLY(&neis, 0);
  igraph_vector_ptr_clear(res);
  IGRAPH_FINALLY(igraph_i_cliques_free_res, res);
    
  /* Will be resized later, if needed. */
  member_storage=Calloc(1, igraph_real_t);
  if (member_storage==0) {
    IGRAPH_ERROR("cliques failed", IGRAPH_ENOMEM);
  }
  IGRAPH_FINALLY(igraph_free, member_storage);
  
  /* Find all 1-cliques: every vertex will be a clique */
  new_member_storage=Calloc(no_of_nodes, igraph_real_t);
  if (new_member_storage==0) {
    IGRAPH_ERROR("cliques failed", IGRAPH_ENOMEM);
  }
  IGRAPH_FINALLY(igraph_free, new_member_storage);
  
  for (i=0; i<no_of_nodes; i++) {
    new_member_storage[i] = i;
  }
  clique_count = no_of_nodes;
  old_clique_count = 0;

  /* Add size 1 cliques if requested */
  if (min_size <= 1) {
    IGRAPH_CHECK(igraph_vector_ptr_resize(res, no_of_nodes));
    igraph_vector_ptr_null(res);
    for (i=0; i<no_of_nodes; i++) {
      igraph_vector_t *p=Calloc(1, igraph_vector_t);
      if (p==0) {
	IGRAPH_ERROR("cliques failed", IGRAPH_ENOMEM);
      }
      IGRAPH_FINALLY(igraph_free, p);
      IGRAPH_CHECK(igraph_vector_init(p, 1));
      VECTOR(*p)[0]=i;
      VECTOR(*res)[i]=p;
      IGRAPH_FINALLY_CLEAN(1);
    }
  }      

  for (i=2; i<=max_size && clique_count > 1; i++) {

    /* Here new_member_storage contains the cliques found in the previous
       iteration. Save this into member_storage, might be needed later  */

    c1=member_storage;
    member_storage=new_member_storage;
    new_member_storage=c1;
    old_clique_count=clique_count;
    
    IGRAPH_ALLOW_INTERRUPTION();
    
    /* Calculate the cliques */
    
    IGRAPH_FINALLY_CLEAN(2);
    IGRAPH_CHECK(igraph_i_find_k_cliques(graph, i, member_storage,
					 &new_member_storage,
					 old_clique_count,
					 &clique_count,
					 &neis,
					 independent_vertices));
    IGRAPH_FINALLY(igraph_free, member_storage);
    IGRAPH_FINALLY(igraph_free, new_member_storage);
    
    /* Add the cliques just found to the result if requested */
    if (i>=min_size && i<=max_size) {
      for (j=0, k=0; j<clique_count; j++, k+=i) {
	igraph_vector_t *p=Calloc(1, igraph_vector_t);
	if (p==0) {
	  IGRAPH_ERROR("cliques failed", IGRAPH_ENOMEM);
	}
	IGRAPH_FINALLY(igraph_free, p);
	IGRAPH_CHECK(igraph_vector_init_copy(p, &new_member_storage[k], i));
	IGRAPH_FINALLY(igraph_vector_destroy, p);
	IGRAPH_CHECK(igraph_vector_ptr_push_back(res, p));
	IGRAPH_FINALLY_CLEAN(2);
      }
    }
    
  } /* i <= max_size && clique_count != 0 */
  
  igraph_free(member_storage);
  igraph_free(new_member_storage);
  igraph_vector_destroy(&neis);
  IGRAPH_FINALLY_CLEAN(4); /* 2 here, +1 is igraph_i_cliques_free_res */
  
  return 0;
}

/**
 * \function igraph_cliques
 * Find all or some cliques in a graph
 *
 * </para><para>
 * Cliques are fully connected subgraphs of a graph.
 *
 * </para><para>
 * If you are only interested in the size of the largest clique in the graph,
 * use \ref igraph_clique_number() instead.
 *
 * \param graph The input graph.
 * \param res Pointer to a pointer vector, the result will be stored
 *   here, ie. \c res will contain pointers to \c igraph_vector_t
 *   objects which contain the indices of vertices involved in a clique.
 *   The pointer vector will be resized if needed but note that the
 *   objects in the pointer vector will not be freed.
 * \param min_size Integer giving the minimum size of the cliques to be
 *   returned. If negative or zero, no lower bound will be used.
 * \param max_size Integer giving the maximum size of the cliques to be
 *   returned. If negative or zero, no upper bound will be used.
 * \return Error code.
 *
 * \sa \ref igraph_largest_cliques() and \ref igraph_clique_number().
 *
 * Time complexity: TODO
 */
int igraph_cliques(const igraph_t *graph, igraph_vector_ptr_t *res,
                   igraph_integer_t min_size, igraph_integer_t max_size) {
  return igraph_i_cliques(graph, res, min_size, max_size, 0);
}

int igraph_i_largest_cliques(const igraph_t *graph, igraph_vector_ptr_t *res,
			     igraph_bool_t independent_vertices) {
  
  igraph_integer_t no_of_nodes;
  igraph_vector_t neis;
  igraph_real_t *member_storage=0, *new_member_storage, *c1;
  long int i, j, k, clique_count, old_clique_count;

  no_of_nodes = igraph_vcount(graph);

  IGRAPH_VECTOR_INIT_FINALLY(&neis, 0);
  igraph_vector_ptr_clear(res);
  IGRAPH_FINALLY(igraph_i_cliques_free_res, res);

  /* Will be resized later, if needed. */
  member_storage=Calloc(1, igraph_real_t);
  if (member_storage==0) {
    IGRAPH_ERROR("cliques failed", IGRAPH_ENOMEM);
  }
  IGRAPH_FINALLY(igraph_free, member_storage);

  new_member_storage=Calloc(no_of_nodes, igraph_real_t);
  if (new_member_storage==0) {
    IGRAPH_ERROR("cliques failed", IGRAPH_ENOMEM);
  }
  IGRAPH_FINALLY(igraph_free, new_member_storage);
  
  for (i=0; i<no_of_nodes; i++) {
    new_member_storage[i] = i;
  }
  clique_count = no_of_nodes;
  old_clique_count = 0;

  for (i=2; i<=no_of_nodes && clique_count > 1; i++) {
    
    c1=member_storage;
    member_storage=new_member_storage;
    new_member_storage=c1;
    old_clique_count=clique_count;
    
    IGRAPH_ALLOW_INTERRUPTION();
    
    /* Calculate the cliques */
    
    IGRAPH_FINALLY_CLEAN(2);
    IGRAPH_CHECK(igraph_i_find_k_cliques(graph, i, member_storage,
					 &new_member_storage,
					 old_clique_count,
					 &clique_count,
					 &neis,
					 independent_vertices));
    IGRAPH_FINALLY(igraph_free, member_storage);
    IGRAPH_FINALLY(igraph_free, new_member_storage);

  }

  /* Where is the result? */
  if (clique_count != 0) {
    c1=new_member_storage;
    i=no_of_nodes;
  } else {
    c1=member_storage;
    i=i-2;
    clique_count=old_clique_count;
  }
  /* Ok, the result is in c1, there are clique_count cliques and
     i is the size of the cliques
  */
  for (j=0, k=0; j<clique_count; j++, k+=i) {
    igraph_vector_t *p=Calloc(1, igraph_vector_t);
    if (p==0) {
      IGRAPH_ERROR("cliques failed", IGRAPH_ENOMEM);
    }
    IGRAPH_FINALLY(igraph_free, p);
    IGRAPH_CHECK(igraph_vector_init_copy(p, &c1[k], i));
    IGRAPH_FINALLY(igraph_vector_destroy, p);
    IGRAPH_CHECK(igraph_vector_ptr_push_back(res, p));
    IGRAPH_FINALLY_CLEAN(2);
  }

  igraph_free(member_storage);
  igraph_free(new_member_storage);
  igraph_vector_destroy(&neis);
  IGRAPH_FINALLY_CLEAN(4); /* 2 here, +1 is igraph_i_cliques_free_res */
  
  return 0;
}

/**
 * \function igraph_largest_cliques
 * \brief Finds the largest clique(s) in a graph.
 * 
 * </para><para>
 * A clique is largest (quite intuitively) if there is no other clique
 * in the graph which contains more vertices. 
 * 
 * </para><para>
 * Note that this is not neccessarily the same as a maximal clique,
 * ie. the largest cliques are always maximal but a maximal clique is
 * not always largest.
 * \param graph The input graph.
 * \param res Pointer to an initialized pointer vector, the result
 *        will be stored here. It will be resized as needed.
 * \return Error code.
 * 
 * \sa \ref igraph_cliques(), \ref igraph_maximal_cliques()
 * 
 * Time complexity: TODO.
 */

int igraph_largest_cliques(const igraph_t *graph, igraph_vector_ptr_t *res) {
  return igraph_i_largest_cliques(graph, res, 0);
}


/**
 * \function igraph_independent_vertex_sets
 * Find all independent vertex sets in a graph
 *
 * </para><para>
 * A vertex set is considered independent if there are no edges between
 * them.
 *
 * </para><para>
 * If you are interested in the size of the largest independent vertex set,
 * use \ref igraph_independence_number() instead.
 *
 * \param graph The input graph.
 * \param res Pointer to a pointer vector, the result will be stored
 *   here, ie. \c res will contain pointers to \c igraph_vector_t
 *   objects which contain the indices of vertices involved in an independent
 *   vertex set. The pointer vector will be resized if needed but note that the
 *   objects in the pointer vector will not be freed.
 * \param min_size Integer giving the minimum size of the sets to be
 *   returned. If negative or zero, no lower bound will be used.
 * \param max_size Integer giving the maximum size of the sets to be
 *   returned. If negative or zero, no upper bound will be used. 
 * \return Error code.
 *
 * \sa \ref igraph_largest_independent_vertex_sets(), 
 * \ref igraph_independence_number().
 *
 * Time complexity: TODO
 */
int igraph_independent_vertex_sets(const igraph_t *graph,
				   igraph_vector_ptr_t *res,
				   igraph_integer_t min_size,
				   igraph_integer_t max_size) {
  return igraph_i_cliques(graph, res, min_size, max_size, 1);
}

/**
 * \function igraph_largest_independent_vertex_sets
 * \brief Finds the largest independent vertex set(s) in a graph.
 * 
 * </para><para>
 * An independent vertex set is largest if there is no other
 * independent vertex set with more vertices in the graph.
 * \param graph The input graph.
 * \param res Pointer to a pointer vector, the result will be stored
 *     here. It will be resized as needed.
 * \return Error code.
 * 
 * \sa \ref igraph_independent_vertex_sets(), \ref
 * igraph_maximal_independent_vertex_sets().
 * 
 * Time complexity: TODO
 */

int igraph_largest_independent_vertex_sets(const igraph_t *graph,
					   igraph_vector_ptr_t *res) {
  return igraph_i_largest_cliques(graph, res, 1);
}

typedef struct igraph_i_max_ind_vsets_data_t {
  igraph_integer_t matrix_size;
  igraph_i_adjlist_t adj_list;         /* Adjacency list of the graph */
  igraph_vector_t deg;                 /* Degrees of individual nodes */
  igraph_set_t* buckets;               /* Bucket array */
  /* The IS value for each node. Still to be explained :) */
  igraph_integer_t* IS;
  igraph_integer_t largest_set_size;   /* Size of the largest set encountered */
} igraph_i_max_ind_vsets_data_t;

int igraph_i_maximal_independent_vertex_sets_backtrack(const igraph_t *graph,
						       igraph_vector_ptr_t *res,
						       igraph_i_max_ind_vsets_data_t *clqdata,
						       igraph_integer_t level) {
  long int v1, v2, v3, c, j, k;
  igraph_vector_t *neis1, *neis2;
  igraph_bool_t f;
  igraph_integer_t j1;
  long int it_state;

  IGRAPH_ALLOW_INTERRUPTION();

  if (level >= clqdata->matrix_size-1) {
    igraph_vector_t *vec;
    igraph_integer_t size=0;
    vec = Calloc(1, igraph_vector_t);
    if (vec == 0)
      IGRAPH_ERROR("igraph_i_maximal_independent_vertex_sets failed", IGRAPH_ENOMEM);
    IGRAPH_VECTOR_INIT_FINALLY(vec, 0);
    if (res) {
      for (v1=0; v1<clqdata->matrix_size; v1++)
	if (clqdata->IS[v1] == 0) {
	  IGRAPH_CHECK(igraph_vector_push_back(vec, v1));
	}
      IGRAPH_CHECK(igraph_vector_ptr_push_back(res, vec));
      size=igraph_vector_size(vec);
    } else {
      for (v1=0, size=0; v1<clqdata->matrix_size; v1++)
	if (clqdata->IS[v1] == 0) size++;
    }
    if (size>clqdata->largest_set_size) clqdata->largest_set_size=size;
    IGRAPH_FINALLY_CLEAN(1);
  } else {
    v1 = level+1;
    /* Count the number of vertices with an index less than v1 that have
     * an IS value of zero */
    neis1 = igraph_i_adjlist_get(&clqdata->adj_list, v1);
    c = 0;
    j = 0;
    while (j<VECTOR(clqdata->deg)[v1] && (v2=VECTOR(*neis1)[j]) <= level) {
      if (clqdata->IS[v2] == 0) c++;
      j++;
    }

    if (c == 0) {
      /* If there are no such nodes... */
      j = 0;
      while (j<VECTOR(clqdata->deg)[v1] && (v2=VECTOR(*neis1)[j]) <= level) {
	clqdata->IS[v2]++;
	j++;
      }
      IGRAPH_CHECK(igraph_i_maximal_independent_vertex_sets_backtrack(graph,res,clqdata,v1));
      j = 0;
      while (j<VECTOR(clqdata->deg)[v1] && (v2=VECTOR(*neis1)[j]) <= level) {
	clqdata->IS[v2]--;
	j++;
      }
    } else {
      /* If there are such nodes, store the count in the IS value of v1 */
      clqdata->IS[v1] = c;
      IGRAPH_CHECK(igraph_i_maximal_independent_vertex_sets_backtrack(graph,res,clqdata,v1));
      clqdata->IS[v1] = 0;
      
      f=1;
      j=0;
      while (j<VECTOR(clqdata->deg)[v1] && (v2=VECTOR(*neis1)[j]) <= level) {
	if (clqdata->IS[v2] == 0) {
	  IGRAPH_CHECK(igraph_set_add(&clqdata->buckets[v1], j));
	  neis2 = igraph_i_adjlist_get(&clqdata->adj_list, v2);
	  k = 0;
	  while (k<VECTOR(clqdata->deg)[v2] && (v3=VECTOR(*neis2)[k])<=level) {
	    clqdata->IS[v3]--;
	    if (clqdata->IS[v3] == 0) f=0;
	    k++;
	  }
	}
	clqdata->IS[v2]++;
	j++;
      }

      if (f) 
	IGRAPH_CHECK(igraph_i_maximal_independent_vertex_sets_backtrack(graph,res,clqdata,v1));

      j=0;
      while (j<VECTOR(clqdata->deg)[v1] && (v2=VECTOR(*neis1)[j]) <= level) {
	clqdata->IS[v2]--;
	j++;
      }
      
      it_state=0;
      while (igraph_set_iterate(&clqdata->buckets[v1], &it_state, &j1)) {
	j=(long)j1;
	v2=VECTOR(*neis1)[j];
	neis2 = igraph_i_adjlist_get(&clqdata->adj_list, v2);
	k = 0;
	while (k<VECTOR(clqdata->deg)[v2] && (v3=VECTOR(*neis2)[k])<=level) {
	  clqdata->IS[v3]++;
	  k++;
	}
      }
      igraph_set_clear(&clqdata->buckets[v1]);
    }
  }

  return 0;
}

void igraph_i_free_set_array(igraph_set_t* array) {
  long int i = 0;
  while (igraph_set_inited(array+i)) {
    igraph_set_destroy(array+i);
    i++;
  }
  Free(array);
}

/**
 * \function igraph_maximal_independent_vertex_sets
 * \brief Find all maximal independent vertex sets of a graph
 *
 * </para><para>
 * A maximal independent vertex set is an independent vertex set which
 * can't be extended any more by adding a new vertex to it.
 *
 * </para><para>
 * The algorithm used here is based on the following paper:
 * S. Tsukiyama, M. Ide, H. Ariyoshi and I. Shirawaka. A new algorithm for
 * generating all the maximal independent sets. SIAM J Computing,
 * 6:505--517, 1977.
 *
 * </para><para>
 * The implementation was originally written by Kevin O'Neill and modified
 * by K M Briggs in the Very Nauty Graph Library. I simply re-wrote it to
 * use igraph's data structures.
 * 
 * </para><para>
 * If you are interested in the size of the largest independent vertex set,
 * use \ref igraph_independence_number() instead.
 *
 * \param graph The input graph.
 * \param res Pointer to a pointer vector, the result will be stored
 *   here, ie. \c res will contain pointers to \c igraph_vector_t
 *   objects which contain the indices of vertices involved in an independent
 *   vertex set. The pointer vector will be resized if needed but note that the
 *   objects in the pointer vector will not be freed.
 * \return Error code.
 *
 * \sa \ref igraph_maximal_cliques(), \ref
 * igraph_independence_number()
 * 
 * Time complexity: TODO.
 */
int igraph_maximal_independent_vertex_sets(const igraph_t *graph,
					   igraph_vector_ptr_t *res) {
  igraph_i_max_ind_vsets_data_t clqdata;
  long int no_of_nodes = igraph_vcount(graph), i;

  clqdata.matrix_size=no_of_nodes;

  IGRAPH_CHECK(igraph_i_adjlist_init(graph, &clqdata.adj_list, IGRAPH_ALL));
  IGRAPH_FINALLY(igraph_i_adjlist_destroy, &clqdata.adj_list);

  clqdata.IS = Calloc(no_of_nodes, igraph_integer_t);
  if (clqdata.IS == 0)
    IGRAPH_ERROR("igraph_maximal_independent_vertex_sets failed", IGRAPH_ENOMEM);
  IGRAPH_FINALLY(igraph_free, clqdata.IS);

  IGRAPH_VECTOR_INIT_FINALLY(&clqdata.deg, no_of_nodes);
  for (i=0; i<no_of_nodes; i++)
    VECTOR(clqdata.deg)[i] = igraph_vector_size(igraph_i_adjlist_get(&clqdata.adj_list, i));

  clqdata.buckets = Calloc(no_of_nodes+1, igraph_set_t);
  if (clqdata.buckets == 0)
    IGRAPH_ERROR("igraph_maximal_independent_vertex_sets failed", IGRAPH_ENOMEM);
  IGRAPH_FINALLY(igraph_i_free_set_array, clqdata.buckets);

  for (i=0; i<no_of_nodes; i++)
    IGRAPH_CHECK(igraph_set_init(&clqdata.buckets[i], 0));

  igraph_vector_ptr_clear(res);
  
  /* Do the show */
  clqdata.largest_set_size=0;
  IGRAPH_CHECK(igraph_i_maximal_independent_vertex_sets_backtrack(graph, res, &clqdata, 0));

  /* Cleanup */
  for (i=0; i<no_of_nodes; i++) igraph_set_destroy(&clqdata.buckets[i]);
  igraph_i_adjlist_destroy(&clqdata.adj_list);
  igraph_vector_destroy(&clqdata.deg);
  igraph_free(clqdata.IS);
  igraph_free(clqdata.buckets);
  IGRAPH_FINALLY_CLEAN(4);
  return 0;
}

/**
 * \function igraph_independence_number
 * \brief Find the independence number of the graph
 *
 * </para><para>
 * The independence number of a graph is the cardinality of the largest
 * independent vertex set.
 *
 * \param graph The input graph.
 * \param no The independence number will be returned to the \c
 *   igraph_integer_t pointed by this variable.
 * \return Error code.
 * 
 * \sa \ref igraph_independent_vertex_sets().
 *
 * Time complexity: TODO.
 */
int igraph_independence_number(const igraph_t *graph, igraph_integer_t *no) {
  igraph_i_max_ind_vsets_data_t clqdata;
  long int no_of_nodes = igraph_vcount(graph), i;

  clqdata.matrix_size=no_of_nodes;

  IGRAPH_CHECK(igraph_i_adjlist_init(graph, &clqdata.adj_list, IGRAPH_ALL));
  IGRAPH_FINALLY(igraph_i_adjlist_destroy, &clqdata.adj_list);

  clqdata.IS = Calloc(no_of_nodes, igraph_integer_t);
  if (clqdata.IS == 0)
    IGRAPH_ERROR("igraph_independence_number failed", IGRAPH_ENOMEM);
  IGRAPH_FINALLY(igraph_free, clqdata.IS);

  IGRAPH_VECTOR_INIT_FINALLY(&clqdata.deg, no_of_nodes);
  for (i=0; i<no_of_nodes; i++)
    VECTOR(clqdata.deg)[i] = igraph_vector_size(igraph_i_adjlist_get(&clqdata.adj_list, i));

  clqdata.buckets = Calloc(no_of_nodes+1, igraph_set_t);
  if (clqdata.buckets == 0)
    IGRAPH_ERROR("igraph_independence_number failed", IGRAPH_ENOMEM);
  IGRAPH_FINALLY(igraph_i_free_set_array, clqdata.buckets);

  for (i=0; i<no_of_nodes; i++)
    IGRAPH_CHECK(igraph_set_init(&clqdata.buckets[i], 0));

  /* Do the show */
  clqdata.largest_set_size=0;
  IGRAPH_CHECK(igraph_i_maximal_independent_vertex_sets_backtrack(graph, 0, &clqdata, 0));
  *no = clqdata.largest_set_size;

  /* Cleanup */
  for (i=0; i<no_of_nodes; i++) igraph_set_destroy(&clqdata.buckets[i]);
  igraph_i_adjlist_destroy(&clqdata.adj_list);
  igraph_vector_destroy(&clqdata.deg);
  igraph_free(clqdata.IS);
  igraph_free(clqdata.buckets);
  IGRAPH_FINALLY_CLEAN(4);

  return 0;
}

/**
 * \function igraph_maximal_cliques
 * \brief Find all maximal cliques of a graph
 *
 * </para><para>
 * A maximal clique is a clique which
 * can't be extended any more by adding a new vertex to it. This is actually
 * implemented by looking for a maximal independent vertex set in the
 * complementer of the graph.
 * 
 * </para><para>
 * If you are only interested in the size of the largest clique in the graph,
 * use \ref igraph_clique_number() instead.
 *
 * \param graph The input graph.
 * \param res Pointer to a pointer vector, the result will be stored
 *   here, ie. \c res will contain pointers to \c igraph_vector_t
 *   objects which contain the indices of vertices involved in a clique.
 *   The pointer vector will be resized if needed but note that the
 *   objects in the pointer vector will not be freed.
 * \return Error code.
 *
 * \sa \ref igraph_maximal_independent_vertex_sets(), \ref
 * igraph_clique_number() 
 * 
 * Time complexity: TODO.
 */
int igraph_maximal_cliques(const igraph_t *graph, igraph_vector_ptr_t *res) {
  igraph_i_max_ind_vsets_data_t clqdata;
  long int no_of_nodes = igraph_vcount(graph), i;

  clqdata.matrix_size=no_of_nodes;

  IGRAPH_CHECK(igraph_i_adjlist_init_complementer(graph, &clqdata.adj_list,
						  IGRAPH_ALL, 0));
  IGRAPH_FINALLY(igraph_i_adjlist_destroy, &clqdata.adj_list);

  clqdata.IS = Calloc(no_of_nodes, igraph_integer_t);
  if (clqdata.IS == 0)
    IGRAPH_ERROR("igraph_maximal_cliques failed", IGRAPH_ENOMEM);
  IGRAPH_FINALLY(igraph_free, clqdata.IS);

  IGRAPH_VECTOR_INIT_FINALLY(&clqdata.deg, no_of_nodes);
  for (i=0; i<no_of_nodes; i++)
    VECTOR(clqdata.deg)[i] = igraph_vector_size(igraph_i_adjlist_get(&clqdata.adj_list, i));

  clqdata.buckets = Calloc(no_of_nodes+1, igraph_set_t);
  if (clqdata.buckets == 0)
    IGRAPH_ERROR("igraph_maximal_cliques failed", IGRAPH_ENOMEM);
  IGRAPH_FINALLY(igraph_i_free_set_array, clqdata.buckets);

  for (i=0; i<no_of_nodes; i++)
    IGRAPH_CHECK(igraph_set_init(&clqdata.buckets[i], 0));

  igraph_vector_ptr_clear(res);
  
  /* Do the show */
  clqdata.largest_set_size=0;
  IGRAPH_CHECK(igraph_i_maximal_independent_vertex_sets_backtrack(graph, res, &clqdata, 0));

  /* Cleanup */
  for (i=0; i<no_of_nodes; i++) igraph_set_destroy(&clqdata.buckets[i]);
  igraph_i_adjlist_destroy(&clqdata.adj_list);
  igraph_vector_destroy(&clqdata.deg);
  igraph_free(clqdata.IS);
  igraph_free(clqdata.buckets);
  IGRAPH_FINALLY_CLEAN(4);
  return 0;
}

/**
 * \function igraph_clique_number
 * \brief Find the clique number of the graph
 *
 * </para><para>
 * The clique number of a graph is the size of the largest clique.
 *
 * \param graph The input graph.
 * \param no The clique number will be returned to the \c igraph_integer_t
 *   pointed by this variable.
 * \return Error code.
 * 
 * \sa \ref igraph_cliques(), \ref igraph_largest_cliques().
 * 
 * Time complexity: TODO.
 */
int igraph_clique_number(const igraph_t *graph, igraph_integer_t *no) {
  igraph_i_max_ind_vsets_data_t clqdata;
  long int no_of_nodes = igraph_vcount(graph), i;

  clqdata.matrix_size=no_of_nodes;

  IGRAPH_CHECK(igraph_i_adjlist_init_complementer(graph, &clqdata.adj_list,
						  IGRAPH_ALL, 0));
  IGRAPH_FINALLY(igraph_i_adjlist_destroy, &clqdata.adj_list);

  clqdata.IS = Calloc(no_of_nodes, igraph_integer_t);
  if (clqdata.IS == 0)
    IGRAPH_ERROR("igraph_clique_number failed", IGRAPH_ENOMEM);
  IGRAPH_FINALLY(igraph_free, clqdata.IS);

  IGRAPH_VECTOR_INIT_FINALLY(&clqdata.deg, no_of_nodes);
  for (i=0; i<no_of_nodes; i++)
    VECTOR(clqdata.deg)[i] = igraph_vector_size(igraph_i_adjlist_get(&clqdata.adj_list, i));

  clqdata.buckets = Calloc(no_of_nodes+1, igraph_set_t);
  if (clqdata.buckets == 0)
    IGRAPH_ERROR("igraph_clique_number failed", IGRAPH_ENOMEM);
  IGRAPH_FINALLY(igraph_i_free_set_array, clqdata.buckets);

  for (i=0; i<no_of_nodes; i++)
    IGRAPH_CHECK(igraph_set_init(&clqdata.buckets[i], 0));

  /* Do the show */
  clqdata.largest_set_size=0;
  IGRAPH_CHECK(igraph_i_maximal_independent_vertex_sets_backtrack(graph, 0, &clqdata, 0));
  *no = clqdata.largest_set_size;

  /* Cleanup */
  for (i=0; i<no_of_nodes; i++) igraph_set_destroy(&clqdata.buckets[i]);
  igraph_i_adjlist_destroy(&clqdata.adj_list);
  igraph_vector_destroy(&clqdata.deg);
  igraph_free(clqdata.IS);
  igraph_free(clqdata.buckets);
  IGRAPH_FINALLY_CLEAN(4);

  return 0;
}