/*

   IGraph library.

   Copyright (C) 2006-2020 The igraph development team

   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_operators.h"

#include "igraph_constructors.h"

#include "igraph_interface.h"

#include "core/fixed_vectorlist.h"

#include "graph/attributes.h"

/**

 * \ingroup structural

 * \function igraph_simplify

 * \brief Removes loop and/or multiple edges from the graph.

 *

 * This function merges parallel edges and removes self-loops, according

 * to the \p multiple and \p loops parameters. Note that this function

 * may change the edge order, even if the input was already a simple graph.

 *

 * \param graph The graph object.

 * \param remove_multiple If true, multiple edges will be removed.

 * \param remove_loops If true, loops (self edges) will be removed.

 * \param edge_comb What to do with the edge attributes. \c NULL means to

 *        discard the edge attributes after the operation, even for edges

 *        that were unaffected. See the igraph manual section about attributes

 *        for details.

 * \return Error code:

 *    \c IGRAPH_ENOMEM if we are out of memory.

 *

 * Time complexity: O(|V|+|E|).

 *

 * \example examples/simple/igraph_simplify.c

 */

igraph_error_t igraph_simplify(igraph_t *graph,

                               igraph_bool_t remove_multiple, igraph_bool_t remove_loops,

                               const igraph_attribute_combination_t *edge_comb) {

    igraph_vector_int_t edges;

    igraph_integer_t no_of_nodes = igraph_vcount(graph);

    igraph_integer_t no_of_edges = igraph_ecount(graph);

    igraph_integer_t edge;

    igraph_bool_t attr = edge_comb && igraph_has_attribute_table();

    igraph_integer_t from, to, pfrom = -1, pto = -2;

    igraph_t res;

    igraph_es_t es;

    igraph_eit_t eit;

    igraph_vector_int_t mergeinto;

    igraph_integer_t actedge;

    /* if we already know there are no multi-edges, they don't need to be removed */

    if (igraph_i_property_cache_has(graph, IGRAPH_PROP_HAS_MULTI) &&

        !igraph_i_property_cache_get_bool(graph, IGRAPH_PROP_HAS_MULTI)) {

        remove_multiple = false;

    }

    /* if we already know there are no loops, they don't need to be removed */

    if (igraph_i_property_cache_has(graph, IGRAPH_PROP_HAS_LOOP) &&

        !igraph_i_property_cache_get_bool(graph, IGRAPH_PROP_HAS_LOOP)) {

        remove_loops = false;

    }

    if (!remove_multiple && !remove_loops)

        /* nothing to do */

    {

        return IGRAPH_SUCCESS;

    }

    if (!remove_multiple) {

        igraph_vector_int_t edges_to_delete;

        /* removing loop edges only, this is simple. No need to combine anything

         * and the whole process can be done in-place */

        IGRAPH_VECTOR_INT_INIT_FINALLY(&edges_to_delete, 0);

        IGRAPH_CHECK(igraph_es_all(&es, IGRAPH_EDGEORDER_ID));

        IGRAPH_FINALLY(igraph_es_destroy, &es);

        IGRAPH_CHECK(igraph_eit_create(graph, es, &eit));

        IGRAPH_FINALLY(igraph_eit_destroy, &eit);

        while (!IGRAPH_EIT_END(eit)) {

            edge = IGRAPH_EIT_GET(eit);

            from = IGRAPH_FROM(graph, edge);

            to = IGRAPH_TO(graph, edge);

            if (from == to) {

                IGRAPH_CHECK(igraph_vector_int_push_back(&edges_to_delete, edge));

            }

            IGRAPH_EIT_NEXT(eit);

        }

        igraph_eit_destroy(&eit);

        igraph_es_destroy(&es);

        IGRAPH_FINALLY_CLEAN(2);

        if (igraph_vector_int_size(&edges_to_delete) > 0) {

            IGRAPH_CHECK(igraph_delete_edges(graph, igraph_ess_vector(&edges_to_delete)));

        }

        igraph_vector_int_destroy(&edges_to_delete);

        IGRAPH_FINALLY_CLEAN(1);

        igraph_i_property_cache_set_bool_checked(graph, IGRAPH_PROP_HAS_LOOP, false);

        return IGRAPH_SUCCESS;

    }

    if (attr) {

        IGRAPH_VECTOR_INT_INIT_FINALLY(&mergeinto, no_of_edges);

    }

    IGRAPH_VECTOR_INT_INIT_FINALLY(&edges, 0);

    IGRAPH_CHECK(igraph_vector_int_reserve(&edges, no_of_edges * 2));

    IGRAPH_CHECK(igraph_es_all(&es, IGRAPH_EDGEORDER_FROM));

    IGRAPH_FINALLY(igraph_es_destroy, &es);

    IGRAPH_CHECK(igraph_eit_create(graph, es, &eit));

    IGRAPH_FINALLY(igraph_eit_destroy, &eit);

    for (actedge = -1; !IGRAPH_EIT_END(eit); IGRAPH_EIT_NEXT(eit)) {

        edge = IGRAPH_EIT_GET(eit);

        from = IGRAPH_FROM(graph, edge);

        to = IGRAPH_TO(graph, edge);

        if (remove_loops && from == to) {

            /* Loop edge to be removed */

            if (attr) {

                VECTOR(mergeinto)[edge] = -1;

            }

        } else if (remove_multiple && from == pfrom && to == pto) {

            /* Multiple edge to be contracted */

            if (attr) {

                VECTOR(mergeinto)[edge] = actedge;

            }

        } else {

            /* Edge to be kept */

            igraph_vector_int_push_back(&edges, from);  /* reserved */

            igraph_vector_int_push_back(&edges, to);  /* reserved */

            if (attr) {

                actedge++;

                VECTOR(mergeinto)[edge] = actedge;

            }

        }

        pfrom = from; pto = to;

    }

    igraph_eit_destroy(&eit);

    igraph_es_destroy(&es);

    IGRAPH_FINALLY_CLEAN(2);

    IGRAPH_CHECK(igraph_create(&res, &edges, no_of_nodes, igraph_is_directed(graph)));

    igraph_vector_int_destroy(&edges);

    IGRAPH_FINALLY_CLEAN(1);

    IGRAPH_FINALLY(igraph_destroy, &res);

    IGRAPH_CHECK(igraph_i_attribute_copy(&res, graph, true, true, /* edges= */ false));

    if (attr) {

        igraph_fixed_vectorlist_t vl;

        IGRAPH_CHECK(igraph_fixed_vectorlist_convert(&vl, &mergeinto, actedge + 1));

        IGRAPH_FINALLY(igraph_fixed_vectorlist_destroy, &vl);

        IGRAPH_CHECK(igraph_i_attribute_combine_edges(graph, &res, &vl.vecs, edge_comb));

        igraph_fixed_vectorlist_destroy(&vl);

        igraph_vector_int_destroy(&mergeinto);

        IGRAPH_FINALLY_CLEAN(2);

    }

    IGRAPH_FINALLY_CLEAN(1);

    igraph_destroy(graph);

    *graph = res;

    /* The cache must be set as the very last step, only after all functions that can

     * potentially return with an error have finished. */

    if (remove_loops) {

        /* Loop edges were removed so we know for sure that there aren't any

         * loop edges now */

        igraph_i_property_cache_set_bool_checked(graph, IGRAPH_PROP_HAS_LOOP, false);

    }

    if (remove_multiple) {

        /* Multi-edges were removed so we know for sure that there aren't any

         * multi-edges now */

        igraph_i_property_cache_set_bool_checked(graph, IGRAPH_PROP_HAS_MULTI, false);

    }

    return IGRAPH_SUCCESS;

}