lwunionfind.c

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/**********************************************************************
 *
 * PostGIS - Spatial Types for PostgreSQL
 * http://postgis.net
 *
 * PostGIS 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.
 *
 * PostGIS 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 PostGIS.  If not, see <http://www.gnu.org/licenses/>.
 *
 **********************************************************************
 *
 * Copyright 2015 Daniel Baston <dbaston@gmail.com>
 *
 **********************************************************************/


#include "liblwgeom.h"
#include "lwunionfind.h"
#include <string.h>

static int cmp_int(const void *a, const void *b);
static int cmp_int_ptr(const void *a, const void *b);

UNIONFIND*
UF_create(uint32_t N)
{
        size_t i;
        UNIONFIND* uf = lwalloc(sizeof(UNIONFIND));
        uf->N = N;
        uf->num_clusters = N;
        uf->clusters = lwalloc(N * sizeof(uint32_t));
        uf->cluster_sizes = lwalloc(N * sizeof(uint32_t));

        for (i = 0; i < N; i++)
        {
                uf->clusters[i] = i;
                uf->cluster_sizes[i] = 1;
        }

        return uf;
}

void
UF_destroy(UNIONFIND* uf)
{
        lwfree(uf->clusters);
        lwfree(uf->cluster_sizes);
        lwfree(uf);
}

uint32_t
UF_find (UNIONFIND* uf, uint32_t i)
{
        uint32_t base = i;
        while (uf->clusters[base] != base) {
                base = uf->clusters[base];
        }

        while (i != base) {
                uint32_t next = uf->clusters[i];
                uf->clusters[i] = base;
                i = next;
        }

        return i;
}

uint32_t
UF_size (UNIONFIND* uf, uint32_t i)
{
    return uf->cluster_sizes[UF_find(uf, i)];
}

void
UF_union(UNIONFIND* uf, uint32_t i, uint32_t j)
{
        uint32_t a = UF_find(uf, i);
        uint32_t b = UF_find(uf, j);

        if (a == b)
        {
                return;
        }

        if (uf->cluster_sizes[a] < uf->cluster_sizes[b] ||
                (uf->cluster_sizes[a] == uf->cluster_sizes[b] && a > b))
        {
                uf->clusters[a] = uf->clusters[b];
                uf->cluster_sizes[b] += uf->cluster_sizes[a];
                uf->cluster_sizes[a] = 0;
        }
        else
        {
                uf->clusters[b] = uf->clusters[a];
                uf->cluster_sizes[a] += uf->cluster_sizes[b];
                uf->cluster_sizes[b] = 0;
        }

        uf->num_clusters--;
}

uint32_t*
UF_ordered_by_cluster(UNIONFIND* uf)
{
        size_t i;
        uint32_t** cluster_id_ptr_by_elem_id = lwalloc(uf->N * sizeof (uint32_t*));
        uint32_t* ordered_ids = lwalloc(uf->N * sizeof (uint32_t));

        for (i = 0; i < uf->N; i++)
        {
                /* Make sure each value in uf->clusters is pointing to the
                 * root of the cluster.
                 * */
                UF_find(uf, i);
                cluster_id_ptr_by_elem_id[i] = &(uf->clusters[i]);
        }

        /* Sort the array of cluster id pointers, so that pointers to the
         * same cluster id are grouped together.
         * */
        qsort(cluster_id_ptr_by_elem_id, uf->N, sizeof (uint32_t*), &cmp_int_ptr);

        /* Recover the input element ids from the cluster id pointers, so
         * we can return element ids grouped by cluster id.
         * */
        for (i = 0; i < uf-> N; i++)
        {
                ordered_ids[i] = (cluster_id_ptr_by_elem_id[i] - uf->clusters);
        }

        lwfree(cluster_id_ptr_by_elem_id);
        return ordered_ids;
}

uint32_t*
UF_get_collapsed_cluster_ids(UNIONFIND* uf, const char* is_in_cluster)
{
        uint32_t* ordered_components = UF_ordered_by_cluster(uf);
        uint32_t* new_ids = lwalloc(uf->N * sizeof(uint32_t));
        uint32_t last_old_id, current_new_id, i;
        char encountered_cluster = LW_FALSE;

        current_new_id = 0; last_old_id = 0;
        for (i = 0; i < uf->N; i++)
        {
                uint32_t j = ordered_components[i];
                if (!is_in_cluster || is_in_cluster[j])
                {
                        uint32_t current_old_id = UF_find(uf, j);
                        if (!encountered_cluster)
                        {
                                encountered_cluster = LW_TRUE;
                                last_old_id = current_old_id;
                        }

                        if (current_old_id != last_old_id)
                                current_new_id++;

                        new_ids[j] = current_new_id;
                        last_old_id = current_old_id;
                }
        }

        lwfree(ordered_components);

        return new_ids;
}

static int
cmp_int(const void *a, const void *b)
{
        if (*((uint32_t*) a) > *((uint32_t*) b))
        {
                return 1;
        }
        else if (*((uint32_t*) a) < *((uint32_t*) b))
        {
                return -1;
        }
        else
        {
                return 0;
        }
}

static int
cmp_int_ptr(const void *a, const void *b)
{
        int val_cmp = cmp_int(*((uint32_t**) a), *((uint32_t**) b));
        if (val_cmp != 0)
        {
                return val_cmp;
        }
        if (a > b)
        {
                return 1;
        }
        if (a < b)
        {
                return -1;
        }
        return 0;
}