bridge.c

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/*!
   \file lib/vector/neta/bridge.c
 
   \brief Network Analysis library - bridges
 
   Computes number of bridges in the graph.
 
   SPDX-FileCopyrightText: 2009-2010 Daniel Bundala
   SPDX-FileCopyrightText: GRASS Development Team
   SPDX-License-Identifier: GPL-2.0-or-later
 
   \author Daniel Bundala (Google Summer of Code 2009)
 */
 
#include <stdio.h>
#include <stdlib.h>
#include <grass/gis.h>
#include <grass/vector.h>
#include <grass/glocale.h>
#include <grass/dgl/graph.h>
 
/*!
   \brief Get number of bridges in the graph.
 
   Bridge is an array containing the indices of the bridges.
 
   \param graph input graph
   \param[out] bridge_list list of bridges
 
   \return number of bridges, -1 on error
 */
int NetA_compute_bridges(dglGraph_s *graph, struct ilist *bridge_list)
{
    int nnodes;
    int bridges = 0;
 
    dglEdgesetTraverser_s
        *current;       /*edge to be processed when the node is visited */
    int *tin, *min_tin; /*time in, and smallest tin over all successors. 0 if
                           not yet visited */
    dglInt32_t *parent; /*edge from parent to the node */
    dglInt32_t **stack; /*stack of nodes */
    dglInt32_t **current_edge; /*current edge for each node */
    dglNodeTraverser_s nt;
    dglInt32_t *current_node;
    int stack_size;
    int i, time;
 
    nnodes = dglGet_NodeCount(graph);
    current = (dglEdgesetTraverser_s *)G_calloc(nnodes + 1,
                                                sizeof(dglEdgesetTraverser_s));
    tin = (int *)G_calloc(nnodes + 1, sizeof(int));
    min_tin = (int *)G_calloc(nnodes + 1, sizeof(int));
    parent = (dglInt32_t *)G_calloc(nnodes + 1, sizeof(dglInt32_t));
    stack = (dglInt32_t **)G_calloc(nnodes + 1, sizeof(dglInt32_t *));
    current_edge = (dglInt32_t **)G_calloc(nnodes + 1, sizeof(dglInt32_t *));
    if (!tin || !min_tin || !parent || !stack || !current) {
        G_fatal_error(_("Out of memory"));
        return -1;
    }
 
    for (i = 1; i <= nnodes; i++) {
        dglEdgeset_T_Initialize(
            &current[i], graph,
            dglNodeGet_OutEdgeset(graph, dglGetNode(graph, i)));
        current_edge[i] = dglEdgeset_T_First(&current[i]);
        tin[i] = 0;
    }
 
    dglNode_T_Initialize(&nt, graph);
 
    time = 0;
    for (current_node = dglNode_T_First(&nt); current_node;
         current_node = dglNode_T_Next(&nt)) {
        dglInt32_t current_id = dglNodeGet_Id(graph, current_node);
 
        if (tin[current_id] == 0) {
            stack[0] = current_node;
            stack_size = 1;
            parent[current_id] = 0;
            while (stack_size) {
                dglInt32_t *node = stack[stack_size - 1];
                dglInt32_t node_id = dglNodeGet_Id(graph, node);
 
                if (tin[node_id] == 0) /*vertex visited for the first time */
                    min_tin[node_id] = tin[node_id] = ++time;
                else { /*return from the recursion */
                    dglInt32_t to = dglNodeGet_Id(
                        graph, dglEdgeGet_Tail(graph, current_edge[node_id]));
                    if (min_tin[to] >
                        tin[node_id]) { /*no path from the subtree above the
                                           current node */
                        Vect_list_append(
                            bridge_list,
                            dglEdgeGet_Id(
                                graph, current_edge[node_id])); /*so it must be
                                                                   a bridge */
                        bridges++;
                    }
                    if (min_tin[to] < min_tin[node_id])
                        min_tin[node_id] = min_tin[to];
                    current_edge[node_id] = dglEdgeset_T_Next(
                        &current[node_id]); /*proceed to the next edge */
                }
                for (; current_edge[node_id];
                     current_edge[node_id] = dglEdgeset_T_Next(
                         &current[node_id])) { /*try next edges */
                    dglInt32_t *to =
                        dglEdgeGet_Tail(graph, current_edge[node_id]);
                    dglInt32_t edge_id =
                        dglEdgeGet_Id(graph, current_edge[node_id]);
                    if (labs(edge_id) == parent[node_id])
                        continue; /*skip edge we used to travel to this node */
                    int to_id = dglNodeGet_Id(graph, to);
 
                    if (tin[to_id]) { /*back edge, cannot be a
                                         bridge/articualtion point */
                        if (tin[to_id] < min_tin[node_id])
                            min_tin[node_id] = tin[to_id];
                    }
                    else { /*forward edge */
                        parent[to_id] = labs(edge_id);
                        stack[stack_size++] = to;
                        break;
                    }
                }
                if (!current_edge[node_id])
                    stack_size--; /*current node completely processed */
            }
        }
    }
 
    dglNode_T_Release(&nt);
    for (i = 1; i <= nnodes; i++)
        dglEdgeset_T_Release(&current[i]);
 
    G_free(current);
    G_free(tin);
    G_free(min_tin);
    G_free(parent);
    G_free(stack);
    G_free(current_edge);
    return bridges;
}