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Simplified implementation of Kruskal's minimum spanning tree algorithm.

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Guus Sliepen 2002-03-12 13:42:23 +00:00
parent d2e0ed533c
commit 2e7db2a693
1 changed files with 17 additions and 35 deletions
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50
src/graph.c
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@ -17,7 +17,7 @@
along with this program; if not, write to the Free Software along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
$Id: graph.c,v 1.1.2.7 2002/02/18 16:25:16 guus Exp $ $Id: graph.c,v 1.1.2.8 2002/03/12 13:42:23 guus Exp $
*/ */
/* We need to generate two trees from the graph: /* We need to generate two trees from the graph:
@ -32,9 +32,7 @@
favour Kruskal's, because we make an extra AVL tree of edges sorted on favour Kruskal's, because we make an extra AVL tree of edges sorted on
weights (metric). That tree only has to be updated when an edge is added or weights (metric). That tree only has to be updated when an edge is added or
removed, and during the MST algorithm we just have go linearly through that removed, and during the MST algorithm we just have go linearly through that
tree, adding safe edges until #edges = #nodes - 1. The implementation here tree.
however is not so fast, because I tried to avoid having to make a forest and
merge trees.
For the SSSP algorithm Dijkstra's seems to be a nice choice. Currently a For the SSSP algorithm Dijkstra's seems to be a nice choice. Currently a
simple breadth-first search is presented here. simple breadth-first search is presented here.
@ -62,20 +60,25 @@
#include "system.h" #include "system.h"
/* Implementation of Kruskal's algorithm. /* Kruskal's minimum spanning tree algorithm.
Running time: O(EN) Running time: O(E)
Please note that sorting on weight is already done by add_edge(). Edges are already sorted on weight.
*/ */
void mst_kruskal(void) void mst_kruskal(void)
{ {
avl_node_t *node, *next; avl_node_t *node;
edge_t *e; edge_t *e;
node_t *n; node_t *n;
connection_t *c; connection_t *c;
int nodes = 0;
int safe_edges = 0; /* Clear MST status on connections */
int skipped;
for(node = connection_tree->head; node; node = node->next)
{
c = (connection_t *)node->data;
c->status.mst = 0;
}
/* Do we have something to do at all? */ /* Do we have something to do at all? */
@ -88,46 +91,25 @@ void mst_kruskal(void)
{ {
n = (node_t *)node->data; n = (node_t *)node->data;
n->status.visited = 0; n->status.visited = 0;
nodes++;
} }
/* Starting point */ /* Starting point */
((edge_t *)edge_weight_tree->head->data)->from.node->status.visited = 1; ((edge_t *)edge_weight_tree->head->data)->from.node->status.visited = 1;
/* Clear MST status on connections */
for(node = connection_tree->head; node; node = node->next)
{
c = (connection_t *)node->data;
c->status.mst = 0;
}
/* Add safe edges */ /* Add safe edges */
for(skipped = 0, node = edge_weight_tree->head; node; node = next) for(node = edge_weight_tree->head; node; node = node->next)
{ {
next = node->next;
e = (edge_t *)node->data; e = (edge_t *)node->data;
if(e->from.node->status.visited == e->to.node->status.visited) if(e->from.node->status.visited && e->to.node->status.visited)
{
skipped = 1;
continue; continue;
}
e->from.node->status.visited = 1; e->from.node->status.visited = 1;
e->to.node->status.visited = 1; e->to.node->status.visited = 1;
if(e->connection) if(e->connection)
e->connection->status.mst = 1; e->connection->status.mst = 1;
safe_edges++;
if(skipped)
{
next = edge_weight_tree->head;
continue;
}
} }
} }