Fix the minimum spanning tree algorithm.

Tinc uses Kruskal's algorithm to calculate a MST. However, this was broken in
commit 6e80da3370249caa1082c23c3ef55f338d1e9e74. Revert back to the working
algorithm from tinc 1.0.

Thanks to Cheng LI for spotting the problem.

diff --git a/src/graph.c b/src/graph.c
index 73db3b2..4cf30eb 100644 (file)
--- a/src/graph.c
+++ b/src/graph.c
@@ -61,7 +61,7 @@
 #include "graph.h"
 
 /* Implementation of Kruskal's algorithm.
-   Running time: O(E)
+   Running time: O(EN)
    Please note that sorting on weight is already done by add_edge().
 */
 
@@ -78,11 +78,24 @@ static void mst_kruskal(void) {
        for splay_each(node_t, n, node_tree)
                n->status.visited = false;
 
+       /* Starting point */
+
+       for splay_each(edge_t, e, edge_weight_tree) {
+               if(e->from->status.reachable) {
+                       e->from->status.visited = true;
+                       break;
+               }
+       }
+
        /* Add safe edges */
 
+       bool skipped = false;
+
        for splay_each(edge_t, e, edge_weight_tree) {
-               if(!e->reverse || (e->from->status.visited && e->to->status.visited))
+               if(!e->reverse || (e->from->status.visited == e->to->status.visited)) {
+                       skipped = true;
                        continue;
+               }
 
                e->from->status.visited = true;
                e->to->status.visited = true;
@@ -93,8 +106,12 @@ static void mst_kruskal(void) {
                if(e->reverse->connection)
                        e->reverse->connection->status.mst = true;
 
-               logger(DEBUG_SCARY_THINGS, LOG_DEBUG, " Adding edge %s - %s weight %d", e->from->name,
-                                  e->to->name, e->weight);
+               logger(DEBUG_SCARY_THINGS, LOG_DEBUG, " Adding edge %s - %s weight %d", e->from->name, e->to->name, e->weight);
+
+               if(skipped) {
+                       skipped = false;
+                       next = edge_weight_tree->head;
+               }
        }
 }