Merge branch '3877-shrink-symmetry-detection-graph' into 'master'

Resolve "Shrink symmetry detection graph"

Closes #3877

See merge request integer/scip!3778

Author: Christopher Hojny

CHANGELOG

@@ -39,6 +39,7 @@ Performance improvements
 ------------------------
 
 - reimplemented SCIPvarGetActiveRepresentatives() by using dense arrays to avoid repeated resorting.
+- simplified symmetry detection graphs in case all edges have the same color
 
 Examples and applications
 -------------------------

src/scip/symmetry_graph.c

@@ -1539,12 +1539,14 @@ SCIP_RETCODE SCIPcomputeSymgraphColors(
    {
       SCIPsort(perm, SYMsortEdges, (void*) graph, graph->nedges);
 
-      /* check whether edges are colored; due to sorting, only check first edge */
-      if( SCIPisInfinity(scip, graph->edgevals[perm[0]]) )
+      /* check for uncolored or uniformly colored edges, which is easy due to sorting */
+      if( SCIPisInfinity(scip, graph->edgevals[perm[0]]) ||
+         SCIPisEQ(scip, graph->edgevals[perm[0]], graph->edgevals[perm[graph->nedges - 1]]) )
       {
-         /* all edges are uncolored */
+         /* when all edges are uncolored or uniformly colored, we can ignore edge colors */
+         graph->uniqueedgetype = TRUE;
          for( i = 0; i < graph->nedges; ++i )
-            graph->edgecolors[perm[i]] = -1;
+            graph->edgecolors[i] = -1;
       }
       else
       {
@@ -1567,10 +1569,6 @@ SCIP_RETCODE SCIPcomputeSymgraphColors(
             prevval = thisval;
          }
 
-         /* check whether all edges are equivalent */
-         if( i == graph->nedges && graph->edgecolors[perm[0]] == graph->edgecolors[perm[i-1]] )
-            graph->uniqueedgetype = TRUE;
-
          /* assign uncolored edges color -1 */
          for( ; i < graph->nedges; ++i )
             graph->edgecolors[perm[i]] = -1;