Ticket #391: 391-2-f6f2a3139f64.patch

lemon/network_simplex.h

@@ -636 +636 @@
     /// The constructor of the class.
     ///
     /// \param graph The digraph the algorithm runs on.
-    /// \param arc_mixing Indicate if the arcs have to be stored in a
+    /// \param arc_mixing Indicate if the arcs will be stored in a
     /// mixed order in the internal data structure.
-    /// In special cases, it could lead to better overall performance,
-    /// but it is usually slower. Therefore it is disabled by default.
-    NetworkSimplex(const GR& graph, bool arc_mixing = false) :
+    /// In general, it leads to similar performance as using the original
+    /// arc order, but it makes the algorithm more robust and for certain
+    /// problem instances, even significantly faster.
+    /// Therefore, it is enabled by default.
+    NetworkSimplex(const GR& graph, bool arc_mixing = true) :
       _graph(graph), _node_id(graph), _arc_id(graph),
       _arc_mixing(arc_mixing),
       MAX(std::numeric_limits<Value>::max()),
@@ -930 +932 @@
       }
       if (_arc_mixing) {
         // Store the arcs in a mixed order
-        int k = std::max(int(std::sqrt(double(_arc_num))), 10);
+        int skip = int(std::sqrt(double(_arc_num * _arc_num) / _node_num));
+        int k = std::max(skip, 3);
         int i = 0, j = 0;
         for (ArcIt a(_graph); a != INVALID; ++a) {
           _arc_id[a] = i;