Fixed calculation of minimal spanning trees of graphs with negative-w…

…eighted edges.

Changelog

@@ -5,7 +5,7 @@ A library for working with graphs in Python
 CHANGELOG
 
 
-Next Release	[??? ??, ????]
+Release 1.8.1	[??? ??, ????]
 
 Enhancements:
 	Shortest-path now executes in O(n*log(n)) instead of O(n^2).
@@ -16,7 +16,8 @@ Fixes:
 	Linking, unlinking and relinking a hypernode to hyperedge now works as expected;
 	Graph comparison does not raise exceptions anymore;
 	Fixed undesired sharing of attribute lists;
-	Fixed reading of XML-stored graphs with edge attributes.
+	Fixed reading of XML-stored graphs with edge attributes;
+	Fixed calculation of minimal spanning trees of graphs with negative-weighted edges.
 
 
 Release 1.8.0	[Oct 01, 2010]

core/pygraph/algorithms/minmax.py

@@ -72,7 +72,7 @@ def minimal_spanning_tree(graph, root=None):
     # Algorithm loop
     while (nroot is not None):
         ledge = _lightest_edge(graph, visited)
-        if (ledge == (-1, -1)):
+        if (ledge == None):
             if (root is not None):
                 break
             nroot = _first_unvisited(graph, visited)
@@ -118,13 +118,13 @@ def _lightest_edge(graph, visited):
     @rtype:  tuple
     @return: Lightest edge in graph going from a visited node to an unvisited one.
     """
-    lightest_edge = (-1, -1)
-    weight = -1
+    lightest_edge = None
+    weight = None
     for each in visited:
         for other in graph[each]:
             if (other not in visited):
                 w = graph.edge_weight((each, other))
-                if (w < weight or weight < 0):
+                if (weight is None or w < weight or weight < 0):
                     lightest_edge = (each, other)
                     weight = w
     return lightest_edge

tests/unittests-minmax.py

@@ -115,15 +115,6 @@ def test_minimal_spanning_tree_on_graph(self):
                          add_spanning_tree(gr2, mst_copy)
                          assert len(depth_first_search(gr2, root=0)[0]) < len_dfs
 
-    def test_minimal_spanning_tree_with_negative_weigths(self):
-        gr = graph()
-        gr.add_nodes((0,1,2,3))
-        gr.add_edge((0,1), wt=-0.4)
-        gr.add_edge((1,2), wt=-0.6)
-        gr.add_edge((2,0), wt=-0.5)
-        gr.add_edge((3,2), wt=-0.1)
-        gr.add_edge((3,1), wt=-0.2)
-        print minimal_spanning_tree(gr, 0)
 
 # shortest path tests