Fix bug in C++ matching code

cpp/mwmatching.h

@@ -1204,12 +1204,12 @@ struct MatchingContext
                                         blossom->subblossoms,
                                         entry_it,
                                         sub_end);
-
-            // Update the base vertex.
-            // We can pull the new base vertex from the entry sub-blossom
-            // since its augmentation has already finished.
-            blossom->base_vertex = entry->base_vertex;
         }
+
+        // Update the base vertex.
+        // We can pull the new base vertex from the entry sub-blossom
+        // since its augmentation has already finished.
+        blossom->base_vertex = entry->base_vertex;
     }
 
     /**
@@ -1259,12 +1259,8 @@ struct MatchingContext
     {
         // Check that the path starts and ends in an unmatched blossom.
         assert(path.edges.size() % 2 == 1);
-        assert(vertex_mate[
+        assert(vertex_mate[vertex_top_blossom[path.edges.front().first]->base_vertex] == NO_VERTEX);
-               vertex_top_blossom[path.edges.front().first]->base_vertex]
+        assert(vertex_mate[vertex_top_blossom[path.edges.back().second]->base_vertex] == NO_VERTEX);
-               == NO_VERTEX);
-        assert(vertex_mate[
-               vertex_top_blossom[path.edges.back().second]->base_vertex]
-               == NO_VERTEX);
 
         // Process the unmatched edges on the augmenting path.
         auto edge_it = path.edges.begin();

cpp/test_mwmatching.cpp

@@ -310,6 +310,21 @@ BOOST_AUTO_TEST_CASE(test48_expand_unlabeled_nested)
     BOOST_TEST(mwmatching::maximum_weight_matching(edges) == expect);
 }
 
+BOOST_AUTO_TEST_CASE(test51_augment_blossom_nested)
+{
+    /*
+     *               19--[1]--17--[4]--11--[7]
+     *              /     |        |
+     *  [3]--15--[2]     19       18
+     *              \     |        |
+     *               21--[5]--19--[6]
+     */
+    EdgeVectorLong edges = {
+        {1, 2, 19}, {1, 4, 17}, {1, 5, 19}, {2, 3, 15}, {2, 5, 21}, {4, 6, 18}, {4, 7, 11}, {5, 6, 19}};
+    Matching expect = {{1, 5}, {2, 3}, {4, 6}};
+    BOOST_TEST(mwmatching::maximum_weight_matching(edges) == expect);
+}
+
 BOOST_AUTO_TEST_CASE(test_fail_bad_input)
 {
     EdgeVectorDouble inf_weight = {{1, 2, std::numeric_limits<double>::infinity()}};

python/test_mwmatching.py

@@ -230,6 +230,19 @@ class TestMaximumWeightMatching(unittest.TestCase):
             mwm([(1,2,12), (1,4,11), (1,5,11), (2,4,12), (2,6,11), (3,4,14), (3,7,14), (4,7,11)]),
             [(1,5), (2,4), (3,7)])
 
+    def test51_augment_blossom_nested(self):
+        """augment nested blossoms with common base vertex"""
+        #
+        #               19--[1]--17--[4]--11--[7]
+        #              /     |        |
+        #  [3]--15--[2]     19       18
+        #              \     |        |
+        #               21--[5]--19--[6]
+        #
+        self.assertEqual(
+            mwm([(1,2,19), (1,4,17), (1,5,19), (2,3,15), (2,5,21), (4,6,18), (4,7,11), (5,6,19)]),
+            [(1,5), (2,3), (4,6)])
+
     def test_fail_bad_input(self):
         """bad input values"""
         with self.assertRaises(TypeError):