Closes #384

demes/demes.py

@@ -2220,7 +2220,7 @@ def collapse_demes(pairs):
                 source = migration["source"]
                 dest = migration["dest"]
                 time_hi = min(self[source].start_time, self[dest].start_time)
-                time_lo = max(self[dest].end_time, self[dest].end_time)
+                time_lo = max(self[source].end_time, self[dest].end_time)
                 if migration["end_time"] == time_lo:
                     del migration["end_time"]
                 if migration["start_time"] == time_hi:

tests/test_demes.py

@@ -4045,6 +4045,23 @@ def test_simplify_symmetric_migrations_branching_demography(self):
         assert len(d["migrations"]) == 1
         assert dict(demes=["a", "b", "c"], rate=0.01) in d["migrations"]
 
+    def test_simplify_symmetric_migrations_demography_with_splits(self):
+        b = Builder()
+        b.add_deme("anc", epochs=[dict(start_size=1, end_time=10)])
+        b.add_deme("a", ancestors=["anc"], epochs=[dict(start_size=1)])
+        b.add_deme("b", ancestors=["anc"], epochs=[dict(start_size=1, end_time=5)])
+        b.add_deme("c", ancestors=["b"], epochs=[dict(start_size=1)])
+        b.add_deme("d", ancestors=["b"], epochs=[dict(start_size=1)])
+        b.add_migration(demes=["a", "b"], rate=0.01)
+        b.add_migration(demes=["a", "c", "d"], rate=0.01)
+        g = b.resolve()
+        d = g.asdict()
+        assert len(d["migrations"]) == 8
+        d = g.asdict_simplified()
+        assert len(d["migrations"]) == 2
+        assert dict(demes=["a", "b"], rate=0.01) in d["migrations"]
+        assert dict(demes=["a", "c", "d"], rate=0.01) in d["migrations"]
+
     def test_invalid_fields(self):
         b = Builder()
         b.add_deme("a", epochs=[dict(start_size=1)])
@@ -4143,3 +4160,6 @@ def test_back_and_forth(self, graph):
         b = Builder.fromdict(graph.asdict_simplified())
         g = b.resolve()
         assert g.isclose(graph)
+
+
+