Fix transitive reduction for parent items with multiple children

Given these dependencies:

A id:1
B p:1
C p:1 id:2
D p:1 p:2

A -> B
A -> C
A -> D
C -> D

The relation A -> D is superfluous (A -> C, C -> D). However, the p:1
tag in D would not be removed, because relation 1 still exists (A -> B).

Therefore a new function is added that identifies p: tags which have no
corresponding edge in the internal dependency graph (rather than looking
at the value of the edge).

test/test_todo_list.py

@@ -367,13 +367,13 @@ class TodoListDependencyTester(TopydoTest):
 class TodoListCleanDependencyTester(TopydoTest):
     def setUp(self):
         super(TodoListCleanDependencyTester, self).setUp()
-
         self.todolist = TodoList([])
+
+    def test_clean_dependencies1(self):
         self.todolist.add("Bar p:1")
         self.todolist.add("Baz p:1 id:2")
         self.todolist.add("Buzz p:2")
 
-    def test_clean_dependencies(self):
         self.todolist.clean_dependencies()
 
         self.assertFalse(self.todolist.todo(1).has_tag('p'))
@@ -381,5 +381,17 @@ class TodoListCleanDependencyTester(TopydoTest):
         self.assertTrue(self.todolist.todo(2).has_tag('id', '2'))
         self.assertTrue(self.todolist.todo(3).has_tag('p', '2'))
 
+    def test_clean_dependencies2(self):
+        self.todolist.add("Foo id:1")
+        self.todolist.add("Bar p:1")
+        self.todolist.add("Baz p:1 id:2")
+        self.todolist.add("Buzz p:1 p:2")
+
+        self.todolist.clean_dependencies()
+
+        self.assertFalse(self.todolist.todo(4).has_tag('p', '1'))
+        self.assertTrue(self.todolist.todo(1).has_tag('id', '1'))
+        self.assertTrue(self.todolist.todo(2).has_tag('p', '1'))
+
 if __name__ == '__main__':
     unittest.main()

topydo/lib/TodoList.py

@@ -203,19 +203,39 @@ class TodoList(TodoListBase):
         graph and removing unused dependency ids from the graph (in that
         order).
         """
-        def clean_by_tag(tag_name):
-            """ Generic function to handle 'p' and 'id' tags. """
-            for todo in [todo for todo in self._todos
-                         if todo.has_tag(tag_name)]:
+        def remove_tag(p_todo, p_tag, p_value):
+            """
+            Removes a tag from a todo item.
+            """
+            p_todo.remove_tag(p_tag, p_value)
+            self.dirty = True
+
+        def clean_parent_relations():
+            """
+            Remove id: tags for todos without child todo items.
+            """
 
-                value = todo.tag_value(tag_name)
+            for todo in [todo for todo in self._todos if todo.has_tag('id')]:
+                value = todo.tag_value('id')
                 if not self._depgraph.has_edge_id(value):
-                    todo.remove_tag(tag_name, value)
-                    self.dirty = True
+                    remove_tag(todo, 'id', value)
+
+        def clean_orphan_relations():
+            """
+            Remove p: tags for todos referring to a parent that is not in the
+            dependency graph anymore.
+            """
+
+            for todo in [todo for todo in self._todos if todo.has_tag('p')]:
+                value = todo.tag_value('p')
+                parent = self.todo_by_dep_id(value)
+
+                if not self._depgraph.has_edge(hash(parent), hash(todo)):
+                    remove_tag(todo, 'p', value)
 
         self._depgraph.transitively_reduce()
-        clean_by_tag('p')
-        clean_by_tag('id')
+        clean_parent_relations()
+        clean_orphan_relations()
 
     def _update_parent_cache(self):
         """