Bug 1383880: add Graph.visit_preorder; r=ahal

MozReview-Commit-ID: BWGqLUuWlN9

--HG--
extra : rebase_source : 84321e62d789faa7972ac92486e9831684f7d50d

taskcluster/taskgraph/graph.py

@@ -78,16 +78,9 @@ class Graph(object):
             new_edges = edges | add_edges
         return Graph(new_nodes, new_edges)
 
-    def visit_postorder(self):
-        """
-        Generate a sequence of nodes in postorder, such that every node is
-        visited *after* any nodes it links to.
-
-        Behavior is undefined (read: it will hang) if the graph contains a
-        cycle.
-        """
+    def _visit(self, reverse):
         queue = collections.deque(sorted(self.nodes))
-        links_by_node = self.links_dict()
+        links_by_node = self.reverse_links_dict() if reverse else self.links_dict()
         seen = set()
         while queue:
             node = queue.popleft()
@@ -101,6 +94,23 @@ class Graph(object):
                 queue.extend(n for n in links if n not in seen)
                 queue.append(node)
 
+    def visit_postorder(self):
+        """
+        Generate a sequence of nodes in postorder, such that every node is
+        visited *after* any nodes it links to.
+
+        Behavior is undefined (read: it will hang) if the graph contains a
+        cycle.
+        """
+        return self._visit(False)
+
+    def visit_preorder(self):
+        """
+        Like visit_postorder, but in reverse: evrey node is visited *before*
+        any nodes it links to.
+        """
+        return self._visit(True)
+
     def links_dict(self):
         """
         Return a dictionary mapping each node to a set of the nodes it links to

taskcluster/taskgraph/test/test_graph.py

@@ -129,6 +129,31 @@ class TestGraph(unittest.TestCase):
         "postorder visit of a disjoint graph satisfies invariant"
         self.assert_postorder(self.disjoint.visit_postorder(), self.disjoint.nodes)
 
+    def assert_preorder(self, seq, all_nodes):
+        seen = set()
+        for e in seq:
+            for l, r, n in self.tree.edges:
+                if r == e:
+                    self.failUnless(l in seen)
+            seen.add(e)
+        self.assertEqual(seen, all_nodes)
+
+    def test_visit_preorder_tree(self):
+        "preorder visit of a tree satisfies invariant"
+        self.assert_preorder(self.tree.visit_preorder(), self.tree.nodes)
+
+    def test_visit_preorder_diamonds(self):
+        "preorder visit of a graph full of diamonds satisfies invariant"
+        self.assert_preorder(self.diamonds.visit_preorder(), self.diamonds.nodes)
+
+    def test_visit_preorder_multi_edges(self):
+        "preorder visit of a graph with duplicate edges satisfies invariant"
+        self.assert_preorder(self.multi_edges.visit_preorder(), self.multi_edges.nodes)
+
+    def test_visit_preorder_disjoint(self):
+        "preorder visit of a disjoint graph satisfies invariant"
+        self.assert_preorder(self.disjoint.visit_preorder(), self.disjoint.nodes)
+
     def test_links_dict(self):
         "link dict for a graph with multiple edges is correct"
         self.assertEqual(self.multi_edges.links_dict(), {