diff -r 000000000000 -r 6474c204b198 gfx/layers/DirectedGraph.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/layers/DirectedGraph.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,116 @@
+/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef GFX_DIRECTEDGRAPH_H
+#define GFX_DIRECTEDGRAPH_H
+
+#include "gfxTypes.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+namespace layers {
+
+template <typename T>
+class DirectedGraph {
+public:
+
+  class Edge {
+    public:
+    Edge(T aFrom, T aTo) : mFrom(aFrom), mTo(aTo) {}
+
+    bool operator==(const Edge& aOther) const
+    {
+      return mFrom == aOther.mFrom && mTo == aOther.mTo;
+    }
+
+    T mFrom;
+    T mTo;
+  };
+
+  class RemoveEdgesToComparator 
+  {
+  public:
+    bool Equals(const Edge& a, T const& b) const { return a.mTo == b; }
+  };
+
+  /**
+   * Add a new edge to the graph.
+   */
+  void AddEdge(Edge aEdge)
+  {
+    NS_ASSERTION(!mEdges.Contains(aEdge), "Adding a duplicate edge!");
+    mEdges.AppendElement(aEdge);
+  }
+
+  void AddEdge(T aFrom, T aTo)
+  {
+    AddEdge(Edge(aFrom, aTo));
+  }
+
+  /**
+   * Get the list of edges.
+   */
+  const nsTArray<Edge>& GetEdgeList() const
+  {
+    return mEdges; 
+  }
+
+  /**
+   * Remove the given edge from the graph.
+   */
+  void RemoveEdge(Edge aEdge)
+  {
+    mEdges.RemoveElement(aEdge);
+  }
+
+  /**
+   * Remove all edges going into aNode.
+   */
+  void RemoveEdgesTo(T aNode)
+  {
+    RemoveEdgesToComparator c;
+    while (mEdges.RemoveElement(aNode, c)) {}
+  }
+  
+  /**
+   * Get the number of edges going into aNode.
+   */
+  unsigned int NumEdgesTo(T aNode)
+  {
+    unsigned int count = 0;
+    for (unsigned int i = 0; i < mEdges.Length(); i++) {
+      if (mEdges.ElementAt(i).mTo == aNode) {
+        count++;
+      }
+    }
+    return count;
+  }
+
+  /**
+   * Get the list of all edges going from aNode
+   */
+  void GetEdgesFrom(T aNode, nsTArray<Edge>& aResult)
+  {
+    for (unsigned int i = 0; i < mEdges.Length(); i++) {
+      if (mEdges.ElementAt(i).mFrom == aNode) {
+        aResult.AppendElement(mEdges.ElementAt(i));
+      }
+    }
+  }
+
+  /**
+   * Get the total number of edges.
+   */
+  unsigned int GetEdgeCount() { return mEdges.Length(); }
+
+private:
+
+  nsTArray<Edge> mEdges;
+};
+
+}
+}
+
+#endif // GFX_DIRECTEDGRAPH_H