3 files changed, 43 insertions, 5 deletions
diff --git a/decoder/hg.cc b/decoder/hg.cc
index de8e5e49..629168ad 100644
--- a/decoder/hg.cc
+++ b/decoder/hg.cc
@@ -111,9 +111,45 @@ void Hypergraph::PushWeightsToGoal(double scale) {
   }
 }
 
-void Hypergraph::PruneEdges(const std::vector<bool>& prune_edge) {
+struct EdgeExistsWeightFunction {
+  EdgeExistsWeightFunction(const std::vector<bool>& prunes) : prunes_(prunes) {}
+  double operator()(const Hypergraph::Edge& edge) const {
+    return (prunes_[edge.id_] ? 0.0 : 1.0);
+  }
+ private:
+  const vector<bool>& prunes_;
+};
+
+void Hypergraph::PruneEdges(const std::vector<bool>& prune_edge, bool run_inside_algorithm) {
   assert(prune_edge.size() == edges_.size());
-  TopologicallySortNodesAndEdges(nodes_.size() - 1, &prune_edge);
+  vector<bool> filtered = prune_edge;
+
+  if (run_inside_algorithm) {
+    const EdgeExistsWeightFunction wf(prune_edge);
+    // use double, not bool since vector<bool> causes problems with the Inside algorithm.
+    // I don't know a good c++ way to resolve this short of template specialization which
+    // I dislike.  If you know of a better way that doesn't involve specialization,
+    // fix this!
+    vector<double> reachable;
+    bool goal_derivable = (0 < Inside<double, EdgeExistsWeightFunction>(*this, &reachable, wf));
+
+    assert(reachable.size() == nodes_.size());
+    for (int i = 0; i < edges_.size(); ++i) {
+      bool prune = prune_edge[i];
+      if (!prune) {
+        const Edge& edge = edges_[i];
+        for (int j = 0; j < edge.tail_nodes_.size(); ++j) {
+          if (!reachable[edge.tail_nodes_[j]]) {
+            prune = true;
+            break;
+          }
+        }
+      }
+      filtered[i] = prune;
+    }
+  }
+ 
+  TopologicallySortNodesAndEdges(nodes_.size() - 1, &filtered);
 }
 
 void Hypergraph::DensityPruneInsideOutside(const double scale,
diff --git a/decoder/hg.h b/decoder/hg.h
index af8d38d2..77d76cc3 100644
--- a/decoder/hg.h
+++ b/decoder/hg.h
@@ -156,7 +156,9 @@ class Hypergraph {
   void RemoveNoncoaccessibleStates(int goal_node_id = -1);
 
   // remove edges from the hypergraph if prune_edge[edge_id] is true
-  void PruneEdges(const std::vector<bool>& prune_edge);
+  // TODO need to investigate why this shouldn't be run for the forest trans
+  // case.  To investigate, change false to true and see where ftrans crashes
+  void PruneEdges(const std::vector<bool>& prune_edge, bool run_inside_algorithm = false);
 
   // if you don't know, use_sum_prod_semiring should be false
   void DensityPruneInsideOutside(const double scale, const bool use_sum_prod_semiring, const double density,
diff --git a/decoder/hg_intersect.cc b/decoder/hg_intersect.cc
index e0e70856..8bd11dd3 100644
--- a/decoder/hg_intersect.cc
+++ b/decoder/hg_intersect.cc
@@ -67,7 +67,7 @@ static bool FastLinearIntersect(const Lattice& target, Hypergraph* hg) {
       }
     }
   }
-  hg->PruneEdges(prune);
+  hg->PruneEdges(prune, true);
   return (cov.size() == target.size());
 }
 
@@ -81,7 +81,7 @@ bool HG::Intersect(const Lattice& target, Hypergraph* hg) {
   const RuleFilter filter(target, 15);   // TODO make configurable
   for (int i = 0; i < rem.size(); ++i)
     rem[i] = filter(*hg->edges_[i].rule_);
-  hg->PruneEdges(rem);
+  hg->PruneEdges(rem, true);
 
   const int nedges = hg->edges_.size();
   const int nnodes = hg->nodes_.size();