check for non-rescorable hypergraphs

3 files changed, 16 insertions, 0 deletions

diff --git a/decoder/decoder.cc b/decoder/decoder.cc
index 354ea2d9..41f36822 100644
--- a/decoder/decoder.cc
+++ b/decoder/decoder.cc
@@ -755,6 +755,11 @@ bool DecoderImpl::Decode(const string& input, DecoderObserver* o) {
     if (!SILENT) cerr << "  *** NODES NOT UNIQUELY IDENTIFIED ***\n";
   }
 
+  if (!forest.ArePreGoalEdgesArity1()) {
+    cerr << "Pre-goal edges are not arity-1. The decoder requires this.\n";
+    abort();
+  }
+
   const bool show_tree_structure=conf.count("show_tree_structure");
   if (!SILENT) forest_stats(forest,"  Init. forest",show_tree_structure,oracle.show_derivation);
   if (conf.count("show_expected_length")) {
diff --git a/decoder/hg.cc b/decoder/hg.cc
index e456fa7c..46543b01 100644
--- a/decoder/hg.cc
+++ b/decoder/hg.cc
@@ -28,6 +28,13 @@ bool Hypergraph::AreNodesUniquelyIdentified() const {
   return true;
 }
 
+bool Hypergraph::ArePreGoalEdgesArity1() const {
+  auto& n = nodes_.back();
+  for (auto eid : n.in_edges_)
+    if (edges_[eid].Arity() != 1) return false;
+  return true;
+}
+
 Hypergraph::Edge const* Hypergraph::ViterbiSortInEdges() {
   NodeProbs nv;
   ComputeNodeViterbi(&nv);
diff --git a/decoder/hg.h b/decoder/hg.h
index 43fb275b..4ed27d87 100644
--- a/decoder/hg.h
+++ b/decoder/hg.h
@@ -268,6 +268,10 @@ public:
   // if all node states are unique, return true
   bool AreNodesUniquelyIdentified() const;
 
+  // the feature function interface assumes that pre-goal edges are
+  // arity 1 (this simplifies the "final transition" feature computation)
+  bool ArePreGoalEdgesArity1() const;
+
   // reserves space in the nodes vector to prevent memory locations
   // from changing
   void ReserveNodes(size_t n, size_t e = 0) {