From c7d1b04980d9d90458625a7f8e92985c7409a78d Mon Sep 17 00:00:00 2001
From: Chris Dyer <cdyer@cs.cmu.edu>
Date: Sun, 29 Jul 2012 21:04:14 -0400
Subject: fix grammar converter to remove edges that cannot exist in any valid
 derivation

---
 decoder/hg_io.cc              |  1 +
 decoder/inside_outside.h      |  4 ----
 decoder/rescore_translator.cc |  1 +
 training/grammar_convert.cc   | 27 +++++++++++++++++++++++++++
 4 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/decoder/hg_io.cc b/decoder/hg_io.cc
index bfb2fb80..8bd40387 100644
--- a/decoder/hg_io.cc
+++ b/decoder/hg_io.cc
@@ -261,6 +261,7 @@ static void WriteRule(const TRule& r, ostream* out) {
 }
 
 bool HypergraphIO::WriteToJSON(const Hypergraph& hg, bool remove_rules, ostream* out) {
+  if (hg.empty()) { *out << "{}\n"; return true; }
   map<const TRule*, int> rid;
   ostream& o = *out;
   rid[NULL] = 0;
diff --git a/decoder/inside_outside.h b/decoder/inside_outside.h
index bb7f9fcc..f73a1d3f 100644
--- a/decoder/inside_outside.h
+++ b/decoder/inside_outside.h
@@ -41,10 +41,6 @@ WeightType Inside(const Hypergraph& hg,
     WeightType* const cur_node_inside_score = &inside_score[i];
     Hypergraph::EdgesVector const& in=hg.nodes_[i].in_edges_;
     const unsigned num_in_edges = in.size();
-    if (num_in_edges == 0) {
-      *cur_node_inside_score = WeightType(1); //FIXME: why not call weight(edge) instead?
-      continue;
-    }
     for (unsigned j = 0; j < num_in_edges; ++j) {
       const Hypergraph::Edge& edge = hg.edges_[in[j]];
       WeightType score = weight(edge);
diff --git a/decoder/rescore_translator.cc b/decoder/rescore_translator.cc
index 5c417393..10192f7a 100644
--- a/decoder/rescore_translator.cc
+++ b/decoder/rescore_translator.cc
@@ -20,6 +20,7 @@ struct RescoreTranslatorImpl {
   bool Translate(const string& input,
                  const vector<double>& weights,
                  Hypergraph* forest) {
+    if (input == "{}") return false;
     if (input.find("{\"rules\"") == 0) {
       istringstream is(input);
       Hypergraph src_cfg_hg;
diff --git a/training/grammar_convert.cc b/training/grammar_convert.cc
index bf8abb26..607a7cb9 100644
--- a/training/grammar_convert.cc
+++ b/training/grammar_convert.cc
@@ -9,6 +9,7 @@
 #include <boost/lexical_cast.hpp>
 #include <boost/program_options.hpp>
 
+#include "inside_outside.h"
 #include "tdict.h"
 #include "filelib.h"
 #include "hg.h"
@@ -69,6 +70,32 @@ void FilterAndCheckCorrectness(int goal, Hypergraph* hg) {
   if (hg->nodes_.size() != old_size) {
     cerr << "Warning! During sorting " << (old_size - hg->nodes_.size()) << " disappeared!\n";
   }
+  vector<double> inside; // inside score at each node
+  double p = Inside<double, TransitionCountWeightFunction>(*hg, &inside);
+  if (!p) {
+    cerr << "Warning! Grammar defines the empty language!\n";
+    hg->clear();
+    return;
+  }
+  vector<bool> prune(hg->edges_.size(), false);
+  int bad_edges = 0;
+  for (unsigned i = 0; i < hg->edges_.size(); ++i) {
+    Hypergraph::Edge& edge = hg->edges_[i];
+    bool bad = false;
+    for (unsigned j = 0; j < edge.tail_nodes_.size(); ++j) {
+      if (!inside[edge.tail_nodes_[j]]) {
+        bad = true;
+        ++bad_edges;
+      }
+    }
+    prune[i] = bad;
+  }
+  cerr << "Removing " << bad_edges << " bad edges from the grammar.\n";
+  for (unsigned i = 0; i < hg->edges_.size(); ++i) {
+    if (prune[i])
+      cerr << "   " << hg->edges_[i].rule_->AsString() << endl;
+  }
+  hg->PruneEdges(prune);
 }
 
 void CreateEdge(const TRulePtr& r, const Hypergraph::TailNodeVector& tail, Hypergraph::Node* head_node, Hypergraph* hg) {
-- 
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