[src] Hopefully make it possible to use empty FST in grammar-fst (#3523)

src/decoder/grammar-fst.cc

@@ -110,12 +110,15 @@ void GrammarFst::InitNonterminalMap() {
 }
 
 
-void GrammarFst::InitEntryArcs(int32 i) {
+bool GrammarFst::InitEntryArcs(int32 i) {
   KALDI_ASSERT(static_cast<size_t>(i) < ifsts_.size());
   const ConstFst<StdArc> &fst = *(ifsts_[i].second);
+  if (fst.NumStates() == 0)
+    return false;  /* this was the empty FST. */
   InitEntryOrReentryArcs(fst, fst.Start(),
                          GetPhoneSymbolFor(kNontermBegin),
                          &(entry_arcs_[i]));
+  return true;
 }
 
 void GrammarFst::InitInstances() {
@@ -345,8 +348,12 @@ GrammarFst::ExpandedState *GrammarFst::ExpandStateUserDefined(
     const ConstFst<StdArc> &child_fst = *(child_instance.fst);
     int32 child_ifst_index = child_instance.ifst_index;
     std::unordered_map<int32, int32> &entry_arcs = entry_arcs_[child_ifst_index];
-    if (entry_arcs.empty())
-      InitEntryArcs(child_ifst_index);
+    if (entry_arcs.empty()) {
+      if (!InitEntryArcs(child_ifst_index)) {
+        // This child-FST was the empty FST.  There are no arcs to expand.
+        continue;
+      }
+    }
     // for explanation of cost_correction, see documentation for CombineArcs().
     float num_entry_arcs = entry_arcs.size(),
         cost_correction = -log(num_entry_arcs);

src/decoder/grammar-fst.h

@@ -217,7 +217,10 @@ class GrammarFst {
   // too much work when it is initialized.  Each call to this function only
   // takes time O(number of left-context phones), which is quite small, but we'd
   // like to avoid that if possible.
-  void InitEntryArcs(int32 i);
+  //
+  // This function returns true if it successfully initialized the
+  // entry_arcs_[i]; and false if it left it empty because
+  bool InitEntryArcs(int32 i);
 
   // sets up instances_ with the top-level instance.
   void InitInstances();