[src] Fixes to grammar-fst code to handle LM-disambig symbols properly (

#3000)

thanks: armando.muscariello@gmail.com

src/decoder/grammar-fst.cc

@@ -443,6 +443,98 @@ void GrammarFst::Read(std::istream &is, bool binary) {
 }
 
 
+/**
+   This utility function input-determinizes a specified state s of the FST
+   'fst'.   (This input-determinizes while treating epsilon as a real symbol,
+   although for the application we expect to use it, there won't be epsilons).
+
+   What this function does is: for any symbol i that appears as the ilabel of
+   more than one arc leaving state s of FST 'fst', it creates an additional
+   state, it creates a new state t with epsilon-input transitions leaving it for
+   each of those multiple arcs leaving state s; it deletes the original arcs
+   leaving state s; and it creates a single arc leaving state s to the newly
+   created state with the ilabel i on it.  It sets the weights as necessary to
+   preserve equivalence and also to ensure that if, prior to this modification,
+   the FST was stochastic when cast to the log semiring (see
+   IsStochasticInLog()), it still will be.  I.e. when interpreted as
+   negative logprobs, the weight from state s to t would be the sum of
+   the weights on the original arcs leaving state s.
+
+   This is used as a very cheap solution when preparing FSTs for the grammar
+   decoder, to ensure that there is only one entry-state to the sub-FST for each
+   phonetic left-context; this keeps the grammar-FST code (i.e. the code that
+   stitches them together) simple.  Of course it will tend to introduce
+   unnecessary epsilons, and if we were careful we might be able to remove
+   some of those, but this wouldn't have a substantial impact on overall
+   decoder performance so we don't bother.
+ */
+static void InputDeterminizeSingleState(StdArc::StateId s,
+                                        VectorFst<StdArc> *fst) {
+  bool was_input_deterministic = true;
+  typedef StdArc Arc;
+  typedef Arc::StateId StateId;
+  typedef Arc::Label Label;
+  typedef Arc::Weight Weight;
+
+  struct InfoForIlabel {
+    std::vector<size_t> arc_indexes;  // indexes of all arcs with this ilabel
+    float tot_cost;  // total cost of all arcs leaving state s for this
+                     // ilabel, summed as if they were negative log-probs.
+    StateId new_state;  // state-id of new state, if any, that we have created
+                        // to remove duplicate symbols with this ilabel.
+    InfoForIlabel(): new_state(-1) { }
+  };
+
+  std::unordered_map<Label, InfoForIlabel> label_map;
+
+  size_t arc_index = 0;
+  for (ArcIterator<VectorFst<Arc> > aiter(*fst, s);
+       !aiter.Done(); aiter.Next(), ++arc_index) {
+    const Arc &arc = aiter.Value();
+    InfoForIlabel &info = label_map[arc.ilabel];
+    if (info.arc_indexes.empty()) {
+      info.tot_cost = arc.weight.Value();
+    } else {
+      info.tot_cost = -kaldi::LogAdd(-info.tot_cost, -arc.weight.Value());
+      was_input_deterministic = false;
+    }
+    info.arc_indexes.push_back(arc_index);
+  }
+
+  if (was_input_deterministic)
+    return;  // Nothing to do.
+
+  // 'new_arcs' will contain the modified list of arcs
+  // leaving state s
+  std::vector<Arc> new_arcs;
+  new_arcs.reserve(arc_index);
+  arc_index = 0;
+  for (ArcIterator<VectorFst<Arc> > aiter(*fst, s);
+       !aiter.Done(); aiter.Next(), ++arc_index) {
+    const Arc &arc = aiter.Value();
+    Label ilabel = arc.ilabel;
+    InfoForIlabel &info = label_map[ilabel];
+    if (info.arc_indexes.size() == 1) {
+      new_arcs.push_back(arc);  // no changes needed
+    } else {
+      if (info.new_state < 0) {
+        info.new_state = fst->AddState();
+        // add arc from state 's' to newly created state.
+        new_arcs.push_back(Arc(ilabel, 0, Weight(info.tot_cost),
+                               info.new_state));
+      }
+      // add arc from new state to original destination of this arc.
+      fst->AddArc(info.new_state, Arc(0, arc.olabel,
+                                      Weight(arc.weight.Value() - info.tot_cost),
+                                      arc.nextstate));
+    }
+  }
+  fst->DeleteArcs(s);
+  for (size_t i = 0; i < new_arcs.size(); i++)
+    fst->AddArc(s, new_arcs[i]);
+}
+
+
 // This class contains the implementation of the function
 // PrepareForGrammarFst(), which is declared in grammar-fst.h.
 class GrammarFstPreparer {
@@ -475,6 +567,12 @@ class GrammarFstPreparer {
           // OK, state s is a special state.
           FixArcsToFinalStates(s);
           MaybeAddFinalProbToState(s);
+          // The following ensures that the start-state of sub-FSTs only has
+          // a single arc per left-context phone (the graph-building recipe can
+          // end up creating more than one if there were disambiguation symbols,
+          // e.g. for langauge model backoff).
+          if (s == fst_->Start() && IsEntryState(s))
+            InputDeterminizeSingleState(s, fst_);
         }
       }
     }
@@ -487,7 +585,7 @@ class GrammarFstPreparer {
 
   // Returns true if state 's' has at least one arc coming out of it with a
   // special nonterminal-related ilabel on it (i.e. an ilabel >=
-  // kNontermBigNumber)
+  // kNontermBigNumber), and false otherwise.
   bool IsSpecialState(StateId s) const;
 
   // This function verifies that state s does not currently have any
@@ -509,6 +607,10 @@ class GrammarFstPreparer {
   // modify this state (by adding input-epsilon arcs), and false otherwise.
   bool NeedEpsilons(StateId s) const;
 
+  // Returns true if state s (which is expected to be the start state, although we
+  // don't check this) has arcs with nonterminal symbols #nonterm_begin.
+  bool IsEntryState(StateId s) const;
+
   // Fixes any final-prob-related problems with this state.  The problem we aim
   // to fix is that there may be arcs with nonterminal symbol #nonterm_end which
   // transition from this state to a state with non-unit final prob.  This
@@ -599,6 +701,24 @@ bool GrammarFstPreparer::IsSpecialState(StateId s) const {
   return false;
 }
 
+bool GrammarFstPreparer::IsEntryState(StateId s) const {
+  int32 big_number = kNontermBigNumber,
+      encoding_multiple = GetEncodingMultiple(nonterm_phones_offset_);
+
+  for (ArcIterator<FST> aiter(*fst_, s ); !aiter.Done(); aiter.Next()) {
+    const Arc &arc = aiter.Value();
+    int32 nonterminal = (arc.ilabel - big_number) /
+        encoding_multiple;
+    // we check that at least one has label with nonterminal equal to #nonterm_begin...
+    // in fact they will all have this value if at least one does, and this was checked
+    // in NeedEpsilons().
+    if (nonterminal == kNontermBegin)
+      return true;
+  }
+  return false;
+}
+
+
 bool GrammarFstPreparer::NeedEpsilons(StateId s) const {
 
   // See the documentation for GetCategoryOfArc() for explanation of what these are.
@@ -647,7 +767,7 @@ bool GrammarFstPreparer::NeedEpsilons(StateId s) const {
     if (nonterminal == GetPhoneSymbolFor(kNontermBegin) &&
         s != fst_->Start()) {
       KALDI_ERR << "#nonterm_begin symbol is present but this is not the "
-          "first arc.  Did you do fstdeterminizestar while compiling?";
+          "first state.  Did you do fstdeterminizestar while compiling?";
     }
     if (nonterminal == GetPhoneSymbolFor(kNontermEnd)) {
       if (fst_->NumArcs(arc.nextstate) != 0 ||

src/decoder/grammar-fst.h

@@ -229,14 +229,15 @@ class GrammarFst {
     an arc-index leaving a particular state in an FST (i.e. an index
     that we could use to Seek() to the matching arc).
 
-      @param [in]  fst  The FST we are looking for state-indexes for
-      @param [in]  entry_state  The state in the FST-- must have arcs with
-                 ilabels decodable as (nonterminal_symbol, left_context_phone).
-                 Will either be the start state (if 'nonterminal_symbol'
-                 corresponds to #nonterm_begin), or an internal state
-                 (if 'nonterminal_symbol' corresponds to #nonterm_reenter).
-                 The arc-indexes of those arcs will be the values
-                 we set in 'phone_to_arc'
+      @param [in]  fst  The FST that is being entered (or reentered)
+      @param [in]  entry_state  The state in 'fst' which is being entered
+                 (or reentered); will be fst.Start() if it's being
+                 entered.  It must have arcs with ilabels decodable as
+                 (nonterminal_symbol, left_context_phone).  Will either be the
+                 start state (if 'nonterminal_symbol' corresponds to
+                 #nonterm_begin), or an internal state (if 'nonterminal_symbol'
+                 corresponds to #nonterm_reenter).  The arc-indexes of those
+                 arcs will be the values we set in 'phone_to_arc'
       @param [in]  nonterminal_symbol  The index in phones.txt of the
                  nonterminal symbol we expect to be encoded in the ilabels
                  of the arcs leaving 'entry_state'.  Will either correspond

src/doc/grammar.dox

@@ -352,7 +352,7 @@ Z_S  243
   The special symbols in CLG.fst will be as follows.
 
   The following special symbols may appear in any CLG graph, top-level or not:
-   - When any graph invokes a sub-graph, there will be n arc with an ilabel
+   - When any graph invokes a sub-graph, there will be an arc with an ilabel
      (</code>\#nonterm:foo</code>, <em>left-context-phone</em>) representing the
      user-specified nonterminal and the actual left-context, which will be
      followed by arcs with ilabels of the form (</code>\#nonterm_reenter</code>,