From 0c9fe1dd72fb2321a5652e0ee66d1c897a3d9f80 Mon Sep 17 00:00:00 2001
From: Chris Dyer <cdyer@cs.cmu.edu>
Date: Tue, 1 Mar 2011 03:05:25 -0500
Subject: support explicit sentence boundary markers with cdec

---
 decoder/ff_klm.cc | 60 +++++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 47 insertions(+), 13 deletions(-)

diff --git a/decoder/ff_klm.cc b/decoder/ff_klm.cc
index 5049f156..5e590053 100644
--- a/decoder/ff_klm.cc
+++ b/decoder/ff_klm.cc
@@ -75,13 +75,19 @@ class KLanguageModelImpl {
     const vector<WordID>& e = rule.e();
     bool context_complete = false;
     for (int j = 0; j < e.size(); ++j) {
-      if (e[j] < 1) {
+      if (e[j] < 1) {   // handle non-terminal substitution
         const void* astate = (ant_states[-e[j]]);
         int unscored_ant_len = UnscoredSize(astate);
         for (int k = 0; k < unscored_ant_len; ++k) {
           const lm::WordIndex cur_word = IthUnscoredWord(k, astate);
-          const lm::ngram::State scopy(state);
-          const double p = ngram_->Score(scopy, cur_word, state);
+          double p = 0;
+          if (cur_word == kSOS_) {
+            if (state.ValidLength() > 0) { p = -100; cerr << rule << endl; }
+            state = ngram_->BeginSentenceState();
+          } else {
+            const lm::ngram::State scopy(state);
+            p = ngram_->Score(scopy, cur_word, state);
+          }
           ++num_scored;
           if (!context_complete) {
             if (num_scored >= order_) context_complete = true;
@@ -99,10 +105,16 @@ class KLanguageModelImpl {
           state = RemnantLMState(astate);
           context_complete = true;
         }
-      } else {
+      } else {   // handle terminal
         const lm::WordIndex cur_word = MapWord(e[j]);
-        const lm::ngram::State scopy(state);
-        const double p = ngram_->Score(scopy, cur_word, state);
+        double p = 0;
+        if (cur_word == kSOS_) {
+          if (state.ValidLength() > 0) p = -100;
+          state = ngram_->BeginSentenceState();
+        } else {
+          const lm::ngram::State scopy(state);
+          p = ngram_->Score(scopy, cur_word, state);
+        }
         ++num_scored;
         if (!context_complete) {
           if (num_scored >= order_) context_complete = true;
@@ -130,11 +142,16 @@ class KLanguageModelImpl {
   //FIXME: this assumes no target words on final unary -> goal rule.  is that ok?
   // for <s> (n-1 left words) and (n-1 right words) </s>
   double FinalTraversalCost(const void* state) {
-    SetRemnantLMState(ngram_->BeginSentenceState(), dummy_state_);
-    SetHasFullContext(1, dummy_state_);
-    SetUnscoredSize(0, dummy_state_);
-    dummy_ants_[1] = state;
-    return LookupWords(*dummy_rule_, dummy_ants_, NULL, NULL);
+    if (add_sos_eos_) {
+      SetRemnantLMState(ngram_->BeginSentenceState(), dummy_state_);
+      SetHasFullContext(1, dummy_state_);
+      SetUnscoredSize(0, dummy_state_);
+      dummy_ants_[1] = state;
+      return LookupWords(*dummy_rule_, dummy_ants_, NULL, NULL);
+    } else {
+      // TODO, figure out whether spans are correct
+      return 0;
+    }
   }
 
   lm::WordIndex MapWord(WordID w) const {
@@ -146,12 +163,18 @@ class KLanguageModelImpl {
 
  public:
   KLanguageModelImpl(const std::string& param) {
+    add_sos_eos_ = true;
+    string fname = param;
+    if (param.find("-x ") == 0) {
+       add_sos_eos_ = false;
+       fname = param.substr(3);
+    }
     lm::ngram::Config conf;
     VMapper vm(&map_);
     conf.enumerate_vocab = &vm; 
-    ngram_ = new Model(param.c_str(), conf);
+    ngram_ = new Model(fname.c_str(), conf);
     order_ = ngram_->Order();
-    cerr << "Loaded " << order_ << "-gram KLM from " << param << " (MapSize=" << map_.size() << ")\n";
+    cerr << "Loaded " << order_ << "-gram KLM from " << fname << " (MapSize=" << map_.size() << ")\n";
     state_size_ = ngram_->StateSize() + 2 + (order_ - 1) * sizeof(lm::WordIndex);
     unscored_size_offset_ = ngram_->StateSize();
     is_complete_offset_ = unscored_size_offset_ + 1;
@@ -162,6 +185,9 @@ class KLanguageModelImpl {
     dummy_ants_.push_back(dummy_state_);
     dummy_ants_.push_back(NULL);
     dummy_rule_.reset(new TRule("[DUMMY] ||| [BOS] [DUMMY] ||| [1] [2] </s> ||| X=0"));
+    kSOS_ = MapWord(TD::Convert("<s>"));
+    assert(kSOS_ > 0);
+    kEOS_ = MapWord(TD::Convert("</s>"));
   }
 
   ~KLanguageModelImpl() {
@@ -172,7 +198,15 @@ class KLanguageModelImpl {
   int ReserveStateSize() const { return state_size_; }
 
  private:
+  lm::WordIndex kSOS_;  // <s> - requires special handling.
+  lm::WordIndex kEOS_;  // </s>
   Model* ngram_;
+  bool add_sos_eos_; // flag indicating whether the hypergraph produces <s> and </s>
+                     // if this is true, FinalTransitionFeatures will "add" <s> and </s>
+                     // if false, FinalTransitionFeatures will score anything with the
+                     // markers in the right place (i.e., the beginning and end of
+                     // the sentence) with 0, and anything else with -100
+
   int order_;
   int state_size_;
   int unscored_size_offset_;
-- 
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