From d5d86d6513e34f0b26030814c9eb516271ce2aec Mon Sep 17 00:00:00 2001
From: Chris Dyer <cdyer@cs.cmu.edu>
Date: Wed, 2 Mar 2011 18:45:51 -0500
Subject: better handling of SOS/EOS markers, part 1

---
 decoder/ff_klm.cc | 43 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 35 insertions(+), 8 deletions(-)

diff --git a/decoder/ff_klm.cc b/decoder/ff_klm.cc
index 9ba2cbaa..a55a0b41 100644
--- a/decoder/ff_klm.cc
+++ b/decoder/ff_klm.cc
@@ -8,6 +8,10 @@
 
 using namespace std;
 
+static const unsigned char HAS_FULL_CONTEXT = 1;
+static const unsigned char HAS_EOS_ON_RIGHT = 2;
+static const unsigned char MASK             = 7;
+
 template <class Model>
 string KLanguageModel<Model>::usage(bool /*param*/,bool /*verbose*/) {
   return "KLanguageModel";
@@ -57,12 +61,24 @@ class KLanguageModelImpl {
     mem[i] = index;
   }
 
-  bool HasFullContext(const void *state) const {
-    return *(static_cast<const char*>(state) + is_complete_offset_);
+  inline bool GetFlag(const void *state, unsigned char flag) const {
+    return (*(static_cast<const char*>(state) + is_complete_offset_) & flag);
+  }
+
+  inline void SetFlag(bool on, unsigned char flag, void *state) const {
+    if (on) {
+      *(static_cast<char*>(state) + is_complete_offset_) |= flag;
+    } else {
+      *(static_cast<char*>(state) + is_complete_offset_) &= (MASK ^ flag);
+    }
+  }
+
+  inline bool HasFullContext(const void *state) const {
+    return GetFlag(state, HAS_FULL_CONTEXT);
   }
 
-  void SetHasFullContext(bool flag, void *state) const {
-    *(static_cast<char*>(state) + is_complete_offset_) = flag;
+  inline void SetHasFullContext(bool flag, void *state) const {
+    SetFlag(flag, HAS_FULL_CONTEXT, state);
   }
 
  public:
@@ -71,6 +87,8 @@ class KLanguageModelImpl {
     double est_sum = 0.0;
     int num_scored = 0;
     int num_estimated = 0;
+    bool saw_eos = false;
+    bool has_some_history = false;
     lm::ngram::State state = ngram_->NullContextState();
     const vector<WordID>& e = rule.e();
     bool context_complete = false;
@@ -82,13 +100,16 @@ class KLanguageModelImpl {
           const lm::WordIndex cur_word = IthUnscoredWord(k, astate);
           double p = 0;
           if (cur_word == kSOS_) {
-            if (state.ValidLength() > 0) { p = -100; }
+            if (has_some_history) { p = -100; }
             state = ngram_->BeginSentenceState();
-            context_complete = true;
+            if (!context_complete && num_scored < (order_ - 2)) num_scored = order_ - 2;
           } else {
             const lm::ngram::State scopy(state);
             p = ngram_->Score(scopy, cur_word, state);
+            if (saw_eos) { p = -100; }
+            saw_eos = (cur_word == kEOS_);
           }
+          has_some_history = true;
           ++num_scored;
           if (!context_complete) {
             if (num_scored >= order_) context_complete = true;
@@ -102,6 +123,7 @@ class KLanguageModelImpl {
             est_sum += p;
           }
         }
+        saw_eos = GetFlag(astate, HAS_EOS_ON_RIGHT);
         if (HasFullContext(astate)) { // this is equivalent to the "star" in Chiang 2007
           state = RemnantLMState(astate);
           context_complete = true;
@@ -110,13 +132,16 @@ class KLanguageModelImpl {
         const lm::WordIndex cur_word = MapWord(e[j]);
         double p = 0;
         if (cur_word == kSOS_) {
-          if (state.ValidLength() > 0) p = -100;
+          if (has_some_history) p = -100;
           state = ngram_->BeginSentenceState();
-          context_complete = true;
+          if (!context_complete && num_scored < (order_ - 2)) num_scored = order_ - 2;
         } else {
           const lm::ngram::State scopy(state);
           p = ngram_->Score(scopy, cur_word, state);
+          if (saw_eos) { p = -100; }
+          saw_eos = (cur_word == kEOS_);
         }
+        has_some_history = true;
         ++num_scored;
         if (!context_complete) {
           if (num_scored >= order_) context_complete = true;
@@ -134,6 +159,7 @@ class KLanguageModelImpl {
     if (pest_sum) *pest_sum = est_sum;
     if (remnant) {
       state.ZeroRemaining();
+      SetFlag(saw_eos, HAS_EOS_ON_RIGHT, remnant);
       SetRemnantLMState(state, remnant);
       SetUnscoredSize(num_estimated, remnant);
       SetHasFullContext(context_complete || (num_scored >= order_), remnant);
@@ -190,6 +216,7 @@ class KLanguageModelImpl {
     kSOS_ = MapWord(TD::Convert("<s>"));
     assert(kSOS_ > 0);
     kEOS_ = MapWord(TD::Convert("</s>"));
+    assert(kEOS_ > 0);
   }
 
   ~KLanguageModelImpl() {
-- 
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