1 files changed, 42 insertions, 2 deletions
diff --git a/klm/lm/blank.hh b/klm/lm/blank.hh
index 639bc98b..4615a09e 100644
--- a/klm/lm/blank.hh
+++ b/klm/lm/blank.hh
@@ -1,12 +1,52 @@
 #ifndef LM_BLANK__
 #define LM_BLANK__
+
 #include <limits>
 
+#include <inttypes.h>
+#include <math.h>
+
 namespace lm {
 namespace ngram {
 
-const float kBlankProb = -std::numeric_limits<float>::quiet_NaN();
-const float kBlankBackoff = std::numeric_limits<float>::infinity();
+/* Suppose "foo bar" appears with zero backoff but there is no trigram
+ * beginning with these words.  Then, when scoring "foo bar", the model could
+ * return out_state containing "bar" or even null context if "bar" also has no
+ * backoff and is never followed by another word.  Then the backoff is set to
+ * kNoExtensionBackoff.  If the n-gram might be extended, then out_state must
+ * contain the full n-gram, in which case kExtensionBackoff is set.  In any
+ * case, if an n-gram has non-zero backoff, the full state is returned so
+ * backoff can be properly charged.  
+ * These differ only in sign bit because the backoff is in fact zero in either
+ * case.   
+ */
+const float kNoExtensionBackoff = -0.0;
+const float kExtensionBackoff = 0.0;
+
+inline void SetExtension(float &backoff) {
+  if (backoff == kNoExtensionBackoff) backoff = kExtensionBackoff;
+}
+
+// This compiles down nicely.  
+inline bool HasExtension(const float &backoff) {
+  typedef union { float f; uint32_t i; } UnionValue;
+  UnionValue compare, interpret;
+  compare.f = kNoExtensionBackoff;
+  interpret.f = backoff;
+  return compare.i != interpret.i;
+}
+
+/* Suppose "foo bar baz quux" appears in the ARPA but not "bar baz quux" or
+ * "baz quux" (because they were pruned).  1.2% of n-grams generated by SRI
+ * with default settings on the benchmark data set are like this.  Since search
+ * proceeds by finding "quux", "baz quux", "bar baz quux", and finally 
+ * "foo bar baz quux" and the trie needs pointer nodes anyway, blanks are
+ * inserted.  The blanks have probability kBlankProb and backoff kBlankBackoff.
+ * A blank is recognized by kBlankProb in the probability field; kBlankBackoff
+ * must be 0 so that inference asseses zero backoff from these blanks.  
+ */
+const float kBlankProb = -std::numeric_limits<float>::infinity();
+const float kBlankBackoff = kNoExtensionBackoff;
 
 } // namespace ngram
 } // namespace lm