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diff --git a/decoder/factored_lexicon_helper.h b/decoder/factored_lexicon_helper.h
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+#ifndef _FACTORED_LEXICON_HELPER_
+#define _FACTORED_LEXICON_HELPER_
+
+#include <cassert>
+#include <vector>
+#include <string>
+#include <map>
+#include "tdict.h"
+#include "sentence_metadata.h"
+
+// when computing features, it can be advantageous to:
+//   1) back off to less specific forms (e.g., less highly inflected forms, POS tags, etc)
+//   2) look at more specific forms (on the source ONLY)
+// this class helps you do both by creating a "corpus" view
+// should probably add a discussion of why the source can be "refined" by this class
+// but not the target. basically, this is because the source is on the right side of
+// the conditioning line in the model, and the target is on the left. the most specific
+// form must always be generated, but the "source" can include arbitrarily large
+// context.
+// this currently only works for sentence input to maintain simplicity of the code and
+// file formats, but there is no reason why it couldn't work with lattices / CFGs
+class FactoredLexiconHelper {
+ public:
+  // default constructor does no mapping
+  FactoredLexiconHelper();
+  // Either filename can be empty or * to indicate no mapping
+  FactoredLexiconHelper(const std::string& srcfile, const std::string& trgmapfile);
+
+  void PrepareForInput(const SentenceMetadata& smeta);
+
+  inline WordID SourceWordAtPosition(const int i) const {
+    if (i < 0) return kNULL;
+    assert(i < cur_src_.size());
+    return Escape(cur_src_[i]);
+  }
+
+  inline WordID CoarsenedTargetWordForTarget(const WordID surface_target) const {
+    if (has_trg_) {
+      const WordWordMap::const_iterator it = trgmap_.find(surface_target);
+      if (it == trgmap_.end()) return surface_target;
+      return Escape(it->second);
+    } else {
+      return Escape(surface_target);
+    }
+  }
+
+ private:
+  inline WordID Escape(WordID word) const {
+    const std::map<WordID,WordID>::const_iterator it = escape_.find(word);
+    if (it == escape_.end()) return word;
+    return it->second;
+  }
+
+  void InitEscape();
+
+  const WordID kNULL;
+  bool has_src_;
+  bool has_trg_;
+  std::vector<std::vector<WordID> > src_;
+  typedef std::map<WordID, WordID> WordWordMap;
+  WordWordMap trgmap_;
+  std::vector<WordID> cur_src_;
+  std::map<WordID,WordID> escape_;
+};
+
+#endif