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diff --git a/klm/lm/left.hh b/klm/lm/left.hh
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+/* Efficient left and right language model state for sentence fragments.
+ * Intended usage:
+ * Store ChartState with every chart entry.  
+ * To do a rule application:
+ * 1. Make a ChartState object for your new entry.  
+ * 2. Construct RuleScore.  
+ * 3. Going from left to right, call Terminal or NonTerminal. 
+ *   For terminals, just pass the vocab id.  
+ *   For non-terminals, pass that non-terminal's ChartState.
+ *     If your decoder expects scores inclusive of subtree scores (i.e. you
+ *     label entries with the highest-scoring path), pass the non-terminal's
+ *     score as prob.  
+ *     If your decoder expects relative scores and will walk the chart later,
+ *     pass prob = 0.0.  
+ *     In other words, the only effect of prob is that it gets added to the
+ *     returned log probability.  
+ * 4. Call Finish.  It returns the log probability.   
+ *
+ * There's a couple more details: 
+ * Do not pass <s> to Terminal as it is formally not a word in the sentence,
+ * only context.  Instead, call BeginSentence.  If called, it should be the
+ * first call after RuleScore is constructed (since <s> is always the
+ * leftmost).
+ *
+ * If the leftmost RHS is a non-terminal, it's faster to call BeginNonTerminal.
+ *
+ * Hashing and sorting comparison operators are provided.   All state objects
+ * are POD.  If you intend to use memcmp on raw state objects, you must call
+ * ZeroRemaining first, as the value of array entries beyond length is
+ * otherwise undefined.  
+ *
+ * Usage is of course not limited to chart decoding.  Anything that generates
+ * sentence fragments missing left context could benefit.  For example, a
+ * phrase-based decoder could pre-score phrases, storing ChartState with each
+ * phrase, even if hypotheses are generated left-to-right.  
+ */
+
+#ifndef LM_LEFT__
+#define LM_LEFT__
+
+#include "lm/max_order.hh"
+#include "lm/model.hh"
+#include "lm/return.hh"
+
+#include "util/murmur_hash.hh"
+
+#include <algorithm>
+
+namespace lm {
+namespace ngram {
+
+struct Left {
+  bool operator==(const Left &other) const {
+    return 
+      (length == other.length) && 
+      pointers[length - 1] == other.pointers[length - 1];
+  }
+
+  int Compare(const Left &other) const {
+    if (length != other.length) return length < other.length ? -1 : 1;
+    if (pointers[length - 1] > other.pointers[length - 1]) return 1;
+    if (pointers[length - 1] < other.pointers[length - 1]) return -1;
+    return 0;
+  }
+
+  bool operator<(const Left &other) const {
+    if (length != other.length) return length < other.length;
+    return pointers[length - 1] < other.pointers[length - 1];
+  }
+
+  void ZeroRemaining() {
+    for (uint64_t * i = pointers + length; i < pointers + kMaxOrder - 1; ++i)
+      *i = 0;
+  }
+
+  unsigned char length;
+  uint64_t pointers[kMaxOrder - 1];
+};
+
+inline size_t hash_value(const Left &left) {
+  return util::MurmurHashNative(&left.length, 1, left.pointers[left.length - 1]);
+}
+
+struct ChartState {
+  bool operator==(const ChartState &other) {
+    return (left == other.left) && (right == other.right) && (full == other.full);
+  }
+
+  int Compare(const ChartState &other) const {
+    int lres = left.Compare(other.left);
+    if (lres) return lres;
+    int rres = right.Compare(other.right);
+    if (rres) return rres;
+    return (int)full - (int)other.full;
+  }
+
+  bool operator<(const ChartState &other) const {
+    return Compare(other) == -1;
+  }
+
+  void ZeroRemaining() {
+    left.ZeroRemaining();
+    right.ZeroRemaining();
+  }
+
+  Left left;
+  bool full;
+  State right;
+};
+
+inline size_t hash_value(const ChartState &state) {
+  size_t hashes[2];
+  hashes[0] = hash_value(state.left);
+  hashes[1] = hash_value(state.right);
+  return util::MurmurHashNative(hashes, sizeof(size_t), state.full);
+}
+
+template <class M> class RuleScore {
+  public:
+    explicit RuleScore(const M &model, ChartState &out) : model_(model), out_(out), left_done_(false), prob_(0.0) {
+      out.left.length = 0;
+      out.right.length = 0;
+    }
+
+    void BeginSentence() {
+      out_.right = model_.BeginSentenceState();
+      // out_.left is empty.
+      left_done_ = true;
+    }
+
+    void Terminal(WordIndex word) {
+      State copy(out_.right);
+      FullScoreReturn ret(model_.FullScore(copy, word, out_.right));
+      prob_ += ret.prob;
+      if (left_done_) return;
+      if (ret.independent_left) {
+        left_done_ = true;
+        return;
+      }
+      out_.left.pointers[out_.left.length++] = ret.extend_left;
+      if (out_.right.length != copy.length + 1)
+        left_done_ = true;
+    }
+
+    // Faster version of NonTerminal for the case where the rule begins with a non-terminal.  
+    void BeginNonTerminal(const ChartState &in, float prob) {
+      prob_ = prob;
+      out_ = in;
+      left_done_ = in.full;
+    }
+
+    void NonTerminal(const ChartState &in, float prob) {
+      prob_ += prob;
+      
+      if (!in.left.length) {
+        if (in.full) {
+          for (const float *i = out_.right.backoff; i < out_.right.backoff + out_.right.length; ++i) prob_ += *i;
+          left_done_ = true;
+          out_.right = in.right;
+        }
+        return;
+      }
+
+      if (!out_.right.length) {
+        out_.right = in.right;
+        if (left_done_) return;
+        if (out_.left.length) {
+          left_done_ = true;
+        } else {
+          out_.left = in.left;
+          left_done_ = in.full;
+        }
+        return;
+      }
+
+      float backoffs[kMaxOrder - 1], backoffs2[kMaxOrder - 1];
+      float *back = backoffs, *back2 = backoffs2;
+      unsigned char next_use;
+      FullScoreReturn ret;
+      ProcessRet(ret = model_.ExtendLeft(out_.right.words, out_.right.words + out_.right.length, out_.right.backoff, in.left.pointers[0], 1, back, next_use));
+      if (!next_use) {
+        left_done_ = true;
+        out_.right = in.right;
+        return;
+      }
+      unsigned char extend_length = 2;
+      for (const uint64_t *i = in.left.pointers + 1; i < in.left.pointers + in.left.length; ++i, ++extend_length) {
+        ProcessRet(ret = model_.ExtendLeft(out_.right.words, out_.right.words + next_use, back, *i, extend_length, back2, next_use));
+        if (!next_use) {
+          left_done_ = true;
+          out_.right = in.right;
+          return;
+        }
+        std::swap(back, back2);
+      }
+
+      if (in.full) {
+        for (const float *i = back; i != back + next_use; ++i) prob_ += *i;
+        left_done_ = true;
+        out_.right = in.right;
+        return;
+      }
+
+      // Right state was minimized, so it's already independent of the new words to the left.  
+      if (in.right.length < in.left.length) {
+        out_.right = in.right;
+        return;
+      }
+
+      // Shift exisiting words down.  
+      for (WordIndex *i = out_.right.words + next_use - 1; i >= out_.right.words; --i) {
+        *(i + in.right.length) = *i;
+      }
+      // Add words from in.right.  
+      std::copy(in.right.words, in.right.words + in.right.length, out_.right.words);
+      // Assemble backoff composed on the existing state's backoff followed by the new state's backoff.  
+      std::copy(in.right.backoff, in.right.backoff + in.right.length, out_.right.backoff);
+      std::copy(back, back + next_use, out_.right.backoff + in.right.length);
+      out_.right.length = in.right.length + next_use;
+    }
+
+    float Finish() {
+      // A N-1-gram might extend left and right but we should still set full to true because it's an N-1-gram.  
+      out_.full = left_done_ || (out_.left.length == model_.Order() - 1);
+      return prob_;
+    }
+
+  private:
+    void ProcessRet(const FullScoreReturn &ret) {
+      prob_ += ret.prob;
+      if (left_done_) return;
+      if (ret.independent_left) {
+        left_done_ = true;
+        return;
+      }
+      out_.left.pointers[out_.left.length++] = ret.extend_left;
+    }
+
+    const M &model_;
+
+    ChartState &out_;
+
+    bool left_done_;
+
+    float prob_;
+};
+
+} // namespace ngram
+} // namespace lm
+
+#endif // LM_LEFT__