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diff --git a/src/ff.h b/src/ff.h
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+#ifndef _FF_H_
+#define _FF_H_
+
+#include <vector>
+
+#include "fdict.h"
+#include "hg.h"
+
+class SentenceMetadata;
+class FeatureFunction;  // see definition below
+
+// if you want to develop a new feature, inherit from this class and
+// override TraversalFeaturesImpl(...).  If it's a feature that returns /
+// depends on context, you may also need to implement
+// FinalTraversalFeatures(...)
+class FeatureFunction {
+ public:
+  FeatureFunction() : state_size_() {}
+  explicit FeatureFunction(int state_size) : state_size_(state_size) {}
+  virtual ~FeatureFunction();
+
+  // returns the number of bytes of context that this feature function will
+  // (maximally) use.  By default, 0 ("stateless" models in Hiero/Joshua).
+  // NOTE: this value is fixed for the instance of your class, you cannot
+  // use different amounts of memory for different nodes in the forest.
+  inline int NumBytesContext() const { return state_size_; }
+
+  // Compute the feature values and (if this applies) the estimates of the
+  // feature values when this edge is used incorporated into a larger context
+  inline void TraversalFeatures(const SentenceMetadata& smeta,
+                                const Hypergraph::Edge& edge,
+                                const std::vector<const void*>& ant_contexts,
+                                SparseVector<double>* features,
+                                SparseVector<double>* estimated_features,
+                                void* out_state) const {
+    TraversalFeaturesImpl(smeta, edge, ant_contexts,
+                          features, estimated_features, out_state);
+    // TODO it's easy for careless feature function developers to overwrite
+    // the end of their state and clobber someone else's memory.  These bugs
+    // will be horrendously painful to track down.  There should be some
+    // optional strict mode that's enforced here that adds some kind of
+    // barrier between the blocks reserved for the residual contexts
+  }
+
+  // if there's some state left when you transition to the goal state, score
+  // it here.  For example, the language model computes the cost of adding
+  // <s> and </s>.
+  virtual void FinalTraversalFeatures(const void* residual_state,
+                                      SparseVector<double>* final_features) const;
+
+ protected:
+  // context is a pointer to a buffer of size NumBytesContext() that the
+  // feature function can write its state to.  It's up to the feature function
+  // to determine how much space it needs and to determine how to encode its
+  // residual contextual information since it is OPAQUE to all clients outside
+  // of the particular FeatureFunction class.  There is one exception:
+  // equality of the contents (i.e., memcmp) is required to determine whether
+  // two states can be combined.
+  virtual void TraversalFeaturesImpl(const SentenceMetadata& smeta,
+                                     const Hypergraph::Edge& edge,
+                                     const std::vector<const void*>& ant_contexts,
+                                     SparseVector<double>* features,
+                                     SparseVector<double>* estimated_features,
+                                     void* context) const = 0;
+
+  // !!! ONLY call this from subclass *CONSTRUCTORS* !!!
+  void SetStateSize(size_t state_size) {
+    state_size_ = state_size;
+  }
+
+ private:
+  int state_size_;
+};
+
+// word penalty feature, for each word on the E side of a rule,
+// add value_
+class WordPenalty : public FeatureFunction {
+ public:
+  WordPenalty(const std::string& param);
+ protected:
+  virtual void TraversalFeaturesImpl(const SentenceMetadata& smeta,
+                                     const Hypergraph::Edge& edge,
+                                     const std::vector<const void*>& ant_contexts,
+                                     SparseVector<double>* features,
+                                     SparseVector<double>* estimated_features,
+                                     void* context) const;
+ private:
+  const int fid_;
+  const double value_;
+};
+
+// this class is a set of FeatureFunctions that can be used to score, rescore,
+// etc. a (translation?) forest
+class ModelSet {
+ public:
+  ModelSet() : state_size_(0) {}
+
+  ModelSet(const std::vector<double>& weights,
+           const std::vector<const FeatureFunction*>& models);
+
+  // sets edge->feature_values_ and edge->edge_prob_
+  // NOTE: edge must not necessarily be in hg.edges_ but its TAIL nodes
+  // must be.
+  void AddFeaturesToEdge(const SentenceMetadata& smeta,
+                         const Hypergraph& hg,
+                         Hypergraph::Edge* edge,
+                         std::string* residual_context,
+                         prob_t* combination_cost_estimate = NULL) const;
+
+  void AddFinalFeatures(const std::string& residual_context,
+                        Hypergraph::Edge* edge) const;
+
+  bool empty() const { return models_.empty(); }
+ private:
+  std::vector<const FeatureFunction*> models_;
+  std::vector<double> weights_; 
+  int state_size_;
+  std::vector<int> model_state_pos_;
+};
+
+#endif