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diff --git a/extractor/rule_extractor.h b/extractor/rule_extractor.h
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+++ b/extractor/rule_extractor.h
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+#ifndef _RULE_EXTRACTOR_H_
+#define _RULE_EXTRACTOR_H_
+
+#include <memory>
+#include <unordered_map>
+#include <vector>
+
+#include "phrase.h"
+
+using namespace std;
+
+namespace extractor {
+
+typedef vector<pair<int, int> > PhraseAlignment;
+
+class Alignment;
+class DataArray;
+class PhraseBuilder;
+class PhraseLocation;
+class Rule;
+class RuleExtractorHelper;
+class Scorer;
+class TargetPhraseExtractor;
+
+/**
+ * Structure containing data about the occurrences of a source-target phrase pair
+ * in the parallel corpus.
+ */
+struct Extract {
+  Extract(const Phrase& source_phrase, const Phrase& target_phrase,
+          double pairs_count, const PhraseAlignment& alignment) :
+      source_phrase(source_phrase), target_phrase(target_phrase),
+      pairs_count(pairs_count), alignment(alignment) {}
+
+  Phrase source_phrase;
+  Phrase target_phrase;
+  double pairs_count;
+  PhraseAlignment alignment;
+};
+
+/**
+ * Component for extracting SCFG rules.
+ */
+class RuleExtractor {
+ public:
+  RuleExtractor(shared_ptr<DataArray> source_data_array,
+                shared_ptr<DataArray> target_data_array,
+                shared_ptr<Alignment> alingment,
+                shared_ptr<PhraseBuilder> phrase_builder,
+                shared_ptr<Scorer> scorer,
+                shared_ptr<Vocabulary> vocabulary,
+                int min_gap_size,
+                int max_rule_span,
+                int max_nonterminals,
+                int max_rule_symbols,
+                bool require_aligned_terminal,
+                bool require_aligned_chunks,
+                bool require_tight_phrases);
+
+  // For testing only.
+  RuleExtractor(shared_ptr<DataArray> source_data_array,
+                shared_ptr<PhraseBuilder> phrase_builder,
+                shared_ptr<Scorer> scorer,
+                shared_ptr<TargetPhraseExtractor> target_phrase_extractor,
+                shared_ptr<RuleExtractorHelper> helper,
+                int max_rule_span,
+                int min_gap_size,
+                int max_nonterminals,
+                int max_rule_symbols,
+                bool require_tight_phrases);
+
+  virtual ~RuleExtractor();
+
+  // Extracts SCFG rules given a source phrase and a set of its occurrences
+  // in the source data.
+  virtual vector<Rule> ExtractRules(const Phrase& phrase,
+                                    const PhraseLocation& location) const;
+
+ protected:
+  RuleExtractor();
+
+ private:
+  // Finds all target phrases that can be aligned with the source phrase for a
+  // particular occurrence in the data.
+  vector<Extract> ExtractAlignments(const Phrase& phrase,
+                                    const vector<int>& matching) const;
+
+  // Extracts all target phrases for a given occurrence of the source phrase in
+  // the data. Constructs a vector of Extracts using these target phrases.
+  void AddExtracts(
+      vector<Extract>& extracts, const Phrase& source_phrase,
+      const unordered_map<int, int>& source_indexes,
+      const vector<pair<int, int> >& target_gaps, const vector<int>& target_low,
+      int target_phrase_low, int target_phrase_high, int sentence_id) const;
+
+  // Adds a leading and/or trailing nonterminal to the source phrase and
+  // extracts target phrases that can be aligned with the extended source
+  // phrase.
+  void AddNonterminalExtremities(
+      vector<Extract>& extracts, const vector<int>& matching,
+      const vector<int>& chunklen, const Phrase& source_phrase,
+      int source_back_low, int source_back_high, const vector<int>& source_low,
+      const vector<int>& source_high, const vector<int>& target_low,
+      const vector<int>& target_high, vector<pair<int, int> > target_gaps,
+      int sentence_id, int source_sent_start, int starts_with_x,
+      int ends_with_x, int extend_left, int extend_right) const;
+
+ private:
+  shared_ptr<DataArray> target_data_array;
+  shared_ptr<DataArray> source_data_array;
+  shared_ptr<PhraseBuilder> phrase_builder;
+  shared_ptr<Scorer> scorer;
+  shared_ptr<TargetPhraseExtractor> target_phrase_extractor;
+  shared_ptr<RuleExtractorHelper> helper;
+  int max_rule_span;
+  int min_gap_size;
+  int max_nonterminals;
+  int max_rule_symbols;
+  bool require_tight_phrases;
+};
+
+} // namespace extractor
+
+#endif