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diff --git a/extractor/rule_extractor_helper.h b/extractor/rule_extractor_helper.h
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+++ b/extractor/rule_extractor_helper.h
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+#ifndef _RULE_EXTRACTOR_HELPER_H_
+#define _RULE_EXTRACTOR_HELPER_H_
+
+#include <memory>
+#include <unordered_map>
+#include <vector>
+
+using namespace std;
+
+namespace extractor {
+
+class Alignment;
+class DataArray;
+
+/**
+ * Helper class for extracting SCFG rules.
+ */
+class RuleExtractorHelper {
+ public:
+  RuleExtractorHelper(shared_ptr<DataArray> source_data_array,
+                      shared_ptr<DataArray> target_data_array,
+                      shared_ptr<Alignment> alignment,
+                      int max_rule_span,
+                      int max_rule_symbols,
+                      bool require_aligned_terminal,
+                      bool require_aligned_chunks,
+                      bool require_tight_phrases);
+
+  virtual ~RuleExtractorHelper();
+
+  // Find the alignment span for each word in the source target sentence pair.
+  virtual void GetLinksSpans(vector<int>& source_low, vector<int>& source_high,
+                             vector<int>& target_low, vector<int>& target_high,
+                             int sentence_id) const;
+
+  // Check if one chunk (all chunks) is aligned at least in one point.
+  virtual bool CheckAlignedTerminals(const vector<int>& matching,
+                                     const vector<int>& chunklen,
+                                     const vector<int>& source_low,
+                                     int source_sent_start) const;
+
+  // Check if the chunks are tight.
+  virtual bool CheckTightPhrases(const vector<int>& matching,
+                                 const vector<int>& chunklen,
+                                 const vector<int>& source_low,
+                                 int source_sent_start) const;
+
+  // Find the target span and the reflected source span for a source phrase
+  // occurrence.
+  virtual bool FindFixPoint(
+      int source_phrase_low, int source_phrase_high,
+      const vector<int>& source_low, const vector<int>& source_high,
+      int& target_phrase_low, int& target_phrase_high,
+      const vector<int>& target_low, const vector<int>& target_high,
+      int& source_back_low, int& source_back_high, int sentence_id,
+      int min_source_gap_size, int min_target_gap_size,
+      int max_new_x, bool allow_low_x, bool allow_high_x,
+      bool allow_arbitrary_expansion) const;
+
+  // Find the gap spans for each nonterminal in the source phrase.
+  virtual bool GetGaps(
+      vector<pair<int, int> >& source_gaps, vector<pair<int, int> >& target_gaps,
+      const vector<int>& matching, const vector<int>& chunklen,
+      const vector<int>& source_low, const vector<int>& source_high,
+      const vector<int>& target_low, const vector<int>& target_high,
+      int source_phrase_low, int source_phrase_high, int source_back_low,
+      int source_back_high, int sentence_id, int source_sent_start,
+      int& num_symbols, bool& met_constraints) const;
+
+  // Get the order of the nonterminals in the target phrase.
+  virtual vector<int> GetGapOrder(const vector<pair<int, int> >& gaps) const;
+
+  // Map each terminal symbol with its position in the source phrase.
+  virtual unordered_map<int, int> GetSourceIndexes(
+      const vector<int>& matching, const vector<int>& chunklen,
+      int starts_with_x, int source_sent_start) const;
+
+ protected:
+  RuleExtractorHelper();
+
+ private:
+  // Find the projection of a source phrase in the target sentence. May also be
+  // used to find the projection of a target phrase in the source sentence.
+  void FindProjection(
+      int source_phrase_low, int source_phrase_high,
+      const vector<int>& source_low, const vector<int>& source_high,
+      int& target_phrase_low, int& target_phrase_high) const;
+
+  shared_ptr<DataArray> source_data_array;
+  shared_ptr<DataArray> target_data_array;
+  shared_ptr<Alignment> alignment;
+  int max_rule_span;
+  int max_rule_symbols;
+  bool require_aligned_terminal;
+  bool require_aligned_chunks;
+  bool require_tight_phrases;
+};
+
+} // namespace extractor
+
+#endif