fix merge conflict

53 files changed, 1298 insertions, 549 deletions

diff --git a/extractor/Makefile.am b/extractor/Makefile.am
index 65a3d436..e5b439f9 100644
--- a/extractor/Makefile.am
+++ b/extractor/Makefile.am
@@ -1,5 +1,5 @@
 
-bin_PROGRAMS = compile run_extractor
+bin_PROGRAMS = compile run_extractor extract
 
 if HAVE_CXX11
 
@@ -15,16 +15,19 @@ EXTRA_PROGRAMS = alignment_test \
     feature_target_given_source_coherent_test \
     grammar_extractor_test \
     matchings_finder_test \
+    matchings_sampler_test \
+    phrase_location_sampler_test \
     phrase_test \
     precomputation_test \
     rule_extractor_helper_test \
     rule_extractor_test \
     rule_factory_test \
-    sampler_test \
     scorer_test \
+    suffix_array_sampler_test \
     suffix_array_test \
     target_phrase_extractor_test \
-    translation_table_test
+    translation_table_test \
+    vocabulary_test
 
 if HAVE_GTEST
   RUNNABLE_TESTS = alignment_test \
@@ -39,16 +42,19 @@ if HAVE_GTEST
     feature_target_given_source_coherent_test \
     grammar_extractor_test \
     matchings_finder_test \
+    matchings_sampler_test \
+    phrase_location_sampler_test \
     phrase_test \
     precomputation_test \
     rule_extractor_helper_test \
     rule_extractor_test \
     rule_factory_test \
-    sampler_test \
     scorer_test \
+    suffix_array_sampler_test \
     suffix_array_test \
     target_phrase_extractor_test \
-    translation_table_test
+    translation_table_test \
+    vocabulary_test
 endif
 
 noinst_PROGRAMS = $(RUNNABLE_TESTS)
@@ -79,6 +85,10 @@ grammar_extractor_test_SOURCES = grammar_extractor_test.cc
 grammar_extractor_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
 matchings_finder_test_SOURCES = matchings_finder_test.cc
 matchings_finder_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
+matchings_sampler_test_SOURCES = matchings_sampler_test.cc
+matchings_sampler_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
+phrase_location_sampler_test_SOURCES = phrase_location_sampler_test.cc
+phrase_location_sampler_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
 phrase_test_SOURCES = phrase_test.cc
 phrase_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
 precomputation_test_SOURCES = precomputation_test.cc
@@ -89,57 +99,31 @@ rule_extractor_test_SOURCES = rule_extractor_test.cc
 rule_extractor_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
 rule_factory_test_SOURCES = rule_factory_test.cc
 rule_factory_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
-sampler_test_SOURCES = sampler_test.cc
-sampler_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
 scorer_test_SOURCES = scorer_test.cc
 scorer_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
+suffix_array_sampler_test_SOURCES = suffix_array_sampler_test.cc
+suffix_array_sampler_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
 suffix_array_test_SOURCES = suffix_array_test.cc
 suffix_array_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
 target_phrase_extractor_test_SOURCES = target_phrase_extractor_test.cc
 target_phrase_extractor_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
 translation_table_test_SOURCES = translation_table_test.cc
 translation_table_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) $(GMOCK_LDFLAGS) $(GMOCK_LIBS) libextractor.a
+vocabulary_test_SOURCES = vocabulary_test.cc
+vocabulary_test_LDADD = $(GTEST_LDFLAGS) $(GTEST_LIBS) libextractor.a
 
-noinst_LIBRARIES = libextractor.a libcompile.a
+noinst_LIBRARIES = libextractor.a
 
 compile_SOURCES = compile.cc
-compile_LDADD = libcompile.a
+compile_LDADD = libextractor.a
 run_extractor_SOURCES = run_extractor.cc
 run_extractor_LDADD = libextractor.a
-
-libcompile_a_SOURCES = \
-  alignment.cc \
-  data_array.cc \
-  phrase_location.cc \
-  precomputation.cc \
-  suffix_array.cc \
-  time_util.cc \
-  translation_table.cc \
-  alignment.h \
-  data_array.h \
-  fast_intersector.h \
-  grammar.h \
-  grammar_extractor.h \
-  matchings_finder.h \
-  matchings_trie.h \
-  phrase.h \
-  phrase_builder.h \
-  phrase_location.h \
-  precomputation.h \
-  rule.h \
-  rule_extractor.h \
-  rule_extractor_helper.h \
-  rule_factory.h \
-  sampler.h \
-  scorer.h \
-  suffix_array.h \
-  target_phrase_extractor.h \
-  time_util.h \
-  translation_table.h \
-  vocabulary.h
+extract_SOURCES = extract.cc
+extract_LDADD = libextractor.a
 
 libextractor_a_SOURCES = \
   alignment.cc \
+  backoff_sampler.cc \
   data_array.cc \
   fast_intersector.cc \
   features/count_source_target.cc \
@@ -153,18 +137,20 @@ libextractor_a_SOURCES = \
   grammar.cc \
   grammar_extractor.cc \
   matchings_finder.cc \
+  matchings_sampler.cc \
   matchings_trie.cc \
   phrase.cc \
   phrase_builder.cc \
   phrase_location.cc \
+  phrase_location_sampler.cc \
   precomputation.cc \
   rule.cc \
   rule_extractor.cc \
   rule_extractor_helper.cc \
   rule_factory.cc \
-  sampler.cc \
   scorer.cc \
   suffix_array.cc \
+  suffix_array_sampler.cc \
   target_phrase_extractor.cc \
   time_util.cc \
   translation_table.cc \
diff --git a/extractor/README.md b/extractor/README.md
index c9db8de8..642fbd1d 100644
--- a/extractor/README.md
+++ b/extractor/README.md
@@ -1,8 +1,14 @@
 C++ implementation of the online grammar extractor originally developed by [Adam Lopez](http://www.cs.jhu.edu/~alopez/).
 
-To run the extractor you need to:
+The grammar extraction takes place in two steps: (a) precomputing a number of data structures and (b) actually extracting the grammars. All the flags below have the same meaning as in the cython implementation.
 
-    cdec/extractor/run_extractor -t <num_threads> -a <alignment> -b <parallel_corpus> -g <grammar_output_path> < <input_sentences> > <sgm_file>
+To compile the data structures you need to run:
+
+    cdec/extractor/compile -a <alignment> -b <parallel_corpus> -c <compile_config_file> -o <compile_directory>
+
+To extract the grammars you need to run:
+
+    cdec/extract/extract -t <num_threads> -c <compile_config_file> -g <grammar_output_path> < <input_sentencs> > <sgm_file>
 
 To run unit tests you need first to configure `cdec` with the [Google Test](https://code.google.com/p/googletest/) and [Google Mock](https://code.google.com/p/googlemock/) libraries:
 
diff --git a/extractor/alignment.cc b/extractor/alignment.cc
index 2278c825..4a7a14f4 100644
--- a/extractor/alignment.cc
+++ b/extractor/alignment.cc
@@ -8,9 +8,7 @@
 #include <vector>
 
 #include <boost/algorithm/string.hpp>
-#include <boost/filesystem.hpp>
 
-namespace fs = boost::filesystem;
 using namespace std;
 
 namespace extractor {
diff --git a/extractor/alignment.h b/extractor/alignment.h
index dc5a8b55..76c27da2 100644
--- a/extractor/alignment.h
+++ b/extractor/alignment.h
@@ -4,13 +4,11 @@
 #include <string>
 #include <vector>
 
-#include <boost/filesystem.hpp>
 #include <boost/serialization/serialization.hpp>
 #include <boost/serialization/split_member.hpp>
 #include <boost/serialization/utility.hpp>
 #include <boost/serialization/vector.hpp>
 
-namespace fs = boost::filesystem;
 using namespace std;
 
 namespace extractor {
diff --git a/extractor/backoff_sampler.cc b/extractor/backoff_sampler.cc
new file mode 100644
index 00000000..891276c6
--- /dev/null
+++ b/extractor/backoff_sampler.cc
@@ -0,0 +1,66 @@
+#include "backoff_sampler.h"
+
+#include "data_array.h"
+#include "phrase_location.h"
+
+namespace extractor {
+
+BackoffSampler::BackoffSampler(
+    shared_ptr<DataArray> source_data_array, int max_samples) :
+    source_data_array(source_data_array), max_samples(max_samples) {}
+
+BackoffSampler::BackoffSampler() {}
+
+PhraseLocation BackoffSampler::Sample(
+    const PhraseLocation& location,
+    const unordered_set<int>& blacklisted_sentence_ids) const {
+  vector<int> samples;
+  int low = GetRangeLow(location), high = GetRangeHigh(location);
+  int last = low - 1;
+  double step = max(1.0, (double) (high - low) / max_samples);
+  for (double num_samples = 0, i = low;
+       num_samples < max_samples && i < high;
+       ++num_samples, i += step) {
+    int sample = round(i);
+    int position = GetPosition(location, sample);
+    int sentence_id = source_data_array->GetSentenceId(position);
+    bool found = false;
+    if (last >= sample ||
+        blacklisted_sentence_ids.count(sentence_id)) {
+      for (double backoff_step = 1; backoff_step < step; ++backoff_step) {
+        double j = i - backoff_step;
+        sample = round(j);
+        if (sample >= 0) {
+          position = GetPosition(location, sample);
+          sentence_id = source_data_array->GetSentenceId(position);
+          if (sample > last && !blacklisted_sentence_ids.count(sentence_id)) {
+            found = true;
+            break;
+          }
+        }
+
+        double k = i + backoff_step;
+        sample = round(k);
+        if (sample < high) {
+          position = GetPosition(location, sample);
+          sentence_id = source_data_array->GetSentenceId(position);
+          if (!blacklisted_sentence_ids.count(sentence_id)) {
+            found = true;
+            break;
+          }
+        }
+      }
+    } else {
+      found = true;
+    }
+
+    if (found) {
+      last = sample;
+      AppendMatching(samples, sample, location);
+    }
+  }
+
+  return PhraseLocation(samples, GetNumSubpatterns(location));
+}
+
+} // namespace extractor
diff --git a/extractor/backoff_sampler.h b/extractor/backoff_sampler.h
new file mode 100644
index 00000000..5c244105
--- /dev/null
+++ b/extractor/backoff_sampler.h
@@ -0,0 +1,41 @@
+#ifndef _BACKOFF_SAMPLER_H_
+#define _BACKOFF_SAMPLER_H_
+
+#include <vector>
+
+#include "sampler.h"
+
+namespace extractor {
+
+class DataArray;
+class PhraseLocation;
+
+class BackoffSampler : public Sampler {
+ public:
+  BackoffSampler(shared_ptr<DataArray> source_data_array, int max_samples);
+
+  BackoffSampler();
+
+  PhraseLocation Sample(
+      const PhraseLocation& location,
+      const unordered_set<int>& blacklisted_sentence_ids) const;
+
+ private:
+  virtual int GetNumSubpatterns(const PhraseLocation& location) const = 0;
+
+  virtual int GetRangeLow(const PhraseLocation& location) const = 0;
+
+  virtual int GetRangeHigh(const PhraseLocation& location) const = 0;
+
+  virtual int GetPosition(const PhraseLocation& location, int index) const = 0;
+
+  virtual void AppendMatching(vector<int>& samples, int index,
+                              const PhraseLocation& location) const = 0;
+
+  shared_ptr<DataArray> source_data_array;
+  int max_samples;
+};
+
+} // namespace extractor
+
+#endif
diff --git a/extractor/compile.cc b/extractor/compile.cc
index 65fdd509..3ee668ce 100644
--- a/extractor/compile.cc
+++ b/extractor/compile.cc
@@ -13,6 +13,7 @@
 #include "suffix_array.h"
 #include "time_util.h"
 #include "translation_table.h"
+#include "vocabulary.h"
 
 namespace ar = boost::archive;
 namespace fs = boost::filesystem;
@@ -29,6 +30,8 @@ int main(int argc, char** argv) {
     ("bitext,b", po::value<string>(), "Parallel text (source ||| target)")
     ("alignment,a", po::value<string>()->required(), "Bitext word alignment")
     ("output,o", po::value<string>()->required(), "Output path")
+    ("config,c", po::value<string>()->required(),
+        "Path where the config file will be generated")
     ("frequent", po::value<int>()->default_value(100),
         "Number of precomputed frequent patterns")
     ("super_frequent", po::value<int>()->default_value(10),
@@ -81,8 +84,12 @@ int main(int argc, char** argv) {
     target_data_array = make_shared<DataArray>(vm["target"].as<string>());
   }
 
+  ofstream config_stream(vm["config"].as<string>());
+
   Clock::time_point start_write = Clock::now();
-  ofstream target_fstream((output_dir / fs::path("target.bin")).string());
+  string target_path = (output_dir / fs::path("target.bin")).string();
+  config_stream << "target = " << target_path << endl;
+  ofstream target_fstream(target_path);
   ar::binary_oarchive target_stream(target_fstream);
   target_stream << *target_data_array;
   Clock::time_point stop_write = Clock::now();
@@ -99,7 +106,9 @@ int main(int argc, char** argv) {
       make_shared<SuffixArray>(source_data_array);
 
   start_write = Clock::now();
-  ofstream source_fstream((output_dir / fs::path("source.bin")).string());
+  string source_path = (output_dir / fs::path("source.bin")).string();
+  config_stream << "source = " << source_path << endl;
+  ofstream source_fstream(source_path);
   ar::binary_oarchive output_stream(source_fstream);
   output_stream << *source_suffix_array;
   stop_write = Clock::now();
@@ -115,7 +124,9 @@ int main(int argc, char** argv) {
       make_shared<Alignment>(vm["alignment"].as<string>());
 
   start_write = Clock::now();
-  ofstream alignment_fstream((output_dir / fs::path("alignment.bin")).string());
+  string alignment_path = (output_dir / fs::path("alignment.bin")).string();
+  config_stream << "alignment = " << alignment_path << endl;
+  ofstream alignment_fstream(alignment_path);
   ar::binary_oarchive alignment_stream(alignment_fstream);
   alignment_stream << *alignment;
   stop_write = Clock::now();
@@ -125,9 +136,12 @@ int main(int argc, char** argv) {
   cerr << "Reading alignment took "
        << GetDuration(start_time, stop_time) << " seconds" << endl;
 
+  shared_ptr<Vocabulary> vocabulary = make_shared<Vocabulary>();
+
   start_time = Clock::now();
   cerr << "Precomputing collocations..." << endl;
   Precomputation precomputation(
+      vocabulary,
       source_suffix_array,
       vm["frequent"].as<int>(),
       vm["super_frequent"].as<int>(),
@@ -138,9 +152,17 @@ int main(int argc, char** argv) {
       vm["min_frequency"].as<int>());
 
   start_write = Clock::now();
-  ofstream precomp_fstream((output_dir / fs::path("precomp.bin")).string());
+  string precomputation_path = (output_dir / fs::path("precomp.bin")).string();
+  config_stream << "precomputation = " << precomputation_path << endl;
+  ofstream precomp_fstream(precomputation_path);
   ar::binary_oarchive precomp_stream(precomp_fstream);
   precomp_stream << precomputation;
+
+  string vocabulary_path = (output_dir / fs::path("vocab.bin")).string();
+  config_stream << "vocabulary = " << vocabulary_path << endl;
+  ofstream vocab_fstream(vocabulary_path);
+  ar::binary_oarchive vocab_stream(vocab_fstream);
+  vocab_stream << *vocabulary;
   stop_write = Clock::now();
   write_duration += GetDuration(start_write, stop_write);
 
@@ -153,7 +175,9 @@ int main(int argc, char** argv) {
   TranslationTable table(source_data_array, target_data_array, alignment);
 
   start_write = Clock::now();
-  ofstream table_fstream((output_dir / fs::path("bilex.bin")).string());
+  string table_path = (output_dir / fs::path("bilex.bin")).string();
+  config_stream << "ttable = " << table_path << endl;
+  ofstream table_fstream(table_path);
   ar::binary_oarchive table_stream(table_fstream);
   table_stream << table;
   stop_write = Clock::now();
diff --git a/extractor/data_array.cc b/extractor/data_array.cc
index 2e4bdafb..9612aa8a 100644
--- a/extractor/data_array.cc
+++ b/extractor/data_array.cc
@@ -5,9 +5,6 @@
 #include <sstream>
 #include <string>
 
-#include <boost/filesystem.hpp>
-
-namespace fs = boost::filesystem;
 using namespace std;
 
 namespace extractor {
@@ -81,7 +78,7 @@ void DataArray::CreateDataArray(const vector<string>& lines) {
 
 DataArray::~DataArray() {}
 
-const vector<int>& DataArray::GetData() const {
+vector<int> DataArray::GetData() const {
   return data;
 }
 
@@ -93,6 +90,18 @@ string DataArray::GetWordAtIndex(int index) const {
   return id2word[data[index]];
 }
 
+vector<int> DataArray::GetWordIds(int index, int size) const {
+  return vector<int>(data.begin() + index, data.begin() + index + size);
+}
+
+vector<string> DataArray::GetWords(int start_index, int size) const {
+  vector<string> words;
+  for (int word_id: GetWordIds(start_index, size)) {
+    words.push_back(id2word[word_id]);
+  }
+  return words;
+}
+
 int DataArray::GetSize() const {
   return data.size();
 }
@@ -118,10 +127,6 @@ int DataArray::GetSentenceId(int position) const {
   return sentence_id[position];
 }
 
-bool DataArray::HasWord(const string& word) const {
-  return word2id.count(word);
-}
-
 int DataArray::GetWordId(const string& word) const {
   auto result = word2id.find(word);
   return result == word2id.end() ? -1 : result->second;
diff --git a/extractor/data_array.h b/extractor/data_array.h
index 2be6a09c..b96901d1 100644
--- a/extractor/data_array.h
+++ b/extractor/data_array.h
@@ -5,13 +5,11 @@
 #include <unordered_map>
 #include <vector>
 
-#include <boost/filesystem.hpp>
 #include <boost/serialization/serialization.hpp>
 #include <boost/serialization/split_member.hpp>
 #include <boost/serialization/string.hpp>
 #include <boost/serialization/vector.hpp>
 
-namespace fs = boost::filesystem;
 using namespace std;
 
 namespace extractor {
@@ -53,7 +51,7 @@ class DataArray {
   virtual ~DataArray();
 
   // Returns a vector containing the word ids.
-  virtual const vector<int>& GetData() const;
+  virtual vector<int> GetData() const;
 
   // Returns the word id at the specified position.
   virtual int AtIndex(int index) const;
@@ -61,15 +59,20 @@ class DataArray {
   // Returns the original word at the specified position.
   virtual string GetWordAtIndex(int index) const;
 
+  // Returns the substring of word ids starting at the specified position and
+  // having the specified length.
+  virtual vector<int> GetWordIds(int start_index, int size) const;
+
+  // Returns the substring of words starting at the specified position and
+  // having the specified length.
+  virtual vector<string> GetWords(int start_index, int size) const;
+
   // Returns the size of the data array.
   virtual int GetSize() const;
 
   // Returns the number of distinct words in the data array.
   virtual int GetVocabularySize() const;
 
-  // Returns whether a word has ever been observed in the data array.
-  virtual bool HasWord(const string& word) const;
-
   // Returns the word id for a given word or -1 if it the word has never been
   // observed.
   virtual int GetWordId(const string& word) const;
diff --git a/extractor/data_array_test.cc b/extractor/data_array_test.cc
index 6c329e34..99f79d91 100644
--- a/extractor/data_array_test.cc
+++ b/extractor/data_array_test.cc
@@ -56,18 +56,26 @@ TEST_F(DataArrayTest, TestGetData) {
   }
 }
 
+TEST_F(DataArrayTest, TestSubstrings) {
+  vector<int> expected_word_ids = {3, 4, 5};
+  vector<string> expected_words = {"are", "mere", "."};
+  EXPECT_EQ(expected_word_ids, source_data.GetWordIds(1, 3));
+  EXPECT_EQ(expected_words, source_data.GetWords(1, 3));
+
+  expected_word_ids = {7, 8};
+  expected_words = {"a", "lot"};
+  EXPECT_EQ(expected_word_ids, target_data.GetWordIds(7, 2));
+  EXPECT_EQ(expected_words, target_data.GetWords(7, 2));
+}
+
 TEST_F(DataArrayTest, TestVocabulary) {
   EXPECT_EQ(9, source_data.GetVocabularySize());
-  EXPECT_TRUE(source_data.HasWord("mere"));
   EXPECT_EQ(4, source_data.GetWordId("mere"));
   EXPECT_EQ("mere", source_data.GetWord(4));
-  EXPECT_FALSE(source_data.HasWord("banane"));
 
   EXPECT_EQ(11, target_data.GetVocabularySize());
-  EXPECT_TRUE(target_data.HasWord("apples"));
   EXPECT_EQ(4, target_data.GetWordId("apples"));
   EXPECT_EQ("apples", target_data.GetWord(4));
-  EXPECT_FALSE(target_data.HasWord("bananas"));
 }
 
 TEST_F(DataArrayTest, TestSentenceData) {
diff --git a/extractor/extract.cc b/extractor/extract.cc
new file mode 100644
index 00000000..387cbe9b
--- /dev/null
+++ b/extractor/extract.cc
@@ -0,0 +1,253 @@
+#include <fstream>
+#include <iostream>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include <boost/archive/binary_iarchive.hpp>
+#include <boost/filesystem.hpp>
+#include <boost/program_options.hpp>
+#include <boost/program_options/variables_map.hpp>
+#include <omp.h>
+
+#include "alignment.h"
+#include "data_array.h"
+#include "features/count_source_target.h"
+#include "features/feature.h"
+#include "features/is_source_singleton.h"
+#include "features/is_source_target_singleton.h"
+#include "features/max_lex_source_given_target.h"
+#include "features/max_lex_target_given_source.h"
+#include "features/sample_source_count.h"
+#include "features/target_given_source_coherent.h"
+#include "grammar.h"
+#include "grammar_extractor.h"
+#include "precomputation.h"
+#include "rule.h"
+#include "scorer.h"
+#include "suffix_array.h"
+#include "time_util.h"
+#include "translation_table.h"
+#include "vocabulary.h"
+
+namespace ar = boost::archive;
+namespace fs = boost::filesystem;
+namespace po = boost::program_options;
+using namespace extractor;
+using namespace features;
+using namespace std;
+
+// Returns the file path in which a given grammar should be written.
+fs::path GetGrammarFilePath(const fs::path& grammar_path, int file_number) {
+  string file_name = "grammar." + to_string(file_number);
+  return grammar_path / file_name;
+}
+
+int main(int argc, char** argv) {
+  po::options_description general_options("General options");
+  int max_threads = 1;
+  #pragma omp parallel
+  max_threads = omp_get_num_threads();
+  string threads_option = "Number of threads used for grammar extraction "
+                          "max(" + to_string(max_threads) + ")";
+  general_options.add_options()
+    ("threads,t", po::value<int>()->required()->default_value(1),
+     threads_option.c_str())
+    ("grammars,g", po::value<string>()->required(), "Grammars output path")
+    ("max_rule_span", po::value<int>()->default_value(15),
+        "Maximum rule span")
+    ("max_rule_symbols", po::value<int>()->default_value(5),
+        "Maximum number of symbols (terminals + nontermals) in a rule")
+    ("min_gap_size", po::value<int>()->default_value(1), "Minimum gap size")
+    ("max_nonterminals", po::value<int>()->default_value(2),
+        "Maximum number of nonterminals in a rule")
+    ("max_samples", po::value<int>()->default_value(300),
+        "Maximum number of samples")
+    ("tight_phrases", po::value<bool>()->default_value(true),
+        "False if phrases may be loose (better, but slower)")
+    ("leave_one_out", po::value<bool>()->zero_tokens(),
+        "do leave-one-out estimation of grammars "
+        "(e.g. for extracting grammars for the training set");
+
+  po::options_description cmdline_options("Command line options");
+  cmdline_options.add_options()
+    ("help", "Show available options")
+    ("config,c", po::value<string>()->required(), "Path to config file");
+  cmdline_options.add(general_options);
+
+  po::options_description config_options("Config file options");
+  config_options.add_options()
+    ("target", po::value<string>()->required(),
+        "Path to target data file in binary format")
+    ("source", po::value<string>()->required(),
+        "Path to source suffix array file in binary format")
+    ("alignment", po::value<string>()->required(),
+        "Path to alignment file in binary format")
+    ("precomputation", po::value<string>()->required(),
+        "Path to precomputation file in binary format")
+    ("vocabulary", po::value<string>()->required(),
+        "Path to vocabulary file in binary format")
+    ("ttable", po::value<string>()->required(),
+        "Path to translation table in binary format");
+  config_options.add(general_options);
+
+  po::variables_map vm;
+  po::store(po::parse_command_line(argc, argv, cmdline_options), vm);
+  if (vm.count("help")) {
+    po::options_description all_options;
+    all_options.add(cmdline_options).add(config_options);
+    cout << all_options << endl;
+    return 0;
+  }
+
+  po::notify(vm);
+
+  ifstream config_stream(vm["config"].as<string>());
+  po::store(po::parse_config_file(config_stream, config_options), vm);
+  po::notify(vm);
+
+  int num_threads = vm["threads"].as<int>();
+  cerr << "Grammar extraction will use " << num_threads << " threads." << endl;
+
+  Clock::time_point read_start_time = Clock::now();
+
+  Clock::time_point start_time = Clock::now();
+  cerr << "Reading target data in binary format..." << endl;
+  shared_ptr<DataArray> target_data_array = make_shared<DataArray>();
+  ifstream target_fstream(vm["target"].as<string>());
+  ar::binary_iarchive target_stream(target_fstream);
+  target_stream >> *target_data_array;
+  Clock::time_point end_time = Clock::now();
+  cerr << "Reading target data took " << GetDuration(start_time, end_time)
+       << " seconds" << endl;
+
+  start_time = Clock::now();
+  cerr << "Reading source suffix array in binary format..." << endl;
+  shared_ptr<SuffixArray> source_suffix_array = make_shared<SuffixArray>();
+  ifstream source_fstream(vm["source"].as<string>());
+  ar::binary_iarchive source_stream(source_fstream);
+  source_stream >> *source_suffix_array;
+  end_time = Clock::now();
+  cerr << "Reading source suffix array took "
+       << GetDuration(start_time, end_time) << " seconds" << endl;
+
+  start_time = Clock::now();
+  cerr << "Reading alignment in binary format..." << endl;
+  shared_ptr<Alignment> alignment = make_shared<Alignment>();
+  ifstream alignment_fstream(vm["alignment"].as<string>());
+  ar::binary_iarchive alignment_stream(alignment_fstream);
+  alignment_stream >> *alignment;
+  end_time = Clock::now();
+  cerr << "Reading alignment took " << GetDuration(start_time, end_time)
+       << " seconds" << endl;
+
+  start_time = Clock::now();
+  cerr << "Reading precomputation in binary format..." << endl;
+  shared_ptr<Precomputation> precomputation = make_shared<Precomputation>();
+  ifstream precomputation_fstream(vm["precomputation"].as<string>());
+  ar::binary_iarchive precomputation_stream(precomputation_fstream);
+  precomputation_stream >> *precomputation;
+  end_time = Clock::now();
+  cerr << "Reading precomputation took " << GetDuration(start_time, end_time)
+       << " seconds" << endl;
+
+  start_time = Clock::now();
+  cerr << "Reading vocabulary in binary format..." << endl;
+  shared_ptr<Vocabulary> vocabulary = make_shared<Vocabulary>();
+  ifstream vocabulary_fstream(vm["vocabulary"].as<string>());
+  ar::binary_iarchive vocabulary_stream(vocabulary_fstream);
+  vocabulary_stream >> *vocabulary;
+  end_time = Clock::now();
+  cerr << "Reading vocabulary took " << GetDuration(start_time, end_time)
+       << " seconds" << endl;
+
+  start_time = Clock::now();
+  cerr << "Reading translation table in binary format..." << endl;
+  shared_ptr<TranslationTable> table = make_shared<TranslationTable>();
+  ifstream ttable_fstream(vm["ttable"].as<string>());
+  ar::binary_iarchive ttable_stream(ttable_fstream);
+  ttable_stream >> *table;
+  end_time = Clock::now();
+  cerr << "Reading translation table took " << GetDuration(start_time, end_time)
+       << " seconds" << endl;
+
+  Clock::time_point read_end_time = Clock::now();
+  cerr << "Total time spent loading data structures into memory: "
+       << GetDuration(read_start_time, read_end_time) << " seconds" << endl;
+
+  Clock::time_point extraction_start_time = Clock::now();
+  // Features used to score each grammar rule.
+  vector<shared_ptr<Feature>> features = {
+      make_shared<TargetGivenSourceCoherent>(),
+      make_shared<SampleSourceCount>(),
+      make_shared<CountSourceTarget>(),
+      make_shared<MaxLexSourceGivenTarget>(table),
+      make_shared<MaxLexTargetGivenSource>(table),
+      make_shared<IsSourceSingleton>(),
+      make_shared<IsSourceTargetSingleton>()
+  };
+  shared_ptr<Scorer> scorer = make_shared<Scorer>(features);
+
+  GrammarExtractor extractor(
+      source_suffix_array,
+      target_data_array,
+      alignment,
+      precomputation,
+      scorer,
+      vocabulary,
+      vm["min_gap_size"].as<int>(),
+      vm["max_rule_span"].as<int>(),
+      vm["max_nonterminals"].as<int>(),
+      vm["max_rule_symbols"].as<int>(),
+      vm["max_samples"].as<int>(),
+      vm["tight_phrases"].as<bool>());
+
+  // Creates the grammars directory if it doesn't exist.
+  fs::path grammar_path = vm["grammars"].as<string>();
+  if (!fs::is_directory(grammar_path)) {
+    fs::create_directory(grammar_path);
+  }
+
+  // Reads all sentences for which we extract grammar rules (the paralellization
+  // is simplified if we read all sentences upfront).
+  string sentence;
+  vector<string> sentences;
+  while (getline(cin, sentence)) {
+    sentences.push_back(sentence);
+  }
+
+  // Extracts the grammar for each sentence and saves it to a file.
+  vector<string> suffixes(sentences.size());
+  bool leave_one_out = vm.count("leave_one_out");
+  #pragma omp parallel for schedule(dynamic) num_threads(num_threads)
+  for (size_t i = 0; i < sentences.size(); ++i) {
+    string suffix;
+    int position = sentences[i].find("|||");
+    if (position != sentences[i].npos) {
+      suffix = sentences[i].substr(position);
+      sentences[i] = sentences[i].substr(0, position);
+    }
+    suffixes[i] = suffix;
+
+    unordered_set<int> blacklisted_sentence_ids;
+    if (leave_one_out) {
+      blacklisted_sentence_ids.insert(i);
+    }
+    Grammar grammar = extractor.GetGrammar(
+        sentences[i], blacklisted_sentence_ids);
+    ofstream output(GetGrammarFilePath(grammar_path, i).c_str());
+    output << grammar;
+  }
+
+  for (size_t i = 0; i < sentences.size(); ++i) {
+    cout << "<seg grammar=" << GetGrammarFilePath(grammar_path, i) << " id=\""
+         << i << "\"> " << sentences[i] << " </seg> " << suffixes[i] << endl;
+  }
+
+  Clock::time_point extraction_stop_time = Clock::now();
+  cerr << "Overall extraction step took "
+       << GetDuration(extraction_start_time, extraction_stop_time)
+       << " seconds" << endl;
+
+  return 0;
+}
diff --git a/extractor/fast_intersector.cc b/extractor/fast_intersector.cc
index a8591a72..0d1fa6d8 100644
--- a/extractor/fast_intersector.cc
+++ b/extractor/fast_intersector.cc
@@ -11,41 +11,22 @@
 
 namespace extractor {
 
-FastIntersector::FastIntersector(shared_ptr<SuffixArray> suffix_array,
-                                 shared_ptr<Precomputation> precomputation,
-                                 shared_ptr<Vocabulary> vocabulary,
-                                 int max_rule_span,
-                                 int min_gap_size) :
+FastIntersector::FastIntersector(
+    shared_ptr<SuffixArray> suffix_array,
+    shared_ptr<Precomputation> precomputation,
+    shared_ptr<Vocabulary> vocabulary,
+    int max_rule_span,
+    int min_gap_size) :
     suffix_array(suffix_array),
+    precomputation(precomputation),
     vocabulary(vocabulary),
     max_rule_span(max_rule_span),
-    min_gap_size(min_gap_size) {
-  Index precomputed_collocations = precomputation->GetCollocations();
-  for (pair<vector<int>, vector<int>> entry: precomputed_collocations) {
-    vector<int> phrase = ConvertPhrase(entry.first);
-    collocations[phrase] = entry.second;
-  }
-}
+    min_gap_size(min_gap_size) {}
 
 FastIntersector::FastIntersector() {}
 
 FastIntersector::~FastIntersector() {}
 
-vector<int> FastIntersector::ConvertPhrase(const vector<int>& old_phrase) {
-  vector<int> new_phrase;
-  new_phrase.reserve(old_phrase.size());
-  shared_ptr<DataArray> data_array = suffix_array->GetData();
-  for (int word_id: old_phrase) {
-    if (word_id < 0) {
-      new_phrase.push_back(word_id);
-    } else {
-      new_phrase.push_back(
-          vocabulary->GetTerminalIndex(data_array->GetWord(word_id)));
-    }
-  }
-  return new_phrase;
-}
-
 PhraseLocation FastIntersector::Intersect(
     PhraseLocation& prefix_location,
     PhraseLocation& suffix_location,
@@ -59,8 +40,9 @@ PhraseLocation FastIntersector::Intersect(
   assert(vocabulary->IsTerminal(symbols.front())
       && vocabulary->IsTerminal(symbols.back()));
 
-  if (collocations.count(symbols)) {
-    return PhraseLocation(collocations[symbols], phrase.Arity() + 1);
+  if (precomputation->Contains(symbols)) {
+    return PhraseLocation(precomputation->GetCollocations(symbols),
+                          phrase.Arity() + 1);
   }
 
   bool prefix_ends_with_x =
diff --git a/extractor/fast_intersector.h b/extractor/fast_intersector.h
index 2819d239..305373dc 100644
--- a/extractor/fast_intersector.h
+++ b/extractor/fast_intersector.h
@@ -12,7 +12,6 @@ using namespace std;
 namespace extractor {
 
 typedef boost::hash<vector<int>> VectorHash;
-typedef unordered_map<vector<int>, vector<int>, VectorHash> Index;
 
 class Phrase;
 class PhraseLocation;
@@ -52,11 +51,6 @@ class FastIntersector {
   FastIntersector();
 
  private:
-  // Uses the vocabulary to convert the phrase from the numberized format
-  // specified by the source data array to the numberized format given by the
-  // vocabulary.
-  vector<int> ConvertPhrase(const vector<int>& old_phrase);
-
   // Estimates the number of computations needed if the prefix/suffix is
   // extended. If the last/first symbol is separated from the rest of the phrase
   // by a nonterminal, then for each occurrence of the prefix/suffix we need to
@@ -85,10 +79,10 @@ class FastIntersector {
   pair<int, int> GetSearchRange(bool has_marginal_x) const;
 
   shared_ptr<SuffixArray> suffix_array;
+  shared_ptr<Precomputation> precomputation;
   shared_ptr<Vocabulary> vocabulary;
   int max_rule_span;
   int min_gap_size;
-  Index collocations;
 };
 
 } // namespace extractor
diff --git a/extractor/fast_intersector_test.cc b/extractor/fast_intersector_test.cc
index 76c3aaea..f2a26ba1 100644
--- a/extractor/fast_intersector_test.cc
+++ b/extractor/fast_intersector_test.cc
@@ -59,15 +59,13 @@ class FastIntersectorTest : public Test {
     }
 
     precomputation = make_shared<MockPrecomputation>();
-    EXPECT_CALL(*precomputation, GetCollocations())
-        .WillRepeatedly(ReturnRef(collocations));
+    EXPECT_CALL(*precomputation, Contains(_)).WillRepeatedly(Return(false));
 
     phrase_builder = make_shared<PhraseBuilder>(vocabulary);
     intersector = make_shared<FastIntersector>(suffix_array, precomputation,
                                                vocabulary, 15, 1);
   }
 
-  Index collocations;
   shared_ptr<MockDataArray> data_array;
   shared_ptr<MockSuffixArray> suffix_array;
   shared_ptr<MockPrecomputation> precomputation;
@@ -82,9 +80,9 @@ TEST_F(FastIntersectorTest, TestCachedCollocation) {
   Phrase phrase = phrase_builder->Build(symbols);
   PhraseLocation prefix_location(15, 16), suffix_location(16, 17);
 
-  collocations[symbols] = expected_location;
-  EXPECT_CALL(*precomputation, GetCollocations())
-      .WillRepeatedly(ReturnRef(collocations));
+  EXPECT_CALL(*precomputation, Contains(symbols)).WillRepeatedly(Return(true));
+  EXPECT_CALL(*precomputation, GetCollocations(symbols)).
+      WillRepeatedly(Return(expected_location));
   intersector = make_shared<FastIntersector>(suffix_array, precomputation,
                                              vocabulary, 15, 1);
 
diff --git a/extractor/grammar_extractor.cc b/extractor/grammar_extractor.cc
index 487abcaf..1dc94c25 100644
--- a/extractor/grammar_extractor.cc
+++ b/extractor/grammar_extractor.cc
@@ -19,10 +19,11 @@ GrammarExtractor::GrammarExtractor(
     shared_ptr<SuffixArray> source_suffix_array,
     shared_ptr<DataArray> target_data_array,
     shared_ptr<Alignment> alignment, shared_ptr<Precomputation> precomputation,
-    shared_ptr<Scorer> scorer, int min_gap_size, int max_rule_span,
+    shared_ptr<Scorer> scorer, shared_ptr<Vocabulary> vocabulary,
+    int min_gap_size, int max_rule_span,
     int max_nonterminals, int max_rule_symbols, int max_samples,
     bool require_tight_phrases) :
-    vocabulary(make_shared<Vocabulary>()),
+    vocabulary(vocabulary),
     rule_factory(make_shared<HieroCachingRuleFactory>(
         source_suffix_array, target_data_array, alignment, vocabulary,
         precomputation, scorer, min_gap_size, max_rule_span, max_nonterminals,
@@ -34,10 +35,12 @@ GrammarExtractor::GrammarExtractor(
     vocabulary(vocabulary),
     rule_factory(rule_factory) {}
 
-Grammar GrammarExtractor::GetGrammar(const string& sentence, const unordered_set<int>& blacklisted_sentence_ids, const shared_ptr<DataArray> source_data_array) {
+Grammar GrammarExtractor::GetGrammar(
+    const string& sentence,
+    const unordered_set<int>& blacklisted_sentence_ids) {
   vector<string> words = TokenizeSentence(sentence);
   vector<int> word_ids = AnnotateWords(words);
-  return rule_factory->GetGrammar(word_ids, blacklisted_sentence_ids, source_data_array);
+  return rule_factory->GetGrammar(word_ids, blacklisted_sentence_ids);
 }
 
 vector<string> GrammarExtractor::TokenizeSentence(const string& sentence) {
diff --git a/extractor/grammar_extractor.h b/extractor/grammar_extractor.h
index ae407b47..0f3069b0 100644
--- a/extractor/grammar_extractor.h
+++ b/extractor/grammar_extractor.h
@@ -15,7 +15,6 @@ class DataArray;
 class Grammar;
 class HieroCachingRuleFactory;
 class Precomputation;
-class Rule;
 class Scorer;
 class SuffixArray;
 class Vocabulary;
@@ -32,6 +31,7 @@ class GrammarExtractor {
       shared_ptr<Alignment> alignment,
       shared_ptr<Precomputation> precomputation,
       shared_ptr<Scorer> scorer,
+      shared_ptr<Vocabulary> vocabulary,
       int min_gap_size,
       int max_rule_span,
       int max_nonterminals,
@@ -45,7 +45,9 @@ class GrammarExtractor {
 
   // Converts the sentence to a vector of word ids and uses the RuleFactory to
   // extract the SCFG rules which may be used to decode the sentence.
-  Grammar GetGrammar(const string& sentence, const unordered_set<int>& blacklisted_sentence_ids, const shared_ptr<DataArray> source_data_array);
+  Grammar GetGrammar(
+      const string& sentence,
+      const unordered_set<int>& blacklisted_sentence_ids);
 
  private:
   // Splits the sentence in a vector of words.
diff --git a/extractor/grammar_extractor_test.cc b/extractor/grammar_extractor_test.cc
index f32a9599..719e90ff 100644
--- a/extractor/grammar_extractor_test.cc
+++ b/extractor/grammar_extractor_test.cc
@@ -41,13 +41,13 @@ TEST(GrammarExtractorTest, TestAnnotatingWords) {
   Grammar grammar(rules, feature_names);
   unordered_set<int> blacklisted_sentence_ids;
   shared_ptr<DataArray> source_data_array;
-  EXPECT_CALL(*factory, GetGrammar(word_ids, blacklisted_sentence_ids, source_data_array))
+  EXPECT_CALL(*factory, GetGrammar(word_ids, blacklisted_sentence_ids))
       .WillOnce(Return(grammar));
 
   GrammarExtractor extractor(vocabulary, factory);
   string sentence = "Anna has many many apples .";
 
-  extractor.GetGrammar(sentence, blacklisted_sentence_ids, source_data_array);
+  extractor.GetGrammar(sentence, blacklisted_sentence_ids);
 }
 
 } // namespace
diff --git a/extractor/matchings_sampler.cc b/extractor/matchings_sampler.cc
new file mode 100644
index 00000000..75a62366
--- /dev/null
+++ b/extractor/matchings_sampler.cc
@@ -0,0 +1,39 @@
+#include "matchings_sampler.h"
+
+#include "data_array.h"
+#include "phrase_location.h"
+
+namespace extractor {
+
+MatchingsSampler::MatchingsSampler(
+    shared_ptr<DataArray> data_array, int max_samples) :
+    BackoffSampler(data_array, max_samples) {}
+
+MatchingsSampler::MatchingsSampler() {}
+
+int MatchingsSampler::GetNumSubpatterns(const PhraseLocation& location) const {
+  return location.num_subpatterns;
+}
+
+int MatchingsSampler::GetRangeLow(const PhraseLocation&) const {
+  return 0;
+}
+
+int MatchingsSampler::GetRangeHigh(const PhraseLocation& location) const {
+  return location.matchings->size() / location.num_subpatterns;
+}
+
+int MatchingsSampler::GetPosition(const PhraseLocation& location,
+                                  int index) const {
+  return (*location.matchings)[index * location.num_subpatterns];
+}
+
+void MatchingsSampler::AppendMatching(vector<int>& samples, int index,
+                                      const PhraseLocation& location) const {
+  int start = index * location.num_subpatterns;
+  copy(location.matchings->begin() + start,
+       location.matchings->begin() + start + location.num_subpatterns,
+       back_inserter(samples));
+}
+
+} // namespace extractor
diff --git a/extractor/matchings_sampler.h b/extractor/matchings_sampler.h
new file mode 100644
index 00000000..ca4fce93
--- /dev/null
+++ b/extractor/matchings_sampler.h
@@ -0,0 +1,31 @@
+#ifndef _MATCHINGS_SAMPLER_H_
+#define _MATCHINGS_SAMPLER_H_
+
+#include "backoff_sampler.h"
+
+namespace extractor {
+
+class DataArray;
+
+class MatchingsSampler : public BackoffSampler {
+ public:
+  MatchingsSampler(shared_ptr<DataArray> data_array, int max_samples);
+
+  MatchingsSampler();
+
+ private:
+   int GetNumSubpatterns(const PhraseLocation& location) const;
+
+   int GetRangeLow(const PhraseLocation& location) const;
+
+   int GetRangeHigh(const PhraseLocation& location) const;
+
+   int GetPosition(const PhraseLocation& location, int index) const;
+
+   void AppendMatching(vector<int>& samples, int index,
+                       const PhraseLocation& location) const;
+};
+
+} // namespace extractor
+
+#endif
diff --git a/extractor/matchings_sampler_test.cc b/extractor/matchings_sampler_test.cc
new file mode 100644
index 00000000..bc927152
--- /dev/null
+++ b/extractor/matchings_sampler_test.cc
@@ -0,0 +1,118 @@
+#include <gtest/gtest.h>
+
+#include <memory>
+
+#include "mocks/mock_data_array.h"
+#include "matchings_sampler.h"
+#include "phrase_location.h"
+
+using namespace std;
+using namespace ::testing;
+
+namespace extractor {
+namespace {
+
+class MatchingsSamplerTest : public Test {
+ protected:
+  virtual void SetUp() {
+    vector<int> locations = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+    location = PhraseLocation(locations, 2);
+
+    data_array = make_shared<MockDataArray>();
+    for (int i = 0; i < 10; ++i) {
+      EXPECT_CALL(*data_array, GetSentenceId(i)).WillRepeatedly(Return(i / 2));
+    }
+  }
+
+  unordered_set<int> blacklisted_sentence_ids;
+  PhraseLocation location;
+  shared_ptr<MockDataArray> data_array;
+  shared_ptr<MatchingsSampler> sampler;
+};
+
+TEST_F(MatchingsSamplerTest, TestSample) {
+  sampler = make_shared<MatchingsSampler>(data_array, 1);
+  vector<int> expected_locations = {0, 1};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  sampler = make_shared<MatchingsSampler>(data_array, 2);
+  expected_locations = {0, 1, 6, 7};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  sampler = make_shared<MatchingsSampler>(data_array, 3);
+  expected_locations = {0, 1, 4, 5, 6, 7};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  sampler = make_shared<MatchingsSampler>(data_array, 7);
+  expected_locations = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+}
+
+TEST_F(MatchingsSamplerTest, TestBackoffSample) {
+  sampler = make_shared<MatchingsSampler>(data_array, 1);
+  blacklisted_sentence_ids = {0};
+  vector<int> expected_locations = {2, 3};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {0, 1, 2, 3};
+  expected_locations = {8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {0, 1, 2, 3, 4};
+  expected_locations = {};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  sampler = make_shared<MatchingsSampler>(data_array, 2);
+  blacklisted_sentence_ids = {0, 3};
+  expected_locations = {2, 3, 4, 5};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  sampler = make_shared<MatchingsSampler>(data_array, 3);
+  blacklisted_sentence_ids = {0, 3};
+  expected_locations = {2, 3, 4, 5, 8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {0, 2, 3};
+  expected_locations = {2, 3, 8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  sampler = make_shared<MatchingsSampler>(data_array, 4);
+  blacklisted_sentence_ids = {0, 1, 2, 3};
+  expected_locations = {8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {1, 3};
+  expected_locations = {0, 1, 4, 5, 8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  sampler = make_shared<MatchingsSampler>(data_array, 7);
+  blacklisted_sentence_ids = {0, 1, 2, 3, 4};
+  expected_locations = {};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {0, 2, 4};
+  expected_locations = {2, 3, 6, 7};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {1, 3};
+  expected_locations = {0, 1, 4, 5, 8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 2),
+            sampler->Sample(location, blacklisted_sentence_ids));
+}
+
+}
+} // namespace extractor
diff --git a/extractor/mocks/mock_data_array.h b/extractor/mocks/mock_data_array.h
index 6f85abb4..98e711d2 100644
--- a/extractor/mocks/mock_data_array.h
+++ b/extractor/mocks/mock_data_array.h
@@ -6,12 +6,13 @@ namespace extractor {
 
 class MockDataArray : public DataArray {
  public:
-  MOCK_CONST_METHOD0(GetData, const vector<int>&());
+  MOCK_CONST_METHOD0(GetData, vector<int>());
   MOCK_CONST_METHOD1(AtIndex, int(int index));
   MOCK_CONST_METHOD1(GetWordAtIndex, string(int index));
+  MOCK_CONST_METHOD2(GetWordIds, vector<int>(int start_index, int size));
+  MOCK_CONST_METHOD2(GetWords, vector<string>(int start_index, int size));
   MOCK_CONST_METHOD0(GetSize, int());
   MOCK_CONST_METHOD0(GetVocabularySize, int());
-  MOCK_CONST_METHOD1(HasWord, bool(const string& word));
   MOCK_CONST_METHOD1(GetWordId, int(const string& word));
   MOCK_CONST_METHOD1(GetWord, string(int word_id));
   MOCK_CONST_METHOD1(GetSentenceLength, int(int sentence_id));
diff --git a/extractor/mocks/mock_matchings_sampler.h b/extractor/mocks/mock_matchings_sampler.h
new file mode 100644
index 00000000..de2009c3
--- /dev/null
+++ b/extractor/mocks/mock_matchings_sampler.h
@@ -0,0 +1,15 @@
+#include <gmock/gmock.h>
+
+#include "phrase_location.h"
+#include "matchings_sampler.h"
+
+namespace extractor {
+
+class MockMatchingsSampler : public MatchingsSampler {
+ public:
+  MOCK_CONST_METHOD2(Sample, PhraseLocation(
+      const PhraseLocation& location,
+      const unordered_set<int>& blacklisted_sentence_ids));
+};
+
+} // namespace extractor
diff --git a/extractor/mocks/mock_precomputation.h b/extractor/mocks/mock_precomputation.h
index 8753343e..5f7aa999 100644
--- a/extractor/mocks/mock_precomputation.h
+++ b/extractor/mocks/mock_precomputation.h
@@ -6,7 +6,8 @@ namespace extractor {
 
 class MockPrecomputation : public Precomputation {
  public:
-  MOCK_CONST_METHOD0(GetCollocations, const Index&());
+  MOCK_CONST_METHOD1(Contains, bool(const vector<int>& pattern));
+  MOCK_CONST_METHOD1(GetCollocations, vector<int>(const vector<int>& pattern));
 };
 
 } // namespace extractor
diff --git a/extractor/mocks/mock_rule_factory.h b/extractor/mocks/mock_rule_factory.h
index 86a084b5..53eb5022 100644
--- a/extractor/mocks/mock_rule_factory.h
+++ b/extractor/mocks/mock_rule_factory.h
@@ -7,7 +7,9 @@ namespace extractor {
 
 class MockHieroCachingRuleFactory : public HieroCachingRuleFactory {
  public:
-  MOCK_METHOD3(GetGrammar, Grammar(const vector<int>& word_ids, const unordered_set<int> blacklisted_sentence_ids, const shared_ptr<DataArray> source_data_array));
+  MOCK_METHOD2(GetGrammar, Grammar(
+      const vector<int>& word_ids,
+      const unordered_set<int>& blacklisted_sentence_ids));
 };
 
 } // namespace extractor
diff --git a/extractor/mocks/mock_sampler.h b/extractor/mocks/mock_sampler.h
index 75c43c27..b2742f62 100644
--- a/extractor/mocks/mock_sampler.h
+++ b/extractor/mocks/mock_sampler.h
@@ -7,7 +7,9 @@ namespace extractor {
 
 class MockSampler : public Sampler {
  public:
-  MOCK_CONST_METHOD1(Sample, PhraseLocation(const PhraseLocation& location));
+  MOCK_CONST_METHOD2(Sample, PhraseLocation(
+      const PhraseLocation& location,
+      const unordered_set<int>& blacklisted_sentence_ids));
 };
 
 } // namespace extractor
diff --git a/extractor/mocks/mock_suffix_array_sampler.h b/extractor/mocks/mock_suffix_array_sampler.h
new file mode 100644
index 00000000..d799b969
--- /dev/null
+++ b/extractor/mocks/mock_suffix_array_sampler.h
@@ -0,0 +1,15 @@
+#include <gmock/gmock.h>
+
+#include "phrase_location.h"
+#include "suffix_array_sampler.h"
+
+namespace extractor {
+
+class MockSuffixArraySampler : public SuffixArrayRangeSampler {
+ public:
+  MOCK_CONST_METHOD2(Sample, PhraseLocation(
+      const PhraseLocation& location,
+      const unordered_set<int>& blacklisted_sentence_ids));
+};
+
+} // namespace extractor
diff --git a/extractor/phrase_location.cc b/extractor/phrase_location.cc
index 13140cac..2c367893 100644
--- a/extractor/phrase_location.cc
+++ b/extractor/phrase_location.cc
@@ -1,5 +1,7 @@
 #include "phrase_location.h"
 
+#include <iostream>
+
 namespace extractor {
 
 PhraseLocation::PhraseLocation(int sa_low, int sa_high) :
diff --git a/extractor/phrase_location_sampler.cc b/extractor/phrase_location_sampler.cc
new file mode 100644
index 00000000..a2eec105
--- /dev/null
+++ b/extractor/phrase_location_sampler.cc
@@ -0,0 +1,34 @@
+#include "phrase_location_sampler.h"
+
+#include "matchings_sampler.h"
+#include "phrase_location.h"
+#include "suffix_array.h"
+#include "suffix_array_sampler.h"
+
+namespace extractor {
+
+PhraseLocationSampler::PhraseLocationSampler(
+    shared_ptr<SuffixArray> suffix_array, int max_samples) {
+  matchings_sampler = make_shared<MatchingsSampler>(
+      suffix_array->GetData(), max_samples);
+  suffix_array_sampler = make_shared<SuffixArrayRangeSampler>(
+      suffix_array, max_samples);
+}
+
+PhraseLocationSampler::PhraseLocationSampler(
+    shared_ptr<MatchingsSampler> matchings_sampler,
+    shared_ptr<SuffixArrayRangeSampler> suffix_array_sampler) :
+    matchings_sampler(matchings_sampler),
+    suffix_array_sampler(suffix_array_sampler) {}
+
+PhraseLocation PhraseLocationSampler::Sample(
+    const PhraseLocation& location,
+    const unordered_set<int>& blacklisted_sentence_ids) const {
+  if (location.matchings == NULL) {
+    return suffix_array_sampler->Sample(location, blacklisted_sentence_ids);
+  } else {
+    return matchings_sampler->Sample(location, blacklisted_sentence_ids);
+  }
+}
+
+} // namespace extractor
diff --git a/extractor/phrase_location_sampler.h b/extractor/phrase_location_sampler.h
new file mode 100644
index 00000000..0e88335e
--- /dev/null
+++ b/extractor/phrase_location_sampler.h
@@ -0,0 +1,35 @@
+#ifndef _PHRASE_LOCATION_SAMPLER_H_
+#define _PHRASE_LOCATION_SAMPLER_H_
+
+#include <memory>
+
+#include "sampler.h"
+
+namespace extractor {
+
+class MatchingsSampler;
+class PhraseLocation;
+class SuffixArray;
+class SuffixArrayRangeSampler;
+
+class PhraseLocationSampler : public Sampler {
+ public:
+  PhraseLocationSampler(shared_ptr<SuffixArray> suffix_array, int max_samples);
+
+  // For testing only.
+  PhraseLocationSampler(
+      shared_ptr<MatchingsSampler> matchings_sampler,
+      shared_ptr<SuffixArrayRangeSampler> suffix_array_sampler);
+
+  PhraseLocation Sample(
+      const PhraseLocation& location,
+      const unordered_set<int>& blacklisted_sentence_ids) const;
+
+ private:
+  shared_ptr<MatchingsSampler> matchings_sampler;
+  shared_ptr<SuffixArrayRangeSampler> suffix_array_sampler;
+};
+
+} // namespace extractor
+
+#endif
diff --git a/extractor/phrase_location_sampler_test.cc b/extractor/phrase_location_sampler_test.cc
new file mode 100644
index 00000000..e7520ce7
--- /dev/null
+++ b/extractor/phrase_location_sampler_test.cc
@@ -0,0 +1,50 @@
+#include <gtest/gtest.h>
+
+#include <memory>
+
+#include "mocks/mock_matchings_sampler.h"
+#include "mocks/mock_suffix_array_sampler.h"
+#include "phrase_location.h"
+#include "phrase_location_sampler.h"
+
+using namespace std;
+using namespace ::testing;
+
+namespace extractor {
+namespace {
+
+class MatchingsSamplerTest : public Test {
+ protected:
+  virtual void SetUp() {
+    matchings_sampler = make_shared<MockMatchingsSampler>();
+    suffix_array_sampler = make_shared<MockSuffixArraySampler>();
+
+    sampler = make_shared<PhraseLocationSampler>(
+        matchings_sampler, suffix_array_sampler);
+  }
+
+  shared_ptr<MockMatchingsSampler> matchings_sampler;
+  shared_ptr<MockSuffixArraySampler> suffix_array_sampler;
+  shared_ptr<PhraseLocationSampler> sampler;
+};
+
+TEST_F(MatchingsSamplerTest, TestSuffixArrayRange) {
+  vector<int> locations = {0, 1, 2, 3};
+  PhraseLocation location(0, 3), result(locations, 2);
+  unordered_set<int> blacklisted_sentence_ids;
+  EXPECT_CALL(*suffix_array_sampler, Sample(location, blacklisted_sentence_ids))
+      .WillOnce(Return(result));
+  EXPECT_EQ(result, sampler->Sample(location, blacklisted_sentence_ids));
+}
+
+TEST_F(MatchingsSamplerTest, TestMatchings) {
+  vector<int> locations = {0, 1, 2, 3};
+  PhraseLocation location(locations, 2), result(locations, 2);
+  unordered_set<int> blacklisted_sentence_ids;
+  EXPECT_CALL(*matchings_sampler, Sample(location, blacklisted_sentence_ids))
+      .WillOnce(Return(result));
+  EXPECT_EQ(result, sampler->Sample(location, blacklisted_sentence_ids));
+}
+
+}
+} // namespace extractor
diff --git a/extractor/precomputation.cc b/extractor/precomputation.cc
index 3b8aed69..3e58e2a9 100644
--- a/extractor/precomputation.cc
+++ b/extractor/precomputation.cc
@@ -5,59 +5,67 @@
 
 #include "data_array.h"
 #include "suffix_array.h"
+#include "time_util.h"
+#include "vocabulary.h"
 
 using namespace std;
 
 namespace extractor {
 
-int Precomputation::FIRST_NONTERMINAL = -1;
-int Precomputation::SECOND_NONTERMINAL = -2;
-
 Precomputation::Precomputation(
-    shared_ptr<SuffixArray> suffix_array, int num_frequent_patterns,
-    int num_super_frequent_patterns, int max_rule_span,
-    int max_rule_symbols, int min_gap_size,
+    shared_ptr<Vocabulary> vocabulary, shared_ptr<SuffixArray> suffix_array,
+    int num_frequent_patterns, int num_super_frequent_patterns,
+    int max_rule_span, int max_rule_symbols, int min_gap_size,
     int max_frequent_phrase_len, int min_frequency) {
-  vector<int> data = suffix_array->GetData()->GetData();
+  Clock::time_point start_time = Clock::now();
+  shared_ptr<DataArray> data_array = suffix_array->GetData();
+  vector<int> data = data_array->GetData();
   vector<vector<int>> frequent_patterns = FindMostFrequentPatterns(
       suffix_array, data, num_frequent_patterns, max_frequent_phrase_len,
       min_frequency);
+  Clock::time_point end_time = Clock::now();
+  cerr << "Finding most frequent patterns took "
+       << GetDuration(start_time, end_time) << " seconds..." << endl;
 
-  // Construct sets containing the frequent and superfrequent contiguous
-  // collocations.
-  unordered_set<vector<int>, VectorHash> frequent_patterns_set;
-  unordered_set<vector<int>, VectorHash> super_frequent_patterns_set;
+  vector<vector<int>> pattern_annotations(frequent_patterns.size());
+  unordered_map<vector<int>, int, VectorHash> frequent_patterns_index;
   for (size_t i = 0; i < frequent_patterns.size(); ++i) {
-    frequent_patterns_set.insert(frequent_patterns[i]);
-    if (i < num_super_frequent_patterns) {
-      super_frequent_patterns_set.insert(frequent_patterns[i]);
-    }
+    frequent_patterns_index[frequent_patterns[i]] = i;
+    pattern_annotations[i] = AnnotatePattern(vocabulary, data_array,
+                                             frequent_patterns[i]);
   }
 
+  start_time = Clock::now();
   vector<tuple<int, int, int>> matchings;
+  vector<vector<int>> annotations;
   for (size_t i = 0; i < data.size(); ++i) {
     // If the sentence is over, add all the discontiguous frequent patterns to
     // the index.
     if (data[i] == DataArray::END_OF_LINE) {
-      AddCollocations(matchings, data, max_rule_span, min_gap_size,
-          max_rule_symbols);
+      UpdateIndex(matchings, annotations, max_rule_span, min_gap_size,
+                  max_rule_symbols);
       matchings.clear();
+      annotations.clear();
       continue;
     }
-    vector<int> pattern;
     // Find all the contiguous frequent patterns starting at position i.
+    vector<int> pattern;
     for (int j = 1; j <= max_frequent_phrase_len && i + j <= data.size(); ++j) {
       pattern.push_back(data[i + j - 1]);
-      if (frequent_patterns_set.count(pattern)) {
-        int is_super_frequent = super_frequent_patterns_set.count(pattern);
-        matchings.push_back(make_tuple(i, j, is_super_frequent));
-      } else {
+      auto it = frequent_patterns_index.find(pattern);
+      if (it == frequent_patterns_index.end()) {
         // If the current pattern is not frequent, any longer pattern having the
         // current pattern as prefix will not be frequent.
         break;
       }
+      int is_super_frequent = it->second < num_super_frequent_patterns;
+      matchings.push_back(make_tuple(i, j, is_super_frequent));
+      annotations.push_back(pattern_annotations[it->second]);
     }
   }
+  end_time = Clock::now();
+  cerr << "Constructing collocations index took "
+       << GetDuration(start_time, end_time) << " seconds..." << endl;
 }
 
 Precomputation::Precomputation() {}
@@ -75,9 +83,9 @@ vector<vector<int>> Precomputation::FindMostFrequentPatterns(
   for (size_t i = 1; i < lcp.size(); ++i) {
     for (int len = lcp[i]; len < max_frequent_phrase_len; ++len) {
       int frequency = i - run_start[len];
-      if (frequency >= min_frequency) {
-        heap.push(make_pair(frequency,
-            make_pair(suffix_array->GetSuffix(run_start[len]), len + 1)));
+      int start = suffix_array->GetSuffix(run_start[len]);
+      if (frequency >= min_frequency && start + len <= data.size()) {
+        heap.push(make_pair(frequency, make_pair(start, len + 1)));
       }
       run_start[len] = i;
     }
@@ -99,8 +107,20 @@ vector<vector<int>> Precomputation::FindMostFrequentPatterns(
   return frequent_patterns;
 }
 
-void Precomputation::AddCollocations(
-    const vector<tuple<int, int, int>>& matchings, const vector<int>& data,
+vector<int> Precomputation::AnnotatePattern(
+    shared_ptr<Vocabulary> vocabulary, shared_ptr<DataArray> data_array,
+    const vector<int>& pattern) const {
+  vector<int> annotation;
+  for (int word_id: pattern) {
+    annotation.push_back(vocabulary->GetTerminalIndex(
+        data_array->GetWord(word_id)));
+  }
+  return annotation;
+}
+
+void Precomputation::UpdateIndex(
+    const vector<tuple<int, int, int>>& matchings,
+    const vector<vector<int>>& annotations,
     int max_rule_span, int min_gap_size, int max_rule_symbols) {
   // Select the leftmost subpattern.
   for (size_t i = 0; i < matchings.size(); ++i) {
@@ -118,16 +138,14 @@ void Precomputation::AddCollocations(
       if (start2 - start1 - size1 >= min_gap_size
           && start2 + size2 - start1 <= max_rule_span
           && size1 + size2 + 1 <= max_rule_symbols) {
-        vector<int> pattern(data.begin() + start1,
-            data.begin() + start1 + size1);
-        pattern.push_back(Precomputation::FIRST_NONTERMINAL);
-        pattern.insert(pattern.end(), data.begin() + start2,
-            data.begin() + start2 + size2);
-        AddStartPositions(collocations[pattern], start1, start2);
+        vector<int> pattern = annotations[i];
+        pattern.push_back(-1);
+        AppendSubpattern(pattern, annotations[j]);
+        AppendCollocation(index[pattern], start1, start2);
 
         // Try extending the binary collocation to a ternary collocation.
         if (is_super2) {
-          pattern.push_back(Precomputation::SECOND_NONTERMINAL);
+          pattern.push_back(-2);
           // Select the rightmost subpattern.
           for (size_t k = j + 1; k < matchings.size(); ++k) {
             int start3, size3, is_super3;
@@ -140,9 +158,8 @@ void Precomputation::AddCollocations(
                 && start3 + size3 - start1 <= max_rule_span
                 && size1 + size2 + size3 + 2 <= max_rule_symbols
                 && (is_super1 || is_super3)) {
-              pattern.insert(pattern.end(), data.begin() + start3,
-                  data.begin() + start3 + size3);
-              AddStartPositions(collocations[pattern], start1, start2, start3);
+              AppendSubpattern(pattern, annotations[k]);
+              AppendCollocation(index[pattern], start1, start2, start3);
               pattern.erase(pattern.end() - size3);
             }
           }
@@ -152,25 +169,35 @@ void Precomputation::AddCollocations(
   }
 }
 
-void Precomputation::AddStartPositions(
-    vector<int>& positions, int pos1, int pos2) {
-  positions.push_back(pos1);
-  positions.push_back(pos2);
+void Precomputation::AppendSubpattern(
+    vector<int>& pattern,
+    const vector<int>& subpattern) {
+  copy(subpattern.begin(), subpattern.end(), back_inserter(pattern));
+}
+
+void Precomputation::AppendCollocation(
+    vector<int>& collocations, int pos1, int pos2) {
+  collocations.push_back(pos1);
+  collocations.push_back(pos2);
+}
+
+void Precomputation::AppendCollocation(
+    vector<int>& collocations, int pos1, int pos2, int pos3) {
+  collocations.push_back(pos1);
+  collocations.push_back(pos2);
+  collocations.push_back(pos3);
 }
 
-void Precomputation::AddStartPositions(
-    vector<int>& positions, int pos1, int pos2, int pos3) {
-  positions.push_back(pos1);
-  positions.push_back(pos2);
-  positions.push_back(pos3);
+bool Precomputation::Contains(const vector<int>& pattern) const {
+  return index.count(pattern);
 }
 
-const Index& Precomputation::GetCollocations() const {
-  return collocations;
+vector<int> Precomputation::GetCollocations(const vector<int>& pattern) const {
+  return index.at(pattern);
 }
 
 bool Precomputation::operator==(const Precomputation& other) const {
-  return collocations == other.collocations;
+  return index == other.index;
 }
 
 } // namespace extractor
diff --git a/extractor/precomputation.h b/extractor/precomputation.h
index 9f0c9424..2b34fc29 100644
--- a/extractor/precomputation.h
+++ b/extractor/precomputation.h
@@ -7,13 +7,11 @@
 #include <tuple>
 #include <vector>
 
-#include <boost/filesystem.hpp>
 #include <boost/functional/hash.hpp>
 #include <boost/serialization/serialization.hpp>
 #include <boost/serialization/utility.hpp>
 #include <boost/serialization/vector.hpp>
 
-namespace fs = boost::filesystem;
 using namespace std;
 
 namespace extractor {
@@ -21,7 +19,9 @@ namespace extractor {
 typedef boost::hash<vector<int>> VectorHash;
 typedef unordered_map<vector<int>, vector<int>, VectorHash> Index;
 
+class DataArray;
 class SuffixArray;
+class Vocabulary;
 
 /**
  * Data structure wrapping an index with all the occurrences of the most
@@ -37,9 +37,9 @@ class Precomputation {
  public:
   // Constructs the index using the suffix array.
   Precomputation(
-      shared_ptr<SuffixArray> suffix_array, int num_frequent_patterns,
-      int num_super_frequent_patterns, int max_rule_span,
-      int max_rule_symbols, int min_gap_size,
+      shared_ptr<Vocabulary> vocabulary, shared_ptr<SuffixArray> suffix_array,
+      int num_frequent_patterns, int num_super_frequent_patterns,
+      int max_rule_span, int max_rule_symbols, int min_gap_size,
       int max_frequent_phrase_len, int min_frequency);
 
   // Creates empty precomputation data structure.
@@ -47,13 +47,13 @@ class Precomputation {
 
   virtual ~Precomputation();
 
-  // Returns a reference to the index.
-  virtual const Index& GetCollocations() const;
+  // Returns whether a pattern is contained in the index of collocations.
+  virtual bool Contains(const vector<int>& pattern) const;
 
-  bool operator==(const Precomputation& other) const;
+  // Returns the list of collocations for a given pattern.
+  virtual vector<int> GetCollocations(const vector<int>& pattern) const;
 
-  static int FIRST_NONTERMINAL;
-  static int SECOND_NONTERMINAL;
+  bool operator==(const Precomputation& other) const;
 
  private:
   // Finds the most frequent contiguous collocations.
@@ -62,25 +62,32 @@ class Precomputation {
       int num_frequent_patterns, int max_frequent_phrase_len,
       int min_frequency);
 
+  vector<int> AnnotatePattern(shared_ptr<Vocabulary> vocabulary,
+                              shared_ptr<DataArray> data_array,
+                              const vector<int>& pattern) const;
+
   // Given the locations of the frequent contiguous collocations in a sentence,
   // it adds new entries to the index for each discontiguous collocation
   // matching the criteria specified in the class description.
-  void AddCollocations(
-      const vector<std::tuple<int, int, int>>& matchings, const vector<int>& data,
+  void UpdateIndex(
+      const vector<tuple<int, int, int>>& matchings,
+      const vector<vector<int>>& annotations,
       int max_rule_span, int min_gap_size, int max_rule_symbols);
 
+  void AppendSubpattern(vector<int>& pattern, const vector<int>& subpattern);
+
   // Adds an occurrence of a binary collocation.
-  void AddStartPositions(vector<int>& positions, int pos1, int pos2);
+  void AppendCollocation(vector<int>& collocations, int pos1, int pos2);
 
   // Adds an occurrence of a ternary collocation.
-  void AddStartPositions(vector<int>& positions, int pos1, int pos2, int pos3);
+  void AppendCollocation(vector<int>& collocations, int pos1, int pos2, int pos3);
 
   friend class boost::serialization::access;
 
   template<class Archive> void save(Archive& ar, unsigned int) const {
-    int num_entries = collocations.size();
+    int num_entries = index.size();
     ar << num_entries;
-    for (pair<vector<int>, vector<int>> entry: collocations) {
+    for (pair<vector<int>, vector<int>> entry: index) {
       ar << entry;
     }
   }
@@ -91,13 +98,13 @@ class Precomputation {
     for (size_t i = 0; i < num_entries; ++i) {
       pair<vector<int>, vector<int>> entry;
       ar >> entry;
-      collocations.insert(entry);
+      index.insert(entry);
     }
   }
 
   BOOST_SERIALIZATION_SPLIT_MEMBER();
 
-  Index collocations;
+  Index index;
 };
 
 } // namespace extractor
diff --git a/extractor/precomputation_test.cc b/extractor/precomputation_test.cc
index e81ece5d..3a98ce05 100644
--- a/extractor/precomputation_test.cc
+++ b/extractor/precomputation_test.cc
@@ -9,6 +9,7 @@
 
 #include "mocks/mock_data_array.h"
 #include "mocks/mock_suffix_array.h"
+#include "mocks/mock_vocabulary.h"
 #include "precomputation.h"
 
 using namespace std;
@@ -23,7 +24,12 @@ class PrecomputationTest : public Test {
   virtual void SetUp() {
     data = {4, 2, 3, 5, 7, 2, 3, 5, 2, 3, 4, 2, 1};
     data_array = make_shared<MockDataArray>();
-    EXPECT_CALL(*data_array, GetData()).WillRepeatedly(ReturnRef(data));
+    EXPECT_CALL(*data_array, GetData()).WillRepeatedly(Return(data));
+    for (size_t i = 0; i < data.size(); ++i) {
+      EXPECT_CALL(*data_array, AtIndex(i)).WillRepeatedly(Return(data[i]));
+    }
+    EXPECT_CALL(*data_array, GetWord(2)).WillRepeatedly(Return("2"));
+    EXPECT_CALL(*data_array, GetWord(3)).WillRepeatedly(Return("3"));
 
     vector<int> suffixes{12, 8, 5, 1, 9, 6, 2, 0, 10, 7, 3, 4, 13};
     vector<int> lcp{-1, 0, 2, 3, 1, 0, 1, 2, 0, 2, 0, 1, 0, 0};
@@ -35,77 +41,98 @@ class PrecomputationTest : public Test {
     }
     EXPECT_CALL(*suffix_array, BuildLCPArray()).WillRepeatedly(Return(lcp));
 
-    precomputation = Precomputation(suffix_array, 3, 3, 10, 5, 1, 4, 2);
+    vocabulary = make_shared<MockVocabulary>();
+    EXPECT_CALL(*vocabulary, GetTerminalIndex("2")).WillRepeatedly(Return(2));
+    EXPECT_CALL(*vocabulary, GetTerminalIndex("3")).WillRepeatedly(Return(3));
+
+    precomputation = Precomputation(vocabulary, suffix_array,
+                                    3, 3, 10, 5, 1, 4, 2);
   }
 
   vector<int> data;
   shared_ptr<MockDataArray> data_array;
   shared_ptr<MockSuffixArray> suffix_array;
+  shared_ptr<MockVocabulary> vocabulary;
   Precomputation precomputation;
 };
 
 TEST_F(PrecomputationTest, TestCollocations) {
-  Index collocations = precomputation.GetCollocations();
-
   vector<int> key = {2, 3, -1, 2};
   vector<int> expected_value = {1, 5, 1, 8, 5, 8, 5, 11, 8, 11};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {2, 3, -1, 2, 3};
   expected_value = {1, 5, 1, 8, 5, 8};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {2, 3, -1, 3};
   expected_value = {1, 6, 1, 9, 5, 9};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {3, -1, 2};
   expected_value = {2, 5, 2, 8, 2, 11, 6, 8, 6, 11, 9, 11};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {3, -1, 3};
   expected_value = {2, 6, 2, 9, 6, 9};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {3, -1, 2, 3};
   expected_value = {2, 5, 2, 8, 6, 8};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {2, -1, 2};
   expected_value = {1, 5, 1, 8, 5, 8, 5, 11, 8, 11};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {2, -1, 2, 3};
   expected_value = {1, 5, 1, 8, 5, 8};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {2, -1, 3};
   expected_value = {1, 6, 1, 9, 5, 9};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
 
   key = {2, -1, 2, -2, 2};
   expected_value = {1, 5, 8, 5, 8, 11};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {2, -1, 2, -2, 3};
   expected_value = {1, 5, 9};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {2, -1, 3, -2, 2};
   expected_value = {1, 6, 8, 5, 9, 11};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {2, -1, 3, -2, 3};
   expected_value = {1, 6, 9};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {3, -1, 2, -2, 2};
   expected_value = {2, 5, 8, 2, 5, 11, 2, 8, 11, 6, 8, 11};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {3, -1, 2, -2, 3};
   expected_value = {2, 5, 9};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {3, -1, 3, -2, 2};
   expected_value = {2, 6, 8, 2, 6, 11, 2, 9, 11, 6, 9, 11};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
   key = {3, -1, 3, -2, 3};
   expected_value = {2, 6, 9};
-  EXPECT_EQ(expected_value, collocations[key]);
+  EXPECT_TRUE(precomputation.Contains(key));
+  EXPECT_EQ(expected_value, precomputation.GetCollocations(key));
 
   // Exceeds max_rule_symbols.
   key = {2, -1, 2, -2, 2, 3};
-  EXPECT_EQ(0, collocations.count(key));
+  EXPECT_FALSE(precomputation.Contains(key));
   // Contains non frequent pattern.
   key = {2, -1, 5};
-  EXPECT_EQ(0, collocations.count(key));
+  EXPECT_FALSE(precomputation.Contains(key));
 }
 
 TEST_F(PrecomputationTest, TestSerialization) {
diff --git a/extractor/rule_factory.cc b/extractor/rule_factory.cc
index 6ae2d792..18a60695 100644
--- a/extractor/rule_factory.cc
+++ b/extractor/rule_factory.cc
@@ -12,6 +12,7 @@
 #include "phrase_builder.h"
 #include "rule.h"
 #include "rule_extractor.h"
+#include "phrase_location_sampler.h"
 #include "sampler.h"
 #include "scorer.h"
 #include "suffix_array.h"
@@ -68,7 +69,8 @@ HieroCachingRuleFactory::HieroCachingRuleFactory(
       target_data_array, alignment, phrase_builder, scorer, vocabulary,
       max_rule_span, min_gap_size, max_nonterminals, max_rule_symbols, true,
       false, require_tight_phrases);
-  sampler = make_shared<Sampler>(source_suffix_array, max_samples);
+  sampler = make_shared<PhraseLocationSampler>(
+      source_suffix_array, max_samples);
 }
 
 HieroCachingRuleFactory::HieroCachingRuleFactory(
@@ -101,7 +103,9 @@ HieroCachingRuleFactory::HieroCachingRuleFactory() {}
 
 HieroCachingRuleFactory::~HieroCachingRuleFactory() {}
 
-Grammar HieroCachingRuleFactory::GetGrammar(const vector<int>& word_ids, const unordered_set<int>& blacklisted_sentence_ids, const shared_ptr<DataArray> source_data_array) {
+Grammar HieroCachingRuleFactory::GetGrammar(
+    const vector<int>& word_ids,
+    const unordered_set<int>& blacklisted_sentence_ids) {
   Clock::time_point start_time = Clock::now();
   double total_extract_time = 0;
   double total_intersect_time = 0;
@@ -193,7 +197,8 @@ Grammar HieroCachingRuleFactory::GetGrammar(const vector<int>& word_ids, const u
       Clock::time_point extract_start = Clock::now();
       if (!state.starts_with_x) {
         // Extract rules for the sampled set of occurrences.
-        PhraseLocation sample = sampler->Sample(next_node->matchings, blacklisted_sentence_ids, source_data_array);
+        PhraseLocation sample = sampler->Sample(
+            next_node->matchings, blacklisted_sentence_ids);
         vector<Rule> new_rules =
             rule_extractor->ExtractRules(next_phrase, sample);
         rules.insert(rules.end(), new_rules.begin(), new_rules.end());
diff --git a/extractor/rule_factory.h b/extractor/rule_factory.h
index df63a9d8..1a9fa2af 100644
--- a/extractor/rule_factory.h
+++ b/extractor/rule_factory.h
@@ -72,7 +72,9 @@ class HieroCachingRuleFactory {
 
   // Constructs SCFG rules for a given sentence.
   // (See class description for more details.)
-  virtual Grammar GetGrammar(const vector<int>& word_ids, const unordered_set<int>& blacklisted_sentence_ids, const shared_ptr<DataArray> source_data_array);
+  virtual Grammar GetGrammar(
+      const vector<int>& word_ids,
+      const unordered_set<int>& blacklisted_sentence_ids);
 
  protected:
   HieroCachingRuleFactory();
diff --git a/extractor/rule_factory_test.cc b/extractor/rule_factory_test.cc
index f26cc567..332c5959 100644
--- a/extractor/rule_factory_test.cc
+++ b/extractor/rule_factory_test.cc
@@ -40,7 +40,7 @@ class RuleFactoryTest : public Test {
         .WillRepeatedly(Return(feature_names));
 
     sampler = make_shared<MockSampler>();
-    EXPECT_CALL(*sampler, Sample(_))
+    EXPECT_CALL(*sampler, Sample(_, _))
         .WillRepeatedly(Return(PhraseLocation(0, 1)));
 
     Phrase phrase;
@@ -77,8 +77,7 @@ TEST_F(RuleFactoryTest, TestGetGrammarDifferentWords) {
 
   vector<int> word_ids = {2, 3, 4};
   unordered_set<int> blacklisted_sentence_ids;
-  shared_ptr<DataArray> source_data_array;
-  Grammar grammar = factory->GetGrammar(word_ids, blacklisted_sentence_ids, source_data_array);
+  Grammar grammar = factory->GetGrammar(word_ids, blacklisted_sentence_ids);
   EXPECT_EQ(feature_names, grammar.GetFeatureNames());
   EXPECT_EQ(7, grammar.GetRules().size());
 }
@@ -97,8 +96,7 @@ TEST_F(RuleFactoryTest, TestGetGrammarRepeatingWords) {
 
   vector<int> word_ids = {2, 3, 4, 2, 3};
   unordered_set<int> blacklisted_sentence_ids;
-  shared_ptr<DataArray> source_data_array;
-  Grammar grammar = factory->GetGrammar(word_ids, blacklisted_sentence_ids, source_data_array);
+  Grammar grammar = factory->GetGrammar(word_ids, blacklisted_sentence_ids);
   EXPECT_EQ(feature_names, grammar.GetFeatureNames());
   EXPECT_EQ(28, grammar.GetRules().size());
 }
diff --git a/extractor/run_extractor.cc b/extractor/run_extractor.cc
index 6f59f0b6..f1aa5e35 100644
--- a/extractor/run_extractor.cc
+++ b/extractor/run_extractor.cc
@@ -5,10 +5,10 @@
 #include <string>
 #include <vector>
 
-#include <omp.h>
 #include <boost/filesystem.hpp>
 #include <boost/program_options.hpp>
 #include <boost/program_options/variables_map.hpp>
+#include <omp.h>
 
 #include "alignment.h"
 #include "data_array.h"
@@ -28,6 +28,7 @@
 #include "suffix_array.h"
 #include "time_util.h"
 #include "translation_table.h"
+#include "vocabulary.h"
 
 namespace fs = boost::filesystem;
 namespace po = boost::program_options;
@@ -77,7 +78,8 @@ int main(int argc, char** argv) {
     ("tight_phrases", po::value<bool>()->default_value(true),
         "False if phrases may be loose (better, but slower)")
     ("leave_one_out", po::value<bool>()->zero_tokens(),
-        "do leave-one-out estimation (e.g. for extracting grammars for the training set)");
+        "do leave-one-out estimation of grammars "
+        "(e.g. for extracting grammars for the training set");
 
   po::variables_map vm;
   po::store(po::parse_command_line(argc, argv, desc), vm);
@@ -98,11 +100,6 @@ int main(int argc, char** argv) {
     return 1;
   }
 
-  bool leave_one_out = false;
-  if (vm.count("leave_one_out")) {
-    leave_one_out = true;
-  }
-
   int num_threads = vm["threads"].as<int>();
   cerr << "Grammar extraction will use " << num_threads << " threads." << endl;
 
@@ -142,11 +139,14 @@ int main(int argc, char** argv) {
   cerr << "Reading alignment took "
        << GetDuration(start_time, stop_time) << " seconds" << endl;
 
+  shared_ptr<Vocabulary> vocabulary = make_shared<Vocabulary>();
+
   // Constructs an index storing the occurrences in the source data for each
   // frequent collocation.
   start_time = Clock::now();
   cerr << "Precomputing collocations..." << endl;
   shared_ptr<Precomputation> precomputation = make_shared<Precomputation>(
+      vocabulary,
       source_suffix_array,
       vm["frequent"].as<int>(),
       vm["super_frequent"].as<int>(),
@@ -174,8 +174,8 @@ int main(int argc, char** argv) {
        << GetDuration(preprocess_start_time, preprocess_stop_time)
        << " seconds" << endl;
 
-  // Features used to score each grammar rule.
   Clock::time_point extraction_start_time = Clock::now();
+  // Features used to score each grammar rule.
   vector<shared_ptr<Feature>> features = {
       make_shared<TargetGivenSourceCoherent>(),
       make_shared<SampleSourceCount>(),
@@ -194,6 +194,7 @@ int main(int argc, char** argv) {
       alignment,
       precomputation,
       scorer,
+      vocabulary,
       vm["min_gap_size"].as<int>(),
       vm["max_rule_span"].as<int>(),
       vm["max_nonterminals"].as<int>(),
@@ -201,9 +202,6 @@ int main(int argc, char** argv) {
       vm["max_samples"].as<int>(),
       vm["tight_phrases"].as<bool>());
 
-  // Releases extra memory used by the initial precomputation.
-  precomputation.reset();
-
   // Creates the grammars directory if it doesn't exist.
   fs::path grammar_path = vm["grammars"].as<string>();
   if (!fs::is_directory(grammar_path)) {
@@ -219,6 +217,7 @@ int main(int argc, char** argv) {
   }
 
   // Extracts the grammar for each sentence and saves it to a file.
+  bool leave_one_out = vm.count("leave_one_out");
   vector<string> suffixes(sentences.size());
   #pragma omp parallel for schedule(dynamic) num_threads(num_threads)
   for (size_t i = 0; i < sentences.size(); ++i) {
@@ -231,8 +230,11 @@ int main(int argc, char** argv) {
     suffixes[i] = suffix;
 
     unordered_set<int> blacklisted_sentence_ids;
-    if (leave_one_out) blacklisted_sentence_ids.insert(i);
-    Grammar grammar = extractor.GetGrammar(sentences[i], blacklisted_sentence_ids, source_data_array);
+    if (leave_one_out) {
+      blacklisted_sentence_ids.insert(i);
+    }
+    Grammar grammar = extractor.GetGrammar(
+        sentences[i], blacklisted_sentence_ids);
     ofstream output(GetGrammarFilePath(grammar_path, i).c_str());
     output << grammar;
   }
diff --git a/extractor/sampler.cc b/extractor/sampler.cc
deleted file mode 100644
index 963afa7a..00000000
--- a/extractor/sampler.cc
+++ /dev/null
@@ -1,75 +0,0 @@
-#include "sampler.h"
-
-#include "phrase_location.h"
-#include "suffix_array.h"
-
-namespace extractor {
-
-Sampler::Sampler(shared_ptr<SuffixArray> suffix_array, int max_samples) :
-    suffix_array(suffix_array), max_samples(max_samples) {}
-
-Sampler::Sampler() {}
-
-Sampler::~Sampler() {}
-
-PhraseLocation Sampler::Sample(const PhraseLocation& location, const unordered_set<int>& blacklisted_sentence_ids, const shared_ptr<DataArray> source_data_array) const {
-  vector<int> sample;
-  int num_subpatterns;
-  if (location.matchings == NULL) {
-    // Sample suffix array range.
-    num_subpatterns = 1;
-    int low = location.sa_low, high = location.sa_high;
-    double step = max(1.0, (double) (high - low) / max_samples);
-    double i = low, last = i;
-    bool found;
-    while (sample.size() < max_samples && i < high) {
-      int x = suffix_array->GetSuffix(Round(i));
-      int id = source_data_array->GetSentenceId(x);
-      if (find(blacklisted_sentence_ids.begin(), blacklisted_sentence_ids.end(), id) != blacklisted_sentence_ids.end()) {
-        found = false;
-        double backoff_step = 1;
-        while (true) {
-          if ((double)backoff_step >= step) break;
-          double j = i - backoff_step;
-          x = suffix_array->GetSuffix(Round(j));
-          id = source_data_array->GetSentenceId(x);
-          if (x >= 0 && j > last && find(blacklisted_sentence_ids.begin(), blacklisted_sentence_ids.end(), id) == blacklisted_sentence_ids.end()) {
-            found = true; last = i; break;
-          }
-          double k = i + backoff_step;
-          x = suffix_array->GetSuffix(Round(k));
-          id = source_data_array->GetSentenceId(x);
-          if (k < min(i+step, (double)high) && find(blacklisted_sentence_ids.begin(), blacklisted_sentence_ids.end(), id) == blacklisted_sentence_ids.end()) {
-            found = true; last = k; break;
-          }
-          if (j <= last && k >= high) break;
-          backoff_step++;
-        }
-      } else {
-        found = true;
-        last = i;
-      }
-      if (found) sample.push_back(x);
-      i += step;
-    }
-  } else {
-    // Sample vector of occurrences.
-    num_subpatterns = location.num_subpatterns;
-    int num_matchings = location.matchings->size() / num_subpatterns;
-    double step = max(1.0, (double) num_matchings / max_samples);
-    for (double i = 0, num_samples = 0;
-         i < num_matchings && num_samples < max_samples;
-         i += step, ++num_samples) {
-      int start = Round(i) * num_subpatterns;
-      sample.insert(sample.end(), location.matchings->begin() + start,
-          location.matchings->begin() + start + num_subpatterns);
-    }
-  }
-  return PhraseLocation(sample, num_subpatterns);
-}
-
-int Sampler::Round(double x) const {
-  return x + 0.5;
-}
-
-} // namespace extractor
diff --git a/extractor/sampler.h b/extractor/sampler.h
index de450c48..3c4e37f1 100644
--- a/extractor/sampler.h
+++ b/extractor/sampler.h
@@ -4,36 +4,20 @@
 #include <memory>
 #include <unordered_set>
 
-#include "data_array.h"
-
 using namespace std;
 
 namespace extractor {
 
 class PhraseLocation;
-class SuffixArray;
 
 /**
- * Provides uniform sampling for a PhraseLocation.
+ * Base sampler class.
  */
 class Sampler {
  public:
-  Sampler(shared_ptr<SuffixArray> suffix_array, int max_samples);
-
-  virtual ~Sampler();
-
-  // Samples uniformly at most max_samples phrase occurrences.
-  virtual PhraseLocation Sample(const PhraseLocation& location, const unordered_set<int>& blacklisted_sentence_ids, const shared_ptr<DataArray> source_data_array) const;
-
- protected:
-  Sampler();
-
- private:
-  // Round floating point number to the nearest integer.
-  int Round(double x) const;
-
-  shared_ptr<SuffixArray> suffix_array;
-  int max_samples;
+  virtual PhraseLocation Sample(
+      const PhraseLocation& location,
+      const unordered_set<int>& blacklisted_sentence_ids) const = 0;
 };
 
 } // namespace extractor
diff --git a/extractor/sampler_test.cc b/extractor/sampler_test.cc
deleted file mode 100644
index 965567ba..00000000
--- a/extractor/sampler_test.cc
+++ /dev/null
@@ -1,80 +0,0 @@
-#include <gtest/gtest.h>
-
-#include <memory>
-
-#include "mocks/mock_suffix_array.h"
-#include "mocks/mock_data_array.h"
-#include "phrase_location.h"
-#include "sampler.h"
-
-using namespace std;
-using namespace ::testing;
-
-namespace extractor {
-namespace {
-
-class SamplerTest : public Test {
- protected:
-  virtual void SetUp() {
-    source_data_array = make_shared<MockDataArray>();
-    EXPECT_CALL(*source_data_array, GetSentenceId(_)).WillRepeatedly(Return(9999));
-    suffix_array = make_shared<MockSuffixArray>();
-    for (int i = 0; i < 10; ++i) {
-      EXPECT_CALL(*suffix_array, GetSuffix(i)).WillRepeatedly(Return(i));
-    }
-  }
-
-  shared_ptr<MockSuffixArray> suffix_array;
-  shared_ptr<Sampler> sampler;
-  shared_ptr<MockDataArray> source_data_array;
-};
-
-TEST_F(SamplerTest, TestSuffixArrayRange) {
-  PhraseLocation location(0, 10);
-  unordered_set<int> blacklist;
-
-  sampler = make_shared<Sampler>(suffix_array, 1);
-  vector<int> expected_locations = {0};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-
-  sampler = make_shared<Sampler>(suffix_array, 2);
-  expected_locations = {0, 5};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-
-  sampler = make_shared<Sampler>(suffix_array, 3);
-  expected_locations = {0, 3, 7};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-
-  sampler = make_shared<Sampler>(suffix_array, 4);
-  expected_locations = {0, 3, 5, 8};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-
-  sampler = make_shared<Sampler>(suffix_array, 100);
-  expected_locations = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-}
-
-TEST_F(SamplerTest, TestSubstringsSample) {
-  vector<int> locations = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
-  unordered_set<int> blacklist;
-  PhraseLocation location(locations, 2);
-
-  sampler = make_shared<Sampler>(suffix_array, 1);
-  vector<int> expected_locations = {0, 1};
-  EXPECT_EQ(PhraseLocation(expected_locations, 2), sampler->Sample(location, blacklist, source_data_array));
-
-  sampler = make_shared<Sampler>(suffix_array, 2);
-  expected_locations = {0, 1, 6, 7};
-  EXPECT_EQ(PhraseLocation(expected_locations, 2), sampler->Sample(location, blacklist, source_data_array));
-
-  sampler = make_shared<Sampler>(suffix_array, 3);
-  expected_locations = {0, 1, 4, 5, 6, 7};
-  EXPECT_EQ(PhraseLocation(expected_locations, 2), sampler->Sample(location, blacklist, source_data_array));
-
-  sampler = make_shared<Sampler>(suffix_array, 7);
-  expected_locations = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
-  EXPECT_EQ(PhraseLocation(expected_locations, 2), sampler->Sample(location, blacklist, source_data_array));
-}
-
-} // namespace
-} // namespace extractor
diff --git a/extractor/sampler_test_blacklist.cc b/extractor/sampler_test_blacklist.cc
deleted file mode 100644
index 3305b990..00000000
--- a/extractor/sampler_test_blacklist.cc
+++ /dev/null
@@ -1,102 +0,0 @@
-#include <gtest/gtest.h>
-
-#include <memory>
-
-#include "mocks/mock_suffix_array.h"
-#include "mocks/mock_data_array.h"
-#include "phrase_location.h"
-#include "sampler.h"
-
-using namespace std;
-using namespace ::testing;
-
-namespace extractor {
-namespace {
-
-class SamplerTestBlacklist : public Test {
- protected:
-  virtual void SetUp() {
-    source_data_array = make_shared<MockDataArray>();
-    for (int i = 0; i < 10; ++i) {
-      EXPECT_CALL(*source_data_array, GetSentenceId(i)).WillRepeatedly(Return(i));
-    }
-    for (int i = -10; i < 0; ++i) {
-      EXPECT_CALL(*source_data_array, GetSentenceId(i)).WillRepeatedly(Return(0));
-    }
-    suffix_array = make_shared<MockSuffixArray>();
-    for (int i = -10; i < 10; ++i) {
-      EXPECT_CALL(*suffix_array, GetSuffix(i)).WillRepeatedly(Return(i));
-    }
-  }
-
-  shared_ptr<MockSuffixArray> suffix_array;
-  shared_ptr<Sampler> sampler;
-  shared_ptr<MockDataArray> source_data_array;
-};
-
-TEST_F(SamplerTestBlacklist, TestSuffixArrayRange) {
-  PhraseLocation location(0, 10);
-  unordered_set<int> blacklist;
-  vector<int> expected_locations;
-   
-  blacklist.insert(0);
-  sampler = make_shared<Sampler>(suffix_array, 1);
-  expected_locations = {1};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-  blacklist.clear();
-  
-  for (int i = 0; i < 9; i++) {
-    blacklist.insert(i);
-  }
-  sampler = make_shared<Sampler>(suffix_array, 1);
-  expected_locations = {9};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-  blacklist.clear();
-
-  blacklist.insert(0);
-  blacklist.insert(5);
-  sampler = make_shared<Sampler>(suffix_array, 2);
-  expected_locations = {1, 4};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-  blacklist.clear();
-
-  blacklist.insert(0);
-  blacklist.insert(1);
-  blacklist.insert(2);
-  blacklist.insert(3);
-  sampler = make_shared<Sampler>(suffix_array, 2);
-  expected_locations = {4, 5};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-  blacklist.clear();
-
-  blacklist.insert(0);
-  blacklist.insert(3);
-  blacklist.insert(7);
-  sampler = make_shared<Sampler>(suffix_array, 3);
-  expected_locations = {1, 2, 6};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-  blacklist.clear();
-
-  blacklist.insert(0);
-  blacklist.insert(3);
-  blacklist.insert(5);
-  blacklist.insert(8);
-  sampler = make_shared<Sampler>(suffix_array, 4);
-  expected_locations = {1, 2, 4, 7};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-  blacklist.clear();
-  
-  blacklist.insert(0);
-  sampler = make_shared<Sampler>(suffix_array, 100);
-  expected_locations = {1, 2, 3, 4, 5, 6, 7, 8, 9};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-  blacklist.clear();
- 
-  blacklist.insert(9);
-  sampler = make_shared<Sampler>(suffix_array, 100);
-  expected_locations = {0, 1, 2, 3, 4, 5, 6, 7, 8};
-  EXPECT_EQ(PhraseLocation(expected_locations, 1), sampler->Sample(location, blacklist, source_data_array));
-}
-
-} // namespace
-} // namespace extractor
diff --git a/extractor/suffix_array.cc b/extractor/suffix_array.cc
index 0cf4d1f6..4a514b12 100644
--- a/extractor/suffix_array.cc
+++ b/extractor/suffix_array.cc
@@ -10,7 +10,6 @@
 #include "phrase_location.h"
 #include "time_util.h"
 
-namespace fs = boost::filesystem;
 using namespace std;
 using namespace chrono;
 
@@ -188,12 +187,12 @@ shared_ptr<DataArray> SuffixArray::GetData() const {
 
 PhraseLocation SuffixArray::Lookup(int low, int high, const string& word,
                                    int offset) const {
-  if (!data_array->HasWord(word)) {
+  int word_id = data_array->GetWordId(word);
+  if (word_id == -1) {
     // Return empty phrase location.
     return PhraseLocation(0, 0);
   }
 
-  int word_id = data_array->GetWordId(word);
   if (offset == 0) {
     return PhraseLocation(word_start[word_id], word_start[word_id + 1]);
   }
diff --git a/extractor/suffix_array.h b/extractor/suffix_array.h
index 8ee454ec..df80c152 100644
--- a/extractor/suffix_array.h
+++ b/extractor/suffix_array.h
@@ -5,12 +5,10 @@
 #include <string>
 #include <vector>
 
-#include <boost/filesystem.hpp>
 #include <boost/serialization/serialization.hpp>
 #include <boost/serialization/split_member.hpp>
 #include <boost/serialization/vector.hpp>
 
-namespace fs = boost::filesystem;
 using namespace std;
 
 namespace extractor {
diff --git a/extractor/suffix_array_sampler.cc b/extractor/suffix_array_sampler.cc
new file mode 100644
index 00000000..4a4ced34
--- /dev/null
+++ b/extractor/suffix_array_sampler.cc
@@ -0,0 +1,40 @@
+#include "suffix_array_sampler.h"
+
+#include "data_array.h"
+#include "phrase_location.h"
+#include "suffix_array.h"
+
+namespace extractor {
+
+SuffixArrayRangeSampler::SuffixArrayRangeSampler(
+    shared_ptr<SuffixArray> source_suffix_array, int max_samples) :
+    BackoffSampler(source_suffix_array->GetData(), max_samples),
+    source_suffix_array(source_suffix_array) {}
+
+SuffixArrayRangeSampler::SuffixArrayRangeSampler() {}
+
+int SuffixArrayRangeSampler::GetNumSubpatterns(const PhraseLocation&) const {
+  return 1;
+}
+
+int SuffixArrayRangeSampler::GetRangeLow(
+    const PhraseLocation& location) const {
+  return location.sa_low;
+}
+
+int SuffixArrayRangeSampler::GetRangeHigh(
+    const PhraseLocation& location) const {
+  return location.sa_high;
+}
+
+int SuffixArrayRangeSampler::GetPosition(
+    const PhraseLocation&, int position) const {
+  return source_suffix_array->GetSuffix(position);
+}
+
+void SuffixArrayRangeSampler::AppendMatching(
+    vector<int>& samples, int index, const PhraseLocation&) const {
+  samples.push_back(source_suffix_array->GetSuffix(index));
+}
+
+} // namespace extractor
diff --git a/extractor/suffix_array_sampler.h b/extractor/suffix_array_sampler.h
new file mode 100644
index 00000000..bb3c2653
--- /dev/null
+++ b/extractor/suffix_array_sampler.h
@@ -0,0 +1,34 @@
+#ifndef _SUFFIX_ARRAY_SAMPLER_H_
+#define _SUFFIX_ARRAY_SAMPLER_H_
+
+#include "backoff_sampler.h"
+
+namespace extractor {
+
+class SuffixArray;
+
+class SuffixArrayRangeSampler : public BackoffSampler {
+ public:
+  SuffixArrayRangeSampler(shared_ptr<SuffixArray> suffix_array,
+                          int max_samples);
+
+  SuffixArrayRangeSampler();
+
+ private:
+  int GetNumSubpatterns(const PhraseLocation& location) const;
+
+  int GetRangeLow(const PhraseLocation& location) const;
+
+  int GetRangeHigh(const PhraseLocation& location) const;
+
+  int GetPosition(const PhraseLocation& location, int index) const;
+
+  void AppendMatching(vector<int>& samples, int index,
+                      const PhraseLocation& location) const;
+
+  shared_ptr<SuffixArray> source_suffix_array;
+};
+
+} // namespace extractor
+
+#endif
diff --git a/extractor/suffix_array_sampler_test.cc b/extractor/suffix_array_sampler_test.cc
new file mode 100644
index 00000000..4b88c027
--- /dev/null
+++ b/extractor/suffix_array_sampler_test.cc
@@ -0,0 +1,114 @@
+#include <gtest/gtest.h>
+
+#include <memory>
+
+#include "mocks/mock_data_array.h"
+#include "mocks/mock_suffix_array.h"
+#include "suffix_array_sampler.h"
+
+using namespace std;
+using namespace ::testing;
+
+namespace extractor {
+namespace {
+
+class SuffixArraySamplerTest : public Test {
+ protected:
+  virtual void SetUp() {
+    data_array = make_shared<MockDataArray>();
+    for (int i = 0; i < 10; ++i) {
+      EXPECT_CALL(*data_array, GetSentenceId(i)).WillRepeatedly(Return(i));
+    }
+
+    suffix_array = make_shared<MockSuffixArray>();
+    EXPECT_CALL(*suffix_array, GetData()).WillRepeatedly(Return(data_array));
+    for (int i = 0; i < 10; ++i) {
+      EXPECT_CALL(*suffix_array, GetSuffix(i)).WillRepeatedly(Return(i));
+    }
+  }
+
+  shared_ptr<MockDataArray> data_array;
+  shared_ptr<MockSuffixArray> suffix_array;
+};
+
+TEST_F(SuffixArraySamplerTest, TestSample) {
+  PhraseLocation location(0, 10);
+  unordered_set<int> blacklisted_sentence_ids;
+
+  SuffixArrayRangeSampler sampler(suffix_array, 1);
+  vector<int> expected_locations = {0};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  sampler = SuffixArrayRangeSampler(suffix_array, 2);
+  expected_locations = {0, 5};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  sampler = SuffixArrayRangeSampler(suffix_array, 3);
+  expected_locations = {0, 3, 7};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  sampler = SuffixArrayRangeSampler(suffix_array, 4);
+  expected_locations = {0, 3, 5, 8};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  sampler = SuffixArrayRangeSampler(suffix_array, 100);
+  expected_locations = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+}
+
+TEST_F(SuffixArraySamplerTest, TestBackoffSample) {
+  PhraseLocation location(0, 10);
+
+  SuffixArrayRangeSampler sampler(suffix_array, 1);
+  unordered_set<int> blacklisted_sentence_ids = {0};
+  vector<int> expected_locations = {1};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {0, 1, 2, 3, 4, 5, 6, 7, 8};
+  expected_locations = {9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  sampler = SuffixArrayRangeSampler(suffix_array, 2);
+  blacklisted_sentence_ids = {0, 5};
+  expected_locations = {1, 4};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {0, 1, 2, 3};
+  expected_locations = {4, 5};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  sampler = SuffixArrayRangeSampler(suffix_array, 3);
+  blacklisted_sentence_ids = {0, 3, 7};
+  expected_locations = {1, 2, 6};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  sampler = SuffixArrayRangeSampler(suffix_array, 4);
+  blacklisted_sentence_ids = {0, 3, 5, 8};
+  expected_locations = {1, 2, 4, 7};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  sampler = SuffixArrayRangeSampler(suffix_array, 100);
+  blacklisted_sentence_ids = {0};
+  expected_locations = {1, 2, 3, 4, 5, 6, 7, 8, 9};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+
+  blacklisted_sentence_ids = {9};
+  expected_locations = {0, 1, 2, 3, 4, 5, 6, 7, 8};
+  EXPECT_EQ(PhraseLocation(expected_locations, 1),
+            sampler.Sample(location, blacklisted_sentence_ids));
+}
+
+}
+} // namespace extractor
diff --git a/extractor/suffix_array_test.cc b/extractor/suffix_array_test.cc
index ba0dbcc3..161edbc0 100644
--- a/extractor/suffix_array_test.cc
+++ b/extractor/suffix_array_test.cc
@@ -21,7 +21,7 @@ class SuffixArrayTest : public Test {
   virtual void SetUp() {
     data = {6, 4, 1, 2, 4, 5, 3, 4, 6, 6, 4, 1, 2};
     data_array = make_shared<MockDataArray>();
-    EXPECT_CALL(*data_array, GetData()).WillRepeatedly(ReturnRef(data));
+    EXPECT_CALL(*data_array, GetData()).WillRepeatedly(Return(data));
     EXPECT_CALL(*data_array, GetVocabularySize()).WillRepeatedly(Return(7));
     EXPECT_CALL(*data_array, GetSize()).WillRepeatedly(Return(13));
     suffix_array = SuffixArray(data_array);
@@ -55,22 +55,18 @@ TEST_F(SuffixArrayTest, TestLookup) {
     EXPECT_CALL(*data_array, AtIndex(i)).WillRepeatedly(Return(data[i]));
   }
 
-  EXPECT_CALL(*data_array, HasWord("word1")).WillRepeatedly(Return(true));
   EXPECT_CALL(*data_array, GetWordId("word1")).WillRepeatedly(Return(6));
   EXPECT_EQ(PhraseLocation(11, 14), suffix_array.Lookup(0, 14, "word1", 0));
 
-  EXPECT_CALL(*data_array, HasWord("word2")).WillRepeatedly(Return(false));
+  EXPECT_CALL(*data_array, GetWordId("word2")).WillRepeatedly(Return(-1));
   EXPECT_EQ(PhraseLocation(0, 0), suffix_array.Lookup(0, 14, "word2", 0));
 
-  EXPECT_CALL(*data_array, HasWord("word3")).WillRepeatedly(Return(true));
   EXPECT_CALL(*data_array, GetWordId("word3")).WillRepeatedly(Return(4));
   EXPECT_EQ(PhraseLocation(11, 13), suffix_array.Lookup(11, 14, "word3", 1));
 
-  EXPECT_CALL(*data_array, HasWord("word4")).WillRepeatedly(Return(true));
   EXPECT_CALL(*data_array, GetWordId("word4")).WillRepeatedly(Return(1));
   EXPECT_EQ(PhraseLocation(11, 13), suffix_array.Lookup(11, 13, "word4", 2));
 
-  EXPECT_CALL(*data_array, HasWord("word5")).WillRepeatedly(Return(true));
   EXPECT_CALL(*data_array, GetWordId("word5")).WillRepeatedly(Return(2));
   EXPECT_EQ(PhraseLocation(11, 13), suffix_array.Lookup(11, 13, "word5", 3));
 
diff --git a/extractor/translation_table.cc b/extractor/translation_table.cc
index 1b1ba112..11e29e1e 100644
--- a/extractor/translation_table.cc
+++ b/extractor/translation_table.cc
@@ -90,13 +90,12 @@ void TranslationTable::IncrementLinksCount(
 
 double TranslationTable::GetTargetGivenSourceScore(
     const string& source_word, const string& target_word) {
-  if (!source_data_array->HasWord(source_word) ||
-      !target_data_array->HasWord(target_word)) {
+  int source_id = source_data_array->GetWordId(source_word);
+  int target_id = target_data_array->GetWordId(target_word);
+  if (source_id == -1 || target_id == -1) {
     return -1;
   }
 
-  int source_id = source_data_array->GetWordId(source_word);
-  int target_id = target_data_array->GetWordId(target_word);
   auto entry = make_pair(source_id, target_id);
   auto it = translation_probabilities.find(entry);
   if (it == translation_probabilities.end()) {
@@ -107,13 +106,12 @@ double TranslationTable::GetTargetGivenSourceScore(
 
 double TranslationTable::GetSourceGivenTargetScore(
     const string& source_word, const string& target_word) {
-  if (!source_data_array->HasWord(source_word) ||
-      !target_data_array->HasWord(target_word)) {
+  int source_id = source_data_array->GetWordId(source_word);
+  int target_id = target_data_array->GetWordId(target_word);
+  if (source_id == -1 || target_id == -1) {
     return -1;
   }
 
-  int source_id = source_data_array->GetWordId(source_word);
-  int target_id = target_data_array->GetWordId(target_word);
   auto entry = make_pair(source_id, target_id);
   auto it = translation_probabilities.find(entry);
   if (it == translation_probabilities.end()) {
diff --git a/extractor/translation_table.h b/extractor/translation_table.h
index 2a37bab7..97620727 100644
--- a/extractor/translation_table.h
+++ b/extractor/translation_table.h
@@ -5,14 +5,12 @@
 #include <string>
 #include <unordered_map>
 
-#include <boost/filesystem.hpp>
 #include <boost/functional/hash.hpp>
 #include <boost/serialization/serialization.hpp>
 #include <boost/serialization/split_member.hpp>
 #include <boost/serialization/utility.hpp>
 
 using namespace std;
-namespace fs = boost::filesystem;
 
 namespace extractor {
 
diff --git a/extractor/translation_table_test.cc b/extractor/translation_table_test.cc
index 606777bd..3cfc0011 100644
--- a/extractor/translation_table_test.cc
+++ b/extractor/translation_table_test.cc
@@ -28,7 +28,7 @@ class TranslationTableTest : public Test {
     vector<int> source_sentence_start = {0, 6, 10, 14};
     shared_ptr<MockDataArray> source_data_array = make_shared<MockDataArray>();
     EXPECT_CALL(*source_data_array, GetData())
-        .WillRepeatedly(ReturnRef(source_data));
+        .WillRepeatedly(Return(source_data));
     EXPECT_CALL(*source_data_array, GetNumSentences())
         .WillRepeatedly(Return(3));
     for (size_t i = 0; i < source_sentence_start.size(); ++i) {
@@ -36,31 +36,25 @@ class TranslationTableTest : public Test {
           .WillRepeatedly(Return(source_sentence_start[i]));
     }
     for (size_t i = 0; i < words.size(); ++i) {
-      EXPECT_CALL(*source_data_array, HasWord(words[i]))
-          .WillRepeatedly(Return(true));
       EXPECT_CALL(*source_data_array, GetWordId(words[i]))
           .WillRepeatedly(Return(i + 2));
     }
-    EXPECT_CALL(*source_data_array, HasWord("d"))
-        .WillRepeatedly(Return(false));
+    EXPECT_CALL(*source_data_array, GetWordId("d")).WillRepeatedly(Return(-1));
 
     vector<int> target_data = {2, 3, 2, 3, 4, 5, 0, 3, 6, 0, 2, 7, 0};
     vector<int> target_sentence_start = {0, 7, 10, 13};
     shared_ptr<MockDataArray> target_data_array = make_shared<MockDataArray>();
     EXPECT_CALL(*target_data_array, GetData())
-        .WillRepeatedly(ReturnRef(target_data));
+        .WillRepeatedly(Return(target_data));
     for (size_t i = 0; i < target_sentence_start.size(); ++i) {
       EXPECT_CALL(*target_data_array, GetSentenceStart(i))
           .WillRepeatedly(Return(target_sentence_start[i]));
     }
     for (size_t i = 0; i < words.size(); ++i) {
-      EXPECT_CALL(*target_data_array, HasWord(words[i]))
-          .WillRepeatedly(Return(true));
       EXPECT_CALL(*target_data_array, GetWordId(words[i]))
           .WillRepeatedly(Return(i + 2));
     }
-    EXPECT_CALL(*target_data_array, HasWord("d"))
-        .WillRepeatedly(Return(false));
+    EXPECT_CALL(*target_data_array, GetWordId("d")).WillRepeatedly(Return(-1));
 
     vector<pair<int, int>> links1 = {
       make_pair(0, 0), make_pair(1, 1), make_pair(2, 2), make_pair(3, 3),
diff --git a/extractor/vocabulary.cc b/extractor/vocabulary.cc
index 15795d1e..c9c2d6f4 100644
--- a/extractor/vocabulary.cc
+++ b/extractor/vocabulary.cc
@@ -8,12 +8,13 @@ int Vocabulary::GetTerminalIndex(const string& word) {
   int word_id = -1;
   #pragma omp critical (vocabulary)
   {
-    if (!dictionary.count(word)) {
+    auto it = dictionary.find(word);
+    if (it != dictionary.end()) {
+      word_id = it->second;
+    } else {
       word_id = words.size();
       dictionary[word] = word_id;
       words.push_back(word);
-    } else {
-      word_id = dictionary[word];
     }
   }
   return word_id;
@@ -34,4 +35,8 @@ string Vocabulary::GetTerminalValue(int symbol) {
   return word;
 }
 
+bool Vocabulary::operator==(const Vocabulary& other) const {
+  return words == other.words && dictionary == other.dictionary;
+}
+
 } // namespace extractor
diff --git a/extractor/vocabulary.h b/extractor/vocabulary.h
index c8fd9411..db092e99 100644
--- a/extractor/vocabulary.h
+++ b/extractor/vocabulary.h
@@ -5,6 +5,10 @@
 #include <unordered_map>
 #include <vector>
 
+#include <boost/serialization/serialization.hpp>
+#include <boost/serialization/string.hpp>
+#include <boost/serialization/vector.hpp>
+
 using namespace std;
 
 namespace extractor {
@@ -14,7 +18,7 @@ namespace extractor {
  *
  * This strucure contains words located in the frequent collocations and words
  * encountered during the grammar extraction time. This dictionary is
- * considerably smaller than the dictionaries in the data arrays (and so is the
+ * considerably smaller than the dictionaries in the data arays (and so is the
  * query time). Note that this is the single data structure that changes state
  * and needs to have thread safe read/write operations.
  *
@@ -38,7 +42,24 @@ class Vocabulary {
   // Returns the word corresponding to the given word id.
   virtual string GetTerminalValue(int symbol);
 
+  bool operator==(const Vocabulary& vocabulary) const;
+
  private:
+  friend class boost::serialization::access;
+
+  template<class Archive> void save(Archive& ar, unsigned int) const {
+    ar << words;
+  }
+
+  template<class Archive> void load(Archive& ar, unsigned int) {
+    ar >> words;
+    for (size_t i = 0; i < words.size(); ++i) {
+      dictionary[words[i]] = i;
+    }
+  }
+
+  BOOST_SERIALIZATION_SPLIT_MEMBER();
+
   unordered_map<string, int> dictionary;
   vector<string> words;
 };
diff --git a/extractor/vocabulary_test.cc b/extractor/vocabulary_test.cc
new file mode 100644
index 00000000..cf5e3e36
--- /dev/null
+++ b/extractor/vocabulary_test.cc
@@ -0,0 +1,45 @@
+#include <gtest/gtest.h>
+
+#include <sstream>
+#include <string>
+#include <vector>
+
+#include <boost/archive/text_iarchive.hpp>
+#include <boost/archive/text_oarchive.hpp>
+
+#include "vocabulary.h"
+
+using namespace std;
+using namespace ::testing;
+namespace ar = boost::archive;
+
+namespace extractor {
+namespace {
+
+TEST(VocabularyTest, TestIndexes) {
+  Vocabulary vocabulary;
+  EXPECT_EQ(0, vocabulary.GetTerminalIndex("zero"));
+  EXPECT_EQ("zero", vocabulary.GetTerminalValue(0));
+
+  EXPECT_EQ(1, vocabulary.GetTerminalIndex("one"));
+  EXPECT_EQ("one", vocabulary.GetTerminalValue(1));
+}
+
+TEST(VocabularyTest, TestSerialization) {
+  Vocabulary vocabulary;
+  EXPECT_EQ(0, vocabulary.GetTerminalIndex("zero"));
+  EXPECT_EQ("zero", vocabulary.GetTerminalValue(0));
+
+  stringstream stream(ios_base::out | ios_base::in);
+  ar::text_oarchive output_stream(stream, ar::no_header);
+  output_stream << vocabulary;
+
+  Vocabulary vocabulary_copy;
+  ar::text_iarchive input_stream(stream, ar::no_header);
+  input_stream >> vocabulary_copy;
+
+  EXPECT_EQ(vocabulary, vocabulary_copy);
+}
+
+} // namespace
+} // namespace extractor