1 files changed, 66 insertions, 0 deletions
diff --git a/extractor/backoff_sampler.cc b/extractor/backoff_sampler.cc
new file mode 100644
index 00000000..28b12909
--- /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_position = 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 position = GetPosition(location, round(i));
+    int sentence_id = source_data_array->GetSentenceId(position);
+    bool found = false;
+    if (last_position >= position ||
+        blacklisted_sentence_ids.count(sentence_id)) {
+      for (double backoff_step = 1; backoff_step < step; ++backoff_step) {
+        double j = i - backoff_step;
+        if (round(j) >= 0) {
+          position = GetPosition(location, round(j));
+          sentence_id = source_data_array->GetSentenceId(position);
+          if (position > last_position &&
+              !blacklisted_sentence_ids.count(sentence_id)) {
+            found = true;
+            last_position = position;
+            break;
+          }
+        }
+
+        double k = i + backoff_step;
+        if (round(k) < high) {
+          position = GetPosition(location, round(k));
+          sentence_id = source_data_array->GetSentenceId(position);
+          if (!blacklisted_sentence_ids.count(sentence_id)) {
+            found = true;
+            last_position = position;
+            break;
+          }
+        }
+      }
+    } else {
+      found = true;
+      last_position = position;
+    }
+
+    if (found) {
+      AppendMatching(samples, position, location);
+    }
+  }
+
+  return PhraseLocation(samples, GetNumSubpatterns(location));
+}
+
+} // namespace extractor