1 files changed, 65 insertions, 0 deletions
diff --git a/lm/builder/interpolate.cc b/lm/builder/interpolate.cc
new file mode 100644
index 000000000..500268069
--- /dev/null
+++ b/lm/builder/interpolate.cc
@@ -0,0 +1,65 @@
+#include "lm/builder/interpolate.hh"
+
+#include "lm/builder/joint_order.hh"
+#include "lm/builder/multi_stream.hh"
+#include "lm/builder/sort.hh"
+#include "lm/lm_exception.hh"
+
+#include <assert.h>
+
+namespace lm { namespace builder {
+namespace {
+
+class Callback {
+  public:
+    Callback(float uniform_prob, const ChainPositions &backoffs) : backoffs_(backoffs.size()), probs_(backoffs.size() + 2) {
+      probs_[0] = uniform_prob;
+      for (std::size_t i = 0; i < backoffs.size(); ++i) {
+        backoffs_.push_back(backoffs[i]);
+      }
+    }
+
+    ~Callback() {
+      for (std::size_t i = 0; i < backoffs_.size(); ++i) {
+        if (backoffs_[i]) {
+          std::cerr << "Backoffs do not match for order " << (i + 1) << std::endl;
+          abort();
+        }
+      }
+    }
+
+    void Enter(unsigned order_minus_1, NGram &gram) {
+      Payload &pay = gram.Value();
+      pay.complete.prob = pay.uninterp.prob + pay.uninterp.gamma * probs_[order_minus_1];
+      probs_[order_minus_1 + 1] = pay.complete.prob;
+      pay.complete.prob = log10(pay.complete.prob);
+      // TODO: this is a hack to skip n-grams that don't appear as context.  Pruning will require some different handling.  
+      if (order_minus_1 < backoffs_.size() && *(gram.end() - 1) != kUNK && *(gram.end() - 1) != kEOS) {
+        pay.complete.backoff = log10(*static_cast<const float*>(backoffs_[order_minus_1].Get()));
+        ++backoffs_[order_minus_1];
+      } else {
+        // Not a context.  
+        pay.complete.backoff = 0.0;
+      }
+    }
+
+    void Exit(unsigned, const NGram &) const {}
+
+  private:
+    FixedArray<util::stream::Stream> backoffs_;
+
+    std::vector<float> probs_;
+};
+} // namespace
+
+Interpolate::Interpolate(uint64_t unigram_count, const ChainPositions &backoffs) 
+  : uniform_prob_(1.0 / static_cast<float>(unigram_count - 1)), backoffs_(backoffs) {}
+
+// perform order-wise interpolation
+void Interpolate::Run(const ChainPositions &positions) {
+  assert(positions.size() == backoffs_.size() + 1);
+  Callback callback(uniform_prob_, backoffs_);
+  JointOrder<Callback, SuffixOrder>(positions, callback);
+}
+
+}} // namespaces