1 files changed, 58 insertions, 0 deletions
diff --git a/lm/filter/wrapper.hh b/lm/filter/wrapper.hh
new file mode 100644
index 000000000..90b07a08f
--- /dev/null
+++ b/lm/filter/wrapper.hh
@@ -0,0 +1,58 @@
+#ifndef LM_FILTER_WRAPPER_H__
+#define LM_FILTER_WRAPPER_H__
+
+#include "util/string_piece.hh"
+
+#include <algorithm>
+#include <string>
+#include <vector>
+
+namespace lm {
+
+// Provide a single-output filter with the same interface as a
+// multiple-output filter so clients code against one interface.
+template <class Binary> class BinaryFilter {
+  public:
+    // Binary modes are just references (and a set) and it makes the API cleaner to copy them.  
+    explicit BinaryFilter(Binary binary) : binary_(binary) {}
+
+    template <class Iterator, class Output> void AddNGram(const Iterator &begin, const Iterator &end, const StringPiece &line, Output &output) {
+      if (binary_.PassNGram(begin, end))
+        output.AddNGram(line);
+    }
+
+    template <class Output> void AddNGram(const StringPiece &ngram, const StringPiece &line, Output &output) {
+      AddNGram(util::TokenIter<util::SingleCharacter, true>(ngram, ' '), util::TokenIter<util::SingleCharacter, true>::end(), line, output);
+    }
+
+    void Flush() const {}
+
+  private:
+    Binary binary_;
+};
+
+// Wrap another filter to pay attention only to context words
+template <class FilterT> class ContextFilter {
+  public:
+    typedef FilterT Filter;
+
+    explicit ContextFilter(Filter &backend) : backend_(backend) {}
+
+    template <class Output> void AddNGram(const StringPiece &ngram, const StringPiece &line, Output &output) {
+      pieces_.clear();
+      // TODO: this copy could be avoided by a lookahead iterator.
+      std::copy(util::TokenIter<util::SingleCharacter, true>(ngram, ' '), util::TokenIter<util::SingleCharacter, true>::end(), std::back_insert_iterator<std::vector<StringPiece> >(pieces_));
+      backend_.AddNGram(pieces_.begin(), pieces_.end() - !pieces_.empty(), line, output);
+    }
+
+    void Flush() const {}
+
+  private:
+    std::vector<StringPiece> pieces_;
+
+    Filter backend_;
+};
+
+} // namespace lm
+
+#endif // LM_FILTER_WRAPPER_H__