treat unk as blank (#299)

sherpa-onnx/csrc/online-recognizer-transducer-impl.h

@@ -57,6 +57,10 @@ class OnlineRecognizerTransducerImpl : public OnlineRecognizerImpl {
         model_(OnlineTransducerModel::Create(config.model_config)),
         sym_(config.model_config.tokens),
         endpoint_(config_.endpoint_config) {
+    if (sym_.contains("<unk>")) {
+      unk_id_ = sym_["<unk>"];
+    }
+
     if (config.decoding_method == "modified_beam_search") {
       if (!config_.lm_config.model.empty()) {
         lm_ = OnlineLM::Create(config.lm_config);
@@ -64,10 +68,10 @@ class OnlineRecognizerTransducerImpl : public OnlineRecognizerImpl {
 
       decoder_ = std::make_unique<OnlineTransducerModifiedBeamSearchDecoder>(
           model_.get(), lm_.get(), config_.max_active_paths,
-          config_.lm_config.scale);
+          config_.lm_config.scale, unk_id_);
     } else if (config.decoding_method == "greedy_search") {
-      decoder_ =
-          std::make_unique<OnlineTransducerGreedySearchDecoder>(model_.get());
+      decoder_ = std::make_unique<OnlineTransducerGreedySearchDecoder>(
+          model_.get(), unk_id_);
     } else {
       SHERPA_ONNX_LOGE("Unsupported decoding method: %s",
                        config.decoding_method.c_str());
@@ -82,13 +86,17 @@ class OnlineRecognizerTransducerImpl : public OnlineRecognizerImpl {
         model_(OnlineTransducerModel::Create(mgr, config.model_config)),
         sym_(mgr, config.model_config.tokens),
         endpoint_(config_.endpoint_config) {
+    if (sym_.contains("<unk>")) {
+      unk_id_ = sym_["<unk>"];
+    }
+
     if (config.decoding_method == "modified_beam_search") {
       decoder_ = std::make_unique<OnlineTransducerModifiedBeamSearchDecoder>(
           model_.get(), lm_.get(), config_.max_active_paths,
-          config_.lm_config.scale);
+          config_.lm_config.scale, unk_id_);
     } else if (config.decoding_method == "greedy_search") {
-      decoder_ =
-          std::make_unique<OnlineTransducerGreedySearchDecoder>(model_.get());
+      decoder_ = std::make_unique<OnlineTransducerGreedySearchDecoder>(
+          model_.get(), unk_id_);
     } else {
       SHERPA_ONNX_LOGE("Unsupported decoding method: %s",
                        config.decoding_method.c_str());
@@ -268,6 +276,7 @@ class OnlineRecognizerTransducerImpl : public OnlineRecognizerImpl {
   std::unique_ptr<OnlineTransducerDecoder> decoder_;
   SymbolTable sym_;
   Endpoint endpoint_;
+  int32_t unk_id_ = -1;
 };
 
 }  // namespace sherpa_onnx

sherpa-onnx/csrc/online-transducer-greedy-search-decoder.cc

@@ -108,7 +108,9 @@ void OnlineTransducerGreedySearchDecoder::Decode(
           static_cast<const float *>(p_logit),
           std::max_element(static_cast<const float *>(p_logit),
                            static_cast<const float *>(p_logit) + vocab_size)));
-      if (y != 0) {
+      // blank id is hardcoded to 0
+      // also, it treats unk as blank
+      if (y != 0 && y != unk_id_) {
         emitted = true;
         r.tokens.push_back(y);
         r.timestamps.push_back(t + r.frame_offset);

sherpa-onnx/csrc/online-transducer-greedy-search-decoder.h

@@ -14,8 +14,9 @@ namespace sherpa_onnx {
 
 class OnlineTransducerGreedySearchDecoder : public OnlineTransducerDecoder {
  public:
-  explicit OnlineTransducerGreedySearchDecoder(OnlineTransducerModel *model)
-      : model_(model) {}
+  OnlineTransducerGreedySearchDecoder(OnlineTransducerModel *model,
+                                      int32_t unk_id)
+      : model_(model), unk_id_(unk_id) {}
 
   OnlineTransducerDecoderResult GetEmptyResult() const override;
 
@@ -26,6 +27,7 @@ class OnlineTransducerGreedySearchDecoder : public OnlineTransducerDecoder {
 
  private:
   OnlineTransducerModel *model_;  // Not owned
+  int32_t unk_id_;
 };
 
 }  // namespace sherpa_onnx

sherpa-onnx/csrc/online-transducer-modified-beam-search-decoder.cc

@@ -155,7 +155,9 @@ void OnlineTransducerModifiedBeamSearchDecoder::Decode(
         float context_score = 0;
         auto context_state = new_hyp.context_state;
 
-        if (new_token != 0) {
+        // blank is hardcoded to 0
+        // also, it treats unk as blank
+        if (new_token != 0 && new_token != unk_id_) {
           new_hyp.ys.push_back(new_token);
           new_hyp.timestamps.push_back(t + frame_offset);
           new_hyp.num_trailing_blanks = 0;

sherpa-onnx/csrc/online-transducer-modified-beam-search-decoder.h

@@ -21,11 +21,12 @@ class OnlineTransducerModifiedBeamSearchDecoder
   OnlineTransducerModifiedBeamSearchDecoder(OnlineTransducerModel *model,
                                             OnlineLM *lm,
                                             int32_t max_active_paths,
-                                            float lm_scale)
+                                            float lm_scale, int32_t unk_id)
       : model_(model),
         lm_(lm),
         max_active_paths_(max_active_paths),
-        lm_scale_(lm_scale) {}
+        lm_scale_(lm_scale),
+        unk_id_(unk_id) {}
 
   OnlineTransducerDecoderResult GetEmptyResult() const override;
 
@@ -45,6 +46,7 @@ class OnlineTransducerModifiedBeamSearchDecoder
 
   int32_t max_active_paths_;
   float lm_scale_;  // used only when lm_ is not nullptr
+  int32_t unk_id_;
 };
 
 }  // namespace sherpa_onnx