Limit number of tokens in fire red asr decoding. (#2459)

@@ -31,7 +31,8 @@ class OfflineFireRedAsrDecoder {
    * @return Return a vector of size `N` containing the decoded results.
    */
   virtual std::vector<OfflineFireRedAsrDecoderResult> Decode(
-      Ort::Value n_layer_cross_k, Ort::Value n_layer_cross_v) = 0;
+      Ort::Value n_layer_cross_k, Ort::Value n_layer_cross_v,
+      int32_t num_feature_frames) = 0;
 };
 
 }  // namespace sherpa_onnx

@@ -16,7 +16,8 @@ namespace sherpa_onnx {
 // Note: this functions works only for batch size == 1 at present
 std::vector<OfflineFireRedAsrDecoderResult>
 OfflineFireRedAsrGreedySearchDecoder::Decode(Ort::Value cross_k,
-                                             Ort::Value cross_v) {
+                                             Ort::Value cross_v,
+                                             int32_t num_feature_frames) {
   const auto &meta_data = model_->GetModelMetadata();
 
   auto memory_info =
@@ -53,7 +54,12 @@ OfflineFireRedAsrGreedySearchDecoder::Decode(Ort::Value cross_k,
                      std::move(cross_v),
                      std::move(offset)};
 
-  for (int32_t i = 0; i < meta_data.max_len; ++i) {
+  // assume at most 6 tokens per second
+  int32_t num_possible_tokens = num_feature_frames / 100 * 6;
+  num_possible_tokens =
+      std::min<int32_t>(num_possible_tokens, meta_data.max_len / 2);
+
+  for (int32_t i = 0; i < num_possible_tokens; ++i) {
     decoder_out = model_->ForwardDecoder(View(&tokens),
                                          std::move(std::get<1>(decoder_out)),
                                          std::move(std::get<2>(decoder_out)),

@@ -18,7 +18,8 @@ class OfflineFireRedAsrGreedySearchDecoder : public OfflineFireRedAsrDecoder {
       : model_(model) {}
 
   std::vector<OfflineFireRedAsrDecoderResult> Decode(
-      Ort::Value cross_k, Ort::Value cross_v) override;
+      Ort::Value cross_k, Ort::Value cross_v,
+      int32_t num_feature_frames) override;
 
  private:
   OfflineFireRedAsrModel *model_;  // not owned

@@ -119,8 +119,8 @@ class OfflineRecognizerFireRedAsrImpl : public OfflineRecognizerImpl {
 
     auto cross_kv = model_->ForwardEncoder(std::move(x), std::move(x_len));
 
-    auto results =
-        decoder_->Decode(std::move(cross_kv.first), std::move(cross_kv.second));
+    auto results = decoder_->Decode(std::move(cross_kv.first),
+                                    std::move(cross_kv.second), num_frames);
 
     auto r = Convert(results[0], symbol_table_);