Add online LSTM transducer model (#25)

sherpa-onnx/csrc/features.h

@@ -0,0 +1,61 @@
+// sherpa/csrc/features.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_FEATURES_H_
+#define SHERPA_ONNX_CSRC_FEATURES_H_
+
+#include <memory>
+#include <mutex>  // NOLINT
+#include <vector>
+
+#include "kaldi-native-fbank/csrc/online-feature.h"
+
+namespace sherpa_onnx {
+
+class FeatureExtractor {
+ public:
+  FeatureExtractor();
+  explicit FeatureExtractor(const knf::FbankOptions &fbank_opts);
+
+  /**
+     @param sampling_rate The sampling_rate of the input waveform. Should match
+                          the one expected by the feature extractor.
+     @param waveform Pointer to a 1-D array of size n
+     @param n Number of entries in waveform
+   */
+  void AcceptWaveform(float sampling_rate, const float *waveform, int32_t n);
+
+  // InputFinished() tells the class you won't be providing any
+  // more waveform.  This will help flush out the last frame or two
+  // of features, in the case where snip-edges == false; it also
+  // affects the return value of IsLastFrame().
+  void InputFinished();
+
+  int32_t NumFramesReady() const;
+
+  // Note: IsLastFrame() will only ever return true if you have called
+  // InputFinished() (and this frame is the last frame).
+  bool IsLastFrame(int32_t frame) const;
+
+  /** Get n frames starting from the given frame index.
+   *
+   * @param frame_index  The starting frame index
+   * @param n  Number of frames to get.
+   * @return Return a 2-D tensor of shape (n, feature_dim).
+   *         which is flattened into a 1-D vector (flattened in in row major)
+   */
+  std::vector<float> GetFrames(int32_t frame_index, int32_t n) const;
+
+  void Reset();
+  int32_t FeatureDim() const { return opts_.mel_opts.num_bins; }
+
+ private:
+  std::unique_ptr<knf::OnlineFbank> fbank_;
+  knf::FbankOptions opts_;
+  mutable std::mutex mutex_;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_FEATURES_H_