Add transducer modified_beam_search for RKNN. (#1949)

sherpa-onnx/csrc/rknn/online-transducer-decoder-rknn.h

@@ -0,0 +1,63 @@
+// sherpa-onnx/csrc/rknn/online-transducer-decoder-rknn.h
+//
+// Copyright (c)  2025  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_RKNN_ONLINE_TRANSDUCER_DECODER_RKNN_H_
+#define SHERPA_ONNX_CSRC_RKNN_ONLINE_TRANSDUCER_DECODER_RKNN_H_
+
+#include <vector>
+
+#include "sherpa-onnx/csrc/hypothesis.h"
+#include "sherpa-onnx/csrc/macros.h"
+
+namespace sherpa_onnx {
+
+struct OnlineTransducerDecoderResultRknn {
+  /// Number of frames after subsampling we have decoded so far
+  int32_t frame_offset = 0;
+
+  /// The decoded token IDs so far
+  std::vector<int64_t> tokens;
+
+  /// number of trailing blank frames decoded so far
+  int32_t num_trailing_blanks = 0;
+
+  /// timestamps[i] contains the output frame index where tokens[i] is decoded.
+  std::vector<int32_t> timestamps;
+
+  // used only by greedy_search
+  std::vector<float> previous_decoder_out;
+
+  // used only in modified beam_search
+  Hypotheses hyps;
+
+  // used only by modified_beam_search
+  std::vector<std::vector<float>> previous_decoder_out2;
+};
+
+class OnlineTransducerDecoderRknn {
+ public:
+  virtual ~OnlineTransducerDecoderRknn() = default;
+
+  /* Return an empty result.
+   *
+   * To simplify the decoding code, we add `context_size` blanks
+   * to the beginning of the decoding result, which will be
+   * stripped by calling `StripPrecedingBlanks()`.
+   */
+  virtual OnlineTransducerDecoderResultRknn GetEmptyResult() const = 0;
+
+  /** Strip blanks added by `GetEmptyResult()`.
+   *
+   * @param r It is changed in-place.
+   */
+  virtual void StripLeadingBlanks(
+      OnlineTransducerDecoderResultRknn * /*r*/) const {}
+
+  virtual void Decode(std::vector<float> encoder_out,
+                      OnlineTransducerDecoderResultRknn *result) const = 0;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_RKNN_ONLINE_TRANSDUCER_DECODER_RKNN_H_