Add CTC HLG decoding using OpenFst (#349)

sherpa-onnx/csrc/offline-ctc-fst-decoder.h

@@ -0,0 +1,33 @@
+// sherpa-onnx/csrc/offline-ctc-fst-decoder.h
+//
+// Copyright (c)  2023  Xiaomi Corporation
+
+#ifndef SHERPA_ONNX_CSRC_OFFLINE_CTC_FST_DECODER_H_
+#define SHERPA_ONNX_CSRC_OFFLINE_CTC_FST_DECODER_H_
+
+#include <memory>
+#include <vector>
+
+#include "fst/fst.h"
+#include "sherpa-onnx/csrc/offline-ctc-decoder.h"
+#include "sherpa-onnx/csrc/offline-ctc-fst-decoder-config.h"
+#include "sherpa-onnx/csrc/parse-options.h"
+
+namespace sherpa_onnx {
+
+class OfflineCtcFstDecoder : public OfflineCtcDecoder {
+ public:
+  explicit OfflineCtcFstDecoder(const OfflineCtcFstDecoderConfig &config);
+
+  std::vector<OfflineCtcDecoderResult> Decode(
+      Ort::Value log_probs, Ort::Value log_probs_length) override;
+
+ private:
+  OfflineCtcFstDecoderConfig config_;
+
+  std::unique_ptr<fst::Fst<fst::StdArc>> fst_;
+};
+
+}  // namespace sherpa_onnx
+
+#endif  // SHERPA_ONNX_CSRC_OFFLINE_CTC_FST_DECODER_H_