Add timestamps for streaming ASR. (#123)

sherpa-onnx/csrc/online-recognizer.cc

@@ -8,11 +8,13 @@
 #include <assert.h>
 
 #include <algorithm>
+#include <iomanip>
 #include <memory>
 #include <sstream>
 #include <utility>
 #include <vector>
 
+#include "nlohmann/json.hpp"
 #include "sherpa-onnx/csrc/file-utils.h"
 #include "sherpa-onnx/csrc/online-transducer-decoder.h"
 #include "sherpa-onnx/csrc/online-transducer-greedy-search-decoder.h"
@@ -22,16 +24,56 @@
 
 namespace sherpa_onnx {
 
+std::string OnlineRecognizerResult::AsJsonString() const {
+  using json = nlohmann::json;
+  json j;
+  j["text"] = text;
+  j["tokens"] = tokens;
+  j["start_time"] = start_time;
+#if 1
+  // This branch chooses number of decimal points to keep in
+  // the return json string
+  std::ostringstream os;
+  os << "[";
+  std::string sep = "";
+  for (auto t : timestamps) {
+    os << sep << std::fixed << std::setprecision(2) << t;
+    sep = ", ";
+  }
+  os << "]";
+  j["timestamps"] = os.str();
+#else
+  j["timestamps"] = timestamps;
+#endif
+
+  j["segment"] = segment;
+  j["is_final"] = is_final;
+
+  return j.dump();
+}
+
 static OnlineRecognizerResult Convert(const OnlineTransducerDecoderResult &src,
-                                      const SymbolTable &sym_table) {
-  std::string text;
-  for (auto t : src.tokens) {
-    text += sym_table[t];
+                                      const SymbolTable &sym_table,
+                                      int32_t frame_shift_ms,
+                                      int32_t subsampling_factor) {
+  OnlineRecognizerResult r;
+  r.tokens.reserve(src.tokens.size());
+  r.timestamps.reserve(src.tokens.size());
+
+  for (auto i : src.tokens) {
+    auto sym = sym_table[i];
+
+    r.text.append(sym);
+    r.tokens.push_back(std::move(sym));
   }
 
-  OnlineRecognizerResult ans;
-  ans.text = std::move(text);
-  return ans;
+  float frame_shift_s = frame_shift_ms / 1000. * subsampling_factor;
+  for (auto t : src.timestamps) {
+    float time = frame_shift_s * t;
+    r.timestamps.push_back(time);
+  }
+
+  return r;
 }
 
 void OnlineRecognizerConfig::Register(ParseOptions *po) {
@@ -169,7 +211,10 @@ class OnlineRecognizer::Impl {
     OnlineTransducerDecoderResult decoder_result = s->GetResult();
     decoder_->StripLeadingBlanks(&decoder_result);
 
-    return Convert(decoder_result, sym_);
+    // TODO(fangjun): Remember to change these constants if needed
+    int32_t frame_shift_ms = 10;
+    int32_t subsampling_factor = 4;
+    return Convert(decoder_result, sym_, frame_shift_ms, subsampling_factor);
   }
 
   bool IsEndpoint(OnlineStream *s) const {