Add timestamps for streaming ASR. (#123)

@@ -126,6 +126,8 @@ if(SHERPA_ONNX_ENABLE_WEBSOCKET)
   include(asio)
 endif()
 
+include(json)
+
 add_subdirectory(sherpa-onnx)
 
 if(SHERPA_ONNX_ENABLE_C_API)

+function(download_json)
+  include(FetchContent)
+
+  set(json_URL  "https://github.com/nlohmann/json/archive/refs/tags/v3.11.2.tar.gz")
+  set(json_URL2 "https://huggingface.co/csukuangfj/sherpa-cmake-deps/resolve/main/json-3.11.2.tar.gz")
+  set(json_HASH "SHA256=d69f9deb6a75e2580465c6c4c5111b89c4dc2fa94e3a85fcd2ffcd9a143d9273")
+
+  # If you don't have access to the Internet,
+  # please pre-download json
+  set(possible_file_locations
+    $ENV{HOME}/Downloads/json-3.11.2.tar.gz
+    ${PROJECT_SOURCE_DIR}/json-3.11.2.tar.gz
+    ${PROJECT_BINARY_DIR}/json-3.11.2.tar.gz
+    /tmp/json-3.11.2.tar.gz
+    /star-fj/fangjun/download/github/json-3.11.2.tar.gz
+  )
+
+  foreach(f IN LISTS possible_file_locations)
+    if(EXISTS ${f})
+      set(json_URL  "${f}")
+      file(TO_CMAKE_PATH "${json_URL}" json_URL)
+      set(json_URL2)
+      break()
+    endif()
+  endforeach()
+
+  FetchContent_Declare(json
+    URL
+      ${json_URL}
+      ${json_URL2}
+    URL_HASH          ${json_HASH}
+  )
+
+  FetchContent_GetProperties(json)
+  if(NOT json_POPULATED)
+    message(STATUS "Downloading json from ${json_URL}")
+    FetchContent_Populate(json)
+  endif()
+  message(STATUS "json is downloaded to ${json_SOURCE_DIR}")
+  include_directories(${json_SOURCE_DIR}/include)
+  # Use #include "nlohmann/json.hpp"
+endfunction()
+
+download_json()

@@ -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));
+  }
+
+  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);
   }
 
-  OnlineRecognizerResult ans;
-  ans.text = std::move(text);
-  return ans;
+  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 {

@@ -7,6 +7,7 @@
 
 #include <memory>
 #include <string>
+#include <vector>
 
 #if __ANDROID_API__ >= 9
 #include "android/asset_manager.h"
@@ -22,10 +23,45 @@
 namespace sherpa_onnx {
 
 struct OnlineRecognizerResult {
+  /// Recognition results.
+  /// For English, it consists of space separated words.
+  /// For Chinese, it consists of Chinese words without spaces.
+  /// Example 1: "hello world"
+  /// Example 2: "你好世界"
   std::string text;
 
-  // TODO(fangjun): Add a method to return a json string
-  std::string ToString() const { return text; }
+  /// Decoded results at the token level.
+  /// For instance, for BPE-based models it consists of a list of BPE tokens.
+  std::vector<std::string> tokens;
+
+  /// timestamps.size() == tokens.size()
+  /// timestamps[i] records the time in seconds when tokens[i] is decoded.
+  std::vector<float> timestamps;
+
+  /// ID of this segment
+  /// When an endpoint is detected, it is incremented
+  int32_t segment = 0;
+
+  /// Starting frame of this segment.
+  /// When an endpoint is detected, it will change
+  float start_time = 0;
+
+  /// True if this is the last segment.
+  bool is_final = false;
+
+  /** Return a json string.
+   *
+   * The returned string contains:
+   *   {
+   *     "text": "The recognition result",
+   *     "tokens": [x, x, x],
+   *     "timestamps": [x, x, x],
+   *     "segment": x,
+   *     "start_time": x,
+   *     "is_final": true|false
+   *   }
+   */
+  std::string AsJsonString() const;
 };
 
 struct OnlineRecognizerConfig {

@@ -34,6 +34,9 @@ OnlineTransducerDecoderResult &OnlineTransducerDecoderResult::operator=(
 
   hyps = other.hyps;
 
+  frame_offset = other.frame_offset;
+  timestamps = other.timestamps;
+
   return *this;
 }
 
@@ -54,6 +57,9 @@ OnlineTransducerDecoderResult &OnlineTransducerDecoderResult::operator=(
   decoder_out = std::move(other.decoder_out);
   hyps = std::move(other.hyps);
 
+  frame_offset = other.frame_offset;
+  timestamps = std::move(other.timestamps);
+
   return *this;
 }
 

@@ -13,12 +13,18 @@
 namespace sherpa_onnx {
 
 struct OnlineTransducerDecoderResult {
+  /// 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;
+
   // Cache decoder_out for endpointing
   Ort::Value decoder_out;
 

@@ -102,16 +102,18 @@ void OnlineTransducerGreedySearchDecoder::Decode(
 
     bool emitted = false;
     for (int32_t i = 0; i < batch_size; ++i, p_logit += vocab_size) {
+      auto &r = (*result)[i];
       auto y = static_cast<int32_t>(std::distance(
           static_cast<const float *>(p_logit),
           std::max_element(static_cast<const float *>(p_logit),
                            static_cast<const float *>(p_logit) + vocab_size)));
       if (y != 0) {
         emitted = true;
-        (*result)[i].tokens.push_back(y);
-        (*result)[i].num_trailing_blanks = 0;
+        r.tokens.push_back(y);
+        r.timestamps.push_back(t + r.frame_offset);
+        r.num_trailing_blanks = 0;
       } else {
-        ++(*result)[i].num_trailing_blanks;
+        ++r.num_trailing_blanks;
       }
     }
     if (emitted) {
@@ -121,6 +123,11 @@ void OnlineTransducerGreedySearchDecoder::Decode(
   }
 
   UpdateCachedDecoderOut(model_->Allocator(), &decoder_out, result);
+
+  // Update frame_offset
+  for (auto &r : *result) {
+    r.frame_offset += num_frames;
+  }
 }
 
 }  // namespace sherpa_onnx

@@ -87,6 +87,7 @@ void OnlineTransducerModifiedBeamSearchDecoder::StripLeadingBlanks(
 
   std::vector<int64_t> tokens(hyp.ys.begin() + context_size, hyp.ys.end());
   r->tokens = std::move(tokens);
+  r->timestamps = std::move(hyp.timestamps);
   r->num_trailing_blanks = hyp.num_trailing_blanks;
 }
 
@@ -148,6 +149,7 @@ void OnlineTransducerModifiedBeamSearchDecoder::Decode(
     float *p_logit = logit.GetTensorMutableData<float>();
 
     for (int32_t b = 0; b < batch_size; ++b) {
+      int32_t frame_offset = (*result)[b].frame_offset;
       int32_t start = hyps_num_split[b];
       int32_t end = hyps_num_split[b + 1];
       LogSoftmax(p_logit, vocab_size, (end - start));
@@ -162,6 +164,7 @@ void OnlineTransducerModifiedBeamSearchDecoder::Decode(
         Hypothesis new_hyp = prev[hyp_index];
         if (new_token != 0) {
           new_hyp.ys.push_back(new_token);
+          new_hyp.timestamps.push_back(t + frame_offset);
           new_hyp.num_trailing_blanks = 0;
         } else {
           ++new_hyp.num_trailing_blanks;
@@ -177,10 +180,12 @@ void OnlineTransducerModifiedBeamSearchDecoder::Decode(
   for (int32_t b = 0; b != batch_size; ++b) {
     auto &hyps = cur[b];
     auto best_hyp = hyps.GetMostProbable(true);
+    auto &r = (*result)[b];
 
-    (*result)[b].hyps = std::move(hyps);
-    (*result)[b].tokens = std::move(best_hyp.ys);
-    (*result)[b].num_trailing_blanks = best_hyp.num_trailing_blanks;
+    r.hyps = std::move(hyps);
+    r.tokens = std::move(best_hyp.ys);
+    r.num_trailing_blanks = best_hyp.num_trailing_blanks;
+    r.frame_offset += num_frames;
   }
 }
 

@@ -196,7 +196,7 @@ void OnlineWebsocketDecoder::Decode() {
     auto result = recognizer_->GetResult(c->s.get());
 
     asio::post(server_->GetConnectionContext(),
-               [this, hdl = c->hdl, str = result.ToString()]() {
+               [this, hdl = c->hdl, str = result.AsJsonString()]() {
                  server_->Send(hdl, str);
                });
     active_.erase(c->hdl);

@@ -102,7 +102,7 @@ for a list of pre-trained models to download.
     recognizer.DecodeStream(s.get());
   }
 
-  std::string text = recognizer.GetResult(s.get()).text;
+  std::string text = recognizer.GetResult(s.get()).AsJsonString();
 
   fprintf(stderr, "Done!\n");
 

@@ -434,7 +434,7 @@ JNIEXPORT jstring JNICALL Java_com_k2fsa_sherpa_onnx_OnlineRecognizer_getResult(
   sherpa_onnx::OnlineStream *s =
       reinterpret_cast<sherpa_onnx::OnlineStream *>(s_ptr);
   sherpa_onnx::OnlineRecognizerResult result = model->GetResult(s);
-  return env->NewStringUTF(result.ToString().c_str());
+  return env->NewStringUTF(result.text.c_str());
 }
 
 SHERPA_ONNX_EXTERN_C