fix audio desync for realsies, stderr to log.txt

src/resource/audio_stream.cpp

@@ -1,5 +1,7 @@
 #include "audio_stream.hpp"
 
+#include <algorithm>
+
 #if defined(__clang__) || defined(__GNUC__)
   #pragma GCC diagnostic push
   #pragma GCC diagnostic ignored "-Wunused-parameter"
@@ -10,16 +12,46 @@ namespace anm2ed
   void AudioStream::callback(void* userData, MIX_Mixer* mixer, const SDL_AudioSpec* spec, float* pcm, int samples)
   {
     auto self = (AudioStream*)userData;
+    if (!self->isFirstCallbackCaptured)
+    {
+      self->firstCallbackCounter = SDL_GetPerformanceCounter();
+      self->isFirstCallbackCaptured = true;
+    }
+    self->callbackSamples = samples;
     self->stream.insert(self->stream.end(), pcm, pcm + samples);
   }
 
   AudioStream::AudioStream(MIX_Mixer* mixer) { MIX_GetMixerFormat(mixer, &spec); }
 
-  void AudioStream::capture_begin(MIX_Mixer* mixer) { MIX_SetPostMixCallback(mixer, callback, this); }
+  void AudioStream::capture_begin(MIX_Mixer* mixer)
+  {
+    stream.clear();
+    callbackSamples = 0;
+    captureStartCounter = SDL_GetPerformanceCounter();
+    firstCallbackCounter = 0;
+    isFirstCallbackCaptured = false;
+    MIX_SetPostMixCallback(mixer, callback, this);
+  }
 
   void AudioStream::capture_end(MIX_Mixer* mixer)
   {
     MIX_SetPostMixCallback(mixer, nullptr, this);
     stream.clear();
   }
-}
+
+  double AudioStream::callback_latency_seconds_get() const
+  {
+    auto freq = std::max(spec.freq, 1);
+    auto channels = std::max(spec.channels, 1);
+    auto framesPerCallback = (double)callbackSamples / (double)channels;
+    return framesPerCallback / (double)freq;
+  }
+
+  double AudioStream::capture_start_delay_seconds_get() const
+  {
+    if (!isFirstCallbackCaptured || captureStartCounter == 0 || firstCallbackCounter < captureStartCounter) return 0.0;
+    auto frequency = SDL_GetPerformanceFrequency();
+    if (frequency == 0) return 0.0;
+    return (double)(firstCallbackCounter - captureStartCounter) / (double)frequency;
+  }
+}