Reduce audio buffer capacity and add active latency recovery

Author: maxolasersquad

src/apple2/SoundCore.cpp

@@ -26,6 +26,16 @@ struct sample_buffer {
     last_value = 0;
   }
 
+  auto skip(size_t len) -> void {
+    size_t filled = get_filled();
+    size_t num = (len < filled) ? len : filled;
+    if (num == 0) {
+      return;
+    }
+    size_t r = read_index.load(std::memory_order_relaxed);
+    read_index.store((r + num) % buffer.size(), std::memory_order_release);
+  }
+
   auto get_filled() const -> size_t {
     size_t r = read_index.load(std::memory_order_relaxed);
     size_t w = write_index.load(std::memory_order_relaxed);
@@ -137,7 +147,7 @@ static sample_buffer* g_mockMixBuffer = nullptr;
 }  // namespace
 
 void SoundCore_Initialize() {
-  constexpr size_t buffer_size = 65536;
+  constexpr size_t buffer_size = 16384;
   if (g_spkrMixBuffer == nullptr) {
     g_spkrMixBuffer = new sample_buffer(buffer_size);
   }
@@ -155,6 +165,15 @@ void SoundCore_Destroy() {
   g_mockMixBuffer = nullptr;
 }
 
+void SoundCore_ClearBuffers() {
+  if (g_spkrMixBuffer != nullptr) {
+    g_spkrMixBuffer->reinit();
+  }
+  if (g_mockMixBuffer != nullptr) {
+    g_mockMixBuffer->reinit();
+  }
+}
+
 void SoundCore_UploadSpeakerSamples(const int16_t* buffer,
                                     uint32_t num_samples) {
   if (g_spkrMixBuffer != nullptr) {
@@ -175,6 +194,22 @@ void SoundCore_GetSamples(int16_t* out, size_t num_samples) {
     return;
   }
 
+  // Active Latency Recovery: skip oldest samples if the buffer backlog is too
+  // large. We allow a cushion of 1024 elements (~23 ms) above the requested
+  // block size to avoid underruns due to thread scheduling jitter.
+  const size_t target_backlog =
+      std::min(num_samples + 1024, static_cast<size_t>(16384 - 2));
+
+  size_t spkr_filled = g_spkrMixBuffer->get_filled();
+  if (spkr_filled > target_backlog) {
+    g_spkrMixBuffer->skip(spkr_filled - target_backlog);
+  }
+
+  size_t mock_filled = g_mockMixBuffer->get_filled();
+  if (mock_filled > target_backlog) {
+    g_mockMixBuffer->skip(mock_filled - target_backlog);
+  }
+
   if (g_spkrMixBuffer->get_filled() == 0 &&
       g_mockMixBuffer->get_filled() == 0) {
     memset(out, 0, num_samples * sizeof(int16_t));

src/apple2/SoundCore.h

@@ -12,6 +12,7 @@ enum { FADE_OUT = 0, FADE_IN = 1 };
 
 void SoundCore_Initialize();
 void SoundCore_Destroy();
+void SoundCore_ClearBuffers();
 
 /**
  * @brief Uploads samples for the built-in Speaker channel.

src/core/LinAppleCore.cpp

@@ -101,6 +101,7 @@ static auto ShouldRunFullSpeed() -> bool {
   } else if (!shouldTurbo && s_wasTurbo) {
     uint32_t elapsed = Linapple_GetTicks() - s_turboStartMs;
     Logger::Perf("Full-speed disk mode disengaged after %ums\n", elapsed);
+    SoundCore_ClearBuffers();
   }
 
   s_wasTurbo = shouldTurbo;
@@ -243,7 +244,8 @@ auto Linapple_RunFrame(uint32_t cycles) -> uint32_t {
 }
 
 void Linapple_SetKeyState(uint8_t apple_code, bool down) {
-  KeyboardEvent_t ev = {apple_code, (uint8_t)(down ? 1 : 0), 0, 0, 0, 0, {0, 0, 0}};
+  KeyboardEvent_t ev = {apple_code, (uint8_t)(down ? 1 : 0), 0, 0, 0, 0,
+                        {0, 0, 0}};
   Peripheral_Command(0, keyboard_cmd_event, &ev, sizeof(ev));
 }