fix(audio): Render audio in small chunks to fix timing gaps

Fixed issue where drum patterns had silence gaps between cycles. The problem
was that audio_render_ahead was rendering audio in large chunks (up to 200ms),
causing the synth internal time to become desynchronized from tracker events.

Problem:
- audio_render_ahead checked buffer fullness, then rendered large chunk
- First call: buffer empty, render 200ms, synth advances by 200ms
- Next 12 calls: buffer > 100ms, do not render, synth state frozen
- Call 13: buffer < 100ms, render more, but tracker triggered events in between
- Events triggered between render calls ended up at wrong synth time position
- Result: Silence gaps between patterns

Solution:
- Changed audio_render_ahead to render in small incremental chunks
- Chunk size: one frame worth of audio (~16.6ms @ 60fps)
- Loop until buffer reaches target lookahead (200ms)
- Synth now advances gradually, staying synchronized with tracker

Result: Synth time stays synchronized with tracker event timing, no gaps.

Testing: All 17 tests pass (100%)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 28 insertions, 21 deletions

diff --git a/src/audio/audio.cc b/src/audio/audio.cc
index 7c0f490..779cb6c 100644
--- a/src/audio/audio.cc
+++ b/src/audio/audio.cc
@@ -73,39 +73,46 @@ void audio_start() {
 
 void audio_render_ahead(float music_time, float dt) {
   // Calculate how much audio is currently buffered
-  const int buffered_samples = g_ring_buffer.available_read();
-  const float buffered_time =
+  int buffered_samples = g_ring_buffer.available_read();
+  float buffered_time =
       (float)buffered_samples / (RING_BUFFER_SAMPLE_RATE * RING_BUFFER_CHANNELS);
 
   // Target: maintain look-ahead buffer
   const float target_lookahead =
       (float)RING_BUFFER_LOOKAHEAD_MS / 1000.0f;
 
-  // Only render if we're below target
-  if (buffered_time < target_lookahead * 0.5f) {  // Refill when half empty
-    // Calculate how many frames to render
-    const float time_to_render = target_lookahead - buffered_time;
-    const int frames_to_render =
-        (int)(time_to_render * RING_BUFFER_SAMPLE_RATE);
+  // Render in small chunks to keep synth time synchronized with tracker
+  // Chunk size: one frame's worth of audio (~16.6ms @ 60fps)
+  const int chunk_frames = (int)(dt * RING_BUFFER_SAMPLE_RATE);
+  const int chunk_samples = chunk_frames * RING_BUFFER_CHANNELS;
 
-    if (frames_to_render > 0) {
-      // Allocate temporary buffer (stereo)
-      const int samples_to_render = frames_to_render * RING_BUFFER_CHANNELS;
-      float* temp_buffer = new float[samples_to_render];
+  // Keep rendering small chunks until buffer is full enough
+  while (buffered_time < target_lookahead) {
+    const int frames_to_render = chunk_frames;
+    if (frames_to_render <= 0) break;
 
-      // Render audio from synth
-      synth_render(temp_buffer, frames_to_render);
+    // Allocate temporary buffer (stereo)
+    const int samples_to_render = frames_to_render * RING_BUFFER_CHANNELS;
+    float* temp_buffer = new float[samples_to_render];
 
-      // Write to ring buffer
-      const int written = g_ring_buffer.write(temp_buffer, samples_to_render);
+    // Render audio from synth (advances synth state incrementally)
+    synth_render(temp_buffer, frames_to_render);
 
-      // Notify backend of frames rendered (for testing/tracking)
-      if (g_audio_backend != nullptr) {
-        g_audio_backend->on_frames_rendered(written / RING_BUFFER_CHANNELS);
-      }
+    // Write to ring buffer
+    const int written = g_ring_buffer.write(temp_buffer, samples_to_render);
 
-      delete[] temp_buffer;
+    // Notify backend of frames rendered (for testing/tracking)
+    if (g_audio_backend != nullptr) {
+      g_audio_backend->on_frames_rendered(written / RING_BUFFER_CHANNELS);
     }
+
+    delete[] temp_buffer;
+
+    // Update buffered time for next iteration
+    buffered_time += (float)written / (RING_BUFFER_SAMPLE_RATE * RING_BUFFER_CHANNELS);
+
+    // Safety: avoid infinite loop if buffer is full
+    if (written < samples_to_render) break;
   }
 }