feat: Add --seek command line option for fast-forward debugging

This feature allows developers to jump to a specific time in the demo sequence (e.g., './demo64k --seek 10.5'). It simulates the game logic, audio state (rendering silent buffers), and visual physics (compute shaders) from t=0 up to the target time before starting real-time playback. Audio initialization is refactored to separate device init and start.

1 files changed, 38 insertions, 0 deletions

diff --git a/src/gpu/effect.cc b/src/gpu/effect.cc
index cfef7f9..040b523 100644
--- a/src/gpu/effect.cc
+++ b/src/gpu/effect.cc
@@ -195,6 +195,44 @@ void MainSequence::render_frame(float global_time, float beat, float peak,
   wgpuTextureRelease(surface_texture.texture);
 }
 
+void MainSequence::simulate_until(float target_time, float step_rate) {
+#ifndef STRIP_ALL
+  // Assuming 128 BPM as per main.cc.
+  // Ideally this should be passed in or shared.
+  const float bpm = 128.0f;
+  const float aspect_ratio = 16.0f / 9.0f; // Dummy aspect
+
+  for (float t = 0.0f; t < target_time; t += step_rate) {
+    WGPUCommandEncoderDescriptor encoder_desc = {};
+    WGPUCommandEncoder encoder = wgpuDeviceCreateCommandEncoder(device, &encoder_desc);
+
+    float beat = fmodf(t * bpm / 60.0f, 1.0f);
+
+    // Update active lists
+    for (auto &entry : sequences_) {
+      if (t >= entry.start_time) {
+        entry.seq->update_active_list(t - entry.start_time);
+      }
+    }
+
+    // Dispatch compute
+    for (auto &entry : sequences_) {
+      if (t >= entry.start_time) {
+        // peak = 0.0f during simulation (no audio analysis)
+        entry.seq->dispatch_compute(encoder, t - entry.start_time, beat, 0.0f, aspect_ratio);
+      }
+    }
+
+    WGPUCommandBufferDescriptor cmd_desc = {};
+    WGPUCommandBuffer commands = wgpuCommandEncoderFinish(encoder, &cmd_desc);
+    wgpuQueueSubmit(queue, 1, &commands);
+  }
+#else
+  (void)target_time;
+  (void)step_rate;
+#endif
+}
+
 void MainSequence::shutdown() {
     for (auto &entry : sequences_) {
         entry.seq->reset();