rework frame limiter

src/device.rs

@@ -262,6 +262,7 @@ impl Device {
                 clock: 0,
                 field: 0,
                 delay: 0,
+                holdover: std::time::Duration::from_secs(0),
                 count_per_scanline: 0,
                 last_vi_time: std::time::Instant::now(),
                 vi_period: std::time::Duration::from_secs_f64(0.0),

src/device/vi.rs

@@ -22,6 +22,7 @@ pub struct Vi {
     pub clock: u64,
     pub delay: u64,
     pub field: u32,
+    pub holdover: std::time::Duration,
     pub count_per_scanline: u64,
     pub last_vi_time: std::time::Instant,
     pub vi_period: std::time::Duration,
@@ -123,9 +124,22 @@ pub fn vertical_interrupt_event(device: &mut device::Device) {
     */
 
     let elapsed = device.vi.last_vi_time.elapsed();
-    if elapsed < device.vi.vi_period {
-        let diff = device.vi.vi_period - elapsed;
-        std::thread::sleep(diff);
+    if elapsed <= device.vi.vi_period {
+        let sleep_time = device.vi.vi_period - elapsed;
+        let remaining_holdover_space = device.vi.vi_period - device.vi.holdover; // holdover can't exceed the vi period
+        if sleep_time <= remaining_holdover_space {
+            device.vi.holdover += sleep_time; // donate all time to the holdover
+        } else {
+            device.vi.holdover += remaining_holdover_space; // max out holdover
+            std::thread::sleep(sleep_time - remaining_holdover_space); // sleep the rest of the time
+        }
+    } else {
+        let over_time = elapsed - device.vi.vi_period; // this is how much we overshot the vi period
+        if over_time <= device.vi.holdover {
+            device.vi.holdover -= over_time; // consume some holdover
+        } else {
+            device.vi.holdover -= device.vi.holdover; // consume all holdover
+        }
     }
     device.vi.last_vi_time = std::time::Instant::now();