Extract TimeStep

src/main.rs

@@ -596,6 +596,54 @@ impl WorldState {
 	}
 }
 
+struct TimeStep {
+	last_frame_time: Instant,
+	// Milliseconds
+	accum: u128,
+	fps_num: u16,
+	fps_den: u16,
+}
+
+impl TimeStep {
+	pub fn new (fps_num: u16, fps_den: u16) -> Self {
+		Self {
+			last_frame_time: Instant::now (),
+			accum: 0,
+			fps_num,
+			fps_den,
+		}
+	}
+	
+	// Automatically gets monotonic system time from Instant
+	// If you need something fancy just rewrite this.
+	// Returns: How many logics steps to run. Typically 0 or 1.
+	
+	pub fn step (&mut self) -> u16 {
+		let frame_time = Instant::now ();
+		
+		let fps_num_128: u128 = self.fps_num.into ();
+		let fps_den_128: u128 = self.fps_den.into ();
+		
+		self.accum += (frame_time - self.last_frame_time).as_millis () * fps_num_128;
+		
+		let mut result = 0;
+		const MAX_FRAMES: u16 = 4;
+		
+		for _ in 0..MAX_FRAMES {
+			if self.accum > fps_den_128 {
+				result += 1;
+				self.accum -= fps_den_128;
+			}
+			else {
+				break;
+			}
+		}
+		
+		self.last_frame_time = frame_time;
+		result
+	}
+}
+
 fn main () {
 	let sdl_context = sdl2::init ().unwrap ();
 	let video_subsystem = sdl_context.video ().unwrap ();
@@ -698,39 +746,18 @@ fn main () {
 		let stride = 4 * num_coords;
 	}
 	
-	let mut last_frame_time = Instant::now ();
-	let mut frame = 0;
-	let mut timestep_accum = Duration::from_millis (0);
-	
+	let mut time_step = TimeStep::new (60, 1000);
 	let mut state = WorldState::new ();
 	
 	let mut event_pump = sdl_context.event_pump ().unwrap ();
 	'running: loop {
-		let frame_time = Instant::now ();
-		
-		// Take 60 steps every 1,000 milliseconds.
-		// I know this is a float, but since we're multiplying it will
-		// be deterministic, since it's a whole number.
-		let fps_num = 60.0;
-		let fps_den = 1000;
-		
-		timestep_accum += (frame_time - last_frame_time).mul_f32 (fps_num);
+		let frames_to_do = time_step.step ();
 		
 		let controller = ControllerState::from_sdl_keyboard (&event_pump.keyboard_state ());
 		
-		for _ in 0..4 {
-			if timestep_accum > Duration::from_millis (fps_den) {
-				frame += 1;
-				timestep_accum -= Duration::from_millis (fps_den);
-				
+		for _ in 0..frames_to_do {
 			state.step (&controller);
 		}
-			else {
-				break;
-			}
-		}
-		
-		last_frame_time = frame_time;
 		
 		let _mouse = event_pump.mouse_state ();