Implement proper clock management

For some reason, sleep causes some issues in release in the egl crate. Maybe
some timing issue ? In debug, the program is way slower than it should be

src/bus.rs

@@ -14,6 +14,7 @@ pub struct Bus<M: Mapper> {
 	ppu: Ppu<M>,
 	apu: Ram<0x1F>,//TODO
 	cartridge: Rc<RefCell<M>>,
+	tick_counter: u8,
 }
 
 impl<M: Mapper> fmt::Debug for Bus<M> {
@@ -119,11 +120,20 @@ impl<M: Mapper> Bus<M> {
 				ppu: Ppu::new(mapper.clone(), screen, pattern_table),
 				apu: Ram::<0x1F>::new(),
 				cartridge: mapper,
+				tick_counter: 0,
 			}
 		}
 
 	pub fn tick(&mut self) {
-		self.ppu.tick();
+
+		if self.tick_counter >= 3 {
+			self.tick_counter = 0;
+			self.ppu.tick();
+		} else {
+			self.tick_counter += 1;
+		}
+
+
 	}
 }
 

src/cpu.rs

@@ -151,6 +151,7 @@ pub struct Cpu<M: Mapper> {
     s: u8,
     p: StatusReg,
     bus: Bus<M>,
+	tick_counter: u8,
 }
 
 impl<M: Mapper> fmt::Debug for Cpu<M> {
@@ -189,16 +190,23 @@ impl<M: Mapper> Cpu<M> {
             s: 0xFD,
             p: StatusReg::from_bits(0x34).unwrap(),
             bus,
+			tick_counter: 0,
         }
     }
 
-    pub fn tick(&mut self) {
+	pub fn tick(&mut self) {
 
-       self.execute();
-       self.bus.tick();
-    }
+		if self.tick_counter >= 11 {
+			self.tick_counter = 0;
+			self.execute();
+		} else {
+			self.tick_counter += 1;
+		}
 
-    fn execute(&mut self) {
+		self.bus.tick();
+	}
+
+	fn execute(&mut self) {
 
         let opcode = self.read_pc_addr();
 //        if self.pc-1 > 0x36D0 && self.pc-1 < 0x36D8 {

src/main.rs

@@ -25,6 +25,12 @@ use canvas::{
 };
 use utils::DisplayBuffer;
 
+///Nanoseconds for 12 periods of the master clock (21.478 MHz). The frequency is divided by 12 since
+///this is the CPU clock. All faster peripherals (i.e. the PPU, will be run whithout timing
+///considerations whithing these 12 periods. This shouldn't affect the system's behavior while
+///improving the clock's precision
+const MASTER_CLOCK_PERIOD_NS: u128 = 559;
+
 struct EmulatorState {
 	pub screen: Rc<RefCell<DisplayBuffer>>,
 	pub screen_sprite: TextureSprite,
@@ -95,15 +101,31 @@ impl Application<EmulatorState> for NesEmulator {
 
 	fn tick(state: &mut EmulatorState, canvas: &mut Canvas) -> Result<(), &'static str> {
 
-		let frame_time = state.start.elapsed().as_secs_f32();
-		state.start = Instant::now();
-		canvas.clear();
-		state.cpu.tick();
+		//save frame start for frame time computation
+		let frame_start = state.start;
 
-		if state.screen.borrow_mut().is_ready {
-			state.screen.borrow_mut().is_ready = false;
+		//tick the system for 1 frame
+		while state.screen.borrow_mut().is_ready == false {
+
+			let tick_period = state.start.elapsed().as_nanos();
+			//this should never fail as id frame_period doesn't fit in a u64, then the substraction
+			//saturated and the resulting 0 now fits
+			let delta = MASTER_CLOCK_PERIOD_NS.saturating_sub(tick_period) as u64;
+			std::thread::sleep(std::time::Duration::from_nanos(delta));
+			state.start = Instant::now();
+
+			for _ in 0..12 {
+				//TODO the cpu is ticked 12 times since MASTER_CLOCK_PERIOD_NS corresponds to 12
+				//clock periods. Further division is done in the function
+				state.cpu.tick();
+			}
 		}
+		state.screen.borrow_mut().is_ready = false;
 
+		//draw frame
+		let frame_time = frame_start.elapsed().as_secs_f32();
+
+		canvas.clear();
 		canvas.draw(&mut state.screen_sprite);
 		canvas.draw(&mut state.pattern_sprite);