use core::cell::RefCell;
use core::ops::Deref;
use cortex_m::interrupt::Mutex;
use cortex_m_rt::exception;
use stm32f4xx_hal::{
    rcc::Clocks,
    time::U32Ext,
    timer::{Timer, Event as TimerEvent},
    stm32::SYST,
};

/// Rate in Hz
const TIMER_RATE: u32 = 500;
/// Interval duration in milliseconds
const TIMER_DELTA: u32 = 1000 / TIMER_RATE;
/// Elapsed time in milliseconds
static TIMER_MS: Mutex<RefCell<u32>> = Mutex::new(RefCell::new(0));

/// Setup SysTick exception
pub fn setup(syst: SYST, clocks: Clocks) {
    
    let timer = Timer::syst(syst, &clocks);
    let mut countdown = timer.start_count_down(TIMER_RATE.hz());
    countdown.listen(TimerEvent::TimeOut);
}

/// SysTick exception (Timer)
#[exception]
fn SysTick() {
    cortex_m::interrupt::free(|cs| {
        *TIMER_MS.borrow(cs)
            .borrow_mut() += TIMER_DELTA;
    });
}

/// Obtain current time in milliseconds
pub fn now() -> u32 {
    cortex_m::interrupt::free(|cs| {
        *TIMER_MS.borrow(cs)
            .borrow()
            .deref()
    })
}

/// block for at least `amount` milliseconds
pub fn sleep(amount: u32) {
    let start = now();
    while now() - start <= amount {}
}