use core::cell::RefCell;
use core::ops::Deref;
use cortex_m::interrupt::Mutex;
use cortex_m::peripheral::syst::SystClkSource;
use cortex_m_rt::exception;
use stm32f4xx_hal::{
    rcc::Clocks,
    pac::SYST,
};

/// Rate in Hz
const TIMER_RATE: u32 = 1000;
/// 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(mut syst: SYST, clocks: Clocks) {    
    syst.set_clock_source(SystClkSource::Core);
    syst.set_reload(clocks.hclk().to_Hz() / TIMER_RATE - 1);
    syst.enable_counter();
    syst.enable_interrupt();
}

/// 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 `amount` milliseconds
pub fn sleep(amount: u32) {
    if amount == 0 {
        return;
    }
    let start = now();
    while now() - start <= amount - 1 {}
}