/// The STM32 line of microcontrollers support various deep-sleep modes which exploit clock-gating
/// to reduce power consumption. `embassy-stm32` provides a low-power executor, [`Executor`] which
/// can use knowledge of which peripherals are currently blocked upon to transparently and safely
/// enter such low-power modes (currently, only `STOP2`) when idle.
///
/// The executor determines which peripherals are active by their RCC state; consequently,
/// low-power states can only be entered if all peripherals have been `drop`'d. There are a few
/// exceptions to this rule:
///
///  * `GPIO`
///  * `RCC`
///
/// Since entering and leaving low-power modes typically incurs a significant latency, the
/// low-power executor will only attempt to enter when the next timer event is at least
/// [`time_driver::MIN_STOP_PAUSE`] in the future.
///
/// Currently there is no macro analogous to `embassy_executor::main` for this executor;
/// consequently one must define their entrypoint manually. Moveover, you must relinquish control
/// of the `RTC` peripheral to the executor. This will typically look like
///
/// ```rust,no_run
/// use embassy_executor::Spawner;
/// use embassy_stm32::low_power::Executor;
/// use embassy_stm32::rtc::{Rtc, RtcConfig};
/// use static_cell::make_static;
///
/// #[cortex_m_rt::entry]
/// fn main() -> ! {
///     Executor::take().run(|spawner| {
///         unwrap!(spawner.spawn(async_main(spawner)));
///     });
/// }
///
/// #[embassy_executor::task]
/// async fn async_main(spawner: Spawner) {
///     // initialize the platform...
///     let mut config = embassy_stm32::Config::default();
///     let p = embassy_stm32::init(config);
///
///     // give the RTC to the executor...
///     let mut rtc = Rtc::new(p.RTC, RtcConfig::default());
///     let rtc = make_static!(rtc);
///     embassy_stm32::low_power::stop_with_rtc(rtc);
///
///     // your application here...
/// }
/// ```
use core::arch::asm;
use core::marker::PhantomData;
use core::sync::atomic::{compiler_fence, Ordering};

use cortex_m::peripheral::SCB;
use embassy_executor::*;

use crate::interrupt;
use crate::time_driver::{get_driver, RtcDriver};

const THREAD_PENDER: usize = usize::MAX;

use crate::rtc::Rtc;

static mut EXECUTOR: Option<Executor> = None;

foreach_interrupt! {
    (RTC, rtc, $block:ident, WKUP, $irq:ident) => {
        #[interrupt]
        unsafe fn $irq() {
            EXECUTOR.as_mut().unwrap().on_wakeup_irq();
        }
    };
}

#[allow(dead_code)]
pub(crate) unsafe fn on_wakeup_irq() {
    EXECUTOR.as_mut().unwrap().on_wakeup_irq();
}

pub fn stop_with_rtc(rtc: &'static Rtc) {
    unsafe { EXECUTOR.as_mut().unwrap() }.stop_with_rtc(rtc)
}

pub fn stop_ready(stop_mode: StopMode) -> bool {
    match unsafe { EXECUTOR.as_mut().unwrap() }.stop_mode() {
        Some(StopMode::Stop2) => true,
        Some(StopMode::Stop1) => stop_mode == StopMode::Stop1,
        None => false,
    }
}

#[non_exhaustive]
#[derive(PartialEq)]
pub enum StopMode {
    Stop1,
    Stop2,
}

/// Thread mode executor, using WFE/SEV.
///
/// This is the simplest and most common kind of executor. It runs on
/// thread mode (at the lowest priority level), and uses the `WFE` ARM instruction
/// to sleep when it has no more work to do. When a task is woken, a `SEV` instruction
/// is executed, to make the `WFE` exit from sleep and poll the task.
///
/// This executor allows for ultra low power consumption for chips where `WFE`
/// triggers low-power sleep without extra steps. If your chip requires extra steps,
/// you may use [`raw::Executor`] directly to program custom behavior.
pub struct Executor {
    inner: raw::Executor,
    not_send: PhantomData<*mut ()>,
    scb: SCB,
    time_driver: &'static RtcDriver,
}

impl Executor {
    /// Create a new Executor.
    pub fn take() -> &'static mut Self {
        critical_section::with(|_| unsafe {
            assert!(EXECUTOR.is_none());

            EXECUTOR = Some(Self {
                inner: raw::Executor::new(THREAD_PENDER as *mut ()),
                not_send: PhantomData,
                scb: cortex_m::Peripherals::steal().SCB,
                time_driver: get_driver(),
            });

            EXECUTOR.as_mut().unwrap()
        })
    }

    unsafe fn on_wakeup_irq(&mut self) {
        self.time_driver.resume_time();
        trace!("low power: resume");
    }

    pub(self) fn stop_with_rtc(&mut self, rtc: &'static Rtc) {
        self.time_driver.set_rtc(rtc);

        rtc.enable_wakeup_line();

        trace!("low power: stop with rtc configured");
    }

    fn stop_mode(&self) -> Option<StopMode> {
        if unsafe { crate::rcc::REFCOUNT_STOP2 == 0 } && unsafe { crate::rcc::REFCOUNT_STOP1 == 0 } {
            Some(StopMode::Stop2)
        } else if unsafe { crate::rcc::REFCOUNT_STOP1 == 0 } {
            Some(StopMode::Stop1)
        } else {
            None
        }
    }

    fn configure_stop(&mut self, _stop_mode: StopMode) {
        // TODO: configure chip-specific settings for stop
    }

    fn configure_pwr(&mut self) {
        self.scb.clear_sleepdeep();

        compiler_fence(Ordering::SeqCst);

        let stop_mode = self.stop_mode();
        if stop_mode.is_none() {
            trace!("low power: not ready to stop");
        } else if self.time_driver.pause_time().is_err() {
            trace!("low power: failed to pause time");
        } else {
            let stop_mode = stop_mode.unwrap();
            match stop_mode {
                StopMode::Stop1 => trace!("low power: stop 1"),
                StopMode::Stop2 => trace!("low power: stop 2"),
            }
            self.configure_stop(stop_mode);

            #[cfg(not(feature = "low-power-debug-with-sleep"))]
            self.scb.set_sleepdeep();
        }
    }

    /// Run the executor.
    ///
    /// The `init` closure is called with a [`Spawner`] that spawns tasks on
    /// this executor. Use it to spawn the initial task(s). After `init` returns,
    /// the executor starts running the tasks.
    ///
    /// To spawn more tasks later, you may keep copies of the [`Spawner`] (it is `Copy`),
    /// for example by passing it as an argument to the initial tasks.
    ///
    /// This function requires `&'static mut self`. This means you have to store the
    /// Executor instance in a place where it'll live forever and grants you mutable
    /// access. There's a few ways to do this:
    ///
    /// - a [StaticCell](https://docs.rs/static_cell/latest/static_cell/) (safe)
    /// - a `static mut` (unsafe)
    /// - a local variable in a function you know never returns (like `fn main() -> !`), upgrading its lifetime with `transmute`. (unsafe)
    ///
    /// This function never returns.
    pub fn run(&'static mut self, init: impl FnOnce(Spawner)) -> ! {
        init(unsafe { EXECUTOR.as_mut().unwrap() }.inner.spawner());

        loop {
            unsafe {
                EXECUTOR.as_mut().unwrap().inner.poll();
                self.configure_pwr();
                asm!("wfe");
            };
        }
    }
}