use core::cell::Cell;
use core::convert::TryInto;
use core::sync::atomic::{compiler_fence, AtomicU32, Ordering};

use embassy::time::{Clock, TICKS_PER_SECOND};
use stm32f4xx_hal::bb;
use stm32f4xx_hal::rcc::Clocks;

use crate::interrupt;
use crate::interrupt::{CriticalSection, Mutex, OwnedInterrupt};

// RTC timekeeping works with something we call "periods", which are time intervals
// of 2^15 ticks. The RTC counter value is 16 bits, so one "overflow cycle" is 2 periods.
//
// A `period` count is maintained in parallel to the RTC hardware `counter`, like this:
// - `period` and `counter` start at 0
// - `period` is incremented on overflow (at counter value 0)
// - `period` is incremented "midway" between overflows (at counter value 0x8000)
//
// Therefore, when `period` is even, counter is in 0..0x7FFF. When odd, counter is in 0x8000..0xFFFF
// This allows for now() to return the correct value even if it races an overflow.
//
// To get `now()`, `period` is read first, then `counter` is read. If the counter value matches
// the expected range for the `period` parity, we're done. If it doesn't, this means that
// a new period start has raced us between reading `period` and `counter`, so we assume the `counter` value
// corresponds to the next period.
//
// `period` is a 32bit integer, so It overflows on 2^32 * 2^15 / 32768 seconds of uptime, which is 136 years.

fn calc_now(period: u32, counter: u16) -> u64 {
    ((period as u64) << 15) + ((counter as u32 ^ ((period & 1) << 15)) as u64)
}

struct AlarmState {
    timestamp: Cell<u64>,
    callback: Cell<Option<(fn(*mut ()), *mut ())>>,
}

impl AlarmState {
    fn new() -> Self {
        Self {
            timestamp: Cell::new(u64::MAX),
            callback: Cell::new(None),
        }
    }
}

// TODO: This is sometimes wasteful, try to find a better way
const ALARM_COUNT: usize = 3;

pub struct RTC<T: Instance> {
    rtc: T,
    irq: T::Interrupt,

    /// Number of 2^23 periods elapsed since boot.
    period: AtomicU32,

    /// Timestamp at which to fire alarm. u64::MAX if no alarm is scheduled.
    alarms: Mutex<[AlarmState; ALARM_COUNT]>,

    clocks: Clocks,
}

impl<T: Instance> RTC<T> {
    pub fn new(rtc: T, irq: T::Interrupt, clocks: Clocks) -> Self {
        Self {
            rtc,
            irq,
            period: AtomicU32::new(0),
            alarms: Mutex::new([AlarmState::new(), AlarmState::new(), AlarmState::new()]),
            clocks,
        }
    }

    pub fn start(&'static self) {
        self.rtc.enable_clock();
        self.rtc.stop_and_reset();

        let multiplier = if T::ppre(&self.clocks) == 1 { 1 } else { 2 };
        let freq = T::pclk(&self.clocks) * multiplier;
        let psc = freq / TICKS_PER_SECOND as u32 - 1;
        let psc: u16 = psc.try_into().unwrap();

        self.rtc.set_psc_arr(psc, u16::MAX);
        // Mid-way point
        self.rtc.set_compare(0, 0x8000);
        self.rtc.set_compare_interrupt(0, true);

        self.irq.set_handler(
            |ptr| unsafe {
                let this = &*(ptr as *const () as *const Self);
                this.on_interrupt();
            },
            self as *const _ as *mut _,
        );
        self.irq.unpend();
        self.irq.enable();

        self.rtc.start();
    }

    fn on_interrupt(&self) {
        if self.rtc.overflow_interrupt_status() {
            self.rtc.overflow_clear_flag();
            self.next_period();
        }

        // Half overflow
        if self.rtc.compare_interrupt_status(0) {
            self.rtc.compare_clear_flag(0);
            self.next_period();
        }

        for n in 1..=ALARM_COUNT {
            if self.rtc.compare_interrupt_status(n) {
                self.rtc.compare_clear_flag(n);
                interrupt::free(|cs| self.trigger_alarm(n, cs));
            }
        }
    }

    fn next_period(&self) {
        interrupt::free(|cs| {
            let period = self.period.fetch_add(1, Ordering::Relaxed) + 1;
            let t = (period as u64) << 15;

            for n in 1..=ALARM_COUNT {
                let alarm = &self.alarms.borrow(cs)[n - 1];
                let at = alarm.timestamp.get();

                let diff = at - t;
                if diff < 0xc000 {
                    self.rtc.set_compare(n, at as u16);
                    self.rtc.set_compare_interrupt(n, true);
                }
            }
        })
    }

    fn trigger_alarm(&self, n: usize, cs: &CriticalSection) {
        self.rtc.set_compare_interrupt(n, false);

        let alarm = &self.alarms.borrow(cs)[n - 1];
        alarm.timestamp.set(u64::MAX);

        // Call after clearing alarm, so the callback can set another alarm.
        if let Some((f, ctx)) = alarm.callback.get() {
            f(ctx);
        }
    }

    fn set_alarm_callback(&self, n: usize, callback: fn(*mut ()), ctx: *mut ()) {
        interrupt::free(|cs| {
            let alarm = &self.alarms.borrow(cs)[n - 1];
            alarm.callback.set(Some((callback, ctx)));
        })
    }

    fn set_alarm(&self, n: usize, timestamp: u64) {
        interrupt::free(|cs| {
            let alarm = &self.alarms.borrow(cs)[n - 1];
            alarm.timestamp.set(timestamp);

            let t = self.now();
            if timestamp <= t {
                self.trigger_alarm(n, cs);
                return;
            }

            let diff = timestamp - t;
            if diff < 0xc000 {
                let safe_timestamp = timestamp.max(t + 3);
                self.rtc.set_compare(n, safe_timestamp as u16);
                self.rtc.set_compare_interrupt(n, true);
            } else {
                self.rtc.set_compare_interrupt(n, false);
            }
        })
    }

    pub fn alarm1(&'static self) -> Alarm<T> {
        Alarm { n: 1, rtc: self }
    }
    pub fn alarm2(&'static self) -> Option<Alarm<T>> {
        if T::REAL_ALARM_COUNT >= 2 {
            Some(Alarm { n: 2, rtc: self })
        } else {
            None
        }
    }
    pub fn alarm3(&'static self) -> Option<Alarm<T>> {
        if T::REAL_ALARM_COUNT >= 3 {
            Some(Alarm { n: 3, rtc: self })
        } else {
            None
        }
    }
}

impl<T: Instance> embassy::time::Clock for RTC<T> {
    fn now(&self) -> u64 {
        let period = self.period.load(Ordering::Relaxed);
        compiler_fence(Ordering::Acquire);
        let counter = self.rtc.counter();
        calc_now(period, counter)
    }
}

pub struct Alarm<T: Instance> {
    n: usize,
    rtc: &'static RTC<T>,
}

impl<T: Instance> embassy::time::Alarm for Alarm<T> {
    fn set_callback(&self, callback: fn(*mut ()), ctx: *mut ()) {
        self.rtc.set_alarm_callback(self.n, callback, ctx);
    }

    fn set(&self, timestamp: u64) {
        self.rtc.set_alarm(self.n, timestamp);
    }

    fn clear(&self) {
        self.rtc.set_alarm(self.n, u64::MAX);
    }
}

mod sealed {
    pub trait Sealed {}
}

pub trait Instance: sealed::Sealed + Sized + 'static {
    type Interrupt: OwnedInterrupt;
    const REAL_ALARM_COUNT: usize;

    fn enable_clock(&self);
    fn set_compare(&self, n: usize, value: u16);
    fn set_compare_interrupt(&self, n: usize, enable: bool);
    fn compare_interrupt_status(&self, n: usize) -> bool;
    fn compare_clear_flag(&self, n: usize);
    fn overflow_interrupt_status(&self) -> bool;
    fn overflow_clear_flag(&self);
    fn set_psc_arr(&self, psc: u16, arr: u16);
    fn stop_and_reset(&self);
    fn start(&self);
    fn counter(&self) -> u16;
    fn ppre(clocks: &Clocks) -> u8;
    fn pclk(clocks: &Clocks) -> u32;
}

mod tim2 {
    use super::*;
    use stm32f4xx_hal::pac::{RCC, TIM2};

    impl sealed::Sealed for TIM2 {}

    impl Instance for TIM2 {
        type Interrupt = interrupt::TIM2Interrupt;
        const REAL_ALARM_COUNT: usize = 3;

        fn enable_clock(&self) {
            // NOTE(unsafe) It will only be used for atomic operations
            unsafe {
                let rcc = &*RCC::ptr();

                bb::set(&rcc.apb1enr, 0);
                bb::set(&rcc.apb1rstr, 0);
                bb::clear(&rcc.apb1rstr, 0);
            }
        }

        fn set_compare(&self, n: usize, value: u16) {
            // NOTE(unsafe) these registers accept all the range of u16 values
            match n {
                0 => self.ccr1.write(|w| unsafe { w.bits(value.into()) }),
                1 => self.ccr2.write(|w| unsafe { w.bits(value.into()) }),
                2 => self.ccr3.write(|w| unsafe { w.bits(value.into()) }),
                3 => self.ccr4.write(|w| unsafe { w.bits(value.into()) }),
                _ => {}
            }
        }

        fn set_compare_interrupt(&self, n: usize, enable: bool) {
            if n > 3 {
                return;
            }
            let bit = n as u8 + 1;
            unsafe {
                if enable {
                    bb::set(&self.dier, bit);
                } else {
                    bb::clear(&self.dier, bit);
                }
            }
        }

        fn compare_interrupt_status(&self, n: usize) -> bool {
            let status = self.sr.read();
            match n {
                0 => status.cc1if().bit_is_set(),
                1 => status.cc2if().bit_is_set(),
                2 => status.cc3if().bit_is_set(),
                3 => status.cc4if().bit_is_set(),
                _ => false,
            }
        }

        fn compare_clear_flag(&self, n: usize) {
            if n > 3 {
                return;
            }
            let bit = n as u8 + 1;
            unsafe {
                bb::clear(&self.sr, bit);
            }
        }

        fn overflow_interrupt_status(&self) -> bool {
            self.sr.read().uif().bit_is_set()
        }

        fn overflow_clear_flag(&self) {
            unsafe {
                bb::clear(&self.sr, 0);
            }
        }

        fn set_psc_arr(&self, psc: u16, arr: u16) {
            // NOTE(unsafe) All u16 values are valid
            self.psc.write(|w| unsafe { w.bits(psc.into()) });
            self.arr.write(|w| unsafe { w.bits(arr.into()) });

            unsafe {
                // Set URS, generate update, clear URS
                bb::set(&self.cr1, 2);
                self.egr.write(|w| w.ug().set_bit());
                bb::clear(&self.cr1, 2);
            }
        }

        fn stop_and_reset(&self) {
            unsafe {
                bb::clear(&self.cr1, 0);
            }
            self.cnt.reset();
        }

        fn start(&self) {
            unsafe { bb::set(&self.cr1, 0) }
        }

        fn counter(&self) -> u16 {
            self.cnt.read().bits() as u16
        }

        fn ppre(clocks: &Clocks) -> u8 {
            clocks.ppre1()
        }

        fn pclk(clocks: &Clocks) -> u32 {
            clocks.pclk1().0
        }
    }
}