use slab::Slab;

use std::{

    convert::TryInto,

    iter,

    num::NonZeroU32,

    time::{Duration, Instant},

};

struct Event<Data> {

    event_id: u32,

    data: Data,

}

#[derive(Clone)]

pub struct Timeout {

    tick_index: u32,

    event_index: u32,

    event_id: u32,

}

pub struct TimedEvents<Data, const MAX_TIMEOUT: usize> {

    events: [Slab<Event<Data>>; MAX_TIMEOUT],

    current_time: Instant,

    current_tick_index: u32,

    next_event_id: u32,

}

impl<Data, const MAX_TIMEOUT: usize> TimedEvents<Data, MAX_TIMEOUT> {

    pub fn new() -> Self {

        Self {

            events: iter::repeat_with(|| Slab::new())

                .take(MAX_TIMEOUT)

                .collect::<Vec<_>>()

                .try_into()

                .ok()

                .unwrap(),

            current_time: Instant::now(),

            current_tick_index: 0,

            next_event_id: 0,

        }

    }

    pub fn set_timeout(&mut self, seconds_delay: NonZeroU32, data: Data) -> Timeout {

        let tick_index = (self.current_tick_index

            + std::cmp::min(seconds_delay.get(), MAX_TIMEOUT as u32))

            % MAX_TIMEOUT as u32;

        let event_id = self.next_event_id;

        self.next_event_id += 1;

        let event = Event { event_id, data };

        let entry = self.events[tick_index as usize].vacant_entry();

        let event_index = entry.key() as u32;

        entry.insert(event);

        Timeout {

            tick_index,

            event_index,

            event_id,

        }

    }

    pub fn cancel_timeout(&mut self, timeout: Timeout) -> Option<Data> {

        let events = &mut self.events[timeout.tick_index as usize];

        if matches!(events.get(timeout.event_index as usize), Some(Event { event_id: id, ..}) if *id == timeout.event_id)

        {

            Some(events.remove(timeout.event_index as usize).data)

        } else {

            None

        }

    }

    pub fn poll(&mut self, time: Instant) -> Vec<Data> {

        let mut result = vec![];

        let second = Duration::from_secs(1);

        while time - self.current_time > second {

            self.current_time += second;

            self.current_tick_index = (self.current_tick_index + 1) % MAX_TIMEOUT as u32;

            result.extend(

                self.events[self.current_tick_index as usize]

                    .drain()

                    .map(|e| e.data),

            );

        }

        result

    }

}

mod test {

    use super::TimedEvents;

    use std::{

        num::NonZeroU32,

        time::{Duration, Instant},

    };

    #[test]

    fn events_test() {

        let mut events = TimedEvents::<u32, 30>::new();

        let now = Instant::now();

        let timeouts = (1..=3)

            .map(|n| events.set_timeout(NonZeroU32::new(n).unwrap(), n))

            .collect::<Vec<_>>();

        let second = Duration::from_secs(1);

        assert_eq!(events.cancel_timeout(timeouts[1].clone()), Some(2));

        assert_eq!(events.poll(now + second), vec![1]);

        assert!(events.poll(now + second).is_empty());

        assert!(events.poll(now + 2 * second).is_empty());

        assert_eq!(events.poll(now + 3 * second), vec![3]);

    }

}