lyra-engine/lyra-game/src/events.rs

use std::{any::{Any, TypeId}, collections::{HashMap, VecDeque}, marker::PhantomData, mem::{self, MaybeUninit}, ops::Range};

use crate::plugin::Plugin;

pub trait Event: Clone + Send + Sync + 'static {}
impl<T: Clone + Send + Sync + 'static> Event for T {}

pub type Events<T> = VecDeque<T>;

#[derive(Clone)]
struct TypeErasedBuffer {
    type_id: TypeId,
    item_size: usize,
    data: Vec<MaybeUninit<u8>>,
    fn_drop_item: fn(item: *mut ()),
}

impl Drop for TypeErasedBuffer {
    fn drop(&mut self) {
        let range = self.data.as_mut_ptr_range();
        let mut current = range.start;
        let end = range.end;

        while current < end {
            // SAFETY: current pointer will either be the start of the buffer, or at the start of
            // the next item
            (self.fn_drop_item)(current.cast());

            // SAFETY: the items are placed right after eachother
            current = unsafe { current.add(self.item_size) };
        }

        // Safety: all of the events were just dropped
        unsafe { self.data.set_len(0) };
    }
}

impl TypeErasedBuffer {
    pub fn new<T: 'static>() -> Self {
        Self {
            type_id: TypeId::of::<T>(),
            item_size: mem::size_of::<T>(),
            data: Default::default(),
            // dropped immediately after the read
            fn_drop_item: |item| unsafe { item.cast::<T>().drop_in_place() },
        }
    }

    pub fn push<T: 'static>(&mut self, value: T) {
        debug_assert!(TypeId::of::<T>() == self.type_id, "The type of the buffer does not match the type that was added to it");

        // reserve enough bytes to store T
        let old_len = self.data.len();
        self.data.reserve(mem::size_of::<T>());

        // Get a pointer to the end of the data.
        // SAFETY: we just reserved enough memory to store the data.
        let end_ptr = unsafe { self.data.as_mut_ptr().add(old_len) };

        unsafe {
            // write the command and its runner into the buffer
            end_ptr.cast::<T>()
                // written unaligned to keep everything packed
                .write_unaligned(value);

            // we wrote to the vec's buffer without using its api, so we need manually
            // set the length of the vec.
            self.data.set_len(old_len + mem::size_of::<T>());
        }
    }

    pub fn len(&self) -> usize {
        self.data.len() / self.item_size
    }
}

pub struct EventQueue {
    events: HashMap<TypeId, TypeErasedBuffer>,
    event_write_queue: HashMap<TypeId, TypeErasedBuffer>,
}

impl Default for EventQueue {
    fn default() -> Self {
        Self::new()
    }
}

impl EventQueue {
    pub fn new() -> Self {
        Self {
            event_write_queue: HashMap::new(),
            events: HashMap::new(),
        }
    }

    /// Trigger an event
    pub fn trigger_event<E>(&mut self, event: E)
    where
        E: Event
    {
        // the compiler wants me to explicit right here for some reason
        /* let default = || RefCell::new(Box::<Events::<E>>::default() as Box<dyn AsSyncAny>);

        // Get, or create, a list of events of this type
        let type_id = event.type_id();
        let mut events = self.event_write_queue.entry(type_id)
            .or_insert_with(default)
            .borrow_mut();

        // downcast resource as an events storage
        let e: &mut Events<E> = events.as_mut().as_any_mut().downcast_mut().unwrap();
        e.push_back(event); */

        let type_id = event.type_id();
        let events = self.event_write_queue.entry(type_id)
            .or_insert_with(|| TypeErasedBuffer::new::<E>());

        events.push(event);
    }

    // Clear events, this should happen at the start of every tick since events are cloned
    // before being given to the reader.
    pub fn update_events(&mut self) {
        self.events.clear();

        // get all keys of events
        let keys: Vec<TypeId> = self.event_write_queue.keys().copied().collect();

        // remove all elements from self.event_write_queue and insert them to events
        for k in keys.into_iter() {
            let v = self.event_write_queue.remove(&k).unwrap();
            self.events.insert(k, v);
        }
    }

    pub fn read_events<E>(&self) -> Option<EventReader<E>>
    where
        E: Event
    {
        self.events.get(&TypeId::of::<E>())
            .map(|event| EventReader::new(event.clone()))
    }
}

pub struct EventReader<E: Event> {
    buf: TypeErasedBuffer,
    range: Option<Range<*mut MaybeUninit<u8>>>,
    _marker: PhantomData<E>,
}

impl<E: Event> EventReader<E> {
    fn new(buffer: TypeErasedBuffer) -> Self {
        Self {
            buf: buffer,
            range: None,
            _marker: PhantomData,
        }
    }

    pub fn len(&self) -> usize {
        self.buf.len()
    }
}

impl<E: Event> Iterator for EventReader<E> {
    type Item = E;

    fn next(&mut self) -> Option<Self::Item> {
        if self.range.is_none() {
            self.range = Some(self.buf.data.as_mut_ptr_range());
        }

        let range = self.range.as_mut().unwrap();

        if range.start < range.end {
            // Retrieve the event in the buffer.
            // SAFETY: current pointer will either be the start of the buffer, or at the start
            // of the next event.
            let event = unsafe { range.start.cast::<E>().read_unaligned() };

            // Advance the pointer to be after this event.
            range.start = unsafe { range.start.add(mem::size_of::<E>()) };

            Some(event)
        } else {
            None
        }
    }
}

#[derive(Default)]
pub struct EventsPlugin;

impl Plugin for EventsPlugin {
    fn setup(&self, game: &mut crate::game::Game) {
        game.world_mut().add_resource(EventQueue::new());
    }
}