specs 0.20.0

use std::marker::PhantomData;

use hibitset::BitSetLike;

use crate::{
    storage::{
        ComponentEvent, DenseVecStorage, SharedGetMutStorage, SyncUnsafeCell, Tracked, TryDefault,
        UnprotectedStorage,
    },
    world::{Component, Index},
};

use shrev::EventChannel;

/// Wrapper storage that tracks modifications, insertions, and removals of
/// components through an `EventChannel`.
///
/// **Note:** Joining over all components of a `FlaggedStorage`
/// mutably will flag all components.
///
/// What you want to instead is to use `restrict_mut()` to first
/// get the entities which contain the component and then conditionally
/// modify the component after a call to `get_mut()` or `get_other_mut()`.
///
/// # Examples
///
/// ```
/// extern crate specs;
///
/// use specs::prelude::*;
///
/// pub struct Comp(u32);
/// impl Component for Comp {
///     // `FlaggedStorage` acts as a wrapper around another storage.
///     // You can put any store inside of here (e.g. HashMapStorage, VecStorage, etc.)
///     //
///     // It also works as `FlaggedStorage<Self>` and defaults to `DenseVecStorage<Self>`
///     // for the inner storage.
///     type Storage = FlaggedStorage<Self, VecStorage<Self>>;
/// }
///
/// pub struct CompSystem {
///     // These keep track of where you left off in the event channel.
///     reader_id: ReaderId<ComponentEvent>,
///
///     // The bitsets you want to populate with modification/insertion events.
///     modified: BitSet,
///     inserted: BitSet,
/// }
///
/// impl<'a> System<'a> for CompSystem {
///     type SystemData = (Entities<'a>, WriteStorage<'a, Comp>);
///     fn run(&mut self, (entities, mut comps): Self::SystemData) {
///         // We want to clear the bitset first so we don't have left over events
///         // from the last frame.
///         //
///         // However, if you want to accumulate changes over a couple frames then you
///         // can only clear it when necessary. (This might be useful if you have some
///         // sort of "tick" system in your game and only want to do operations every
///         // 1/4th of a second or something)
///         //
///         // It is not okay to only read the events in an interval though as that could
///         // leave behind events which would end up growing the event ring buffer to
///         // extreme sizes.
///         self.modified.clear();
///         self.inserted.clear();
///
///         // Here we can populate the bitsets by iterating over the events.
///         // You can also just iterate over the events without using a bitset which will
///         // give you an ordered history of the events (which is good for caches and synchronizing
///         // other storages, but this allows us to use them in joins with components.
///         {
///             let events = comps.channel()
///                 .read(&mut self.reader_id);
///             for event in events {
///                 match event {
///                     ComponentEvent::Modified(id) => { self.modified.add(*id); },
///                     ComponentEvent::Inserted(id) => { self.inserted.add(*id); },
///                     _ => { },
///                 };
///             }
///         }
///
///         // Iterates over all components like normal.
///         for comp in (&comps).join() {
///             // ...
///         }
///
///         // **Never do this**
///         // This will flag all components as modified regardless of whether the inner loop
///         // actually modified the component.
///         //
///         // Only do this if you have other filters, like some other components to filter
///         // out the ones you want to modify.
///         for comp in (&mut comps).join() {
///             // ...
///         }
///
///         // Instead you will want to restrict the amount of components iterated over, either through
///         // other components in the join, or by using `RestrictedStorage` and only getting the component
///         // mutably when you are sure you need to modify it.
/// #        let condition = true;
///         for (entity, mut comps) in (&entities, &mut comps.restrict_mut()).join() {
///             if condition { // check whether this component should be modified.
///                  let mut comp = comps.get_mut();
///                  // ...
///             }
///         }
///
///         // To iterate over the modified components:
///         for comp in (&comps, &self.modified).join() {
///             // ...
///         }
///
///         // To iterate over all inserted/modified components;
///         for comp in (&comps, &self.modified & &self.inserted).join() {
///             // ...
///         }
///     }
/// }
///
/// fn main() {
///     let mut world = World::new();
///     world.register::<Comp>();
///
///     // You will want to register the system `ReaderId`s
///     // before adding/modifying/removing any entities and components.
///     //
///     // Otherwise you won't receive any of the modifications until
///     // you start tracking them.
///     let mut comp_system = {
///         let mut comps = world.write_storage::<Comp>();
///         CompSystem {
///             reader_id: comps.register_reader(),
///             modified: BitSet::new(),
///             inserted: BitSet::new(),
///         }
///     };
///
///     world.create_entity().with(Comp(19u32)).build();
///
///     {
///         let mut comps = world.write_storage::<Comp>();
///         let events = comps.channel().read(&mut comp_system.reader_id);
///         assert_eq!(events.len(), 1);
///     }
///
///     #[cfg(feature = "storage-event-control")]
///     {
///         world.write_storage::<Comp>().set_event_emission(false);
///         world.create_entity().with(Comp(19u32)).build();
///
///         {
///             let mut comps = world.write_storage::<Comp>();
///             let events = comps.channel().read(&mut comp_system.reader_id);
///             assert_eq!(events.len(), 0);
///         }
///
///         world.write_storage::<Comp>().set_event_emission(true);
///         world.create_entity().with(Comp(19u32)).build();
///
///         {
///             let mut comps = world.write_storage::<Comp>();
///             let events = comps.channel().read(&mut comp_system.reader_id);
///             assert_eq!(events.len(), 1);
///         }
///     }
/// }
/// ```
pub struct FlaggedStorage<C, T = DenseVecStorage<C>> {
    channel: SyncUnsafeCell<EventChannel<ComponentEvent>>,
    storage: T,
    #[cfg(feature = "storage-event-control")]
    event_emission: bool,
    phantom: PhantomData<C>,
}

impl<C, T> FlaggedStorage<C, T> {
    #[cfg(feature = "storage-event-control")]
    fn emit_event(&self) -> bool {
        self.event_emission
    }

    #[cfg(not(feature = "storage-event-control"))]
    fn emit_event(&self) -> bool {
        true
    }
}

impl<C, T> Default for FlaggedStorage<C, T>
where
    T: TryDefault,
{
    fn default() -> Self {
        FlaggedStorage {
            channel: SyncUnsafeCell::new(EventChannel::<ComponentEvent>::default()),
            storage: T::unwrap_default(),
            #[cfg(feature = "storage-event-control")]
            event_emission: true,
            phantom: PhantomData,
        }
    }
}

impl<C: Component, T: UnprotectedStorage<C>> UnprotectedStorage<C> for FlaggedStorage<C, T> {
    type AccessMut<'a> = <T as UnprotectedStorage<C>>::AccessMut<'a> where T: 'a;

    unsafe fn clean<B>(&mut self, has: B)
    where
        B: BitSetLike,
    {
        // SAFETY: Requirements passed to caller.
        unsafe { self.storage.clean(has) };
    }

    unsafe fn get(&self, id: Index) -> &C {
        // SAFETY: Requirements passed to caller.
        unsafe { self.storage.get(id) }
    }

    unsafe fn get_mut(&mut self, id: Index) -> <T as UnprotectedStorage<C>>::AccessMut<'_> {
        if self.emit_event() {
            self.channel
                .get_mut()
                .single_write(ComponentEvent::Modified(id));
        }
        // SAFETY: Requirements passed to caller.
        unsafe { self.storage.get_mut(id) }
    }

    unsafe fn insert(&mut self, id: Index, comp: C) {
        if self.emit_event() {
            self.channel
                .get_mut()
                .single_write(ComponentEvent::Inserted(id));
        }
        // SAFETY: Requirements passed to caller.
        unsafe { self.storage.insert(id, comp) };
    }

    unsafe fn remove(&mut self, id: Index) -> C {
        if self.emit_event() {
            self.channel
                .get_mut()
                .single_write(ComponentEvent::Removed(id));
        }
        // SAFETY: Requirements passed to caller.
        unsafe { self.storage.remove(id) }
    }
}

impl<C: Component, T: SharedGetMutStorage<C>> SharedGetMutStorage<C> for FlaggedStorage<C, T> {
    unsafe fn shared_get_mut(&self, id: Index) -> <T as UnprotectedStorage<C>>::AccessMut<'_> {
        if self.emit_event() {
            let channel_ptr = self.channel.get();
            // SAFETY: Caller required to ensure references returned from other
            // safe methods such as Tracked::channel are no longer alive. This
            // storage is not marked with a `DistinctStorage` impl.
            unsafe { &mut *channel_ptr }.single_write(ComponentEvent::Modified(id));
        }
        // SAFETY: Requirements passed to caller.
        unsafe { self.storage.shared_get_mut(id) }
    }
}

impl<C, T> Tracked for FlaggedStorage<C, T> {
    fn channel(&self) -> &EventChannel<ComponentEvent> {
        let channel_ptr = self.channel.get();
        // SAFETY: The only place that mutably accesses the channel via a shared
        // reference is the impl of `SharedGetMut::shared_get_mut` which
        // requires callers to avoid calling other methods with `&self` while
        // references returned there are still in use (and to ensure references
        // from methods like this no longer exist).
        unsafe { &*channel_ptr }
    }

    fn channel_mut(&mut self) -> &mut EventChannel<ComponentEvent> {
        self.channel.get_mut()
    }

    #[cfg(feature = "storage-event-control")]
    fn set_event_emission(&mut self, emit: bool) {
        self.event_emission = emit;
    }

    #[cfg(feature = "storage-event-control")]
    fn event_emission(&self) -> bool {
        self.event_emission
    }
}