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use std::ops::{Deref, DerefMut};

use shrev::{EventChannel, ReaderId};

use join::Join;
use storage::{MaskedStorage, Storage};
use world::{Component, Index};

/// `UnprotectedStorage`s that track modifications, insertions, and
/// removals of components.
pub trait Tracked {
    /// Event channel tracking modified/inserted/removed components.
    fn channel(&self) -> &EventChannel<ComponentEvent>;
    /// Mutable event channel tracking modified/inserted/removed components.
    fn channel_mut(&mut self) -> &mut EventChannel<ComponentEvent>;
}

#[derive(Debug, Copy, Clone)]
/// Component storage events received from a `FlaggedStorage` or any storage that implements
/// `Tracked`.
pub enum ComponentEvent {
    /// An insertion event, note that a modification event will be triggered if the entity
    /// already had a component and had a new one inserted.
    Inserted(Index),
    /// A modification event, this will be sent any time a component is accessed mutably so be
    /// careful with joins over `&mut storages` as it could potentially flag all of them.
    Modified(Index),
    /// A removal event.
    Removed(Index),
}

impl<'e, T, D> Storage<'e, T, D>
where
    T: Component,
    T::Storage: Tracked,
    D: Deref<Target = MaskedStorage<T>>,
{
    /// Returns the event channel tracking modified components.
    pub fn channel(&self) -> &EventChannel<ComponentEvent> {
        unsafe { self.open() }.1.channel()
    }
}

impl<'e, T, D> Storage<'e, T, D>
where
    T: Component,
    T::Storage: Tracked,
    D: DerefMut<Target = MaskedStorage<T>>,
{
    /// Returns the event channel for insertions/removals/modifications of this storage's
    /// components.
    pub fn channel_mut(&mut self) -> &mut EventChannel<ComponentEvent> {
        unsafe { self.open() }.1.channel_mut()
    }

    /// Starts tracking component events. Note that this reader id should be used every frame,
    /// otherwise events will pile up and memory use by the event channel will grow waiting
    /// for this reader.
    pub fn register_reader(&mut self) -> ReaderId<ComponentEvent> {
        self.channel_mut().register_reader()
    }

    /// Flags an index with a `ComponentEvent`.
    pub fn flag(&mut self, event: ComponentEvent) {
        self.channel_mut().single_write(event);
    }
}