use std::marker::PhantomData;
use std::sync::{Arc, Weak};

use crate::component::{
    Component,
    Metadata as ComponentMetadata,
    Sequence as ComponentSequence,
};
use crate::system::{NoInitParamFlag, Param as SystemParam, System};
use crate::uid::{Kind as UidKind, Uid};
use crate::{ActionQueue, World};

/// Used to to queue up actions for a [`World`] to perform.
#[derive(Debug)]
pub struct Actions<'world>
{
    action_queue: &'world ActionQueue,
    action_queue_weak: Weak<ActionQueue>,
}

impl<'world> Actions<'world>
{
    /// Queues up a entity to spawn at the end of the current tick.
    pub fn spawn<Comps: ComponentSequence>(&mut self, components: Comps)
    {
        self.action_queue.push(Action::Spawn(
            components.into_vec(),
            EventIds { ids: Comps::added_event_ids() },
        ));
    }

    /// Queues up despawning a entity at the end of the current tick.
    pub fn despawn(&mut self, entity_uid: Uid)
    {
        debug_assert_eq!(entity_uid.kind(), UidKind::Entity);

        self.action_queue.push(Action::Despawn(entity_uid));
    }

    /// Queues up adding component(s) to a entity at the end of the current tick.
    pub fn add_components<Comps>(&mut self, entity_uid: Uid, components: Comps)
    where
        Comps: ComponentSequence,
    {
        debug_assert_eq!(entity_uid.kind(), UidKind::Entity);

        self.action_queue.push(Action::AddComponents(
            entity_uid,
            components.into_vec(),
            EventIds { ids: Comps::added_event_ids() },
        ));
    }

    /// Queues up removing component(s) from a entity at the end of the current tick.
    pub fn remove_components<Comps>(&mut self, entity_uid: Uid)
    where
        Comps: ComponentSequence,
    {
        debug_assert_eq!(entity_uid.kind(), UidKind::Entity);

        self.action_queue.push(Action::RemoveComponents(
            entity_uid,
            Comps::metadata(),
            EventIds { ids: Comps::removed_event_ids() },
        ));
    }

    /// Stops the [`World`]. The world will finish the current tick and that tick will be
    /// the last.
    pub fn stop(&mut self)
    {
        self.action_queue.push(Action::Stop);
    }

    /// Returns a struct which holds a weak reference to the [`World`] that `Actions`
    /// references and that can be used to aquire a new `Actions` instance if the
    /// referenced [`World`] is still alive.
    #[must_use]
    pub fn to_weak_ref(&self) -> WeakRef
    {
        WeakRef {
            action_queue: self.action_queue_weak.clone(),
        }
    }

    fn new(action_queue: &'world Arc<ActionQueue>) -> Self
    {
        Self {
            action_queue,
            action_queue_weak: Arc::downgrade(action_queue),
        }
    }
}

unsafe impl<'world> SystemParam<'world> for Actions<'world>
{
    type Flags = NoInitParamFlag;
    type Input = ();

    fn initialize<SystemImpl>(
        _system: &mut impl System<'world, SystemImpl>,
        _input: Self::Input,
    )
    {
    }

    fn new<SystemImpl>(
        _system: &'world impl System<'world, SystemImpl>,
        world: &'world World,
    ) -> Self
    {
        Self::new(&world.data.action_queue)
    }
}

#[derive(Debug, Clone)]
pub struct WeakRef
{
    action_queue: Weak<ActionQueue>,
}

impl WeakRef
{
    /// Returns a struct which can be used to retrieve a [`Actions`].
    ///
    /// Returns [`None`] if the referenced [`World`] has been dropped.
    #[must_use]
    pub fn access(&self) -> Option<Ref<'_>>
    {
        Some(Ref {
            action_queue: self.action_queue.upgrade()?,
            _pd: PhantomData,
        })
    }
}

/// Intermediate between [`Actions`] and [`WeakRef`]. Contains a strong reference to
/// a world which is not allowed direct access to.
#[derive(Debug, Clone)]
pub struct Ref<'weak_ref>
{
    action_queue: Arc<ActionQueue>,
    _pd: PhantomData<&'weak_ref ()>,
}

impl<'weak_ref> Ref<'weak_ref>
{
    #[must_use]
    pub fn to_actions(&self) -> Actions<'_>
    {
        Actions::new(&self.action_queue)
    }
}

#[derive(Debug)]
pub(crate) struct EventIds
{
    pub(crate) ids: Vec<Uid>,
}

/// A action for a [`System`] to perform.
#[derive(Debug)]
pub(crate) enum Action
{
    Spawn(Vec<Box<dyn Component>>, EventIds),
    Despawn(Uid),
    AddComponents(Uid, Vec<Box<dyn Component>>, EventIds),
    RemoveComponents(Uid, Vec<ComponentMetadata>, EventIds),
    Stop,
}