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

use crate::component::{Component, Sequence as ComponentSequence};
use crate::lock::{Lock, WriteGuard};
use crate::system::{NoInitParamFlag, Param as SystemParam, System};
use crate::{ActionQueue, WorldData};

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

impl<'world> Actions<'world>
{
    /// Adds a spawning a new entity to the action queue.
    pub fn spawn<Comps: ComponentSequence>(&mut self, components: Comps)
    {
        self.action_queue.push(Action::Spawn(components.into_vec()));
    }

    /// Adds stopping the loop in [`Engine::event_loop`] at the next opportune time to the
    /// action queue.
    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<Lock<ActionQueue>>) -> Self
    {
        Self {
            action_queue: action_queue
                .write_nonblock()
                .expect("Failed to aquire read-write action queue lock"),
            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_data: &'world WorldData,
    ) -> Self
    {
        Self::new(&world_data.action_queue)
    }

    fn is_compatible<Other: SystemParam<'world>>() -> bool
    {
        let other_comparable = Other::get_comparable();

        other_comparable.downcast_ref::<Comparable>().is_none()
    }

    fn get_comparable() -> Box<dyn Any>
    {
        Box::new(())
    }
}

#[derive(Debug, Clone)]
pub struct WeakRef
{
    action_queue: Weak<Lock<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<Lock<ActionQueue>>,
    _pd: PhantomData<&'weak_ref ()>,
}

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

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

struct Comparable;