use std::{cell::RefMut, collections::VecDeque, mem, ptr::{self, NonNull}};

use unique::Unique;

use crate::{system::FnArgFetcher, Access, Bundle, Entities, Entity, World};

pub trait Command {
    fn run(self, world: &mut World) -> anyhow::Result<()>;
}

impl<F> Command for F
where
    F: FnOnce(&mut World) -> anyhow::Result<()>
{
    fn run(self, world: &mut World) -> anyhow::Result<()> {
        self(world)
    }
}

type RunCommand = unsafe fn(cmd: Unique<()>, world: &mut World) -> anyhow::Result<()>;

#[derive(Default)]
pub struct CommandQueue(VecDeque<(RunCommand, Unique<()>)>);

pub struct Commands<'a, 'b> {
    queue: &'b mut CommandQueue,
    entities: &'a mut Entities,
}

impl<'a, 'b> Commands<'a, 'b> {
    pub fn new(queue: &'b mut CommandQueue, world: &'a mut World) -> Self {
        Self {
            queue,
            entities: &mut world.entities,
        }
    }

    /// Add a command to the end of the command queue
    pub fn add<C: Command>(&mut self, mut cmd: C) {
        // get an owned pointer to the command, then forget it to ensure its destructor isn't ran
        let ptr = Unique::from(&mut cmd).cast::<()>();
        mem::forget(cmd);

        let run_fn = |cmd_ptr: Unique<()>, world: &mut World| unsafe {
            let cmd = cmd_ptr.cast::<C>();
            let cmd = ptr::read(cmd.as_ptr());
            cmd.run(world)?;

            Ok(())
        };

        self.queue.0.push_back((run_fn, ptr));
    }

    pub fn spawn<B: Bundle>(&mut self, mut bundle: B) -> Entity {
        let e = self.entities.reserve();

        let bundle_ptr = Unique::from(&mut bundle);
        mem::forget(bundle);
        //let bundle_box = Box::new(bundle);
        
        self.add(move |world: &mut World| {
            let bundle = unsafe { ptr::read(bundle_ptr.as_ptr()) };
            world.spawn_into(e, bundle);
            Ok(())
        });

        e
    }

    /// Execute all commands in the queue, in order of insertion
    pub fn execute(&mut self, world: &mut World) -> anyhow::Result<()> {
        while let Some((cmd_fn, cmd_ptr)) = self.queue.0.pop_front() {
            unsafe {
                cmd_fn(cmd_ptr, world)?;
            }
        }

        Ok(())
    }
}

impl FnArgFetcher for Commands<'_, '_> {
    type State = CommandQueue;
    type Arg<'a, 'state> = Commands<'a, 'state>;

    fn world_access(&self) -> Access {
        Access::Write
    }

    unsafe fn get<'a, 'state>(state: &'state mut Self::State, mut world_ptr: ptr::NonNull<World>) -> Self::Arg<'a, 'state> {
        let world = world_ptr.as_mut();
        Commands::new(state, world)
    }

    fn create_state(_: NonNull<World>) -> Self::State {
        CommandQueue::default()
    }

    fn apply_deferred<'a>(mut state: Self::State, mut world_ptr: NonNull<World>) {
        let world = unsafe { world_ptr.as_mut() };

        let mut cmds = Commands::new(&mut state, world);
        // safety: Commands has a mut borrow to entities in the world
        let world = unsafe { world_ptr.as_mut() };
        cmds.execute(world).unwrap()
    }
}

pub fn execute_deferred_commands(world: &mut World, mut commands: RefMut<Commands>) -> anyhow::Result<()> {
    commands.execute(world)?;

    Ok(())
}