use std::any::type_name;
use std::fmt::Debug;
use std::marker::PhantomData;
use std::mem::transmute;
use std::slice::Iter as SliceIter;

use seq_macro::seq;

use crate::entity::Handle as EntityHandle;
use crate::event::Emitted as EmittedEvent;
use crate::pair::Pair;
use crate::system::{
    Callbacks,
    NoCallbacks,
    Param,
    ReturnValue as SystemReturnValue,
    TypeErased as TypeErasedSystem,
};
use crate::uid::Uid;
use crate::util::Array;
use crate::{Component, World};

pub trait Observed
{
    type Events: Array<Pair<Uid, Uid>>;

    fn events() -> Self::Events;
}

impl<Relation, Target> Observed for Pair<Relation, Target>
where
    Relation: Component,
    Target: Component,
{
    type Events = [Pair<Uid, Uid>; 1];

    fn events() -> Self::Events
    {
        [Pair::builder()
            .relation::<Relation>()
            .target::<Target>()
            .build()]
    }
}

/// Observer system.
pub trait Observer<'world, Impl>
{
    type Callbacks: Callbacks;
    type ObservedEvents: Array<Pair<Uid, Uid>>;

    fn observed_events() -> Self::ObservedEvents;

    fn finish_observer(self) -> (TypeErasedSystem, Self::Callbacks);
}

pub struct Observe<'world, ObservedT: Observed>
{
    _pd: PhantomData<ObservedT>,
    world: &'world World,
    emitted_event: EmittedEvent<'world>,
}

impl<'world, ObservedT: Observed> Observe<'world, ObservedT>
{
    pub fn new(world: &'world World, emitted_event: EmittedEvent<'world>) -> Self
    {
        Self {
            _pd: PhantomData,
            world,
            emitted_event,
        }
    }

    #[must_use]
    pub fn event(&self) -> Uid
    {
        self.emitted_event.event
    }
}

impl<ObservedT: Observed> Observe<'_, ObservedT>
{
    #[must_use]
    pub fn iter(&self) -> ObserveIter<'_, ObservedT>
    {
        ObserveIter {
            world: self.world,
            inner: self.emitted_event.match_ids.iter(),
            _pd: PhantomData,
        }
    }
}

impl<'a, ObservedT: Observed> IntoIterator for &'a Observe<'_, ObservedT>
{
    type IntoIter = ObserveIter<'a, ObservedT>;
    type Item = <Self::IntoIter as Iterator>::Item;

    fn into_iter(self) -> Self::IntoIter
    {
        self.iter()
    }
}

pub struct ObserveIter<'observe, ObservedT: Observed>
{
    world: &'observe World,
    inner: SliceIter<'observe, Uid>,
    _pd: PhantomData<ObservedT>,
}

impl<'observe, ObservedT: Observed> Iterator for ObserveIter<'observe, ObservedT>
{
    type Item = EventMatch<'observe, ObservedT>;

    fn next(&mut self) -> Option<Self::Item>
    {
        let match_id = *self.inner.next()?;

        Some(EventMatch {
            world: self.world,
            id: match_id,
            _pd: PhantomData,
        })
    }
}

/// A event match.
#[derive(Debug)]
pub struct EventMatch<'world, ObservedT: Observed>
{
    world: &'world World,
    id: Uid,
    _pd: PhantomData<ObservedT>,
}

impl<'world, ObservedT: Observed> EventMatch<'world, ObservedT>
{
    #[must_use]
    pub fn entity_id(&self) -> Uid
    {
        self.id
    }

    /// Attempts to get the entity with the id of this match.
    #[must_use]
    pub fn try_get_entity(&self) -> Option<EntityHandle<'world>>
    {
        self.world.get_entity(self.id)
    }
}

macro_rules! impl_observer {
    ($c: tt) => {
        seq!(I in 0..$c {
            impl<'world, ObservedT, Func, Ret, #(TParam~I,)*> Observer<
                'world,
                fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret
            > for Func
            where
                ObservedT: Observed,
                Func: Fn(Observe<'world, ObservedT>, #(TParam~I,)*) -> Ret + Copy + 'static,
                Ret: SystemReturnValue,
                #(TParam~I: Param<'world, Input = ()>,)*
            {
                type Callbacks = NoCallbacks;
                type ObservedEvents = ObservedT::Events;

                fn observed_events() -> Self::ObservedEvents
                {
                    ObservedT::events()
                }

                fn finish_observer(self) -> (TypeErasedSystem, NoCallbacks)
                {
                    #![allow(unused)]

                    crate::util::const_assert!(
                        size_of::<Func>() == 0,
                        "System function is not zero-sized (not function pointer)"
                    );

                    let system = TypeErasedSystem {
                        run: |world, metadata, emitted_event| {
                            // SAFETY: The caller of TypeErased::run ensures the lifetime
                            // is correct
                            let world = unsafe { &*std::ptr::from_ref(world) };

                            let emitted_event =
                                emitted_event.expect("No event info passed to observer");

                            // SAFETY: The caller of TypeErased::run ensures the lifetime
                            // is correct
                            let emitted_event = unsafe {
                                transmute::<EmittedEvent<'_>, EmittedEvent<'_>>(
                                    emitted_event
                                )
                            };

                            let func = unsafe { std::mem::zeroed::<Func>() };

                            func(Observe::new(world, emitted_event), #({
                                TParam~I::new(world, &metadata)
                            },)*).into_result()
                        },
                        name: type_name::<Func>()
                    };

                    (system, NoCallbacks)
                }
            }
        });
    };
}

seq!(C in 0..16 {
    impl_observer!(C);
});