/*
 * Event Bus
 * Copyright (C) 2018 Soni L.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

/// Registers an event hook.
///
/// Usage: `register_hook!(bus, priority, type, handler)`
#[macro_export]
macro_rules! register_hook {
    ($b:expr, $p:expr, $t:ty, $h:expr) => {
        {
            // hygiene
            let bus: &$crate::EventBus = $b;
            let hook = $h;
            let pri = $p;
            {
                static EVENT_ID: ::std::sync::atomic::AtomicUsize = std::sync::atomic::ATOMIC_USIZE_INIT;
                static EVENT_ID_INIT: ::std::sync::Once = std::sync::ONCE_INIT;
                // no generics allowed.
                static _DUMMY: ::std::marker::PhantomData<dyn Fn(&mut $t) + Send + Sync> = ::std::marker::PhantomData;
                EVENT_ID_INIT.call_once(|| {
                    EVENT_ID.store($crate::get_event_id::<$t>(), std::sync::atomic::Ordering::Relaxed);
                });
                let id = EVENT_ID.load(std::sync::atomic::Ordering::Relaxed);
                $crate::register::<$t, _>(bus, pri, hook, id)
            }
        }
    }
}

/// Posts an event.
///
/// Usage: `post_event!(bus, event, type, type, ...)`
#[macro_export]
macro_rules! post_event {
    // prefer (bus, ty, ty, ty, ..., evt) but rust is just bad at parsing
    ($b:expr, $e:expr, $($t:ty),+) => {
        {
            // hygiene
            let bus: &$crate::EventBus = $b;
            // event setup, this may evaluate to any type.
            let event: &mut _ = $e;
            {
                // it is a logic error for an event's cancellability to change based on its value.
                static CANCELLABLE: ::std::sync::atomic::AtomicBool = ::std::sync::atomic::ATOMIC_BOOL_INIT;
                static CANCELLABLE_INIT: ::std::sync::Once = ::std::sync::ONCE_INIT;
                CANCELLABLE_INIT.call_once(|| {
                    CANCELLABLE.store(<$crate::Event>::cancellable(event), std::sync::atomic::Ordering::Relaxed);
                });
                let cancellable = CANCELLABLE.load(std::sync::atomic::Ordering::Relaxed);

                #[allow(unused_mut)]
                let mut event_handlers = ::std::iter::empty::<(i32, Box<dyn Fn(&mut _)>)>();
                $(
                let handlers;
                #[allow(unused_mut)]
                let mut event_handlers = {
                    // event type setup
                    static EVENT_ID: ::std::sync::atomic::AtomicUsize = ::std::sync::atomic::ATOMIC_USIZE_INIT;
                    static EVENT_ID_INIT: ::std::sync::Once = ::std::sync::ONCE_INIT;
                    // no generics allowed.
                    static _DUMMY: ::std::marker::PhantomData<dyn Fn(&mut $t) + Send + Sync> = ::std::marker::PhantomData;
                    EVENT_ID_INIT.call_once(|| {
                        EVENT_ID.store($crate::get_event_id::<$t>(), std::sync::atomic::Ordering::Relaxed);
                    });
                    let id = EVENT_ID.load(std::sync::atomic::Ordering::Relaxed);

                    // handler retrieval and invokation
                    {
                        let subevent: &mut $t = event;
                        handlers = $crate::get_post_targets::<$t>(bus, subevent, id);
                    }
                    $crate::type_hint_trick(event, event_handlers, handlers.iter().cloned(), |f| move |evt| f(evt))
                };
                )+

                for (_pri, fun) in event_handlers {
                    fun(event);
                    if cancellable && <_ as $crate::Event>::cancelled(event) {
                        break;
                    }
                }
                cancellable && <_ as $crate::Event>::cancelled(event)
            }
        }
    }
}

/// Basic trait for defining an event.
pub trait Event: 'static {
    /// Returns whether this event is cancellable.
    ///
    /// When this is true, `cancelled` and `cancel` should also be implemented.
    ///
    /// Note: While this method does take a `&self`, it's a logic error for an event's
    /// cancellability to change based on its value. The `&self` is just for trait objectification.
    /// You have been warned!
    fn cancellable(&self) -> bool {
        false
    }

    /// Returns whether this event has been cancelled.
    fn cancelled(&self) -> bool {
        false
    }

    /// Sets whether this event is cancelled.
    ///
    /// # Panics
    ///
    /// Panics if this event is not cancellable.
    fn set_cancelled(&mut self, cancel: bool) {
        let _ = cancel;
        panic!("not cancellable");
    }
}

/// An event bus.
pub struct EventBus {
    id: usize,
    dropper: Mutex<LinkedList<Box<dyn Fn(usize) + Send + Sync>>>,
}

#[doc(hidden)]
pub fn type_hint_trick<T: Event + Sized, A, B, C: Event + ?Sized, E>(_event: &mut T, event_handlers: A, handlers: B, convert: fn(::std::sync::Arc<dyn Fn(&mut C) + Send + Sync>) -> E) -> impl ::std::iter::Iterator<Item=(i32, Box<dyn for<'a> Fn(&'a mut T) + 'static>)> where
    A: ::std::iter::Iterator<Item=(i32, Box<dyn Fn(&mut T)>)>,
    B: ::std::iter::Iterator<Item=(i32, ::std::sync::Arc<dyn Fn(&mut C) + Send + Sync>)>,
    E: Fn(&mut T) + 'static,
    {
    itertools::merge_join_by(event_handlers, handlers, |l, r| l.0.cmp(&r.0))
        .map(move |eob: itertools::EitherOrBoth<(i32, Box<dyn Fn(&mut T)>), (i32, ::std::sync::Arc<dyn Fn(&mut C) + Send + Sync>)>| ({
            eob.as_ref().left().map(|v| v.0).or_else(|| eob.as_ref().right().map(|v| v.0)).unwrap_or(0)
        }, Box::new(move |evt: &mut _| {
            eob.as_ref().left().map(|x| (x.1)(evt));
            eob.as_ref().right().map(|x| convert(x.1.clone())(evt));
        }) as Box<_>))
}

#[macro_use]
extern crate lazy_static;
extern crate anymap;
extern crate itertools;

//use std::marker::PhantomData;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::ATOMIC_USIZE_INIT;
use std::sync::atomic::Ordering as AtomicOrdering;
use std::sync::Mutex;
use std::collections::LinkedList;
use std::any::Any;
use std::sync::Arc;
use std::sync::RwLock;
use std::mem;

/* only exists as a module to keep us from accidentally misusing these */
mod id_map {
    use std::marker::PhantomData;
    use std::sync::atomic::AtomicUsize;
    use std::sync::atomic::ATOMIC_USIZE_INIT;
    use std::sync::atomic::Ordering as AtomicOrdering;
    use std::sync::Mutex;
    use std::sync::Arc;
    use super::Event;
    use super::Handlers;
    lazy_static! {
        static ref EVENT_ID_MAP: Mutex<::anymap::Map<::anymap::any::Any + Send + Sync>> = Mutex::new(::anymap::Map::new());
    }
    struct EventId<T: Event + ?Sized> {
        id: usize,
        _t: PhantomData<dyn Fn(&mut T) + Send + Sync>
    }

    pub fn get_event_id<T: Event + ?Sized>() -> usize {
        static EVENT_ID_COUNT: AtomicUsize = ATOMIC_USIZE_INIT;
        EVENT_ID_MAP.lock().expect("failed to allocate event id").entry::<EventId<T>>().or_insert_with(|| {
            let handlers: Arc<Handlers<T>> = Default::default();
            Arc::make_mut(&mut super::HANDLERS.write().expect("???")).push(handlers);
            EventId { id: EVENT_ID_COUNT.fetch_add(1, AtomicOrdering::SeqCst), _t: PhantomData }
        }).id
    }
}

/// Low-level event handling function, returns the event ID for use with get_post_targets.
///
/// This function is (extremely) slow!
pub fn get_event_id<T: Event + ?Sized>() -> usize {
    id_map::get_event_id::<T>()
}

type RwArcVec<T> = RwLock<Arc<Vec<T>>>;

struct Handlers<T: Event + ?Sized>(RwArcVec<RwArcVec<(i32, Arc<dyn Fn(&mut T) + Send + Sync>)>>);

impl<T: Event + ?Sized> Default for Handlers<T> {
    fn default() -> Self {
        Handlers(Default::default())
    }
}

lazy_static! {
    static ref HANDLERS: RwArcVec<Arc<dyn Any + Send + Sync>> = Default::default();
}

/// Low-level event handling function,  returns the handlers that should be called to post the
/// event.
///
/// This is useful if you want to simulate inheritance.
///
/// # Panics
///
/// Panics if `id` doesn't match the given `T` (but not if the given `id` doesn't exist).
pub fn get_post_targets<T: Event + ?Sized>(bus: &EventBus, _event: &mut T, id: usize) -> Arc<Vec<(i32, Arc<dyn Fn(&mut T) + Send + Sync>)>> {
    let target: Option<Arc<Vec<_>>> = HANDLERS.read().expect("failed to lock for reading").get(id).map(|arc| Arc::clone(arc))
        .map(|any| Arc::downcast::<Handlers<T>>(any).expect("Wrong id"))
        .and_then(|handlers| handlers.0.read().expect("failed to lock for reading").get(bus.id).map(|hooks| Arc::clone(&*hooks.read().expect("failed to lock for reading"))));
    target.unwrap_or_else(|| Arc::new(Vec::new()))
}

/// Low-level event handling function, registers an event.
/// 
/// This function is kinda slow. It doesn't block currently executing hooks (and, in fact, may be
/// safely called from a running hook), but does block threads from starting hook execution.
///
/// # Panics
///
/// Panics if `id` doesn't match the given `T`, or if `id` doesn't exist.
pub fn register<T: Event + ?Sized, F: for<'a> Fn(&'a mut T) + Send + Sync + 'static>(bus: &EventBus, priority: i32, handler: F, id: usize) {
    HANDLERS.read().expect("failed to lock for reading").get(id).map(|arc| Arc::clone(arc))
        .map(|any| Arc::downcast::<Handlers<T>>(any).expect("Wrong id"))
        .map(|handlers| {
            let mut lock = handlers.0.read().expect("failed to lock for reading");
            if lock.len() <= id {
                mem::drop(lock);
                let mut wlock = handlers.0.write().expect("failed to lock for writing");
                if wlock.len() <= id { // another thread may have touched this
                    let vec = Arc::get_mut(&mut wlock).expect("failed to mutate busses");
                    let count = id - vec.len() + 1;
                    vec.reserve_exact(count);
                    for _ in 0..count {
                        vec.push(Default::default());
                    }
                }
                // there's no way to downgrade a write lock, but as long as we never shrink the
                // vecs it should be fine.
                mem::drop(wlock);
                lock = handlers.0.read().expect("failed to lock for reading");
            }
            let option = lock.get(bus.id);
            // last RwLock
            let hooks = option.expect("failed to make vecs");
            let hook: (i32, Arc<dyn Fn(&mut T) + Send + Sync + 'static>) = (priority, Arc::new(handler));
            let mut wlock = hooks.write().expect("failed to lock for writing");
            let vec = Arc::make_mut(&mut wlock);
            // would be nice if Result had a .coalesce()
            let pos = match vec.binary_search_by(|probe| probe.0.cmp(&priority)) { Ok(p) => p, Err(p) => p };
            vec.insert(pos, hook);
        }).expect("No such id");
}

static BUS_ID: AtomicUsize = ATOMIC_USIZE_INIT;

lazy_static! {
    // this is the closest to high-performance we can get with std
    // would be nicer if this didn't lock the whole list
    static ref FREED_BUS_IDS: Mutex<LinkedList<usize>> = Mutex::new(LinkedList::new());
}

impl EventBus {
    /// Makes a new EventBus.
    pub fn new() -> EventBus {
        EventBus {
            id: FREED_BUS_IDS.lock().unwrap().pop_front().unwrap_or_else(|| BUS_ID.fetch_add(1, AtomicOrdering::SeqCst)),
            dropper: Default::default(),
        }
    }
}

impl Drop for EventBus {
    fn drop(&mut self) {
        // TODO
    }
}

// test with good error messages
#[cfg(test)]
mod useful_test {
    use super::{Event, EventBus};

    struct MyEvent {
        i: i32
    }

    trait DynEvent : Event {
    }

    impl DynEvent for MyEvent {
    }

    impl Event for MyEvent {
    }

    fn add_handler(e: &mut MyEvent) {
        /* adds 1 */
        e.i += 1;
    }

    fn no_handler(e: &mut MyEvent) {
        /* does nothing */
        e.i += 0;
    }

    #[test]
    fn test_usage_internal() {
        let event_bus = EventBus::new();
        //let handler_id = event_bus.register(add_handler, 0);
        register_hook!(&event_bus, 0, MyEvent, add_handler);
        let mut event = MyEvent { i: 3 };
        assert_eq!(event.i, 3);
        post_event!(&event_bus, &mut event, MyEvent);
        post_event!(&event_bus, &mut event, dyn DynEvent);
        assert_eq!(event.i, 4);
        register_hook!(&event_bus, 1, MyEvent, no_handler);
        post_event!(&event_bus, &mut event, MyEvent);
        assert_eq!(event.i, 5);
        //event_bus.unregister(handler_id);
        post_event!(&event_bus, &mut event, MyEvent);
        //assert_eq!(event.i, 5);
    }
}