use crate::utils::copyhashmap::CopyHashMap;
use crate::utils::numcell::NumCell;
use std::cell::{Cell, RefCell};
use std::collections::VecDeque;
use std::rc::Rc;
use thiserror::Error;
use uapi::{c, Errno, OwnedFd};

#[derive(Debug, Error)]
pub enum EventLoopError {
    #[error("Could not create an epoll fd: {0}")]
    CreateFailed(crate::utils::oserror::OsError),
    #[error("epoll_wait failed: {0}")]
    WaitFailed(crate::utils::oserror::OsError),
    #[error("A dispatcher returned a fatal error: {0}")]
    DispatcherError(Box<dyn std::error::Error>),
    #[error("Could not insert an fd to wait on: {0}")]
    InsertFailed(crate::utils::oserror::OsError),
    #[error("Could not modify an fd to wait on: {0}")]
    ModifyFailed(crate::utils::oserror::OsError),
    #[error("Could not remove an fd to wait on: {0}")]
    RemoveFailed(crate::utils::oserror::OsError),
    #[error("Entry is not registered")]
    NoEntry,
    #[error("Event loop is already destroyed")]
    Destroyed,
}

#[derive(Copy, Clone, Debug, Ord, PartialOrd, Eq, PartialEq, Hash)]
pub struct EventLoopId(u64);

pub trait EventLoopDispatcher {
    fn dispatch(self: Rc<Self>, events: i32) -> Result<(), Box<dyn std::error::Error>>;
}

#[derive(Clone)]
struct Entry {
    fd: Option<i32>,
    dispatcher: Rc<dyn EventLoopDispatcher>,
}

pub struct EventLoop {
    destroyed: Cell<bool>,
    epoll: OwnedFd,
    next_id: NumCell<u64>,
    entries: CopyHashMap<u64, Entry>,
    scheduled: RefCell<VecDeque<u64>>,
}

impl EventLoop {
    pub fn new() -> Result<Rc<Self>, EventLoopError> {
        let epoll = match uapi::epoll_create1(c::EPOLL_CLOEXEC) {
            Ok(e) => e,
            Err(e) => return Err(EventLoopError::CreateFailed(e.into())),
        };
        Ok(Rc::new(Self {
            destroyed: Cell::new(false),
            epoll,
            next_id: NumCell::new(1),
            entries: CopyHashMap::new(),
            scheduled: RefCell::new(Default::default()),
        }))
    }

    fn check_destroyed(&self) -> Result<(), EventLoopError> {
        if self.destroyed.get() {
            return Err(EventLoopError::Destroyed);
        }
        Ok(())
    }

    pub fn id(&self) -> EventLoopId {
        EventLoopId(self.next_id.fetch_add(1))
    }

    pub fn stop(&self) {
        self.destroyed.set(true);
        self.entries.clear();
    }

    pub fn insert(
        &self,
        id: EventLoopId,
        fd: Option<i32>,
        events: i32,
        dispatcher: Rc<dyn EventLoopDispatcher>,
    ) -> Result<(), EventLoopError> {
        self.check_destroyed()?;
        let id = id.0;
        if let Some(fd) = fd {
            let event = c::epoll_event {
                events: events as _,
                u64: id,
            };
            if let Err(e) = uapi::epoll_ctl(self.epoll.raw(), c::EPOLL_CTL_ADD, fd, Some(&event)) {
                return Err(EventLoopError::InsertFailed(e.into()));
            }
        }
        self.entries.set(id, Entry { fd, dispatcher });
        Ok(())
    }

    pub fn modify(&self, id: EventLoopId, events: i32) -> Result<(), EventLoopError> {
        self.check_destroyed()?;
        let id = id.0;
        let entry = match self.entries.get(&id) {
            Some(e) => e,
            None => return Err(EventLoopError::NoEntry),
        };
        if let Some(fd) = entry.fd {
            let event = c::epoll_event {
                events: events as _,
                u64: id,
            };
            if let Err(e) = uapi::epoll_ctl(self.epoll.raw(), c::EPOLL_CTL_MOD, fd, Some(&event)) {
                return Err(EventLoopError::ModifyFailed(e.into()));
            }
        }
        Ok(())
    }

    pub fn remove(&self, id: EventLoopId) -> Result<(), EventLoopError> {
        self.check_destroyed()?;
        let id = id.0;
        let entry = match self.entries.remove(&id) {
            Some(e) => e,
            None => return Err(EventLoopError::NoEntry),
        };
        if let Some(fd) = entry.fd {
            if let Err(e) = uapi::epoll_ctl(self.epoll.raw(), c::EPOLL_CTL_DEL, fd, None) {
                if e.0 != c::ENOENT {
                    return Err(EventLoopError::RemoveFailed(e.into()));
                }
            }
        }
        Ok(())
    }

    pub fn schedule(&self, id: EventLoopId) -> Result<(), EventLoopError> {
        self.check_destroyed()?;
        self.scheduled.borrow_mut().push_back(id.0);
        Ok(())
    }

    pub fn run(&self) -> Result<(), EventLoopError> {
        let res = self.run_();
        self.stop();
        res
    }

    fn run_(&self) -> Result<(), EventLoopError> {
        self.check_destroyed()?;
        let mut buf = [c::epoll_event { events: 0, u64: 0 }; 16];
        while !self.destroyed.get() {
            while let Some(id) = self.scheduled.borrow_mut().pop_front() {
                if self.destroyed.get() {
                    break;
                }
                if let Some(entry) = self.entries.get(&id) {
                    if let Err(e) = entry.dispatcher.clone().dispatch(0) {
                        return Err(EventLoopError::DispatcherError(e));
                    }
                }
            }
            let num = match uapi::epoll_wait(self.epoll.raw(), &mut buf, -1) {
                Ok(n) => n,
                Err(Errno(c::EINTR)) => continue,
                Err(e) => return Err(EventLoopError::WaitFailed(e.into())),
            };
            for event in &buf[..num] {
                if self.destroyed.get() {
                    break;
                }
                let id = event.u64;
                let entry = match self.entries.get(&id) {
                    Some(d) => d,
                    None => {
                        log::warn!(
                            "Client {} created an event but has already been removed",
                            id,
                        );
                        continue;
                    }
                };
                if let Err(e) = entry.dispatcher.clone().dispatch(event.events as i32) {
                    return Err(EventLoopError::DispatcherError(e));
                }
            }
        }
        Ok(())
    }
}