use std::{cell::RefCell, collections::HashMap, rc::Rc, sync::Arc, time::Duration};

#[cfg(unix)]
use std::os::unix::io::{AsFd, AsRawFd, BorrowedFd as Borrowed, RawFd as Raw};

#[cfg(windows)]
use std::os::windows::io::{AsRawSocket, AsSocket, BorrowedSocket as Borrowed, RawSocket as Raw};

use polling::{Event, Events, PollMode, Poller};

use crate::sources::timer::TimerWheel;
use crate::token::TokenInner;
use crate::RegistrationToken;

/// Possible modes for registering a file descriptor
#[derive(Copy, Clone, Debug)]
pub enum Mode {
    /// Single event generation
    ///
    /// This FD will be disabled as soon as it has generated one event.
    ///
    /// The user will need to use `LoopHandle::update()` to re-enable it if
    /// desired.
    OneShot,

    /// Level-triggering
    ///
    /// This FD will report events on every poll as long as the requested interests
    /// are available.
    Level,

    /// Edge-triggering
    ///
    /// This FD will report events only when it *gains* one of the requested interests.
    /// it must thus be fully processed before it'll generate events again.
    ///
    /// This mode is not supported on certain platforms, and an error will be returned
    /// if it is used.
    ///
    /// ## Supported Platforms
    ///
    /// As of the time of writing, the platforms that support edge triggered polling are
    /// as follows:
    ///
    /// - Linux/Android
    /// - macOS/iOS/tvOS/watchOS
    /// - FreeBSD/OpenBSD/NetBSD/DragonflyBSD
    Edge,
}

/// Interest to register regarding the file descriptor
#[derive(Copy, Clone, Debug)]
pub struct Interest {
    /// Wait for the FD to be readable
    pub readable: bool,

    /// Wait for the FD to be writable
    pub writable: bool,
}

impl Interest {
    /// Shorthand for empty interest
    pub const EMPTY: Interest = Interest {
        readable: false,
        writable: false,
    };

    /// Shorthand for read interest
    pub const READ: Interest = Interest {
        readable: true,
        writable: false,
    };

    /// Shorthand for write interest
    pub const WRITE: Interest = Interest {
        readable: false,
        writable: true,
    };

    /// Shorthand for read and write interest
    pub const BOTH: Interest = Interest {
        readable: true,
        writable: true,
    };
}

/// Readiness for a file descriptor notification
#[derive(Copy, Clone, Debug)]
pub struct Readiness {
    /// Is the FD readable
    pub readable: bool,

    /// Is the FD writable
    pub writable: bool,

    /// Is the FD in an error state
    pub error: bool,
}

impl Readiness {
    /// Shorthand for empty readiness
    pub const EMPTY: Readiness = Readiness {
        readable: false,
        writable: false,
        error: false,
    };
}

#[derive(Debug)]
pub(crate) struct PollEvent {
    pub(crate) readiness: Readiness,
    pub(crate) token: Token,
}

/// Factory for creating tokens in your registrations
///
/// When composing event sources, each sub-source needs to
/// have its own token to identify itself. This factory is
/// provided to produce such unique tokens.

#[derive(Debug)]
pub struct TokenFactory {
    next_token: TokenInner,
}

impl TokenFactory {
    pub(crate) fn new(token: TokenInner) -> TokenFactory {
        TokenFactory {
            next_token: token.forget_sub_id(),
        }
    }

    /// Get the "raw" registration token of this TokenFactory
    pub(crate) fn registration_token(&self) -> RegistrationToken {
        RegistrationToken::new(self.next_token.forget_sub_id())
    }

    /// Produce a new unique token
    pub fn token(&mut self) -> Token {
        let token = self.next_token;
        self.next_token = token.increment_sub_id();
        Token { inner: token }
    }
}

/// A token (for implementation of the [`EventSource`](crate::EventSource) trait)
///
/// This token is produced by the [`TokenFactory`] and is used when calling the
/// [`EventSource`](crate::EventSource) implementations to process event, in order
/// to identify which sub-source produced them.
///
/// You should forward it to the [`Poll`] when registering your file descriptors.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Token {
    pub(crate) inner: TokenInner,
}

/// The polling system
///
/// This type represents the polling system of calloop, on which you
/// can register your file descriptors. This interface is only accessible in
/// implementations of the [`EventSource`](crate::EventSource) trait.
///
/// You only need to interact with this type if you are implementing your
/// own event sources, while implementing the [`EventSource`](crate::EventSource) trait.
/// And even in this case, you can often just use the [`Generic`](crate::generic::Generic) event
/// source and delegate the implementations to it.
pub struct Poll {
    /// The handle to wepoll/epoll/kqueue/... used to poll for events.
    pub(crate) poller: Arc<Poller>,

    /// The buffer of events returned by the poller.
    events: RefCell<Events>,

    /// The sources registered as level triggered.
    ///
    /// Some platforms that `polling` supports do not support level-triggered events. As of the time
    /// of writing, this only includes Solaris and illumos. To work around this, we emulate level
    /// triggered events by keeping this map of file descriptors.
    ///
    /// One can emulate level triggered events on top of oneshot events by just re-registering the
    /// file descriptor every time it is polled. However, this is not ideal, as it requires a
    /// system call every time. It's better to use the intergrated system, if available.
    level_triggered: Option<RefCell<HashMap<usize, (Raw, polling::Event)>>>,

    pub(crate) timers: Rc<RefCell<TimerWheel>>,
}

impl std::fmt::Debug for Poll {
    #[cfg_attr(feature = "nightly_coverage", coverage(off))]
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.write_str("Poll { ... }")
    }
}

impl Poll {
    pub(crate) fn new() -> crate::Result<Poll> {
        Self::new_inner(false)
    }

    fn new_inner(force_fallback_lt: bool) -> crate::Result<Poll> {
        let poller = Poller::new()?;
        let level_triggered = if poller.supports_level() && !force_fallback_lt {
            None
        } else {
            Some(RefCell::new(HashMap::new()))
        };

        Ok(Poll {
            poller: Arc::new(poller),
            events: RefCell::new(Events::new()),
            timers: Rc::new(RefCell::new(TimerWheel::new())),
            level_triggered,
        })
    }

    pub(crate) fn poll(&self, mut timeout: Option<Duration>) -> crate::Result<Vec<PollEvent>> {
        let now = std::time::Instant::now();

        // Adjust the timeout for the timers.
        if let Some(next_timeout) = self.timers.borrow().next_deadline() {
            if next_timeout <= now {
                timeout = Some(Duration::ZERO);
            } else if let Some(deadline) = timeout {
                timeout = Some(std::cmp::min(deadline, next_timeout - now));
            } else {
                timeout = Some(next_timeout - now);
            }
        };

        let mut events = self.events.borrow_mut();
        events.clear();
        self.poller.wait(&mut events, timeout)?;

        // Convert `polling` events to `calloop` events.
        let level_triggered = self.level_triggered.as_ref().map(RefCell::borrow);
        let mut poll_events = events
            .iter()
            .map(|ev| {
                // If we need to emulate level-triggered events...
                if let Some(level_triggered) = level_triggered.as_ref() {
                    // ...and this event is from a level-triggered source...
                    if let Some((source, interest)) = level_triggered.get(&ev.key) {
                        // ...then we need to re-register the source.
                        // SAFETY: The source is valid.
                        self.poller
                            .modify(unsafe { Borrowed::borrow_raw(*source) }, *interest)?;
                    }
                }

                Ok(PollEvent {
                    readiness: Readiness {
                        readable: ev.readable,
                        writable: ev.writable,
                        error: false,
                    },
                    token: Token {
                        inner: TokenInner::from(ev.key),
                    },
                })
            })
            .collect::<std::io::Result<Vec<_>>>()?;

        drop(events);

        // Update 'now' as some time may have elapsed in poll()
        let now = std::time::Instant::now();
        let mut timers = self.timers.borrow_mut();
        while let Some((_, token)) = timers.next_expired(now) {
            poll_events.push(PollEvent {
                readiness: Readiness {
                    readable: true,
                    writable: false,
                    error: false,
                },
                token,
            });
        }

        Ok(poll_events)
    }

    /// Register a new file descriptor for polling
    ///
    /// The file descriptor will be registered with given interest,
    /// mode and token. This function will fail if given a
    /// bad file descriptor or if the provided file descriptor is already
    /// registered.
    ///
    /// # Safety
    ///
    /// The registered source must not be dropped before it is unregistered.
    ///
    /// # Leaking tokens
    ///
    /// If your event source is dropped without being unregistered, the token
    /// passed in here will remain on the heap and continue to be used by the
    /// polling system even though no event source will match it.
    pub unsafe fn register(
        &self,
        #[cfg(unix)] fd: impl AsFd,
        #[cfg(windows)] fd: impl AsSocket,
        interest: Interest,
        mode: Mode,
        token: Token,
    ) -> crate::Result<()> {
        let raw = {
            #[cfg(unix)]
            {
                fd.as_fd().as_raw_fd()
            }

            #[cfg(windows)]
            {
                fd.as_socket().as_raw_socket()
            }
        };

        let ev = cvt_interest(interest, token);

        // SAFETY: See invariant on function.
        unsafe {
            self.poller
                .add_with_mode(raw, ev, cvt_mode(mode, self.poller.supports_level()))?;
        }

        // If this is level triggered and we're emulating level triggered mode...
        if let (Mode::Level, Some(level_triggered)) = (mode, self.level_triggered.as_ref()) {
            // ...then we need to keep track of the source.
            let mut level_triggered = level_triggered.borrow_mut();
            level_triggered.insert(ev.key, (raw, ev));
        }

        Ok(())
    }

    /// Update the registration for a file descriptor
    ///
    /// This allows you to change the interest, mode or token of a file
    /// descriptor. Fails if the provided fd is not currently registered.
    ///
    /// See note on [`register()`](Self::register()) regarding leaking.
    pub fn reregister(
        &self,
        #[cfg(unix)] fd: impl AsFd,
        #[cfg(windows)] fd: impl AsSocket,
        interest: Interest,
        mode: Mode,
        token: Token,
    ) -> crate::Result<()> {
        let (borrowed, raw) = {
            #[cfg(unix)]
            {
                (fd.as_fd(), fd.as_fd().as_raw_fd())
            }

            #[cfg(windows)]
            {
                (fd.as_socket(), fd.as_socket().as_raw_socket())
            }
        };

        let ev = cvt_interest(interest, token);
        self.poller
            .modify_with_mode(borrowed, ev, cvt_mode(mode, self.poller.supports_level()))?;

        // If this is level triggered and we're emulating level triggered mode...
        if let (Mode::Level, Some(level_triggered)) = (mode, self.level_triggered.as_ref()) {
            // ...then we need to keep track of the source.
            let mut level_triggered = level_triggered.borrow_mut();
            level_triggered.insert(ev.key, (raw, ev));
        }

        Ok(())
    }

    /// Unregister a file descriptor
    ///
    /// This file descriptor will no longer generate events. Fails if the
    /// provided file descriptor is not currently registered.
    pub fn unregister(
        &self,
        #[cfg(unix)] fd: impl AsFd,
        #[cfg(windows)] fd: impl AsSocket,
    ) -> crate::Result<()> {
        let (borrowed, raw) = {
            #[cfg(unix)]
            {
                (fd.as_fd(), fd.as_fd().as_raw_fd())
            }

            #[cfg(windows)]
            {
                (fd.as_socket(), fd.as_socket().as_raw_socket())
            }
        };
        self.poller.delete(borrowed)?;

        if let Some(level_triggered) = self.level_triggered.as_ref() {
            let mut level_triggered = level_triggered.borrow_mut();
            level_triggered.retain(|_, (source, _)| *source != raw);
        }

        Ok(())
    }

    /// Get a thread-safe handle which can be used to wake up the `Poll`.
    pub(crate) fn notifier(&self) -> Notifier {
        Notifier(self.poller.clone())
    }

    /// Get a reference to the poller.
    pub(crate) fn poller(&self) -> &Arc<Poller> {
        &self.poller
    }
}

/// Thread-safe handle which can be used to wake up the `Poll`.
#[derive(Clone)]
pub(crate) struct Notifier(Arc<Poller>);

impl Notifier {
    pub(crate) fn notify(&self) -> crate::Result<()> {
        self.0.notify()?;

        Ok(())
    }
}

fn cvt_interest(interest: Interest, tok: Token) -> Event {
    let mut event = Event::none(tok.inner.into());
    event.readable = interest.readable;
    event.writable = interest.writable;
    event
}

fn cvt_mode(mode: Mode, supports_other_modes: bool) -> PollMode {
    if !supports_other_modes {
        return PollMode::Oneshot;
    }

    match mode {
        Mode::Edge => PollMode::Edge,
        Mode::Level => PollMode::Level,
        Mode::OneShot => PollMode::Oneshot,
    }
}