//! Watcher implementation for the inotify Linux API
//!
//! The inotify API provides a mechanism for monitoring filesystem events.  Inotify can be used to
//! monitor individual files, or to monitor directories.  When a directory is monitored, inotify
//! will return events for the directory itself, and for files inside the directory.

use super::event::*;
use super::{Config, Error, EventFn, RecursiveMode, Result, Watcher};
use crossbeam_channel::{bounded, unbounded, Sender};
use inotify as inotify_sys;
use inotify_sys::{EventMask, Inotify, WatchDescriptor, WatchMask};
use std::collections::HashMap;
use std::env;
use std::ffi::OsStr;
use std::fs::metadata;
use std::os::unix::io::AsRawFd;
use std::path::{Path, PathBuf};
use std::sync::Mutex;
use std::thread;
use std::time::Duration;
use walkdir::WalkDir;

const INOTIFY: mio::Token = mio::Token(0);
const MESSAGE: mio::Token = mio::Token(1);

// The EventLoop will set up a mio::Poll and use it to wait for the following:
//
// -  messages telling it what to do
//
// -  events telling it that something has happened on one of the watched files.
struct EventLoop {
    running: bool,
    poll: mio::Poll,
    event_loop_tx: mio_extras::channel::Sender<EventLoopMsg>,
    event_loop_rx: mio_extras::channel::Receiver<EventLoopMsg>,
    inotify: Option<Inotify>,
    event_fn: Box<dyn EventFn>,
    watches: HashMap<PathBuf, (WatchDescriptor, WatchMask, bool)>,
    paths: HashMap<WatchDescriptor, PathBuf>,
    rename_event: Option<Event>,
}

/// Watcher implementation based on inotify
pub struct INotifyWatcher(Mutex<mio_extras::channel::Sender<EventLoopMsg>>);

enum EventLoopMsg {
    AddWatch(PathBuf, RecursiveMode, Sender<Result<()>>),
    RemoveWatch(PathBuf, Sender<Result<()>>),
    Shutdown,
    RenameTimeout(usize),
    Configure(Config, Sender<Result<bool>>),
}

#[inline]
fn send_pending_rename_event(rename_event: &mut Option<Event>, event_fn: &dyn EventFn) {
    if let Some(e) = rename_event.take() {
        event_fn(Ok(e));
    }
}

#[inline]
fn add_watch_by_event(
    path: &Option<PathBuf>,
    event: &inotify_sys::Event<&OsStr>,
    watches: &HashMap<PathBuf, (WatchDescriptor, WatchMask, bool)>,
    add_watches: &mut Vec<PathBuf>,
) {
    if let Some(ref path) = *path {
        if event.mask.contains(EventMask::ISDIR) {
            if let Some(parent_path) = path.parent() {
                if let Some(&(_, _, is_recursive)) = watches.get(parent_path) {
                    if is_recursive {
                        add_watches.push(path.to_owned());
                    }
                }
            }
        }
    }
}

#[inline]
fn remove_watch_by_event(
    path: &Option<PathBuf>,
    watches: &HashMap<PathBuf, (WatchDescriptor, WatchMask, bool)>,
    remove_watches: &mut Vec<PathBuf>,
) {
    if let Some(ref path) = *path {
        if watches.contains_key(path) {
            remove_watches.push(path.to_owned());
        }
    }
}

impl EventLoop {
    pub fn new(inotify: Inotify, event_fn: Box<dyn EventFn>) -> Result<Self> {
        let (event_loop_tx, event_loop_rx) = mio_extras::channel::channel::<EventLoopMsg>();
        let poll = mio::Poll::new()?;
        poll.register(
            &event_loop_rx,
            MESSAGE,
            mio::Ready::readable(),
            mio::PollOpt::edge(),
        )?;

        let inotify_fd = inotify.as_raw_fd();
        let evented_inotify = mio::unix::EventedFd(&inotify_fd);
        poll.register(
            &evented_inotify,
            INOTIFY,
            mio::Ready::readable(),
            mio::PollOpt::edge(),
        )?;

        let event_loop = EventLoop {
            running: true,
            poll,
            event_loop_tx,
            event_loop_rx,
            inotify: Some(inotify),
            event_fn,
            watches: HashMap::new(),
            paths: HashMap::new(),
            rename_event: None,
        };
        Ok(event_loop)
    }

    // Run the event loop.
    pub fn run(self) {
        thread::spawn(|| self.event_loop_thread());
    }

    fn event_loop_thread(mut self) {
        let mut events = mio::Events::with_capacity(16);
        loop {
            // Wait for something to happen.
            self.poll.poll(&mut events, None).expect("poll failed");

            // Process whatever happened.
            for event in &events {
                self.handle_event(&event);
            }

            // Stop, if we're done.
            if !self.running {
                break;
            }
        }
    }

    fn channel(&self) -> mio_extras::channel::Sender<EventLoopMsg> {
        self.event_loop_tx.clone()
    }

    // Handle a single event.
    fn handle_event(&mut self, event: &mio::Event) {
        match event.token() {
            MESSAGE => {
                // The channel is readable - handle messages.
                self.handle_messages()
            }
            INOTIFY => {
                // inotify has something to tell us.
                self.handle_inotify()
            }
            _ => unreachable!(),
        }
    }

    fn handle_messages(&mut self) {
        while let Ok(msg) = self.event_loop_rx.try_recv() {
            match msg {
                EventLoopMsg::AddWatch(path, recursive_mode, tx) => {
                    let _ = tx.send(self.add_watch(path, recursive_mode.is_recursive(), true));
                }
                EventLoopMsg::RemoveWatch(path, tx) => {
                    let _ = tx.send(self.remove_watch(path, false));
                }
                EventLoopMsg::Shutdown => {
                    let _ = self.remove_all_watches();
                    if let Some(inotify) = self.inotify.take() {
                        let _ = inotify.close();
                    }
                    self.running = false;
                    break;
                }
                EventLoopMsg::RenameTimeout(cookie) => {
                    let current_cookie = self.rename_event.as_ref().and_then(|e| e.tracker());
                    // send pending rename event only if the rename event for which the timer has been created hasn't been handled already; otherwise ignore this timeout
                    if current_cookie == Some(cookie) {
                        send_pending_rename_event(&mut self.rename_event, &*self.event_fn);
                    }
                }
                EventLoopMsg::Configure(config, tx) => {
                    self.configure_raw_mode(config, tx);
                }
            }
        }
    }

    fn configure_raw_mode(&mut self, _config: Config, tx: Sender<Result<bool>>) {
        tx.send(Ok(false))
            .expect("configuration channel disconnected");
    }

    fn handle_inotify(&mut self) {
        let mut add_watches = Vec::new();
        let mut remove_watches = Vec::new();

        if let Some(ref mut inotify) = self.inotify {
            let mut buffer = [0; 1024];
            match inotify.read_events(&mut buffer) {
                Ok(events) => {
                    for event in events {
                        if event.mask.contains(EventMask::Q_OVERFLOW) {
                            let ev = Ok(Event::new(EventKind::Other).set_flag(Flag::Rescan));
                            (self.event_fn)(ev);
                        }

                        let path = match event.name {
                            Some(name) => self.paths.get(&event.wd).map(|root| root.join(&name)),
                            None => self.paths.get(&event.wd).cloned(),
                        };

                        if event.mask.contains(EventMask::MOVED_FROM) {
                            send_pending_rename_event(&mut self.rename_event, &*self.event_fn);
                            remove_watch_by_event(&path, &self.watches, &mut remove_watches);
                            self.rename_event = Some(
                                Event::new(EventKind::Modify(ModifyKind::Name(RenameMode::From)))
                                    .add_some_path(path.clone())
                                    .set_tracker(event.cookie as usize),
                            );
                        } else {
                            let mut evs = Vec::new();
                            if event.mask.contains(EventMask::MOVED_TO) {
                                if let Some(e) = self.rename_event.take() {
                                    if e.tracker() == Some(event.cookie as usize) {
                                        (self.event_fn)(Ok(e.clone()));
                                        evs.push(
                                            Event::new(EventKind::Modify(ModifyKind::Name(
                                                RenameMode::To,
                                            )))
                                            .set_tracker(event.cookie as usize)
                                            .add_some_path(path.clone()),
                                        );
                                        evs.push(
                                            Event::new(EventKind::Modify(ModifyKind::Name(
                                                RenameMode::Both,
                                            )))
                                            .set_tracker(event.cookie as usize)
                                            .add_some_path(e.paths.first().cloned())
                                            .add_some_path(path.clone()),
                                        );
                                    } else {
                                        // TODO should it be rename?
                                        evs.push(
                                            Event::new(EventKind::Create(
                                                if event.mask.contains(EventMask::ISDIR) {
                                                    CreateKind::Folder
                                                } else {
                                                    CreateKind::File
                                                },
                                            ))
                                            .add_some_path(path.clone()),
                                        );
                                    }
                                } else {
                                    // TODO should it be rename?
                                    evs.push(
                                        Event::new(EventKind::Create(
                                            if event.mask.contains(EventMask::ISDIR) {
                                                CreateKind::Folder
                                            } else {
                                                CreateKind::File
                                            },
                                        ))
                                        .add_some_path(path.clone()),
                                    );
                                }
                                add_watch_by_event(&path, &event, &self.watches, &mut add_watches);
                            }
                            if event.mask.contains(EventMask::MOVE_SELF) {
                                evs.push(
                                    Event::new(EventKind::Modify(ModifyKind::Name(
                                        RenameMode::From,
                                    )))
                                    .add_some_path(path.clone()),
                                );
                                // TODO stat the path and get to new path
                                // - emit To and Both events
                                // - change prefix for further events
                            }
                            if event.mask.contains(EventMask::CREATE) {
                                evs.push(
                                    Event::new(EventKind::Create(
                                        if event.mask.contains(EventMask::ISDIR) {
                                            CreateKind::Folder
                                        } else {
                                            CreateKind::File
                                        },
                                    ))
                                    .add_some_path(path.clone()),
                                );
                                add_watch_by_event(&path, &event, &self.watches, &mut add_watches);
                            }
                            if event.mask.contains(EventMask::DELETE_SELF)
                                || event.mask.contains(EventMask::DELETE)
                            {
                                evs.push(
                                    Event::new(EventKind::Remove(
                                        if event.mask.contains(EventMask::ISDIR) {
                                            RemoveKind::Folder
                                        } else {
                                            RemoveKind::File
                                        },
                                    ))
                                    .add_some_path(path.clone()),
                                );
                                remove_watch_by_event(&path, &self.watches, &mut remove_watches);
                            }
                            if event.mask.contains(EventMask::MODIFY) {
                                evs.push(
                                    Event::new(EventKind::Modify(ModifyKind::Data(
                                        DataChange::Any,
                                    )))
                                    .add_some_path(path.clone()),
                                );
                            }
                            if event.mask.contains(EventMask::CLOSE_WRITE) {
                                evs.push(
                                    Event::new(EventKind::Access(AccessKind::Close(
                                        AccessMode::Write,
                                    )))
                                    .add_some_path(path.clone()),
                                );
                            }
                            if event.mask.contains(EventMask::CLOSE_NOWRITE) {
                                evs.push(
                                    Event::new(EventKind::Access(AccessKind::Close(
                                        AccessMode::Read,
                                    )))
                                    .add_some_path(path.clone()),
                                );
                            }
                            if event.mask.contains(EventMask::ATTRIB) {
                                evs.push(
                                    Event::new(EventKind::Modify(ModifyKind::Metadata(
                                        MetadataKind::Any,
                                    )))
                                    .add_some_path(path.clone()),
                                );
                            }
                            if event.mask.contains(EventMask::OPEN) {
                                evs.push(
                                    Event::new(EventKind::Access(AccessKind::Open(
                                        AccessMode::Any,
                                    )))
                                    .add_some_path(path.clone()),
                                );
                            }

                            if !evs.is_empty() {
                                send_pending_rename_event(&mut self.rename_event, &*self.event_fn);
                            }

                            for ev in evs {
                                (self.event_fn)(Ok(ev));
                            }
                        }
                    }

                    // When receiving only the first part of a move event (IN_MOVED_FROM) it is unclear
                    // whether the second part (IN_MOVED_TO) will arrive because the file or directory
                    // could just have been moved out of the watched directory. So it's necessary to wait
                    // for possible subsequent events in case it's a complete move event but also to make sure
                    // that the first part of the event is handled in a timely manner in case no subsequent events arrive.
                    // TODO: don't do this here, instead leave it entirely to the debounce
                    // -> related to some rename events being reported as creates.

                    if let Some(ref rename_event) = self.rename_event {
                        let event_loop_tx = self.event_loop_tx.clone();
                        let cookie = rename_event.tracker().unwrap(); // unwrap is safe because rename_event is always set with some cookie
                        thread::spawn(move || {
                            thread::sleep(Duration::from_millis(10)); // wait up to 10 ms for a subsequent event
                            event_loop_tx
                                .send(EventLoopMsg::RenameTimeout(cookie))
                                .unwrap();
                        });
                    }
                }
                Err(e) => {
                    (self.event_fn)(Err(Error::io(e)));
                }
            }
        }

        for path in remove_watches {
            self.remove_watch(path, true).ok();
        }

        for path in add_watches {
            self.add_watch(path, true, false).ok();
        }
    }

    fn add_watch(&mut self, path: PathBuf, is_recursive: bool, mut watch_self: bool) -> Result<()> {
        // If the watch is not recursive, or if we determine (by stat'ing the path to get its
        // metadata) that the watched path is not a directory, add a single path watch.
        if !is_recursive || !metadata(&path).map_err(Error::io)?.is_dir() {
            return self.add_single_watch(path, false, true);
        }

        for entry in WalkDir::new(path)
            .follow_links(true)
            .into_iter()
            .filter_map(filter_dir)
        {
            self.add_single_watch(entry.path().to_path_buf(), is_recursive, watch_self)?;
            watch_self = false;
        }

        Ok(())
    }

    fn add_single_watch(
        &mut self,
        path: PathBuf,
        is_recursive: bool,
        watch_self: bool,
    ) -> Result<()> {
        let mut watchmask = WatchMask::ATTRIB
            | WatchMask::CREATE
            | WatchMask::DELETE
            | WatchMask::CLOSE_WRITE
            | WatchMask::MODIFY
            | WatchMask::MOVED_FROM
            | WatchMask::MOVED_TO;

        if watch_self {
            watchmask.insert(WatchMask::DELETE_SELF);
            watchmask.insert(WatchMask::MOVE_SELF);
        }

        if let Some(&(_, old_watchmask, _)) = self.watches.get(&path) {
            watchmask.insert(old_watchmask);
            watchmask.insert(WatchMask::MASK_ADD);
        }

        if let Some(ref mut inotify) = self.inotify {
            match inotify.add_watch(&path, watchmask) {
                Err(e) => Err(Error::io(e)),
                Ok(w) => {
                    watchmask.remove(WatchMask::MASK_ADD);
                    self.watches
                        .insert(path.clone(), (w.clone(), watchmask, is_recursive));
                    self.paths.insert(w, path);
                    Ok(())
                }
            }
        } else {
            Ok(())
        }
    }

    fn remove_watch(&mut self, path: PathBuf, remove_recursive: bool) -> Result<()> {
        match self.watches.remove(&path) {
            None => return Err(Error::watch_not_found()),
            Some((w, _, is_recursive)) => {
                if let Some(ref mut inotify) = self.inotify {
                    inotify.rm_watch(w.clone()).map_err(Error::io)?;
                    self.paths.remove(&w);

                    if is_recursive || remove_recursive {
                        let mut remove_list = Vec::new();
                        for (w, p) in &self.paths {
                            if p.starts_with(&path) {
                                inotify.rm_watch(w.clone()).map_err(Error::io)?;
                                self.watches.remove(p);
                                remove_list.push(w.clone());
                            }
                        }
                        for w in remove_list {
                            self.paths.remove(&w);
                        }
                    }
                }
            }
        }
        Ok(())
    }

    fn remove_all_watches(&mut self) -> Result<()> {
        if let Some(ref mut inotify) = self.inotify {
            for w in self.paths.keys() {
                inotify.rm_watch(w.clone()).map_err(Error::io)?;
            }
            self.watches.clear();
            self.paths.clear();
        }
        Ok(())
    }
}

/// return `DirEntry` when it is a directory
fn filter_dir(e: walkdir::Result<walkdir::DirEntry>) -> Option<walkdir::DirEntry> {
    if let Ok(e) = e {
        if let Ok(metadata) = e.metadata() {
            if metadata.is_dir() {
                return Some(e);
            }
        }
    }
    None
}

impl INotifyWatcher {
    fn from_event_fn(event_fn: Box<dyn EventFn>) -> Result<Self> {
        let inotify = Inotify::init()?;
        let event_loop = EventLoop::new(inotify, event_fn)?;
        let channel = event_loop.channel();
        event_loop.run();
        Ok(INotifyWatcher(Mutex::new(channel)))
    }

    fn watch_inner(&mut self, path: &Path, recursive_mode: RecursiveMode) -> Result<()> {
        let pb = if path.is_absolute() {
            path.to_owned()
        } else {
            let p = env::current_dir().map_err(Error::io)?;
            p.join(path)
        };
        let (tx, rx) = unbounded();
        let msg = EventLoopMsg::AddWatch(pb, recursive_mode, tx);

        // we expect the event loop to live and reply => unwraps must not panic
        self.0.lock().unwrap().send(msg).unwrap();
        rx.recv().unwrap()
    }

    fn unwatch_inner(&mut self, path: &Path) -> Result<()> {
        let pb = if path.is_absolute() {
            path.to_owned()
        } else {
            let p = env::current_dir().map_err(Error::io)?;
            p.join(path)
        };
        let (tx, rx) = unbounded();
        let msg = EventLoopMsg::RemoveWatch(pb, tx);

        // we expect the event loop to live and reply => unwraps must not panic
        self.0.lock().unwrap().send(msg).unwrap();
        rx.recv().unwrap()
    }
}

impl Watcher for INotifyWatcher {
    fn new_immediate<F: EventFn>(event_fn: F) -> Result<INotifyWatcher> {
        INotifyWatcher::from_event_fn(Box::new(event_fn))
    }

    fn watch<P: AsRef<Path>>(&mut self, path: P, recursive_mode: RecursiveMode) -> Result<()> {
        self.watch_inner(path.as_ref(), recursive_mode)
    }

    fn unwatch<P: AsRef<Path>>(&mut self, path: P) -> Result<()> {
        self.unwatch_inner(path.as_ref())
    }

    fn configure(&mut self, config: Config) -> Result<bool> {
        let (tx, rx) = bounded(1);
        self.0.lock()?.send(EventLoopMsg::Configure(config, tx))?;
        rx.recv()?
    }
}

impl Drop for INotifyWatcher {
    fn drop(&mut self) {
        // we expect the event loop to live => unwrap must not panic
        self.0.lock().unwrap().send(EventLoopMsg::Shutdown).unwrap();
    }
}