use std::path::{Path, PathBuf};
use std::sync::mpsc;
use std::thread;
use std::time::Duration;
use std::{collections::HashSet, io};

use crossbeam_channel::Receiver;
use notify::{watcher, DebouncedEvent, RecommendedWatcher, RecursiveMode, Watcher};

use crate::{DirEntry, Metadata, ReadDir, VfsBackend, VfsEvent};

/// `VfsBackend` that uses `std::fs` and the `notify` crate.
pub struct StdBackend {
    watcher: RecommendedWatcher,
    watcher_receiver: Receiver<VfsEvent>,
    watches: HashSet<PathBuf>,
}

impl StdBackend {
    pub fn new() -> io::Result<StdBackend> {
        let (notify_tx, notify_rx) = mpsc::channel();
        let watcher = watcher(notify_tx, Duration::from_millis(50)).map_err(io::Error::other)?;

        let (tx, rx) = crossbeam_channel::unbounded();

        thread::spawn(move || {
            for event in notify_rx {
                match event {
                    DebouncedEvent::Create(path) => {
                        tx.send(VfsEvent::Create(path))?;
                    }
                    DebouncedEvent::Write(path) => {
                        tx.send(VfsEvent::Write(path))?;
                    }
                    DebouncedEvent::Remove(path) => {
                        tx.send(VfsEvent::Remove(path))?;
                    }
                    DebouncedEvent::Rename(from, to) => {
                        tx.send(VfsEvent::Remove(from))?;
                        tx.send(VfsEvent::Create(to))?;
                    }
                    _ => {}
                }
            }

            Result::<(), crossbeam_channel::SendError<VfsEvent>>::Ok(())
        });

        Ok(Self {
            watcher,
            watcher_receiver: rx,
            watches: HashSet::new(),
        })
    }
}

impl VfsBackend for StdBackend {
    fn read(&mut self, path: &Path) -> io::Result<Vec<u8>> {
        fs_err::read(path)
    }

    fn write(&mut self, path: &Path, data: &[u8]) -> io::Result<()> {
        fs_err::write(path, data)
    }

    fn exists(&mut self, path: &Path) -> io::Result<bool> {
        std::fs::exists(path)
    }

    fn read_dir(&mut self, path: &Path) -> io::Result<ReadDir> {
        let entries: Result<Vec<_>, _> = fs_err::read_dir(path)?.collect();
        let mut entries = entries?;

        entries.sort_by_cached_key(|entry| entry.file_name());

        let inner = entries
            .into_iter()
            .map(|entry| Ok(DirEntry { path: entry.path() }));

        Ok(ReadDir {
            inner: Box::new(inner),
        })
    }

    fn create_dir(&mut self, path: &Path) -> io::Result<()> {
        fs_err::create_dir(path)
    }

    fn create_dir_all(&mut self, path: &Path) -> io::Result<()> {
        fs_err::create_dir_all(path)
    }

    fn remove_file(&mut self, path: &Path) -> io::Result<()> {
        fs_err::remove_file(path)
    }

    fn remove_dir_all(&mut self, path: &Path) -> io::Result<()> {
        fs_err::remove_dir_all(path)
    }

    fn metadata(&mut self, path: &Path) -> io::Result<Metadata> {
        let inner = fs_err::metadata(path)?;

        Ok(Metadata {
            is_file: inner.is_file(),
        })
    }

    fn canonicalize(&mut self, path: &Path) -> io::Result<PathBuf> {
        dunce::canonicalize(path)
    }

    fn event_receiver(&self) -> crossbeam_channel::Receiver<VfsEvent> {
        self.watcher_receiver.clone()
    }

    fn watch(&mut self, path: &Path) -> io::Result<()> {
        if self.watches.contains(path)
            || path
                .ancestors()
                .any(|ancestor| self.watches.contains(ancestor))
        {
            Ok(())
        } else {
            self.watches.insert(path.to_path_buf());
            self.watcher
                .watch(path, RecursiveMode::Recursive)
                .map_err(io::Error::other)
        }
    }

    fn unwatch(&mut self, path: &Path) -> io::Result<()> {
        // `Vfs::commit_event` calls this for every removed path, but only the roots passed to
        // `watch` are watched. Forwarding any other path to notify made its FSEvents backend on
        // macOS restart the event stream, and changes made during the restart were lost.
        if self.watches.remove(path) {
            self.watcher.unwatch(path).map_err(io::Error::other)
        } else {
            Ok(())
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use std::fs;
    use std::time::Instant;

    /// `Vfs::commit_event` calls `unwatch` for every removed path, though only the roots passed
    /// to `watch` are watched. Forwarding those paths to notify made its macOS (FSEvents) backend
    /// restart its event stream each time, and events that happened during a restart were never
    /// delivered: deleting files while others were being created lost some of the creations.
    #[test]
    fn unwatching_paths_that_were_never_watched_loses_no_events() {
        let dir = tempfile::tempdir().unwrap();
        let root = dunce::canonicalize(dir.path()).unwrap();
        let (old, new) = (root.join("old"), root.join("new"));
        fs::create_dir(&old).unwrap();
        fs::create_dir(&new).unwrap();
        for i in 0..100 {
            fs::write(old.join(format!("{i}.txt")), "").unwrap();
        }

        let mut backend = StdBackend::new().unwrap();
        backend.watch(&root).unwrap();
        thread::sleep(Duration::from_millis(500));
        let events = backend.event_receiver();
        while events.try_recv().is_ok() {}

        for i in 0..100 {
            let removed = old.join(format!("{i}.txt"));
            fs::remove_file(&removed).unwrap();
            let _ = backend.unwatch(&removed);
            fs::write(new.join(format!("{i}.txt")), "").unwrap();
            // Paced so the OS does not coalesce the burst into a rescan, which notify 4 drops on
            // its own; this test is about the events unwatch loses.
            thread::sleep(Duration::from_millis(5));
        }

        let mut created = HashSet::new();
        let deadline = Instant::now() + Duration::from_secs(5);
        while created.len() < 100 && Instant::now() < deadline {
            if let Ok(VfsEvent::Create(path) | VfsEvent::Write(path)) =
                events.recv_timeout(Duration::from_millis(100))
            {
                if path.parent() == Some(new.as_path()) {
                    created.insert(path);
                }
            }
        }
        assert_eq!(created.len(), 100, "file creations were not reported");
    }
}