//! Local game directory monitor.

use std::{
    collections::HashSet,
    path::{Component, Path, PathBuf},
    sync::Arc,
    time::Duration,
};

use notify::{Config, Event, RecommendedWatcher, RecursiveMode, Watcher};
use tokio::sync::{RwLock, mpsc::UnboundedSender};

use crate::{
    PeerEvent,
    config::LOCAL_GAME_FALLBACK_SCAN_SECS,
    context::Ctx,
    handlers::update_and_announce_games,
    local_games::{
        is_ignored_game_root_name,
        is_local_dir_name,
        rescan_local_game,
        scan_local_library,
    },
};

struct WatchState {
    watcher: RecommendedWatcher,
    game_dir: PathBuf,
    watched: HashSet<PathBuf>,
}

#[derive(Clone, Default)]
struct RescanGate {
    running: Arc<RwLock<HashSet<String>>>,
    pending: Arc<RwLock<HashSet<String>>>,
}

/// Monitors the local game directory for changes.
pub async fn run_local_game_monitor(
    tx_notify_ui: UnboundedSender<PeerEvent>,
    ctx: Ctx,
) -> eyre::Result<()> {
    log::info!("Starting notify-based local game directory monitor");

    let (watch_tx, mut watch_rx) = tokio::sync::mpsc::unbounded_channel::<notify::Result<Event>>();
    let mut watch_state = build_watch_state(&ctx, watch_tx.clone()).await;
    let gate = RescanGate::default();
    let mut fallback_interval =
        tokio::time::interval(Duration::from_secs(LOCAL_GAME_FALLBACK_SCAN_SECS));

    loop {
        tokio::select! {
            () = ctx.shutdown.cancelled() => return Ok(()),
            _ = fallback_interval.tick() => {
                run_fallback_scan(&ctx, &tx_notify_ui).await;
                reconcile_watch_state(&ctx, &mut watch_state, watch_tx.clone()).await;
            }
            Some(event) = watch_rx.recv() => {
                handle_watch_event(
                    &ctx,
                    &tx_notify_ui,
                    &gate,
                    event,
                ).await;
                reconcile_watch_state(&ctx, &mut watch_state, watch_tx.clone()).await;
            }
        }
    }
}

async fn build_watch_state(
    ctx: &Ctx,
    watch_tx: tokio::sync::mpsc::UnboundedSender<notify::Result<Event>>,
) -> Option<WatchState> {
    let game_dir = ctx.game_dir.read().await.clone();
    let mut fs_watcher = match RecommendedWatcher::new(
        move |result| {
            let _ = watch_tx.send(result);
        },
        Config::default(),
    ) {
        Ok(watcher) => watcher,
        Err(err) => {
            log::warn!("Filesystem watcher unavailable; falling back to periodic scans: {err}");
            return None;
        }
    };

    let watched_paths = match watch_game_roots(&mut fs_watcher, &game_dir).await {
        Ok(paths) => paths,
        Err(err) => {
            log::warn!(
                "Failed to initialize filesystem watcher for {}: {err}; falling back to periodic scans",
                game_dir.display()
            );
            return None;
        }
    };

    Some(WatchState {
        watcher: fs_watcher,
        game_dir,
        watched: watched_paths,
    })
}

async fn reconcile_watch_state(
    ctx: &Ctx,
    watch_state: &mut Option<WatchState>,
    watch_tx: tokio::sync::mpsc::UnboundedSender<notify::Result<Event>>,
) {
    let current_game_dir = ctx.game_dir.read().await.clone();
    if watch_state
        .as_ref()
        .is_none_or(|state| state.game_dir != current_game_dir)
    {
        *watch_state = build_watch_state(ctx, watch_tx).await;
        return;
    }

    if let Some(state) = watch_state
        && let Err(err) = reconcile_game_root_watches(state).await
    {
        log::warn!(
            "Failed to reconcile filesystem watches for {}: {err}",
            state.game_dir.display()
        );
    }
}

async fn watch_game_roots(
    watcher: &mut RecommendedWatcher,
    game_dir: &Path,
) -> eyre::Result<HashSet<PathBuf>> {
    let mut watched_paths = HashSet::new();
    watch_path(watcher, game_dir, &mut watched_paths)?;

    for root in list_game_roots(game_dir).await? {
        watch_path(watcher, &root, &mut watched_paths)?;
    }

    Ok(watched_paths)
}

async fn reconcile_game_root_watches(state: &mut WatchState) -> eyre::Result<()> {
    let desired = {
        let mut desired = HashSet::from([state.game_dir.clone()]);
        desired.extend(list_game_roots(&state.game_dir).await?);
        desired
    };

    let stale_paths = state
        .watched
        .difference(&desired)
        .cloned()
        .collect::<Vec<_>>();
    for path in stale_paths {
        if let Err(err) = state.watcher.unwatch(&path) {
            log::debug!("Failed to unwatch {}: {err}", path.display());
        }
        state.watched.remove(&path);
    }

    let new_paths = desired
        .difference(&state.watched)
        .cloned()
        .collect::<Vec<_>>();
    for path in new_paths {
        watch_path(&mut state.watcher, &path, &mut state.watched)?;
    }

    Ok(())
}

fn watch_path(
    watcher: &mut RecommendedWatcher,
    path: &Path,
    watched_paths: &mut HashSet<PathBuf>,
) -> notify::Result<()> {
    watcher.watch(path, RecursiveMode::NonRecursive)?;
    watched_paths.insert(path.to_path_buf());
    Ok(())
}

async fn list_game_roots(game_dir: &Path) -> eyre::Result<Vec<PathBuf>> {
    let mut roots = Vec::new();
    let mut entries = match tokio::fs::read_dir(game_dir).await {
        Ok(entries) => entries,
        Err(err) if err.kind() == std::io::ErrorKind::NotFound => return Ok(roots),
        Err(err) => return Err(err.into()),
    };

    while let Some(entry) = entries.next_entry().await? {
        if !entry.file_type().await?.is_dir() {
            continue;
        }
        let Some(name) = entry.file_name().to_str().map(ToOwned::to_owned) else {
            continue;
        };
        if is_ignored_game_root_name(&name) {
            continue;
        }
        roots.push(entry.path());
    }
    Ok(roots)
}

async fn handle_watch_event(
    ctx: &Ctx,
    tx_notify_ui: &UnboundedSender<PeerEvent>,
    gate: &RescanGate,
    event: notify::Result<Event>,
) {
    let event = match event {
        Ok(event) => event,
        Err(err) => {
            log::warn!("Filesystem watcher event error: {err}");
            return;
        }
    };

    let game_dir = ctx.game_dir.read().await.clone();
    let ids = event
        .paths
        .iter()
        .filter_map(|path| game_id_from_event_path(&game_dir, path))
        .collect::<HashSet<_>>();

    for id in ids {
        if ctx.active_operations.read().await.contains_key(&id) {
            log::debug!("Dropping filesystem event for {id}: operation active");
            continue;
        }
        queue_rescan(ctx, tx_notify_ui, gate, id).await;
    }
}

async fn queue_rescan(
    ctx: &Ctx,
    tx_notify_ui: &UnboundedSender<PeerEvent>,
    gate: &RescanGate,
    id: String,
) {
    {
        let mut running = gate.running.write().await;
        if running.contains(&id) {
            gate.pending.write().await.insert(id);
            return;
        }
        running.insert(id.clone());
    }

    let ctx = ctx.clone();
    let tx_notify_ui = tx_notify_ui.clone();
    let gate = gate.clone();
    ctx.task_tracker.clone().spawn(async move {
        run_gated_rescan(ctx, tx_notify_ui, gate, id).await;
    });
}

async fn run_gated_rescan(
    ctx: Ctx,
    tx_notify_ui: UnboundedSender<PeerEvent>,
    gate: RescanGate,
    id: String,
) {
    loop {
        gate.pending.write().await.remove(&id);

        if ctx.active_operations.read().await.contains_key(&id) {
            break;
        }

        let game_dir = ctx.game_dir.read().await.clone();
        let catalog = ctx.catalog.read().await.clone();
        match rescan_local_game(&game_dir, &catalog, &id).await {
            Ok(scan) => update_and_announce_games(&ctx, &tx_notify_ui, scan).await,
            Err(err) => log::error!("Failed to rescan local game {id}: {err}"),
        }

        if !gate.pending.write().await.remove(&id) {
            break;
        }
    }

    gate.running.write().await.remove(&id);
}

async fn run_fallback_scan(ctx: &Ctx, tx_notify_ui: &UnboundedSender<PeerEvent>) {
    let game_dir = ctx.game_dir.read().await.clone();
    let catalog = ctx.catalog.read().await.clone();
    match scan_local_library(&game_dir, &catalog).await {
        Ok(scan) => update_and_announce_games(ctx, tx_notify_ui, scan).await,
        Err(err) => log::error!("Failed to scan local games directory: {err}"),
    }
}

fn game_id_from_event_path(game_dir: &Path, path: &Path) -> Option<String> {
    let relative = path.strip_prefix(game_dir).ok()?;
    let mut components = relative.components();
    let game_id = component_name(components.next()?)?;
    if is_ignored_game_root_name(game_id) {
        return None;
    }

    if let Some(second) = components.next().and_then(component_name)
        && should_ignore_game_child(second)
    {
        return None;
    }

    Some(game_id.to_string())
}

fn component_name(component: Component<'_>) -> Option<&str> {
    match component {
        Component::Normal(name) => name.to_str(),
        _ => None,
    }
}

fn should_ignore_game_child(name: &str) -> bool {
    is_local_dir_name(name)
        || name.starts_with(".local.")
        || name.starts_with(".version.ini.")
        || name == ".lanspread"
        || name == ".lanspread.json"
        || name == ".sync"
        || name == ".softlan_game_installed"
}

#[cfg(test)]
mod tests {
    use super::{game_id_from_event_path, should_ignore_game_child};

    #[test]
    fn event_paths_map_to_top_level_game_id() {
        let root = std::path::Path::new("/games");
        assert_eq!(
            game_id_from_event_path(root, std::path::Path::new("/games/aoe2/version.ini"))
                .as_deref(),
            Some("aoe2")
        );
        assert_eq!(
            game_id_from_event_path(root, std::path::Path::new("/games/aoe2/local/save.dat")),
            None
        );
        assert_eq!(
            game_id_from_event_path(root, std::path::Path::new("/games/.lanspread/index.json")),
            None
        );
    }

    #[test]
    fn event_ignore_list_covers_reserved_names() {
        for name in [
            "local",
            ".local.installing",
            ".local.backup",
            ".version.ini.tmp",
            ".version.ini.discarded",
            ".lanspread",
            ".lanspread.json",
            ".sync",
            ".softlan_game_installed",
        ] {
            assert!(should_ignore_game_child(name));
        }
        assert!(!should_ignore_game_child("version.ini"));
        assert!(!should_ignore_game_child("game.eti"));
    }
}