//! Live config reloader. Handles hooks and filesystem wathcers
//!
//! # Notes
//!  - The path passed to `watch()` MUST exist, or child will panic (TODO). But the path can be removed afterwards.
//!  - When `Oneesan` is dropped, it signals child worker to gracefully shutdown
//!  - When child worker panics, waiting on `Oneesan` can cause deadlock.
//!  - The filename passed to `watch()` is canonicalised.
//! ## Hook format
//!  - File paths are all relative to their root.
//!  - Appending a directory path with `/` will cause events in file within the directory path being dispatched to also be dispatched to this hook. Otherwise, only events on the directory itself will dispatch.
//!  - The empty string matches only the root directory
//!  - A single `/` string will match everything.
//! # TODO
//!  - Make child not panic if `watch()` is called on non-existant path
//!  - Change hook from just string path prefix to pattern match with optional regex capability. (For finding and dispatching on global/local config files differently)
#![allow(unused_imports)] // For when hotreload feature disabled

use super::*;
use std::{
    path::{
	PathBuf,
	Path,
    },
    sync::Arc,
    fmt,
    marker::{
	Send,Sync,
    },
    ops::{
	Deref,
    },
};
use tokio::{
    sync::{
	RwLock,
	Mutex,
	oneshot,
	mpsc::{
	    self,
	    error::SendTimeoutError,
	},
    },
    task::{
	JoinHandle,
    },
    time,
};
use futures::{
    future::{
	join_all,
    },
};
use notify::{
    Watcher,
    RecursiveMode,
    RecommendedWatcher,
    event,
};

/// An event to be passed to `Context`.
pub type Event = event::EventKind;

/// Max event backlog (not used if `unlimited_watcher` is set)
const GLOBAL_BACKLOG: usize = config::build::hot::BACKLOG;  //100;
/// Timeout for hook dispatches in seconds. Only set if `watcher_timeout` feature is enabled.
const GLOBAL_TIMEOUT: u64 = config::build::hot::TIMEOUT;

/// Decontruct a notify event into event kind and full paths.
#[inline]
fn deconstruct_event<P: AsRef<Path>>(root: P,event: event::Event) -> (Vec<PathBuf>, Event)
{
    let root = root.as_ref();
    (event.paths.into_iter().map(|x| {
	// Remove root
	x.strip_prefix(root).unwrap_or(&x).to_owned()
    }).collect(), event.kind)
}

/// An event filter function
pub type EventFilterFn = Box<dyn Fn(&Event) -> bool + Send + Sync>;

/// Represents one path hook
pub struct Hook {  
    path: PathBuf,
    sender: Mutex<mpsc::Sender<Event>>,
    filter: Option<EventFilterFn>,
}

impl std::fmt::Debug for Hook
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
    {
	write!(f, "(path: {:?}, sender: {:?}, filter: {})", self.path, self.sender, self.filter.is_some())
    }
}


/// Check if `two` is a partial match of `one`.
#[inline]
fn is_path_match<P,Q>(one: P, two: Q) -> bool
where P: AsRef<Path>,
      Q: AsRef<Path>
{
    let (one, two) = (one.as_ref(), two.as_ref());

    if one.as_os_str() == "/" {
	return true; //match-all case
    }
    
    //debug!("Checking {:?} ?== {:?}", one,two);
    //println!(" {:?} {:?} {} {}", one, two, one.to_str().map(|x| x.ends_with("/")).unwrap_or(false), two.starts_with(one));
    one == two || {
	one.to_str().map(|x| x.ends_with("/")).unwrap_or(false) && two.starts_with(one)
    }
}

impl Hook {
    /// Optional timeout for dispatches. None for infinite wait. A high or `None` value can cause event backlogs to overflow when receivers are not taking events properly.
    #[cfg(feature="watcher_timeout")]
    const DISPATCH_TIMEOUT: Option<time::Duration> = Some(time::Duration::from_secs(GLOBAL_TIMEOUT));
    #[cfg(not(feature="watcher_timeout"))]
    const DISPATCH_TIMEOUT: Option<time::Duration> = None;
    const DISPATCH_MAX_BACKLOG: usize = 16;

    /// Create a new `Hook` object, with a path, and an optional filter function.
    /// Return the hook and the receiver for the hook.
    pub fn new<F,P>(path: P, filter: Option<F>) -> (Self, mpsc::Receiver<Event>)
    where F: Fn(&Event) -> bool + Send + Sync + 'static,
	  P: AsRef<Path>
    {
	let (tx, rx) = mpsc::channel(Self::DISPATCH_MAX_BACKLOG);
	
	(Self{
	    path: path.as_ref().to_owned(),//std::fs::canonicalize(&path).unwrap_or(path.as_ref().to_owned()),
	    sender: Mutex::new(tx),
	    filter: filter.map(|f| Box::new(f) as EventFilterFn),
	}, rx)
    }
    
    /// Force an event dispatch on this hook.
    pub async fn dispatch(&self, event: Event) -> Result<(), SendTimeoutError<Event>>
    {
	debug!(" dispatching on {:?}", &self.path);
	let mut lock =self.sender.lock().await;

	match &Self::DISPATCH_TIMEOUT {
	    Some(timeout) => lock.send_timeout(event, timeout.clone()).await?,
	    None => lock.send(event).await.map_err(|e| mpsc::error::SendTimeoutError::Closed(e.0))?,
	};
	Ok(())
    }

    /// Try to dispatch on this hook, if needed.
    pub async fn try_dispatch(&self, path: impl AsRef<Path>, event: &Event) -> Result<bool, SendTimeoutError<Event>>
    {
	Ok(if is_path_match(&self.path, path) {
	    self.dispatch(event.to_owned()).await?;
	    true
	} else {
	    false
	})
    }

    /// Try to dispatch on one of many paths
    pub async fn try_dispatch_many<T,P>(&self, paths: T, event: &Event) -> Result<bool, SendTimeoutError<Event>>
    where T: IntoIterator<Item=P>,
	  P: AsRef<Path>
    {
	for path in paths.into_iter() {
	    if is_path_match(&self.path, path) {
		self.dispatch(event.to_owned()).await?;
		return Ok(true);
	    }
	}
	Ok(false)
    }
}

/// A hook container for `Oneesan`
#[derive(Debug, Clone)]
pub struct HookContainer(Arc<RwLock<Vec<Hook>>>);

impl HookContainer
{
    /// Add a hook to this container. The worker will be pushed the new hook.
    /// 
    /// Dropping the receiver will remove the hook.
    /// # Note
    /// This function does not check the path, and may add hooks that are never fired.
    pub async fn hook<F,P>(&self, path: P, filter: Option<F>) -> mpsc::Receiver<Event>
    where F: Fn(&Event) -> bool + Send + Sync + 'static,
	  P: AsRef<Path>
	
    {
	let mut hooks = self.0.write().await;
	let (hook, recv) = Hook::new(path,filter);
	hooks.push(hook);
	recv
    }
}

/// A watcher context, we hook specific paths here, to be dispatched to on file change
#[derive(Debug)]
pub struct Oneesan {
    path: PathBuf,

    hooks: HookContainer,
    shutdown: PhantomDrop<oneshot::Sender<()>, fn (oneshot::Sender<()>) -> ()>,
    handle: JoinHandle<()>,

}

impl Deref for Oneesan
{
    type Target = HookContainer;
    fn deref(&self) -> &Self::Target
    {
	&self.hooks
    }
}

impl Oneesan
{
    /// Add a hook to this container if `path` is inside this instance. The worker will be pushed the new hook if the check passes, if not, will return `None`.
    ///
    /// Dropping the receiver will remove the hook.
    pub async fn try_hook<F,P>(&self, path: P, filter: Option<F>) -> Option<mpsc::Receiver<Event>>
    where F: Fn(&Event) -> bool + Send + Sync + 'static,
	  P: AsRef<Path>
    {
	let path = path.as_ref();
	if self.path.starts_with(&path) {
	    Some(self.hook(path, filter).await)
	} else {
	    None
	}
    }
    /// Join this instance
    pub fn shutdown(self) -> JoinHandle<()>
    {
	self.handle
    }
}

#[cfg(feature="watcher")]
/// Start watching this path for changes of files
pub fn watch(path: impl AsRef<Path>) -> Oneesan
{
    let path = path.as_ref().to_owned();
    let hooks = Arc::new(RwLock::new(Vec::new()));
    let (shutdown, mut shutdown_rx) = oneshot::channel();
    
    let handle = {
	let path=std::fs::canonicalize(&path).unwrap_or_else(|_| path.clone());
	let hooks = Arc::clone(&hooks);
	tokio::spawn(async move {
	    #[cfg(feature="watcher_unlimited")]
	    let (tx,mut rx) = mpsc::unbounded_channel();
	    #[cfg(not(feature="watcher_unlimited"))]
	    let (tx,mut rx) = mpsc::channel(GLOBAL_BACKLOG);


	    let mut watcher: RecommendedWatcher = Watcher::new_immediate(move |res| {
		#[cfg(not(feature="watcher_unlimited"))]
		let mut tx = tx.clone();
		match res {
		    Ok(event) => if let Err(err) = {
			#[cfg(feature="watcher_unlimited")] {tx.send(event)}
			#[cfg(not(feature="watcher_unlimited"))] {tx.try_send(event)}
		    }
		    {warn!("Watcher failed to pass event: {}", err)},
		    Err(e) => error!("Watcher returned error: {}", e),
		}
	    }).expect("Failed to initialise watcher");
	    
	    debug!("Watcher initialised, starting for path {:?}", &path);
	    watcher.watch(&path, RecursiveMode::Recursive).expect("Failed to start watcher");

	    let work = async {
		while let Some(event) = rx.recv().await {
		    //debug!(" -> {:?}", event);
		    let (paths, event) = deconstruct_event(&path, event);

		    let closed: Vec<usize> = { //XXX: Change to generational arena
			let hooks = hooks.read().await;
			
			let mut i=0;
			join_all(hooks.iter().map(|hook: &Hook| hook.try_dispatch_many(&paths[..], &event))).await.into_iter().filter_map(|res| {
			    (match res {
				Err(SendTimeoutError::Closed(_)) => Some(i), //If channel closed, hook is ded, remove it.
				_ => None,
			    }, i+=1).0
			}).collect()
		    };
		    if closed.len() > 0 {
			let mut hooks = hooks.write().await;
			for index in closed.into_iter() {
			    debug!("Closing dead hook {}", index);
			    hooks.remove(index);
			}
		    }
		}
	    };
	    tokio::pin!(work);
	    info!("Watcher up");
	    tokio::select!{
		_ = &mut shutdown_rx => {
		    info!("Received shutdown, closing gracefully");
		},
		_ = work => {debug!("Worker stopped on it's own, notify watcher has been dropped.")},
	    };
	    debug!("Watcher going down for shutdown now");
	})
    };
    
    let output = Oneesan {
	hooks: HookContainer(hooks),
	path: std::fs::canonicalize(&path).unwrap_or(path),
	shutdown: PhantomDrop::new(shutdown, |shutdown| {
	    if !shutdown.is_closed()
	    {
		shutdown.send(()).expect("mpsc fatal");
	    }
	}),
	handle,
    };
    

    output
}

pub mod filter
{
    use super::*;

    #[inline(always)]
    const fn all(_: &Event) -> bool {true}
    pub const ALL: Option<fn (&Event) -> bool> = None;
}