//! Handles spawning and restarting service task(s)
use super::*;
use tokio::sync::RwLock;
use std::mem::MaybeUninit;
use std::ops;
use futures::prelude::*;
use tokio::time;
use std::{fmt, error};

const SUPERVISOR_BACKLOG: usize = 32;

mod dispatch; pub use dispatch::*;

TODO: This all needs redoing when i'm actually lucid. This part seems okay but the rest of `service` needs to go and be replaced by something like this

/// Signal the shutdown method to the supervisor.
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy)]
pub enum SupervisorControl
{
    /// Normal working
    Initialise,
    /// Signal the subtask(s) to shut down, then wait for them and exit, with an optional timeout.
    ///
    /// # Notes
    /// If the timeout expires while waiting, then the mode is switched to `Drop`.
    Signal(Option<time::Duration>),
    /// Drop all handles and pipes to subtask(s) then immediately exit.
    Drop,
    /// Restart any and all subtask(s)
    Restart,

    /// Set the max task limit. Default is 0.
    TaskLimit(usize),
}

impl Default for SupervisorControl
{
    #[inline]
    fn default() -> Self
    {
	Self::Initialise
    }
}

/// Supervisor responsible for spawning the state handler service.
#[derive(Debug)]
pub(super) struct Supervisor
{
    /// Handle for the supervisor task itself
    handle: JoinHandle<ExitStatus>,

    /// Watch sender for signalling shutdowns the supervisor task itself
    shutdown: watch::Sender<SupervisorControl>,

    /// The pipe to send requests to the supervisor's subtasks
    pipe: mpsc::Sender<ServiceRequest>,

    /// The initial receiver created from `broadcast_root`.
    broadcast_receiver: broadcast::Receiver<ServiceEvent>,

    /// Data shared between the supervisor's task and its controller instance here.
    shared: Arc<SupervisorShared>,
}

/// Object shared btweeen the Supervisor control instance and its supervisor task.
#[derive(Debug)]
struct SupervisorShared
{
    /// this is for filtering specific messages to specific subscribers
    sub: RwLock<BTreeMap<ServiceEventKind, SESet<ServiceSubID>>>,
    
    broadcast_root: broadcast::Sender<ServiceEvent>,
    state: state::State,
}

/// A subscriber to supervisor task(s) event pump
#[derive(Debug)]
pub struct Subscriber
{
    id: ServiceSubID,

    /// For directed messages
    spec: mpsc::Receiver<ServiceEvent>,
    /// For broadcast messages
    broad: broadcast::Receiver<ServiceEvent>,
}


impl Supervisor
{
    /// Attempt to send a control signal to the supervisor itself
    pub fn signal_control(&self, sig: SupervisorControl) -> Result<(), watch::error::SendError<SupervisorControl>>
    {
	self.shutdown.broadcast(sig)?;
	Ok(())
    }
    
    /// Drop all communications with background worker and wait for it to complete
    pub async fn join_now(self) -> eyre::Result<()>
    {
	let handle = {
	    let Self { handle, ..} = self; // drop everything else
	    handle
	};
	handle.await?;

	Ok(())
    }

    /// Check if the background worker has not been dropped.
    ///
    /// If this returns false it usually indicates a fatal error.
    pub fn is_alive(&self) -> bool
    {
	Arc::strong_count(&self.shared) > 1
    }
    
    /// Create a new supervisor for this state.
    pub fn new(state: state::State) -> Self
    {
	let shutdown = watch::channel(Default::default());
	let pipe = mpsc::channel(SUPERVISOR_BACKLOG);
	let (broadcast_root, broadcast_receiver) = broadcast::channel(SUPERVISOR_BACKLOG);

	let shared = Arc::new(SupervisorShared{
	    broadcast_root,
	    state,
	});

	let (shutdown_0, shutdown_1) = shutdown;
	let (pipe_0, pipe_1) = pipe;
	
	Self {
	    shutdown: shutdown_0,
	    pipe: pipe_0,
	    broadcast_receiver,
	    shared: Arc::clone(&shared),

	    handle: tokio::spawn(async move {
		let shared = shared;
		ExitStatus::from(service_fn(SupervisorTaskState {
		    shared,
		    recv: pipe_1,
		    shutdown: shutdown_1,
		}).await.or_else(|err| err.into_own_result()))
	    }),
	}
    }
}

/// The state held by the running superviser service
#[derive(Debug)]
struct SupervisorTaskState
{
    shutdown: watch::Receiver<SupervisorControl>,
    recv: mpsc::Receiver<ServiceRequest>,
    shared: Arc<SupervisorShared>,
}

/// Detached supervisor server
async fn service_fn(SupervisorTaskState {shared, mut recv, mut shutdown}: SupervisorTaskState) -> Result<(), ServiceTerminationError>
{
    impl Default for TerminationKind
    {
	#[inline]
	fn default() -> Self
	{
	    Self::Graceful
	}
    }

    // The command stream to dispatch to the worker tasks
    let command_dispatch = async {
	while let Some(req) = recv.recv().await {
	    todo!("Dispatch to child(s)");
	}
	TerminationKind::Graceful
    };
    tokio::pin!(command_dispatch);

    // The signal stream to be handled here
    let signal_stream = async {
	while let Some(value) = shutdown.recv().await
	{
	    use SupervisorControl::*;
	    match value {
		Initialise => info!("Initialised"),
		Signal(None) => return TerminationKind::SignalHup,
		Signal(Some(to)) => return TerminationKind::SignalTimeout(to),
		Drop => return TerminationKind::Immediate,
		Restart => todo!("not implemented"),
		TaskLimit(_limit) => todo!("not implemented"),
	    }
	}
	TerminationKind::Graceful
    };
    tokio::pin!(signal_stream);

    //loop {
    tokio::select! {
	sd_kind = &mut signal_stream => {
	    // We received a signal
	    
	    Err(ServiceTerminationError::Signal(sd_kind))
	}
	disp_end = &mut command_dispatch => {
	    // The command dispatch exited first, the logical error is `Graceful`. But it's not really an error, so...
	    disp_end.into()
	}
    }
    //    }
       
}
/// The mannor in which the supervisor exited.
#[derive(Debug)]
pub enum TerminationKind
{
    /// The child task(s) were signalled to stop and they were waited on.
    SignalHup,
    /// If there was a timeout specified, and that timeout expired, the message will be `SignalTimeout` instead of `SignalHup`.
    SignalTimeout(time::Duration),
    /// Immediately drop everything and exit
    Immediate,
    /// A non-signalled shutdown. There were no more watchers for the shutdown channel.
    Graceful,
}

impl TerminationKind
{
    /// Convert `TerminationKind::Graceful` into a non-error
    fn strip_grace(self) -> Result<(), Self>
    {
	match self {
	    Self::Graceful => Ok(()),
	    e => Err(e),
	}
    }
}

impl error::Error for TerminationKind{}
impl fmt::Display for TerminationKind
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
    {
	match self {
	    Self::SignalHup => write!(f, "children were signalled to shut down and compiled"),
	    Self::SignalTimeout(to) => write!(f, "children were signalled to shut but did not do so within the {:?} timeout", to),
	    Self::Immediate => write!(f, "children were dropped and an immediate exit was made"),
	    Self::Graceful => write!(f, "a graceful shutdown order was issued and compiled with"),
	}
    }
}


/// The error returned on a failed service termination.
#[derive(Debug)]
#[non_exhaustive]
pub enum ServiceTerminationError
{
    /// Was terminated by a signal.
    Signal(TerminationKind),
    /// Was terminated by a panic.
    Panic,
    /// There were no more commands being sent through, and the worker gracefully shut down.
    Interest,
}

impl From<TerminationKind> for Result<(), ServiceTerminationError>
{
    fn from(from: TerminationKind) -> Self
    {
	ServiceTerminationError::Signal(from).into_own_result()
    }
}


impl ServiceTerminationError
{
    fn into_own_result(self) -> Result<(), Self>
    {
	match self {
	    Self::Signal(term) => term.strip_grace().map_err(Self::Signal),
	    x => Err(x),
	}
    }
}

impl error::Error for ServiceTerminationError
{
    fn source(&self) -> Option<&(dyn error::Error + 'static)>
    {
	Some(match &self {
	    Self::Signal(ts) => ts,
	    _ => return None,
	})
    }
}
impl fmt::Display for ServiceTerminationError
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
    {
	match self {
	    Self::Signal(_) => write!(f, "shut down by signal"),
	    Self::Panic => write!(f, "shut down by panic. this is usually fatal"),
	    Self::Interest => write!(f, "all communications with this service stopped"),
	}
    }
}