//! Buffering and resource pooling utilities
//!
//!
use super::*;

pub use bytes::{
    Buf, BufMut,
    Bytes, BytesMut,
};
use std::{
    fmt, error,
    sync::Arc,
    pin::Pin,
};


#[cfg(feature="_loan")]
const _TODO_REWORK_THIS_INTERFACE_ITS_OVERCOMPLICATED_SHITE: () = {
    type OwnedResult<S, T, E> = Result<T, (E, S)>;

    #[derive(Debug, Clone, PartialEq, Eq, Hash)]
    enum MaybeRef<'r, T>
    {
	Owned(T),
	Borrowed(&'r T),
    }

    #[derive(Debug)]
    struct Loan<'owner, O: Loaner>
    {
	owner: MaybeRef<'owner, O>,
	item: O::Item<'owner>,
    }

    impl<'o, O> Loaned<'o, O> for Loan<'o, O>
    where O: Loaner
    {
	fn try_release(self) -> Result<(), <O as Loaner>::Error> {
	    self.item.try_release()
	}
	fn try_release_owned(self: Arc<Self>) -> OwnedResult<Arc<Self>, (), <O as Loaner>::Error> {
	    
	}
    }

    pub trait Loaned<'owner, Owner: ?Sized + 'owner>: Sized
    where Owner: Loaner
    {
	//fn owner(&self) -> Option<&Owner>;
	
	fn try_release(self) -> Result<(), Owner::Error>;
	fn try_release_owned(self: Arc<Self>) -> OwnedResult<Arc<Self>, (), Owner::Error>
	where Self: 'owner; // XXX: What? 'owner or 'static??? I think it's `'owner` == `'static` but how tf do I write that?
    }

    pub trait Loaner {
	type Item<'owner>: Loaned<'owner, Self>
	where Self: 'owner;
	type Error: error::Error + Send + 'static;

	fn try_loan(&self) -> Result<Self::Item<'_>, Self::Error>;
	fn try_loan_owned(self: Arc<Self>) -> OwnedResult<Arc<Self>, Self::Item<'static>, Self::Error>
	where Self: 'static;

	fn try_release(&self, item: Self::Item<'_>) -> OwnedResult<Self::Item<'_>, (), Self::Error>;
    }

    compile_error!("This `trait Loan` design isn't going to fucking work ofc...")
};

/// Thread pooling
#[cfg(feature="threads")]
pub mod thread_pool {
    use super::*;

    use std::{
	mem::{
	    self,
	    ManuallyDrop,
	},
	thread::{
	    self,
	    Thread, ThreadId,
	},
	ops::Drop,
	sync::{Arc, Weak,},
	
    };
    use parking_lot::{
	Condvar,
	Mutex,
    };
    use crossbeam::{
	queue::ArrayQueue,
	channel,
    };

    //TODO: Imlement a oneshot channel: `oneshot::{Sender, Receiver`; works like Tokio's `sync::oneshot`, but is sync, not async: Can use `{Sender,Receiver}slot: {Arc, Weak}<Mutex<MaybeUninit<T>>>, `waiter: {Weak/_PinnedPtrIntoArc<Pin<slot>>(avoids 2nd alloc)/Weak,Arc<_, A = SenderSharedBumpAllocator*>,Arc}<Condvar>` (NOTE: Bump allocator crate name `bumpalo`.)

    //TODO: Implement `ThreadPool<'scope = 'static>` like below, but instead use `crossbeam::channel` mpsc to send commands to unparked threads (i.e. loaned threads from the pool that have an active user handle.)
    // The design will be far simpler, since the Arc/Weak, send+recv stuff will be handled fine by `channel` on both ends. The only thing we need dtors for is `force_push()`ing threads back into their pool's ringbuffer, and for force-unparking threads from a pool that are removed (same as loan, except sender should be dropped before unpark happens.)

    #[cfg(feature="_thread-pool-no-mpsc")] 
    const _: () = {
	#[derive(Debug)]
	pub enum ThreadHandleKind<'scope, T>
	{
	    Detached(Option<ThreadId>),
	    Attached(thread::JoinHandle<T>),
	    Scoped(thread::ScopedJoinHandle<'scope, T>),
	}

	impl<'scope, T> ThreadHandleKind<'scope, T>
	{
	    /// Get a reference to the attached thread (if there is one.)
	    #[inline] 
	    pub fn thread(&self) -> Option<&Thread>
	    {
		Some(match self {
		    Self::Scoped(s) => s.thread(),
		    Self::Attached(s) => s.thread(),
		    _ => return None,
		})
	    }

	    /// Is this handle attached to a thread?
	    ///
	    /// If it is *attached*, this means the thread *should not* outlive the instance (unless the instance is `'static`.)
	    #[inline] 
	    pub const fn is_attached(&self) -> bool
	    {
		match self {
		    Self::Attached(_) |
		    Self::Scoped(_) => true,
		    Self::Detached(_) => false,
		}
	    }

	    /// Is this handle no longer referring to *any* thread?
	    ///
	    /// In a *poisoned* state, the handle is useless.
	    #[inline] 
	    pub const fn is_poisoned(&self) -> bool
	    {
		match self {
		    Self::Detached(None) => true,
		    _ => false,
		}
	    }

	    /// Get the thread id of the handle, if there is a referred to thread.
	    #[inline] 
	    pub fn id(&self) -> Option<ThreadId>
	    {
		Some(match self {
		    Self::Scoped(x) => x.thread().id(),
		    Self::Attached(x) => x.thread().id(),
		    Self::Detached(Some(id)) => *id,
		    _ => return None
		})
	    }
	}
	
	/// Represents a raw thread handle
	#[derive(Debug)]
	#[repr(transparent)]
	pub struct ThreadHandle<'scope, T>(ThreadHandleKind<'scope, T>);

	//TODO: forward `ThreadHandle<'s, T>` methods to `ThreadHandleKind<'s, T>`: `thread(), id(), is_attached()`. Add methods `is_running(), join(), try_join(),` etc...
	
	impl<'s, T> Drop for ThreadHandle<'s, T>
	{
	    #[inline] 
	    fn drop(&mut self) {
		match mem::replace(&mut self.0, ThreadHandleKind::Detached(None)) {
		    ThreadHandleKind::Attached(a) => drop(a.join()), // Attempt join, ignore panic.
		    ThreadHandleKind::Scoped(s) => drop(s.join().unwrap_or_resume()), // Attempt join, carry panic back through into scope.
		    _ => (),
		}
	    }
	}

	#[derive(Debug)]
	enum PooledSendContext<T: Send, F: ?Sized + Send>
	{
	    /// An execute directive `Err(Some(func))` is taken by the thread and replaced by `Ok(func())`.
	    ///
	    /// # Direction
	    /// Bidirectional.
	    /// (Handle -> Thread; Thread -> Handle)
	    Execute(Result<T, Option<Box<F>>>),
	    /// Pass a caught panic within an `Execute(Err(Some(F)))` payload back from the thread.
	    ///
	    /// # Direction
	    /// Thread -> Handle
	    Panic(UnwindPayload),
	    /// When the loaned `PooledThread` is moved back into the pool, tell the thread to park itself again.
	    ///
	    /// # Direction
	    /// Handle -> Thread
	    Park,

	    /// The default invariant used for when an operation has been taken by a thread.
	    ///
	    /// # When `Taken`.
	    /// The replacement is always `Taken` after a `take(&mut self)` that contains a value.
	    /// The `Park` directive will not be replaced to `Taken`.
	    Taken,

	    //NOTE: `Close` variant not needed because `handle`  will have a `Weak` reference to `callback_passage`; when the upgrade fails, the thread will exit.
	}

	impl<T: Send, F: ?Sized + Send> PooledSendContext<T, F>
	{
	    pub fn take(&mut self) -> Option<Self>
	    {
		Some(match self {
		    //XXX: I don't think not replacing cloneable variants is a good or useful default. (see below V)
		    //// Clone non-data-holding variants.
		    //Self::Park => Self::Park,
		    //// If the ctx has already been taken, return `None`.
		    //Self::Taken => return None,
		    // Move out of data-holding variants.
		    _ => mem::replace(self, Self::Taken)
		})
	    }
	}

	/// A loanable thread that belongs to a pool.
	///
	/// # Pooling
	/// When taken from a `ThreadPool`, the object is *moved* out of the thread-pool's ring-buffer and then pinned in return (See `PooledThread`.)
	///
	/// To return the thread to the pool, the `callback_passage` writes a `Park` command to the thread, unpins itself, and then on drop moves itself back into the start of the ring-buffer of the thread pool `owner`
	#[derive(Debug)]
	struct PooledThreadHandle<'scope, T, F: ?Sized = dyn FnOnce() -> T + Send + 'scope>
	where F: FnOnce() -> T + Send + 'scope,
	      T: Send + 'scope
	{
	    
	    /// Passes `F` to `handle` and retrieves `T`.
	    callback_passage: Arc<(Condvar, Mutex<PooledSendContext<T, F>>)>,
	    /// The backing thread, which when started parks itself.
	    /// The `PooledThread` object will unpark the thread when the object is loaned (moved) out of the pool, and re-parked when enter
	    ///
	    /// Will wait on `callback_passage?.0` for `Execute` directives, pass them back to the handle, and when a `Park` directive is recieved (i.e. when moved back into the pool) the thread will park itself and wait to be unparked when moved out of the pool or closed again.
	    ///
	    /// If `callback_message` is dropped, the thread will exit when unparked (dropped from pool.)
	    //TODO: Ensure unpark on drop?
	    handle: ThreadHandle<'scope, Result<(), ()>>,
	}

	impl<'scope, T, F:?Sized> PooledThreadHandle<'scope, T, F>
	where F: FnOnce() -> T + Send + 'scope,
	      T: Send + 'scope
	{
	    fn send_to_thread_raw(&self, message: PooledSendContext<T, F>) -> bool
	    {
		let mut ctx = self.callback_passage.1.lock();
		*ctx = message;
		self.callback_passage.0.notify_one()
	    }
	    fn send_from_thread_raw(&self, message: PooledSendContext<T, F>)
	    {
		let mut ctx = self.callback_passage.1.lock();
		*ctx = message;
	    }
	    
	    fn read_in_thread_raw(&self, wait_for: bool) -> Option<PooledSendContext<T, F>>
	    {
		let mut ctx = self.callback_passage.1.lock(); // NOTE: Should we fair lock here?
		// Wait on condvar if `wait_for` is true.
		if wait_for {
		    self.callback_passage.0.wait(&mut ctx);
		}
		// Then replace the ctx with `Taken`.
		ctx.take()
	    }
	    fn try_read_from_thread_raw(&self) -> Option<PooledSendContext<T, F>>
	    {
		let mut ctx = self.callback_passage.1.try_lock()?;
		ctx.take()
	    }
	    fn read_from_thread_raw(&self) -> Option<PooledSendContext<T, F>>
	    {
		let mut ctx = self.callback_passage.1.lock();
		ctx.take()
	    }
	}

	#[derive(Debug)]
	pub struct PooledThread<'scope, 'pool: 'scope, T, F: ?Sized = dyn FnOnce() -> T + Send + 'scope>
	where F: FnOnce() -> T + Send + 'scope,
	      T: Send + 'scope
	{
	    /// The owner to return `thread` to on drop.
	    owner: Option<&'pool ThreadPool<'scope, T, F>>,
	    /// The pinned thread handle that is returned on drop.
	    thread: ManuallyDrop<PooledThreadHandle<'scope, T, F>>, //XXX: Pin<> needed (or even useful at all) here?
	}

	impl<'scope, 'pool: 'scope, T, F:?Sized> Drop for PooledThread<'scope, 'pool, T, F>
	where F: FnOnce() -> T + Send + 'scope,
	      T: Send + 'scope
	{
	    fn drop(&mut self) {
		let Some(loaned_from) = self.owner else {
		    // There is no owner.
		    // Instead, unpark the thread after drop.
		    let PooledThreadHandle { callback_passage, handle } = unsafe { ManuallyDrop::take(&mut self.thread) };
		    
		    // Drop the Arc, so when `thread` wakes from unpark, it will fail to acquire context.
		    drop(callback_passage);
		    
		    if let Some(thread) = handle.0.thread() {
			thread.unpark();
		    }
		    
		    return todo!("How do we know *when* to unpark? How can we unpark *after* dropping the Arc?");
		};

		// There is an owner. Tell the thread to park itself.
		let mut handle = unsafe { ManuallyDrop::take(&mut self.thread) };
		handle.send_to_thread_raw(PooledSendContext::Park);

		// Read the message object, take it from the thread after it notifies us it is about to park.
		let ctx = {
		    let mut ctx = handle.callback_passage.1.lock();
		    // Before thread parks, it should signal condvar, and callback_passage should not be unique again yet.
		    handle.callback_passage.0.wait_while(&mut ctx, |_| Arc::strong_count(&handle.callback_passage) > 1);
		    ctx.take()
		};

		// Force insert back into thred pool.
		if let Some( PooledThreadHandle { callback_passage, handle }) = loaned_from.pool_queue.force_push(handle) {
		    // If one is removed, drop the context..
		    drop(callback_passage);
		    // Then force-unpark the thread.
		    match handle.0.thread() {
			Some(thread) => thread.unpark(),
			None => (),
		    }
		}
		
		todo!("Should we force `join()` here? Or allow non-scoped handles to be forgotten?")
	    }
	}
	#[derive(Debug)]
	pub struct ThreadPool<'scope, T, F: ?Sized = dyn FnOnce() -> T + Send + 'scope>
	where F: FnOnce() -> T + Send + 'scope,
	      T: Send + 'scope
	{
	    pool_queue: ArrayQueue<PooledThreadHandle<'scope, T, F>>,
	}
    };

    //TODO: Build a ThreadPool<'scope = 'static> using crossbeam::ArrayQueue<...> (XXX: See phone for details on how pool should use un/parking for loaning; and CondVar for passing tasks.) Also, add a `Loan` trait with assoc type `Item<'owner>` and methods `try_loan(&self) -> Result<Self::Item<'_>, ...>`, `return(&self, item: Self::Item<'_>) -> Result<(), ...>` and `try_loan_owned(self: Arc<Self>) -> `MovedResult<Arc<Self>, OwnedLoan<'static, Self>, ...>`  (struct OwnedLoad<'scope, L>{ item: Option<L::Item<'scope>> } )`, etc.
}