//! `Channel<T>`: Move objects between parent and child processes.
use super::*;
use bytes::{
    Buf,
    BufMut,
    Bytes,
};
use std::marker::PhantomData;
use std::{fmt, error};
use std::io::{
    self,
    Read,
    Write,
    Cursor,
};
use serde::{
    Serialize,
    de::DeserializeOwned,
};

use sys::pipe;

/// A value that can be sent through a serial `Channel`
///
/// Bitwise channels can take `ChannelBitwiseValue` objects.
pub trait ChannelSerialValue: Serialize + DeserializeOwned{}

impl<T: ?Sized> ChannelSerialValue for T
where T: Serialize + DeserializeOwned{}

/// A value that can be sent through a bitwise `Channel`
///
/// Serial channels require `ChannelSerialValue`.
pub trait ChannelBitwiseValue: Send + Unpin{}

impl<T: ?Sized> ChannelBitwiseValue for T
where T: Send + Unpin{}

//TODO: We can create a non-stream version of this using a memory mapped ring-buffer on an fd shared between parent and child. (i.e: map two pages, the first and second one pointing to the start of the file). But that might be unneeded or too much work for this project's scope.

/// Controls how objects are sent through the streams.
pub trait TransferMethod
{
    /// The returned buffer that is build from `send`.
    ///
    /// `recv` takes any `impl Buf` object.
    type Buffer: Buf;
    /// An optional internal state object to hold any additional data needed.
    /// This can be set to `()`.
    type State;

    /// Error for the send/recv conversion.
    /// Must be able to convert into `channel::Error`.
    type Error: Into<Error>;

    /// Serialise the object `T` to a byte buffer for sending.
    fn send<T>(state: &mut Self::State, obj: &T) -> Result<Self::Buffer, Error>;
    /// Read the object `T` from a byte buffer that was read from the pipe.
    ///
    /// The buffer's size is handled by the channel.
    fn recv<T, B: Buf>(state: &mut Self::State, buffer: B) -> Result<T, Error>;
}

/// Serialize objects deeply to send them.
#[derive(Debug)]
pub struct TransferSerial;

impl TransferMethod for TransferSerial
{
    type Buffer = Cursor<Vec<u8>>;
    type State = ();
    type Error = Error;

    fn send<T>(_: &mut Self::State, obj: &T) -> Result<Self::Buffer, Error> {
	todo!()
    }
    fn recv<T, B: Buf>(_: &mut Self::State, buffer: B) -> Result<T, Error> {
	todo!()
    }
}

/// Send objects through the stream as bitwise copied (unsafe.)
#[derive(Debug)]
pub struct TransferBitwise;

impl TransferMethod for TransferBitwise
{
    type Buffer = Bytes;
    type State = ();
    type Error = std::convert::Infallible;

    fn send<T>(_: &mut Self::State, obj: &T) -> Result<Self::Buffer, Error> {
	let bytes = unsafe {
	    std::slice::from_raw_parts(obj as *const T as *const _, std::mem::size_of::<T>())
	};
	Ok(bytes.into())
    }
    fn recv<T, B: Buf>(_: &mut Self::State, mut buffer: B) -> Result<T, Error> {
	use std::mem::{
	    self,
	    MaybeUninit,
	};
	let mut value: MaybeUninit<T> = MaybeUninit::uninit();
	unsafe {
	    let slice = std::slice::from_raw_parts_mut(&mut value as *mut MaybeUninit<T> as *mut _, mem::size_of::<T>());
	    buffer.copy_to_slice(slice);

	    Ok(value.assume_init())
	}
    }
}

/// Send objects of type `T` through parent-child streams.
#[derive(Debug)]
pub struct Sender<T, M: ?Sized + TransferMethod = TransferSerial>
{
    tx: pipe::WriteHalf,

    state: M::State,
    
    _data: PhantomData<Vec<T>>,
    _method: PhantomData<M>,
}


/// Receive objects of type `T` from a parent-child stream.
#[derive(Debug)]
pub struct Receiver<T, M: ?Sized + TransferMethod = TransferSerial>
{
    rx: pipe::ReadHalf,
    
    state: M::State,

    _data: PhantomData<Vec<T>>,
    _mathod: PhantomData<M>,
}

#[inline] pub fn pair<T: ChannelSerialValue>() -> (Sender<T, TransferSerial>, Receiver<T, TransferSerial>)
{
    Sender::<_, TransferSerial>::new()
}

// TODO: TransferBitwise is very unsafe, document that here.
#[inline] pub unsafe fn pair_bitwise<T: ChannelBitwiseValue>() -> (Sender<T, TransferBitwise>, Receiver<T, TransferBitwise>)
{
    Sender::<_, TransferBitwise>::new()
}

// TODO: What bounds should we have on this, if any? Send? Unpin? Maybe even restricted to Copy.
impl<T: ChannelBitwiseValue> Sender<T, TransferBitwise>
{
    // TODO: TransferBitwise is very unsafe, document that here.
    pub unsafe fn new() -> (Self, Receiver<T, TransferBitwise>)
    {
	todo!()
    }
}

impl<T: ChannelSerialValue> Sender<T, TransferSerial>
{
    pub fn new() -> (Self, Receiver<T, TransferSerial>)
    {
	todo!()
    }
}

/// The kind of channel error
#[derive(Debug)]
#[non_exhaustive]
pub enum ErrorKind
{
    Unknown,
    
    Serialisation(serde_cbor::Error),
    Deserialisation(serde_cbor::Error),
    
    IO(io::Error),
}

/// An error in a channel operation.
#[derive(Debug)]
pub struct Error(Box<(ErrorKind, Option<bool>)>);

impl From<(ErrorKind, bool)> for Error
{
    #[inline] fn from((k, s): (ErrorKind, bool)) -> Self
    {
	let from = (k, Some(s));
	Self(Box::new(from))
    }
}

impl From<ErrorKind> for Error
{
    fn from(from: ErrorKind) -> Self
    {
	Self(Box::new((from, None)))
    }
}

impl From<std::convert::Infallible> for Error
{
    #[inline] fn from(from: std::convert::Infallible) -> Self
    {
	match from{}
    }
}

remove! {
    /// The kind of channel error
    #[derive(Debug)]
    #[non_exhaustive]
    pub enum ErrorKind
    {
	IO(io::Error),
	
	Unknown,
    }

    /// An error on a channel operation
    #[derive(Debug)]
    pub struct Error<T>(Box<(ErrorKind, bool, Option<T>)>);

    impl fmt::Display for ErrorKind
    {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
	{
	    todo!()
	}
    }


    impl<T: fmt::Debug> fmt::Display for Error<T>
    {
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
	{
	    write!(f, "channel<{}>: failed to ", std::any::type_name::<T>())?;
	    match (self.0.1, self.0.2.as_ref()) {
		(true, None) => write!(f, "send"),
		(false, None) => write!(f, "recv"),
		(true, Some(val)) => write!(f, "send (value [{:?}])", val),
		(false, Some(val)) => write!(f, "recv (value [{:?}])", val),
	    }?;
	    write!(f, ": {}", self.0.0)
	}
    }

    impl<T> Error<T>
    {
	pub fn recv(kind: ErrorKind, value: Option<T>) -> Self
	{
	    Self(Box::new((
		kind,
		false,
		value
	    )))
	}
	pub fn send(kind: ErrorKind, value: Option<T>) -> Self
	{
	    Self(Box::new((
		kind,
		true,
		value
	    )))
	}
    }
}