//! Message passing things
use super::*;
use tokio::{
    sync::{
	watch,
    },
};
use std::{
    task::{Poll, Context},
    pin::Pin,
    fmt,
    error,
};
use futures::{
    future::Future,
};

#[derive(Debug)]
pub struct InitError;
#[derive(Debug)]
pub struct InitWaitError;

impl error::Error for InitError{}
impl fmt::Display for InitError
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
    {
	write!(f, "failed to set init value")
    }
}

impl error::Error for InitWaitError{}
impl fmt::Display for InitWaitError
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result
    {
	write!(f, "failed to receive init value")
    }
}

/// Provides a method of waiting on and setting a single initialisation.
///
/// In general, it should only be set once, as multiple sets do nothing but hog `Arc`s.
/// Dropping the `Initialiser` after waiting or setting should generally be done immediately.
/// Choose the `into_wait()` and `set()` varients over the non-consuming ones.
#[derive(Clone, Debug)]
pub struct Initialiser
{
    tx: Arc<watch::Sender<bool>>,
    rx: watch::Receiver<bool>
}

impl Initialiser
{
    /// Create a new, unset initialiser
    pub fn new() -> Self
    {
	let (tx, rx) = watch::channel(false);
	Self {
	    tx: Arc::new(tx),
	    rx,
	}
    }

    /// Create a pre-set initialiser. Calls to `wait()` will immediately resolve.
    pub fn new_set() -> Self
    {
	let (tx, rx) = watch::channel(true);
	Self {
	    tx: Arc::new(tx),
	    rx,
	}
    }

    /// Consume into a future that completes when init is set.
    pub fn into_wait(self) -> impl Future<Output=Result<(), InitWaitError>> + 'static
    {
	let mut rx = self.rx;
	async move {
	    if !*rx.borrow() {
		while !rx.recv().await.ok_or_else(|| InitWaitError)? {
		    //tokio::task::yield_now().await;
		}
		Ok(())
	    } else {
		Ok(())
	    }
	}
    }

    /// Clone into a future that completes when init is set.
    ///
    /// This method does not clone any `Arc`s and is prefered to `self.clone().into_wait()`.
    /// Use this when the `Initialiser` you want to wait on is behind a shared reference.
    pub fn clone_into_wait(&self) -> impl Future<Output=Result<(), InitWaitError>> + 'static
    {
	let mut rx = self.rx.clone();
	async move {
	    if !*rx.borrow() {
		while !rx.recv().await.ok_or_else(|| InitWaitError)? {
		    //tokio::task::yield_now().await;
		}
		Ok(())
	    } else {
		Ok(())
	    }
	}
    }

    /// Completes when init is set
    pub async fn wait(&mut self) -> Result<(), InitWaitError>
    {
	if !*self.rx.borrow() {
	    while !self.rx.recv().await.ok_or_else(|| InitWaitError)? {
		//tokio::task::yield_now().await;
	    }
	    Ok(())
	} else {
	    Ok(())
	}
    }

    /// Is init set?
    pub fn is_set(&self) -> bool
    {
	*self.rx.borrow()
    }

    /// Consume and set init if it's not already set
    pub fn set(self) -> Result<(), InitError>
    {
	if !*self.rx.borrow() {
	    self.tx.broadcast(true).map_err(|_| InitError)
	} else {
	    Ok(())
	}
    }
    
    /// Set init without consuming.
    ///
    /// # Note
    /// It is prefered to use `set()`, as this method may make `Arc`s hang around longer than they need to.
    /// Calling this multiple times is useless.
    pub fn set_in_place(&self) -> Result<(), InitError>
    {
	if !*self.rx.borrow() {
	    self.tx.broadcast(true).map_err(|_| InitError)
	} else {
	    Ok(())
	}
    }
}

impl Future for Initialiser
{
    type Output = Result<(), InitWaitError>;
    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
	let uhh = self.wait();
	tokio::pin!(uhh);
	uhh.poll(cx)
    }
}