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//! A lightweight actor model inspired framework to build
//! customizable componets with message-based intercommunications.

#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]

use async_io::block_on;
use flume::{bounded, unbounded, Sender};
use futures_lite::future::pending;
use once_cell::sync::Lazy;
use std::{
    panic::catch_unwind,
    process::abort,
    ptr::NonNull,
    sync::atomic::{AtomicUsize, Ordering},
    thread,
};

const MAX_REFCOUNT: usize = (isize::MAX) as usize;

/// An async executor.
pub type Executor<'a> = async_executor::Executor<'a>;

/// A default executor.
pub static DEFAULT_EXECUTOR: Lazy<Executor<'_>> = Lazy::new(|| {
    let num_threads = num_cpus::get();
    for n in 1..=num_threads {
        thread::Builder::new()
            .name(format!("appliance-{}", n))
            .spawn(|| loop {
                catch_unwind(|| block_on(DEFAULT_EXECUTOR.run(pending::<()>()))).ok();
            })
            .expect("cannot spawn an appliance executor thread");
    }
    Executor::new()
});

/// An opaqueing error.
#[derive(Debug)]
pub enum Error {
    /// Exposes message handling errors.
    HandlingFailure,
}

#[repr(C)]
struct Inner {
    strong: AtomicUsize,
}

unsafe impl Send for Inner {}
unsafe impl Sync for Inner {}

/// A stateful entity that only allows to
/// interact with via handling messages.
#[derive(Debug)]
pub struct Appliance<M> {
    ptr: NonNull<Inner>,
    messages_in: Sender<M>,
}

unsafe impl<M> Send for Appliance<M> {}
unsafe impl<M> Sync for Appliance<M> {}

impl<M> Clone for Appliance<M> {
    fn clone(&self) -> Self {
        // NB: See std::sync::Arc source code for details.
        let old_size = self.inner().strong.fetch_add(1, Ordering::Relaxed);
        if old_size > MAX_REFCOUNT {
            abort();
        }
        Appliance {
            ptr: self.ptr,
            messages_in: self.messages_in.clone(),
        }
    }
}

impl<M> Drop for Appliance<M> {
    #[inline]
    fn drop(&mut self) {
        self.inner().strong.fetch_sub(1, Ordering::Release);
    }
}

impl<M> Appliance<M> {
    /// Returns the number of strong pointers (clones) of a given appliance.
    pub fn strong_count(this: &Self) -> usize {
        this.inner().strong.load(Ordering::SeqCst)
    }

    #[inline]
    fn inner(&self) -> &Inner {
        unsafe { self.ptr.as_ref() }
    }
}

impl<'a, M: Send + 'a> Appliance<M> {
    /// Creates a new appliance.
    pub fn new<S: Send + 'a>(
        executor: &Executor<'a>,
        mut state: S,
        handler: impl Fn(&mut S, M) + Send + 'a,
        message_bus_size: Option<usize>,
    ) -> Self {
        let (messages_in, messages_out) = if let Some(mbs) = message_bus_size {
            bounded(mbs)
        } else {
            unbounded()
        };
        let boxed_inner = Box::new(Inner {
            strong: AtomicUsize::new(1),
        });
        let ptr = Box::leak(boxed_inner).into();
        let appliance = Appliance { ptr, messages_in };
        executor
            .spawn(async move {
                loop {
                    match messages_out.recv_async().await {
                        Ok(message) => handler(&mut state, message),
                        Err(_) => return,
                    }
                }
            })
            .detach();
        appliance
    }

    /// Processes a message on the current appliance.
    pub fn handle(&self, message: M) -> Result<(), Error> {
        match self.messages_in.try_send(message) {
            Err(_) => Err(Error::HandlingFailure),
            Ok(_) => Ok(()),
        }
    }
}