// Copyright 2018 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use std::fmt;
use std::marker::Unpin;
use std::mem;
use std::pin::Pin;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::Arc;

use crate::executor::{EHandle, PacketReceiver, ReceiverRegistration};
use fuchsia_zircon::{self as zx, AsHandleRef};
use futures::task::{AtomicWaker, Waker};
use futures::{Future, Poll};

struct OnSignalsReceiver {
    maybe_signals: AtomicUsize,
    task: AtomicWaker,
}

impl OnSignalsReceiver {
    fn get_signals(&self, lw: &Waker) -> Poll<zx::Signals> {
        let mut signals = self.maybe_signals.load(Ordering::Relaxed);
        if signals == 0 {
            // No signals were received-- register to receive a wakeup when they arrive.
            self.task.register(lw);
            // Check again for signals after registering for a wakeup in case signals
            // arrived between registering and the initial load of signals
            signals = self.maybe_signals.load(Ordering::SeqCst);
        }
        if signals == 0 {
            Poll::Pending
        } else {
            Poll::Ready(zx::Signals::from_bits_truncate(signals as u32))
        }
    }

    fn set_signals(&self, signals: zx::Signals) {
        self.maybe_signals
            .store(signals.bits() as usize, Ordering::SeqCst);
        self.task.wake();
    }
}

impl PacketReceiver for OnSignalsReceiver {
    fn receive_packet(&self, packet: zx::Packet) {
        let observed = if let zx::PacketContents::SignalOne(p) = packet.contents() {
            p.observed()
        } else {
            return;
        };

        self.set_signals(observed);
    }
}

/// A future that completes when some set of signals become available on a Handle.
#[must_use = "futures do nothing unless polled"]
pub struct OnSignals(Result<ReceiverRegistration<OnSignalsReceiver>, zx::Status>);

impl OnSignals {
    /// Creates a new `OnSignals` object which will receive notifications when
    /// any signals in `signals` occur on `handle`.
    pub fn new<T>(handle: &T, signals: zx::Signals) -> Self
    where
        T: AsHandleRef,
    {
        let ehandle = EHandle::local();
        let receiver = ehandle.register_receiver(Arc::new(OnSignalsReceiver {
            maybe_signals: AtomicUsize::new(0),
            task: AtomicWaker::new(),
        }));

        let res = handle.wait_async_handle(
            receiver.port(),
            receiver.key(),
            signals,
            zx::WaitAsyncOpts::Once,
        );

        OnSignals(res.map(|()| receiver).map_err(Into::into))
    }
}

impl Unpin for OnSignals {}

impl Future for OnSignals {
    type Output = Result<zx::Signals, zx::Status>;
    fn poll(mut self: Pin<&mut Self>, lw: &Waker) -> Poll<Self::Output> {
        let reg = self
            .0
            .as_mut()
            .map_err(|e| mem::replace(e, zx::Status::OK))?;
        reg.receiver().get_signals(lw).map(Ok)
    }
}

impl fmt::Debug for OnSignals {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "OnSignals")
    }
}