1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135

use std::collections::VecDeque;
use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll, Waker};

use futures::lock::BiLock;

#[cfg(test)]
mod tests;

pub fn channel<T>(max_len: usize) -> (Sender<T>, Receiver<T>) {
    let inner =
        Inner { queue: Default::default(), max_len, sender_waker: None, receiver_waker: None };

    let (sender, receiver) = BiLock::new(inner);
    (Sender(sender), Receiver(receiver))
}

#[derive(Debug)]
pub struct Receiver<T>(BiLock<Inner<T>>);

#[derive(Debug)]
pub struct Sender<T>(BiLock<Inner<T>>);

impl<T> Receiver<T>
where
    T: Unpin,
{
    pub fn recv<'a>(&'a mut self, should_block: bool) -> impl Future<Output = Option<T>> + 'a {
        Receive { lock: &self.0, should_block }
    }

    pub async fn len(&self) -> (usize, usize) {
        let locked = self.0.lock().await;
        (locked.queue.len(), locked.max_len)
    }
}

impl<T> Sender<T>
where
    T: Unpin,
{
    pub fn send<'a>(
        &'a mut self,
        item: T,
        should_block: bool,
    ) -> impl Future<Output = Result<(), T>> + 'a {
        Send { lock: &self.0, should_block, item: Some(item) }
    }

    pub async fn len(&self) -> (usize, usize) {
        let locked = self.0.lock().await;
        (locked.queue.len(), locked.max_len)
    }
}

#[derive(Debug)]
struct Inner<T> {
    queue: VecDeque<T>,
    max_len: usize,
    sender_waker: Option<Waker>,
    receiver_waker: Option<Waker>,
}

#[pin_project::pin_project]
struct Receive<'a, T> {
    lock: &'a BiLock<Inner<T>>,
    should_block: bool,
}

#[pin_project::pin_project]
struct Send<'a, T> {
    lock: &'a BiLock<Inner<T>>,
    should_block: bool,
    item: Option<T>,
}

impl<'a, T> Future for Receive<'a, T>
where
    T: Unpin,
{
    type Output = Option<T>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let this = self.project();

        let mut locked = futures::ready!(this.lock.poll_lock(cx));
        let _ = locked.receiver_waker.take();

        match (locked.queue.pop_front(), this.should_block) {
            (Some(item), _) => {
                locked.sender_waker.take().map(Waker::wake);
                Poll::Ready(Some(item))
            },
            (None, false) => Poll::Ready(None),
            (None, true) => {
                let should_be_none = locked.receiver_waker.replace(cx.waker().to_owned());
                assert!(should_be_none.is_none());
                Poll::Pending
            },
        }
    }
}

impl<'a, T> Future for Send<'a, T>
where
    T: Unpin,
{
    type Output = Result<(), T>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let this = self.project();

        let mut locked = futures::ready!(this.lock.poll_lock(cx));
        let _ = locked.sender_waker.take();

        match (locked.queue.len() < locked.max_len, this.should_block) {
            (true, _) => {
                let item = this.item.take().expect("Item empty");
                locked.queue.push_back(item);
                locked.receiver_waker.take().map(Waker::wake);
                Poll::Ready(Ok(()))
            },
            (false, false) => {
                let item = this.item.take().expect("Item empty");
                Poll::Ready(Err(item))
            },
            (false, true) => {
                let should_be_none = locked.sender_waker.replace(cx.waker().to_owned());
                assert!(should_be_none.is_none());
                Poll::Pending
            },
        }
    }
}