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
136
137
138
139
140
141
142
143
144
145

use futures::{Async, Future};
use std::mem;

pub struct WhileDriving<A, B: Future> {
    inner: WhileDrivingInner<A, B>,
}

enum WhileDrivingInner<A, B: Future> {
    Driving(A, B),
    Drove(A, Result<B::Item, B::Error>),
    Finished,
}

impl<A, B: Future> WhileDriving<A, B> {
    pub fn new(a: A, b: B) -> WhileDriving<A, B> {
        WhileDriving {
            inner: WhileDrivingInner::Driving(a, b),
        }
    }
}

impl<A, B> Future for WhileDriving<A, B>
where
    A: Future,
    B: Future,
{
    type Item = (A::Item, Finish<B>);
    type Error = (A::Error, Finish<B>);

    fn poll(&mut self) -> Result<Async<(A::Item, Finish<B>)>, (A::Error, Finish<B>)> {
        let inner = mem::replace(&mut self.inner, WhileDrivingInner::Finished);
        match inner {
            WhileDrivingInner::Driving(mut a, mut b) => {
                match a.poll() {
                    Ok(Async::Ready(x)) => {
                        let finish = Finish {
                            state: FinishState::Inner(FinishInner::Running(b)),
                        };
                        Ok(Async::Ready((x, finish)))
                    },
                    Ok(Async::NotReady) => {
                        match b.poll() {
                            Ok(Async::Ready(x)) => {
                                self.inner = WhileDrivingInner::Drove(a, Ok(x));
                            },
                            Ok(Async::NotReady) => {
                                self.inner = WhileDrivingInner::Driving(a, b);
                            },
                            Err(e) => {
                                self.inner = WhileDrivingInner::Drove(a, Err(e));
                            },
                        }
                        Ok(Async::NotReady)
                    },
                    Err(e) => {
                        let finish = Finish {
                            state: FinishState::Inner(FinishInner::Running(b)),
                        };
                        Err((e, finish))
                    },
                }
            },
            WhileDrivingInner::Drove(mut a, res) => {
                match a.poll() {
                    Ok(Async::Ready(x)) => {
                        let finish = Finish {
                            state: FinishState::Inner(FinishInner::Ran(res)),
                        };
                        Ok(Async::Ready((x, finish)))
                    },
                    Ok(Async::NotReady) => {
                        self.inner = WhileDrivingInner::Drove(a, res);
                        Ok(Async::NotReady)
                    },
                    Err(e) => {
                        let finish = Finish {
                            state: FinishState::Inner(FinishInner::Ran(res)),
                        };
                        Err((e, finish))
                    },
                }
            },
            WhileDrivingInner::Finished => {
                panic!("poll() called on WhileDriving which has already finished");
            },
        }
    }
}

/// Future yielded by `WhileDriving` which wraps the future it was driving. Can be resolved to
/// whatever the original future would resolved to or unpacked using `into_inner`.
pub struct Finish<B: Future> {
    state: FinishState<B>,
}

impl<B: Future> Finish<B> {
    pub fn into_inner(self) -> FinishInner<B> {
        match self.state {
            FinishState::Inner(inner) => inner,
            FinishState::Finished => {
                panic!("into_inner() called on Finish which has already finished");
            },
        }
    }
}

pub enum FinishInner<B: Future> {
    Running(B),
    Ran(Result<B::Item, B::Error>),
}

enum FinishState<B: Future> {
    Inner(FinishInner<B>),
    Finished,
}

impl<B: Future> Future for Finish<B> {
    type Item = B::Item;
    type Error = B::Error;

    fn poll(&mut self) -> Result<Async<B::Item>, B::Error> {
        let state = mem::replace(&mut self.state, FinishState::Finished);
        match state {
            FinishState::Inner(FinishInner::Running(mut b)) => {
                match b.poll() {
                    Ok(Async::Ready(x)) => {
                        Ok(Async::Ready(x))
                    },
                    Ok(Async::NotReady) => {
                        self.state = FinishState::Inner(FinishInner::Running(b));
                        Ok(Async::NotReady)
                    },
                    Err(e) => {
                        Err(e)
                    },
                }
            },
            FinishState::Inner(FinishInner::Ran(res)) => Ok(Async::Ready(res?)),
            FinishState::Finished => {
                panic!("poll() called on Finish which has already finished");
            },
        }
    }
}