use std::pin::Pin;
use std::task::{Context, Poll};
use std::time::{Duration, Instant};

use futures_core::future::Future;
use futures_core::stream::Stream;
use futures_timer::Delay;

/// Creates a new stream that yields at a set interval.
///
/// The stream first yields after `dur`, and continues to yield every
/// `dur` after that. The stream accounts for time elapsed between calls, and
/// will adjust accordingly to prevent time skews.
///
/// Each interval may be slightly longer than the specified duration, but never
/// less.
///
/// Note that intervals are not intended for high resolution timers, but rather
/// they will likely fire some granularity after the exact instant that they're
/// otherwise indicated to fire at.
///
/// See also: [`task::sleep`].
///
/// [`task::sleep`]: ../task/fn.sleep.html
///
/// # Examples
///
/// Basic example:
///
/// ```no_run
/// use async_std::prelude::*;
/// use async_std::stream;
/// use std::time::Duration;
///
/// # fn main() -> std::io::Result<()> { async_std::task::block_on(async {
/// #
/// let mut interval = stream::interval(Duration::from_secs(4));
/// while let Some(_) = interval.next().await {
///     println!("prints every four seconds");
/// }
/// #
/// # Ok(()) }) }
/// ```
#[cfg(feature = "unstable")]
#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
pub fn interval(dur: Duration) -> Interval {
    Interval {
        delay: Delay::new(dur),
        interval: dur,
    }
}

/// A stream representing notifications at fixed interval
///
/// This stream is created by the [`interval`] function. See its
/// documentation for more.
///
/// [`interval`]: fn.interval.html
#[cfg(feature = "unstable")]
#[cfg_attr(feature = "docs", doc(cfg(unstable)))]
#[derive(Debug)]
pub struct Interval {
    delay: Delay,
    interval: Duration,
}

impl Stream for Interval {
    type Item = ();

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        if Pin::new(&mut self.delay).poll(cx).is_pending() {
            return Poll::Pending;
        }
        let when = Instant::now();
        let next = next_interval(when, Instant::now(), self.interval);
        self.delay.reset(next);
        Poll::Ready(Some(()))
    }
}

/// Converts Duration object to raw nanoseconds if possible
///
/// This is useful to divide intervals.
///
/// While technically for large duration it's impossible to represent any
/// duration as nanoseconds, the largest duration we can represent is about
/// 427_000 years. Large enough for any interval we would use or calculate in
/// tokio.
fn duration_to_nanos(dur: Duration) -> Option<u64> {
    dur.as_secs()
        .checked_mul(1_000_000_000)
        .and_then(|v| v.checked_add(u64::from(dur.subsec_nanos())))
}

fn next_interval(prev: Instant, now: Instant, interval: Duration) -> Instant {
    let new = prev + interval;
    if new > now {
        return new;
    }

    let spent_ns = duration_to_nanos(now.duration_since(prev)).expect("interval should be expired");
    let interval_ns =
        duration_to_nanos(interval).expect("interval is less that 427 thousand years");
    let mult = spent_ns / interval_ns + 1;
    assert!(
        mult < (1 << 32),
        "can't skip more than 4 billion intervals of {:?} \
         (trying to skip {})",
        interval,
        mult
    );
    prev + interval * (mult as u32)
}

#[cfg(test)]
mod test {
    use super::next_interval;
    use std::cmp::Ordering;
    use std::time::{Duration, Instant};

    struct Timeline(Instant);

    impl Timeline {
        fn new() -> Timeline {
            Timeline(Instant::now())
        }
        fn at(&self, millis: u64) -> Instant {
            self.0 + Duration::from_millis(millis)
        }
        fn at_ns(&self, sec: u64, nanos: u32) -> Instant {
            self.0 + Duration::new(sec, nanos)
        }
    }

    fn dur(millis: u64) -> Duration {
        Duration::from_millis(millis)
    }

    // The math around Instant/Duration isn't 100% precise due to rounding
    // errors, see #249 for more info
    fn almost_eq(a: Instant, b: Instant) -> bool {
        match a.cmp(&b) {
            Ordering::Equal => true,
            Ordering::Greater => a - b < Duration::from_millis(1),
            Ordering::Less => b - a < Duration::from_millis(1),
        }
    }

    #[test]
    fn norm_next() {
        let tm = Timeline::new();
        assert!(almost_eq(
            next_interval(tm.at(1), tm.at(2), dur(10)),
            tm.at(11)
        ));
        assert!(almost_eq(
            next_interval(tm.at(7777), tm.at(7788), dur(100)),
            tm.at(7877)
        ));
        assert!(almost_eq(
            next_interval(tm.at(1), tm.at(1000), dur(2100)),
            tm.at(2101)
        ));
    }

    #[test]
    fn fast_forward() {
        let tm = Timeline::new();
        assert!(almost_eq(
            next_interval(tm.at(1), tm.at(1000), dur(10)),
            tm.at(1001)
        ));
        assert!(almost_eq(
            next_interval(tm.at(7777), tm.at(8888), dur(100)),
            tm.at(8977)
        ));
        assert!(almost_eq(
            next_interval(tm.at(1), tm.at(10000), dur(2100)),
            tm.at(10501)
        ));
    }

    /// TODO: this test actually should be successful, but since we can't
    ///       multiply Duration on anything larger than u32 easily we decided
    ///       to allow it to fail for now
    #[test]
    #[should_panic(expected = "can't skip more than 4 billion intervals")]
    fn large_skip() {
        let tm = Timeline::new();
        assert_eq!(
            next_interval(tm.at_ns(0, 1), tm.at_ns(25, 0), Duration::new(0, 2)),
            tm.at_ns(25, 1)
        );
    }
}