use crate::time::{sleep_until, Duration, Instant, Sleep};

use crate::util::trace;

use std::future::{poll_fn, Future};

use std::panic::Location;

use std::pin::Pin;

use std::task::{ready, Context, Poll};

/// Creates new [`Interval`] that yields with interval of `period`. The first

/// tick completes immediately. The default [`MissedTickBehavior`] is

/// [`Burst`](MissedTickBehavior::Burst), but this can be configured

/// by calling [`set_missed_tick_behavior`](Interval::set_missed_tick_behavior).

///

/// An interval will tick indefinitely. At any time, the [`Interval`] value can

/// be dropped. This cancels the interval.

///

/// This function is equivalent to

/// [`interval_at(Instant::now(), period)`](interval_at).

///

/// # Panics

///

/// This function panics if `period` is zero.

///

/// # Examples

///

/// ```

/// use tokio::time::{self, Duration};

///

/// #[tokio::main]

/// async fn main() {

///     let mut interval = time::interval(Duration::from_millis(10));

///

///     interval.tick().await; // ticks immediately

///     interval.tick().await; // ticks after 10ms

///     interval.tick().await; // ticks after 10ms

///

///     // approximately 20ms have elapsed.

/// }

/// ```

///

/// A simple example using `interval` to execute a task every two seconds.

///

/// The difference between `interval` and [`sleep`] is that an [`Interval`]

/// measures the time since the last tick, which means that [`.tick().await`]

/// may wait for a shorter time than the duration specified for the interval

/// if some time has passed between calls to [`.tick().await`].

///

/// If the tick in the example below was replaced with [`sleep`], the task

/// would only be executed once every three seconds, and not every two

/// seconds.

///

/// ```

/// use tokio::time;

///

/// async fn task_that_takes_a_second() {

///     println!("hello");

///     time::sleep(time::Duration::from_secs(1)).await

/// }

///

/// #[tokio::main]

/// async fn main() {

///     let mut interval = time::interval(time::Duration::from_secs(2));

///     for _i in 0..5 {

///         interval.tick().await;

///         task_that_takes_a_second().await;

///     }

/// }

/// ```

///

/// [`sleep`]: crate::time::sleep()

/// [`.tick().await`]: Interval::tick

#[track_caller]

pub fn interval(period: Duration) -> Interval {

    assert!(period > Duration::new(0, 0), "`period` must be non-zero.");

    internal_interval_at(Instant::now(), period, trace::caller_location())

}

/// Creates new [`Interval`] that yields with interval of `period` with the

/// first tick completing at `start`. The default [`MissedTickBehavior`] is

/// [`Burst`](MissedTickBehavior::Burst), but this can be configured

/// by calling [`set_missed_tick_behavior`](Interval::set_missed_tick_behavior).

///

/// An interval will tick indefinitely. At any time, the [`Interval`] value can

/// be dropped. This cancels the interval.

///

/// # Panics

///

/// This function panics if `period` is zero.

///

/// # Examples

///

/// ```

/// use tokio::time::{interval_at, Duration, Instant};

///

/// #[tokio::main]

/// async fn main() {

///     let start = Instant::now() + Duration::from_millis(50);

///     let mut interval = interval_at(start, Duration::from_millis(10));

///

///     interval.tick().await; // ticks after 50ms

///     interval.tick().await; // ticks after 10ms

///     interval.tick().await; // ticks after 10ms

///

///     // approximately 70ms have elapsed.

/// }

/// ```

#[track_caller]

pub fn interval_at(start: Instant, period: Duration) -> Interval {

    assert!(period > Duration::new(0, 0), "`period` must be non-zero.");

    internal_interval_at(start, period, trace::caller_location())

}

#[cfg_attr(not(all(tokio_unstable, feature = "tracing")), allow(unused_variables))]

fn internal_interval_at(

    start: Instant,

    period: Duration,

    location: Option<&'static Location<'static>>,

) -> Interval {

    #[cfg(all(tokio_unstable, feature = "tracing"))]

    let resource_span = {

        let location = location.expect("should have location if tracing");

        tracing::trace_span!(

            parent: None,

            "runtime.resource",

            concrete_type = "Interval",

            kind = "timer",

            loc.file = location.file(),

            loc.line = location.line(),

            loc.col = location.column(),

        )

    };

    #[cfg(all(tokio_unstable, feature = "tracing"))]

    let delay = resource_span.in_scope(|| Box::pin(sleep_until(start)));

    #[cfg(not(all(tokio_unstable, feature = "tracing")))]

    let delay = Box::pin(sleep_until(start));

    Interval {

        delay,

        period,

        missed_tick_behavior: MissedTickBehavior::default(),

        #[cfg(all(tokio_unstable, feature = "tracing"))]

        resource_span,

    }

}

/// Defines the behavior of an [`Interval`] when it misses a tick.

///

/// Sometimes, an [`Interval`]'s tick is missed. For example, consider the

/// following:

///

/// ```

/// use tokio::time::{self, Duration};

/// # async fn task_that_takes_one_to_three_millis() {}

///

/// #[tokio::main]

/// async fn main() {

///     // ticks every 2 milliseconds

///     let mut interval = time::interval(Duration::from_millis(2));

///     for _ in 0..5 {

///         interval.tick().await;

///         // if this takes more than 2 milliseconds, a tick will be delayed

///         task_that_takes_one_to_three_millis().await;

///     }

/// }

/// ```

///

/// Generally, a tick is missed if too much time is spent without calling

/// [`Interval::tick()`].

///

/// By default, when a tick is missed, [`Interval`] fires ticks as quickly as it

/// can until it is "caught up" in time to where it should be.

/// `MissedTickBehavior` can be used to specify a different behavior for

/// [`Interval`] to exhibit. Each variant represents a different strategy.

///

/// Note that because the executor cannot guarantee exact precision with timers,

/// these strategies will only apply when the delay is greater than 5

/// milliseconds.

#[derive(Debug, Clone, Copy, PartialEq, Eq)]

pub enum MissedTickBehavior {

    /// Ticks as fast as possible until caught up.

    ///

    /// When this strategy is used, [`Interval`] schedules ticks "normally" (the

    /// same as it would have if the ticks hadn't been delayed), which results

    /// in it firing ticks as fast as possible until it is caught up in time to

    /// where it should be. Unlike [`Delay`] and [`Skip`], the ticks yielded

    /// when `Burst` is used (the [`Instant`]s that [`tick`](Interval::tick)

    /// yields) aren't different than they would have been if a tick had not

    /// been missed. Like [`Skip`], and unlike [`Delay`], the ticks may be

    /// shortened.

    ///

    /// This looks something like this:

    /// ```text

    /// Expected ticks: |     1     |     2     |     3     |     4     |     5     |     6     |

    /// Actual ticks:   | work -----|          delay          | work | work | work -| work -----|

    /// ```

    ///

    /// In code:

    ///

    /// ```

    /// use tokio::time::{interval, Duration};

    /// # async fn task_that_takes_200_millis() {}

    ///

    /// # #[tokio::main(flavor = "current_thread")]

    /// # async fn main() {

    /// let mut interval = interval(Duration::from_millis(50));

    ///

    /// // First tick resolves immediately after creation

    /// interval.tick().await;

    ///

    /// task_that_takes_200_millis().await;

    /// // The `Interval` has missed a tick

    ///

    /// // Since we have exceeded our timeout, this will resolve immediately

    /// interval.tick().await;

    ///

    /// // Since we are more than 100ms after the start of `interval`, this will

    /// // also resolve immediately.

    /// interval.tick().await;

    ///

    /// // Also resolves immediately, because it was supposed to resolve at

    /// // 150ms after the start of `interval`

    /// interval.tick().await;

    ///

    /// // Resolves immediately

    /// interval.tick().await;

    ///

    /// // Since we have gotten to 200ms after the start of `interval`, this

    /// // will resolve after 50ms

    /// interval.tick().await;

    /// # }

    /// ```

    ///

    /// This is the default behavior when [`Interval`] is created with

    /// [`interval`] and [`interval_at`].

    ///

    /// [`Delay`]: MissedTickBehavior::Delay

    /// [`Skip`]: MissedTickBehavior::Skip

    Burst,

    /// Tick at multiples of `period` from when [`tick`] was called, rather than

    /// from `start`.

    ///

    /// When this strategy is used and [`Interval`] has missed a tick, instead

    /// of scheduling ticks to fire at multiples of `period` from `start` (the

    /// time when the first tick was fired), it schedules all future ticks to

    /// happen at a regular `period` from the point when [`tick`] was called.

    /// Unlike [`Burst`] and [`Skip`], ticks are not shortened, and they aren't

    /// guaranteed to happen at a multiple of `period` from `start` any longer.

    ///

    /// This looks something like this:

    /// ```text

    /// Expected ticks: |     1     |     2     |     3     |     4     |     5     |     6     |

    /// Actual ticks:   | work -----|          delay          | work -----| work -----| work -----|

    /// ```

    ///

    /// In code:

    ///

    /// ```

    /// use tokio::time::{interval, Duration, MissedTickBehavior};

    /// # async fn task_that_takes_more_than_50_millis() {}

    ///

    /// # #[tokio::main(flavor = "current_thread")]

    /// # async fn main() {

    /// let mut interval = interval(Duration::from_millis(50));

    /// interval.set_missed_tick_behavior(MissedTickBehavior::Delay);

    ///

    /// task_that_takes_more_than_50_millis().await;

    /// // The `Interval` has missed a tick

    ///

    /// // Since we have exceeded our timeout, this will resolve immediately

    /// interval.tick().await;

    ///

    /// // But this one, rather than also resolving immediately, as might happen

    /// // with the `Burst` or `Skip` behaviors, will not resolve until

    /// // 50ms after the call to `tick` up above. That is, in `tick`, when we

    /// // recognize that we missed a tick, we schedule the next tick to happen

    /// // 50ms (or whatever the `period` is) from right then, not from when

    /// // were *supposed* to tick

    /// interval.tick().await;

    /// # }

    /// ```

    ///

    /// [`Burst`]: MissedTickBehavior::Burst

    /// [`Skip`]: MissedTickBehavior::Skip

    /// [`tick`]: Interval::tick

    Delay,

    /// Skips missed ticks and tick on the next multiple of `period` from

    /// `start`.

    ///

    /// When this strategy is used, [`Interval`] schedules the next tick to fire

    /// at the next-closest tick that is a multiple of `period` away from

    /// `start` (the point where [`Interval`] first ticked). Like [`Burst`], all

    /// ticks remain multiples of `period` away from `start`, but unlike

    /// [`Burst`], the ticks may not be *one* multiple of `period` away from the

    /// last tick. Like [`Delay`], the ticks are no longer the same as they

    /// would have been if ticks had not been missed, but unlike [`Delay`], and

    /// like [`Burst`], the ticks may be shortened to be less than one `period`

    /// away from each other.

    ///

    /// This looks something like this:

    /// ```text

    /// Expected ticks: |     1     |     2     |     3     |     4     |     5     |     6     |

    /// Actual ticks:   | work -----|          delay          | work ---| work -----| work -----|

    /// ```

    ///

    /// In code:

    ///

    /// ```

    /// use tokio::time::{interval, Duration, MissedTickBehavior};

    /// # async fn task_that_takes_75_millis() {}

    ///

    /// # #[tokio::main(flavor = "current_thread")]

    /// # async fn main() {

    /// let mut interval = interval(Duration::from_millis(50));

    /// interval.set_missed_tick_behavior(MissedTickBehavior::Skip);

    ///

    /// task_that_takes_75_millis().await;

    /// // The `Interval` has missed a tick

    ///

    /// // Since we have exceeded our timeout, this will resolve immediately

    /// interval.tick().await;

    ///

    /// // This one will resolve after 25ms, 100ms after the start of

    /// // `interval`, which is the closest multiple of `period` from the start

    /// // of `interval` after the call to `tick` up above.

    /// interval.tick().await;

    /// # }

    /// ```

    ///

    /// [`Burst`]: MissedTickBehavior::Burst

    /// [`Delay`]: MissedTickBehavior::Delay

    Skip,

}

impl MissedTickBehavior {

    /// If a tick is missed, this method is called to determine when the next tick should happen.

    fn next_timeout(&self, timeout: Instant, now: Instant, period: Duration) -> Instant {

        match self {

            Self::Burst => timeout + period,

            Self::Delay => now + period,

            Self::Skip => {

                now + period

                    - Duration::from_nanos(

                        ((now - timeout).as_nanos() % period.as_nanos())

                            .try_into()

                            // This operation is practically guaranteed not to

                            // fail, as in order for it to fail, `period` would

                            // have to be longer than `now - timeout`, and both

                            // would have to be longer than 584 years.

                            //

                            // If it did fail, there's not a good way to pass

                            // the error along to the user, so we just panic.

                            .expect(

                                "too much time has elapsed since the interval was supposed to tick",

                            ),

                    )

            }

        }

    }

}

impl Default for MissedTickBehavior {

    /// Returns [`MissedTickBehavior::Burst`].

    ///

    /// For most usecases, the [`Burst`] strategy is what is desired.

    /// Additionally, to preserve backwards compatibility, the [`Burst`]

    /// strategy must be the default. For these reasons,

    /// [`MissedTickBehavior::Burst`] is the default for [`MissedTickBehavior`].

    /// See [`Burst`] for more details.

    ///

    /// [`Burst`]: MissedTickBehavior::Burst

    fn default() -> Self {

        Self::Burst

    }

}

/// Interval returned by [`interval`] and [`interval_at`].

///

/// This type allows you to wait on a sequence of instants with a certain

/// duration between each instant. Unlike calling [`sleep`] in a loop, this lets

/// you count the time spent between the calls to [`sleep`] as well.

///

/// An `Interval` can be turned into a `Stream` with [`IntervalStream`].

///

/// [`IntervalStream`]: https://docs.rs/tokio-stream/latest/tokio_stream/wrappers/struct.IntervalStream.html

/// [`sleep`]: crate::time::sleep()

#[derive(Debug)]

pub struct Interval {

    /// Future that completes the next time the `Interval` yields a value.

    delay: Pin<Box<Sleep>>,

    /// The duration between values yielded by `Interval`.

    period: Duration,

    /// The strategy `Interval` should use when a tick is missed.

    missed_tick_behavior: MissedTickBehavior,

    #[cfg(all(tokio_unstable, feature = "tracing"))]

    resource_span: tracing::Span,

}

impl Interval {

    /// Completes when the next instant in the interval has been reached.

    ///

    /// # Cancel safety

    ///

    /// This method is cancellation safe. If `tick` is used as the branch in a `tokio::select!` and

    /// another branch completes first, then no tick has been consumed.

    ///

    /// # Examples

    ///

    /// ```

    /// use tokio::time;

    ///

    /// use std::time::Duration;

    ///

    /// #[tokio::main]

    /// async fn main() {

    ///     let mut interval = time::interval(Duration::from_millis(10));

    ///

    ///     interval.tick().await;

    ///     // approximately 0ms have elapsed. The first tick completes immediately.

    ///     interval.tick().await;

    ///     interval.tick().await;

    ///

    ///     // approximately 20ms have elapsed.

    /// }

    /// ```

    pub async fn tick(&mut self) -> Instant {

        #[cfg(all(tokio_unstable, feature = "tracing"))]

        let resource_span = self.resource_span.clone();

        #[cfg(all(tokio_unstable, feature = "tracing"))]

        let instant = trace::async_op(

            || poll_fn(|cx| self.poll_tick(cx)),

            resource_span,

            "Interval::tick",

            "poll_tick",

            false,

        );

        #[cfg(not(all(tokio_unstable, feature = "tracing")))]

        let instant = poll_fn(|cx| self.poll_tick(cx));

        instant.await

    }

    /// Polls for the next instant in the interval to be reached.

    ///

    /// This method can return the following values:

    ///

    ///  * `Poll::Pending` if the next instant has not yet been reached.

    ///  * `Poll::Ready(instant)` if the next instant has been reached.

    ///

    /// When this method returns `Poll::Pending`, the current task is scheduled

    /// to receive a wakeup when the instant has elapsed. Note that on multiple

    /// calls to `poll_tick`, only the [`Waker`](std::task::Waker) from the

    /// [`Context`] passed to the most recent call is scheduled to receive a

    /// wakeup.

    pub fn poll_tick(&mut self, cx: &mut Context<'_>) -> Poll<Instant> {

        // Wait for the delay to be done

        ready!(Pin::new(&mut self.delay).poll(cx));

        // Get the time when we were scheduled to tick

        let timeout = self.delay.deadline();

        let now = Instant::now();

        // If a tick was not missed, and thus we are being called before the

        // next tick is due, just schedule the next tick normally, one `period`

        // after `timeout`

        //

        // However, if a tick took excessively long and we are now behind,

        // schedule the next tick according to how the user specified with

        // `MissedTickBehavior`

        let next = if now > timeout + Duration::from_millis(5) {

            self.missed_tick_behavior

                .next_timeout(timeout, now, self.period)

        } else {

            timeout

                .checked_add(self.period)

                .unwrap_or_else(Instant::far_future)

        };

        // When we arrive here, the internal delay returned `Poll::Ready`.

        // Reset the delay but do not register it. It should be registered with

        // the next call to [`poll_tick`].

        self.delay.as_mut().reset_without_reregister(next);

        // Return the time when we were scheduled to tick

        Poll::Ready(timeout)

    }

    /// Resets the interval to complete one period after the current time.

    ///

    /// This method ignores [`MissedTickBehavior`] strategy.

    ///

    /// This is equivalent to calling `reset_at(Instant::now() + period)`.

    ///

    /// # Examples

    ///

    /// ```

    /// use tokio::time;

    ///

    /// use std::time::Duration;

    ///

    /// #[tokio::main]

    /// async fn main() {

    ///     let mut interval = time::interval(Duration::from_millis(100));

    ///

    ///     interval.tick().await;

    ///

    ///     time::sleep(Duration::from_millis(50)).await;

    ///     interval.reset();

    ///

    ///     interval.tick().await;

    ///     interval.tick().await;

    ///

    ///     // approximately 250ms have elapsed.

    /// }

    /// ```

    pub fn reset(&mut self) {

        self.delay.as_mut().reset(Instant::now() + self.period);

    }

    /// Resets the interval immediately.

    ///

    /// This method ignores [`MissedTickBehavior`] strategy.

    ///

    /// This is equivalent to calling `reset_at(Instant::now())`.

    ///

    /// # Examples

    ///

    /// ```

    /// use tokio::time;

    ///

    /// use std::time::Duration;

    ///

    /// #[tokio::main]

    /// async fn main() {

    ///     let mut interval = time::interval(Duration::from_millis(100));

    ///

    ///     interval.tick().await;

    ///

    ///     time::sleep(Duration::from_millis(50)).await;

    ///     interval.reset_immediately();

    ///

    ///     interval.tick().await;

    ///     interval.tick().await;

    ///

    ///     // approximately 150ms have elapsed.

    /// }

    /// ```

    pub fn reset_immediately(&mut self) {

        self.delay.as_mut().reset(Instant::now());

    }

    /// Resets the interval after the specified [`std::time::Duration`].

    ///

    /// This method ignores [`MissedTickBehavior`] strategy.

    ///

    /// This is equivalent to calling `reset_at(Instant::now() + after)`.

    ///

    /// # Examples

    ///

    /// ```

    /// use tokio::time;

    ///

    /// use std::time::Duration;

    ///

    /// #[tokio::main]

    /// async fn main() {

    ///     let mut interval = time::interval(Duration::from_millis(100));

    ///     interval.tick().await;

    ///

    ///     time::sleep(Duration::from_millis(50)).await;

    ///

    ///     let after = Duration::from_millis(20);

    ///     interval.reset_after(after);

    ///

    ///     interval.tick().await;

    ///     interval.tick().await;

    ///

    ///     // approximately 170ms have elapsed.

    /// }

    /// ```

    pub fn reset_after(&mut self, after: Duration) {

        self.delay.as_mut().reset(Instant::now() + after);

    }

    /// Resets the interval to a [`crate::time::Instant`] deadline.

    ///

    /// Sets the next tick to expire at the given instant. If the instant is in

    /// the past, then the [`MissedTickBehavior`] strategy will be used to

    /// catch up. If the instant is in the future, then the next tick will

    /// complete at the given instant, even if that means that it will sleep for

    /// longer than the duration of this [`Interval`]. If the [`Interval`] had

    /// any missed ticks before calling this method, then those are discarded.

    ///

    /// # Examples

    ///

    /// ```

    /// use tokio::time::{self, Instant};

    ///

    /// use std::time::Duration;

    ///

    /// #[tokio::main]

    /// async fn main() {

    ///     let mut interval = time::interval(Duration::from_millis(100));

    ///     interval.tick().await;

    ///

    ///     time::sleep(Duration::from_millis(50)).await;

    ///

    ///     let deadline = Instant::now() + Duration::from_millis(30);

    ///     interval.reset_at(deadline);

    ///

    ///     interval.tick().await;

    ///     interval.tick().await;

    ///

    ///     // approximately 180ms have elapsed.

    /// }

    /// ```

    pub fn reset_at(&mut self, deadline: Instant) {

        self.delay.as_mut().reset(deadline);

    }

    /// Returns the [`MissedTickBehavior`] strategy currently being used.

    pub fn missed_tick_behavior(&self) -> MissedTickBehavior {

        self.missed_tick_behavior

    }

    /// Sets the [`MissedTickBehavior`] strategy that should be used.

    pub fn set_missed_tick_behavior(&mut self, behavior: MissedTickBehavior) {

        self.missed_tick_behavior = behavior;

    }

    /// Returns the period of the interval.

    pub fn period(&self) -> Duration {

        self.period

    }

}