//! Declare the `RetryTime` enumeration and related code.

use derive_more::{From, Into};

use std::{

    cmp::Ordering,

    time::{Duration, Instant},

};

use strum::EnumDiscriminants;

/// A description of when an operation may be retried.

///

/// # Retry times values are contextual.

///

/// Note that retrying is necessarily contextual, depending on what exactly

/// we're talking about retrying.

///

/// For an example of how context matters:  suppose that we try to build a

/// circuit, and encounter a failure extending to the second hop.  If we try to

/// build a circuit _through the same path_ immediately, it's likely to fail

/// again.  But if we try to build a circuit through a different path, then

/// there's no reason to expect that same kind of error.

///

/// Thus, the same inner error condition ("failed to extend to the nth hop") can

/// indicate either a "Retry after waiting for a while" or "Retry immediately."

///

/// # Retry times depend on what we think might change.

///

/// Whether retrying will help depends on what we think is likely to change in

/// the near term.

///

/// For example, we generally assume an unreachable relay has some likelihood of

/// becoming reachable in the near future, and therefore connecting to such a

/// relay is worth retrying.

///

/// On the other hand, we _don't_ assume that the network is changing wildly

/// over time.  Thus, if there is currently no relay that supports delivering

/// traffic to port 23 (telnet), we say that building a request for such a relay

/// is not retriable, even though technically such a relay might appear in the

/// next consensus.

#[non_exhaustive]

// We define a discriminant type so we can simplify loose_cmp.

#[strum_discriminants(derive(Ord, PartialOrd))]

// We don't want to expose RetryTimeDiscriminants.

#[strum_discriminants(vis())]

pub enum RetryTime {

    /// The operation can be retried immediately, and no delay is needed.

    ///

    /// The recipient of this `RetryTime` variant may retry the operation

    /// immediately without waiting.

    ///

    /// This case should be used cautiously: it risks making code retry in a

    /// loop without delay.  It should only be used for error conditions that

    /// are necessarily produced via a process that itself introduces a delay.

    /// (For example, this case is suitable for errors caused by a remote

    /// timeout.)

    Immediate,

    /// The operation can be retried after a short delay, to prevent overloading

    /// the network.  

    ///

    /// The recipient of this `RetryTime` variant should delay a short amount of

    /// time before retrying.  The amount of time to delay should be randomized,

    /// and should tend to grow larger the more failures there have been

    /// recently for the given operation.  (The `RetryDelay` type from

    /// `tor-basic-utils` is suitable for managing this calculation.)

    ///

    /// This case should be used for problems that tend to be "self correcting",

    /// such as remote server failures (the server might come back up).

    AfterWaiting,

    /// The operation can be retried after a particular delay.

    ///

    /// The recipient of this `RetryTime` variant should wait for at least the

    /// given duration before retrying the operation.

    ///

    /// This case should only be used if there is some reason not to return

    /// `AfterWaiting`: for example, if the implementor is providing their own

    /// back-off algorithm instead of using `RetryDelay.`

    ///

    /// (This is a separate variant from `At`, since the constructor may not

    /// have convenient access to (a mocked view of) the current time.  If you

    /// know that the current time is `now`, then `After(d)` is equivalent to

    /// `At(now + d)`.)

    After(Duration),

    /// The operation can be retried at some particular time in the future.

    ///

    /// The recipient of this this `RetryTime` variant should wait until the

    /// current time (as returned by `Instant::now` or `SleepProvider::now` as

    /// appropriate) is at least this given instant.

    ///

    /// This case is appropriate for when we have a failure condition caused by

    /// waiting for multiple other timeouts.  (For example, if we believe that

    /// all our guards are down, then we won't be able to try getting a guard

    /// until the next time guard is scheduled to be marked as retriable.)

    At(Instant),

    /// Retrying is unlikely to make this operation succeed, unless something

    /// else is fixed first.

    ///

    /// The recipient of this `RetryTime` variant should generally give up, and

    /// stop retrying the given operation.

    ///

    /// We don't mean "literally" that the operation will never succeed: only

    /// that retrying it in the near future without fixing the underlying cause

    /// is unlikely to help.

    ///

    /// This case is appropriate for issues like misconfiguration, internal

    /// errors, and requests for operations that the network doesn't support.

    ///

    /// This case is also appropriate for a problem that is "technically"

    /// retriable, but where any resolution is likelier to take days or weeks

    /// instead  of minutes or hours.

    Never,

}

/// A `RetryTime` wrapped so that it compares according to [`RetryTime::loose_cmp`]

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

pub struct LooseCmpRetryTime(RetryTime);

/// Trait for an error object that can tell us when the operation which

/// generated it can be retried.

pub trait HasRetryTime {

    /// Return the time when the operation that gave this error can be retried.

    ///

    /// See all caveats and explanations on [`RetryTime`].

    fn retry_time(&self) -> RetryTime;

    /// Return an absolute retry when the operation that gave this error can be

    /// retried.

    ///

    /// Requires that `now` is the current time, and `choose_delay` is a

    /// function to choose a delay for [`RetryTime::AfterWaiting`].

}

/// An absolute [`RetryTime`].

///

/// Unlike `RetryTime`, this type always denotes a particular instant in time.

/// You can derive it using [`RetryTime::absolute`].

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

#[allow(clippy::exhaustive_enums)]

pub enum AbsRetryTime {

    /// See [`RetryTime::Immediate`].

    Immediate,

    /// See [`RetryTime::At`].

    At(Instant),

    /// See [`RetryTime::Never`].

    Never,

}

impl AbsRetryTime {

    /// Construct an AbsRetryTime representing `base` + `plus`.

        }

}

impl RetryTime {

    /// Convert this [`RetryTime`] in to an absolute time.

    ///

    /// Requires that `now` is the current time, and `choose_delay` is a

    /// function to choose a delay for [`RetryTime::AfterWaiting`].

        }

    /// Convert all the provided `items` into [`AbsRetryTime`] values, and

    /// return the earliest one.

    ///

    /// Requires that `now` is the current time, and `choose_delay` is a

    /// function to choose a delay for [`RetryTime::AfterWaiting`].

    ///

    /// Differs from `items.map(AbsRetryTime::absolute(now,

    /// choose_delay)).min()` in that it calls `choose_delay` at most once.

    /// Return the "approximately earliest" item for an iterator of retry times.

    ///

    /// This is necessarily an approximation, since we can't be sure what time

    /// will be chosen if the retry is supposed to happen at a random time, and

    /// therefore cannot tell whether `AfterWaiting` comes before or after

    /// particular `At` and `After` instances.

    ///

    /// If you need an exact answer, use earliest_absolute.

    /// A loose-but-total comparison operator, suitable for choosing a retry

    /// time when multiple attempts have failed.

    ///

    /// If you need an absolute comparison operator, convert to [`AbsRetryTime`] first.

    ///

    /// See also:

    /// [`LooseCmpRetryTime`], a wrapper for `RetryTime` that uses this comparison.

        use RetryTime as RT;

            // When we have the same type with an internal embedded duration or time,

            // we compare based on the duration or time.

            // Otherwise, we compare based on discriminant type.

            //

            // This can't do a perfect "apples-to-apples" comparison for

            // `AfterWaiting` vs `At` vs `After`, but at least it imposes a

            // total order.

        }

}

impl Ord for LooseCmpRetryTime {

}

impl PartialOrd for LooseCmpRetryTime {

}

#[cfg(test)]

mod test {

    // @@ begin test lint list maintained by maint/add_warning @@

    #![allow(clippy::bool_assert_comparison)]

    #![allow(clippy::clone_on_copy)]

    #![allow(clippy::dbg_macro)]

    #![allow(clippy::mixed_attributes_style)]

    #![allow(clippy::print_stderr)]

    #![allow(clippy::print_stdout)]

    #![allow(clippy::single_char_pattern)]

    #![allow(clippy::unwrap_used)]

    #![allow(clippy::unchecked_duration_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use super::*;

    #[test]

    fn comparison() {

        use RetryTime as RT;

        let sec = Duration::from_secs(1);

        let now = Instant::now();

        let sorted = vec![

            RT::Immediate,

            RT::AfterWaiting,

            RT::After(sec * 10),

            RT::After(sec * 20),

            RT::At(now),

            RT::At(now + sec * 30),

            RT::Never,

        ];

        // Verify that these objects are actually in loose-cmp sorted order.

        for (i, a) in sorted.iter().enumerate() {

            for (j, b) in sorted.iter().enumerate() {

                assert_eq!(a.loose_cmp(b), i.cmp(&j));

            }

        }

    }

    #[test]

    fn abs_comparison() {

        use AbsRetryTime as ART;

        let sec = Duration::from_secs(1);

        let now = Instant::now();

        let sorted = vec![

            ART::Immediate,

            ART::At(now),

            ART::At(now + sec * 30),

            ART::Never,

        ];

        // Verify that these objects are actually in loose-cmp sorted order.

        for (i, a) in sorted.iter().enumerate() {

            for (j, b) in sorted.iter().enumerate() {

                assert_eq!(a.cmp(b), i.cmp(&j));

            }

        }

    }

    #[test]

    fn earliest_absolute() {

        let sec = Duration::from_secs(1);

        let now = Instant::now();

        let times = vec![RetryTime::AfterWaiting, RetryTime::Never];

        let earliest = RetryTime::earliest_absolute(times.into_iter(), now, || sec);

        assert_eq!(

            earliest.expect("no absolute time"),

            AbsRetryTime::At(now + sec)

        );

    }

    #[test]

    fn abs_from_sum() {

        let base = Instant::now();

        let delta = Duration::from_secs(1);

        assert_eq!(

            AbsRetryTime::from_sum(base, delta),

            AbsRetryTime::At(base + delta)

        );

        assert_eq!(

            AbsRetryTime::from_sum(base, Duration::MAX),

            AbsRetryTime::Never

        );

    }

}