//! An implementation of the "decorrelated jitter" algorithm for scheduling retries.

//!

//! See [`RetryDelay`] for more information.

use std::time::Duration;

use crate::RngExt as _;

use rand::Rng;

/// An implementation for retrying a remote operation based on a [decorrelated

/// jitter] schedule.

///

/// The algorithm used here has several desirable properties:

///    * It is randomized, so that multiple timeouts don't have a danger of

///      getting synchronized with each other and hammering the same servers all

///      at once.

///    * It tends on average to wait longer and longer over time, so that if the

///      server is down, it won't get pummeled by a zillion failing clients

///      when it comes back up.

///    * It has a chance of retrying promptly, which results in better client

///      performance on average.

///

/// For a more full specification, see [`dir-spec.txt`].

///

/// [decorrelated jitter]:

///     https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/

/// [`dir-spec.txt`]: https://spec.torproject.org/dir-spec

#[derive(Clone, Debug)]

pub struct RetryDelay {

    /// The last delay that this retry delay returned (in msec), or 0

    /// if this never returned a delay.

    last_delay_ms: u32,

    /// The lowest allowable delay (in msec).

    low_bound_ms: u32,

}

/// Lowest possible lower bound, in milliseconds.

// We're doing this in MS, and Tor does it in seconds, so I'm

// multiplying the minimum by 1000 here.

const MIN_LOW_BOUND: u32 = 1000;

/// Largest possible lower bound, in milliseconds.

const MAX_LOW_BOUND: u32 = u32::MAX - 1;

/// Maximum amount to multiply the previous delay by.

const MAX_DELAY_MULT: u32 = 3;

impl RetryDelay {

    /// Construct a new RetryDelay from a given base delay in

    /// milliseconds.

    ///

    /// The base delay defines the lowest possible interval that can

    /// be returned.

    ///

    /// # Limitations

    ///

    /// If the base delay is less than 1000, a base delay of 1000 is

    /// used instead, since that's what the C tor implementation does.

    /// Construct a new RetryDelay from a given base delay.

    ///

    /// See from_msec for more information.

    /// Helper: Return a lower and upper bound for the next delay to

    /// be yielded.

    ///

    /// Values are in milliseconds.

    ///

    /// The return value `(low, high)` is guaranteed to have `low < high`.

    /// Return the next delay to be used (in milliseconds), according

    /// to a given random number generator.

    /// Return the next delay to be used (as a [`Duration`]),

    /// according to a given random number generator.

    /// Return this [`RetryDelay`] to its original state.

}

impl Default for RetryDelay {

}

#[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::*;

    use crate::test_rng::testing_rng;

    #[test]

    fn init() {

        let rd = RetryDelay::from_msec(2000);

        assert_eq!(rd.last_delay_ms, 0);

        assert_eq!(rd.low_bound_ms, 2000);

        let rd = RetryDelay::from_msec(0);

        assert_eq!(rd.last_delay_ms, 0);

        assert_eq!(rd.low_bound_ms, 1000);

        let rd = RetryDelay::from_duration(Duration::new(1, 500_000_000));

        assert_eq!(rd.last_delay_ms, 0);

        assert_eq!(rd.low_bound_ms, 1500);

    }

    #[test]

    fn bounds() {

        let mut rd = RetryDelay::from_msec(1000);

        assert_eq!(rd.delay_bounds(), (1000, 1001));

        rd.last_delay_ms = 1500;

        assert_eq!(rd.delay_bounds(), (1000, 4500));

        rd.last_delay_ms = 3_000_000_000;

        assert_eq!(rd.delay_bounds(), (1000, u32::MAX));

        rd.reset();

        assert_eq!(rd.delay_bounds(), (1000, 1001));

    }

    #[test]

    fn rng() {

        let mut rd = RetryDelay::from_msec(50);

        let real_low_bound = std::cmp::max(50, MIN_LOW_BOUND);

        let mut rng = testing_rng();

        for _ in 1..100 {

            let (b_lo, b_hi) = rd.delay_bounds();

            assert!(b_lo == real_low_bound);

            assert!(b_hi > b_lo);

            let delay = rd.next_delay(&mut rng).as_millis() as u32;

            assert_eq!(delay, rd.last_delay_ms);

            assert!(delay >= b_lo);

            assert!(delay < b_hi);

        }

    }

}