//! Helpers for tracking whether a tunnel or circuit is still active.

use derive_deftly::Deftly;

use std::{num::NonZeroUsize, time::Instant};

/// An object to track whether a tunnel or circuit should still be considered active.

///

/// The "active" status of a tunnel depends on whether it is in use for streams,

/// and if not, how much time has passed since it was last in use for streams.

///

/// # Ordering and aggregation

///

/// We rely on the ordering for `TunnelActivity` structs.

/// In particular, we depend on the property that a "more active"

/// `TunnelActivity` is greater than a less active one.

///

/// Specifically,

/// - a TunnelActivity with streams is "more active" than one without streams.

/// - a TunnelActivity that was last used for streams recently is "more active"

///   than one that was last used for streams more time ago.

/// - a TunnelActivity that was ever in use for streams is "more active"

///   than one that has never been used.

///

/// For implementation convenience, we do not generally keep a TunnelActivity

/// for an entire tunnel.

/// Instead, we keep a separate TunnelActivity for each hop of each circuit.

/// When we need to find the TunnelActivity of the tunnel as a whole,

/// we look for the TunnelActivity of the "most active" hop.

/// This _does not_ give an accurate count of all the streams on the

/// tunnel, but we don't generally care about that.

///

/// > We could instead _add_ the TunnelActivity for each hop,

/// > but that would be a bit more implementation effort to little benefit,

/// > and we'd need to avoid counting conflux join points twice.

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

pub(crate) struct TunnelActivity {

    /// Actual activity for the associated tunnel.

    inner: Inner,

}

/// Inner enumeration used to implement [`TunnelActivity`].

///

/// This is a separate type to keep it private.

//

// NOTE: Don't re-order these: we rely on the speicific behavior of

// derive(PartialOrd) in order to get the behavior we want.

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

enum Inner {

    /// The tunnel has never been used for streams.

    #[default]

    NeverUsed,

    /// The tunnel was in use for streams, but has no streams right now.

    Disused {

        /// The time at which the last stream was closed.

        ///

        /// See note on [`TunnelActivity::disused_since`].

        since: Instant,

    },

    /// The tunnel has open streams.

    InUse {

        /// The number of open streams on this tunnel.

        n_open_streams: NonZeroUsize,

    },

}

/// A zero-sized token type returned for each call to [`TunnelActivity::inc_streams()`].

///

/// The caller is responsible for passing this object to [`TunnelActivity::dec_streams()`]

/// when the stream is no longer in use.

/// Otherwise, this type will panic when it is dropped.

#[derive(Debug, Deftly)]

#[must_use]

#[derive_deftly_adhoc]

pub(crate) struct InTunnelActivity {

    /// Prevent this type from being created from other modules.

    _prevent_create: (),

}

impl Drop for InTunnelActivity {

    }

}

// Assert that no member of InTunnelActivity actually has meaningful drop semantics.

//

// (This lets us call std::mem::forget() below with confidence.)

derive_deftly::derive_deftly_adhoc! {

    InTunnelActivity:

    const _ : () = {

        $(

            assert!(! std::mem::needs_drop::<$ftype>());

        )

    };

}

impl InTunnelActivity {

    /// Consume this token safely, without triggering its drop panic.

    ///

    /// Calling this method directly will invalidate the corresponding TunnelActivity's counter.

    /// Instead, you should usually pass this to [`TunnelActivity::dec_streams`]

}

impl TunnelActivity {

    /// Construct a new TunnelActivity for a tunnel that has never been used.

    /// Increase the number of streams on this tunnel by one.

    /// Decrease the number of streams on this tunnel by one.

        };

    /// Return the number of open streams on this tunnel.

    ///

    /// (But see note on [`TunnelActivity`] documentation)

        }

    /// Return the time at which this tunnel was last in use.

    ///

    /// Returns None if the tunnel has open streams right now,

    /// or if it has never had any open streams.

    ///

    /// # A note about time

    ///

    /// The returned Instant value is a direct result of an earlier call to `Instant::now()`.

    /// It is not affected by any runtime mocking.

        }

}

#[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_time_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

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

    use std::time::Duration;

    use super::*;

    use rand::seq::SliceRandom as _;

    use tor_basic_utils::test_rng::testing_rng;

    #[test]

    fn ordering() {

        use Inner::*;

        let t1 = Instant::now();

        let t2 = t1 + Duration::new(60, 0);

        let t3 = t2 + Duration::new(120, 0);

        let sorted = vec![

            NeverUsed,

            NeverUsed,

            Disused { since: t1 },

            Disused { since: t2 },

            Disused { since: t3 },

            InUse {

                n_open_streams: 5.try_into().unwrap(),

            },

            InUse {

                n_open_streams: 10.try_into().unwrap(),

            },

        ];

        let mut scrambled = sorted.clone();

        let mut rng = testing_rng();

        for _ in 0..=8 {

            scrambled.shuffle(&mut rng);

            scrambled.sort();

            assert_eq!(&scrambled, &sorted);

        }

    }

}