nautilus-network 0.62.0

// -------------------------------------------------------------------------------------------------
//  Copyright (C) 2015-2026 Nautech Systems Pty Ltd. All rights reserved.
//  https://nautechsystems.io
//
//  Licensed under the GNU Lesser General Public License Version 3.0 (the "License");
//  You may not use this file except in compliance with the License.
//  You may obtain a copy of the License at https://www.gnu.org/licenses/lgpl-3.0.en.html
//
//  Unless required by applicable law or agreed to in writing, software
//  distributed under the License is distributed on an "AS IS" BASIS,
//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//  See the License for the specific language governing permissions and
//  limitations under the License.
// -------------------------------------------------------------------------------------------------

use std::{num::NonZeroU32, time::Duration};

use super::nanos::Nanos;

/// A rate-limiting quota.
///
/// Quotas are expressed in a positive number of "cells" (the maximum number of positive decisions /
/// allowed items until the rate limiter needs to replenish) and the amount of time for the rate
/// limiter to replenish a single cell.
///
/// Neither the number of cells nor the replenishment unit of time may be zero.
///
/// # Burst Sizes
/// There are multiple ways of expressing the same quota: a quota given as `Quota::per_second(1)`
/// allows, on average, the same number of cells through as a quota given as `Quota::per_minute(60)`.
/// The quota of `Quota::per_minute(60)` has a burst size of 60 cells, meaning it is
/// possible to accommodate 60 cells in one go, after which the equivalent of a minute of inactivity
/// is required for the burst allowance to be fully restored.
///
/// Burst size gets really important when you construct a rate limiter that should allow multiple
/// elements through at one time (using [`RateLimiter.check_n`](struct.RateLimiter.html#method.check_n)
/// and its related functions): Only
/// at most as many cells can be let through in one call as are given as the burst size.
///
/// In other words, the burst size is the maximum number of cells that the rate limiter will ever
/// allow through without replenishing them.
#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub struct Quota {
    pub(crate) max_burst: NonZeroU32,
    pub(crate) replenish_1_per: Duration,
}

/// Constructors for Quotas
impl Quota {
    /// Constructs a quota for a number of cells per second. The given number of cells is also
    /// assumed to be the maximum burst size.
    ///
    /// Returns `None` if `max_burst` is so large that the replenish interval rounds to zero
    /// nanoseconds (i.e. `max_burst > 1_000_000_000`).
    #[must_use]
    pub const fn per_second(max_burst: NonZeroU32) -> Option<Self> {
        let replenish_interval_ns = Duration::from_secs(1).as_nanos() / (max_burst.get() as u128);
        if replenish_interval_ns == 0 {
            return None;
        }
        Some(Self {
            max_burst,
            replenish_1_per: Duration::from_nanos(replenish_interval_ns as u64),
        })
    }

    /// Constructs a quota for a number of cells per 60‑second period. The given number of cells
    /// is also assumed to be the maximum burst size.
    #[must_use]
    pub const fn per_minute(max_burst: NonZeroU32) -> Self {
        let replenish_interval_ns = Duration::from_mins(1).as_nanos() / (max_burst.get() as u128);
        Self {
            max_burst,
            replenish_1_per: Duration::from_nanos(replenish_interval_ns as u64),
        }
    }

    /// Constructs a quota for a number of cells per 60‑minute period. The given number of cells
    /// is also assumed to be the maximum burst size.
    #[must_use]
    pub const fn per_hour(max_burst: NonZeroU32) -> Self {
        let replenish_interval_ns = Duration::from_hours(1).as_nanos() / (max_burst.get() as u128);
        Self {
            max_burst,
            replenish_1_per: Duration::from_nanos(replenish_interval_ns as u64),
        }
    }

    /// Constructs a quota that replenishes one cell in a given interval.
    ///
    /// If the time interval is zero, returns `None`.
    #[must_use]
    pub const fn with_period(replenish_1_per: Duration) -> Option<Self> {
        if replenish_1_per.as_nanos() == 0 {
            None
        } else {
            Some(Self {
                max_burst: NonZeroU32::MIN,
                replenish_1_per,
            })
        }
    }

    /// Adjusts the maximum burst size for a quota to construct a rate limiter with a capacity
    /// for at most the given number of cells.
    #[must_use]
    pub const fn allow_burst(self, max_burst: NonZeroU32) -> Self {
        Self { max_burst, ..self }
    }
}

/// Retrieving information about a quota
impl Quota {
    /// The time it takes for a rate limiter with an exhausted burst budget to replenish
    /// a single element.
    #[must_use]
    pub const fn replenish_interval(&self) -> Duration {
        self.replenish_1_per
    }

    /// The maximum number of cells that can be allowed in one burst.
    #[must_use]
    pub const fn burst_size(&self) -> NonZeroU32 {
        self.max_burst
    }

    /// The time it takes to replenish the entire maximum burst size.
    ///
    /// Saturates at [`Duration::MAX`] if the full duration cannot be represented.
    #[must_use]
    pub const fn burst_size_replenished_in(&self) -> Duration {
        self.replenish_1_per.saturating_mul(self.max_burst.get())
    }
}

impl Quota {
    /// A way to reconstruct a Quota from an in-use Gcra.
    ///
    /// This is useful mainly for [`crate::middleware::RateLimitingMiddleware`]
    /// where custom code may want to construct information based on
    /// the amount of burst balance remaining.
    ///
    /// # Panics
    ///
    /// Panics if the division result is 0 or exceeds `u32::MAX`.
    pub(crate) fn from_gcra_parameters(t: Nanos, tau: Nanos) -> Self {
        let t_u64 = t.as_u64();
        let tau_u64 = tau.as_u64();

        // Validate division won't be zero or overflow
        assert!(t_u64 != 0, "Invalid GCRA parameter: t cannot be zero");

        let division_result = tau_u64 / t_u64;
        assert!(
            division_result != 0,
            "Invalid GCRA parameters: tau/t results in zero burst capacity"
        );
        assert!(
            u32::try_from(division_result).is_ok(),
            "Invalid GCRA parameters: tau/t exceeds u32::MAX"
        );

        // We've verified the result is non-zero and fits in u32
        let max_burst = NonZeroU32::new(division_result as u32)
            .expect("Division result should be non-zero after validation");
        let replenish_1_per = t.into();
        Self {
            max_burst,
            replenish_1_per,
        }
    }
}