opthash 0.10.4

Rust implementations of Elastic Hashing and Funnel Hashing
Documentation 
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use super::config::GROUP_SIZE;

/// Capacity and reserve-fraction arithmetic shared by both maps.
pub(crate) mod capacity {
    use crate::ReserveFraction;

    #[inline]
    pub(crate) const fn max_insertions(capacity: usize, reserve: ReserveFraction) -> usize {
        capacity.saturating_sub(reserve.floor_reserved(capacity))
    }

    #[inline]
    pub(crate) fn capacity_for(
        start: usize,
        needed: usize,
        reserve: ReserveFraction,
    ) -> Option<usize> {
        let mut cap = start;
        while max_insertions(cap, reserve) < needed {
            cap = cap.checked_mul(2)?;
        }
        Some(cap)
    }

    #[inline]
    pub(crate) fn tombstone_cleanup_threshold(capacity: usize) -> usize {
        let half = capacity / 2;
        let cap_three_quarters = capacity.saturating_mul(3) / 4;
        let hysteresis = half.saturating_add(super::GROUP_SIZE.saturating_mul(4));
        hysteresis.min(cap_three_quarters).max(half)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    const fn reserve(reserve_exponent: u32) -> crate::ReserveFraction {
        match crate::ReserveFraction::from_exponent(reserve_exponent) {
            Ok(reserve) => reserve,
            Err(_) => panic!("test exponent must be positive"),
        }
    }

    #[test]
    fn max_insertions_reserves_floor_fraction() {
        assert_eq!(capacity::max_insertions(0, reserve(2)), 0);
        assert_eq!(capacity::max_insertions(16, reserve(2)), 12);
        assert_eq!(capacity::max_insertions(100, reserve(1)), 50);
    }

    #[test]
    fn capacity_for_finds_smallest_doubling_fit() {
        assert_eq!(capacity::capacity_for(16, 12, reserve(2)), Some(16));
        assert_eq!(capacity::capacity_for(16, 13, reserve(2)), Some(32));
        assert_eq!(capacity::capacity_for(16, 0, reserve(2)), Some(16));
    }

    #[test]
    fn capacity_for_returns_none_on_overflow() {
        assert_eq!(capacity::capacity_for(1, usize::MAX, reserve(1)), None);
    }

    #[test]
    fn tombstone_cleanup_threshold_stays_between_half_and_three_quarters() {
        for &cap in &[0usize, GROUP_SIZE, 128, 1024, 1_000_000] {
            let threshold = capacity::tombstone_cleanup_threshold(cap);
            assert!(threshold >= cap / 2);
            assert!(threshold <= cap * 3 / 4 || cap == 0);
        }
    }
}