o3 0.36.2

shared-nothing primitives
Documentation 
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use std::marker;

use crate::depot::queue::round::{self, phase};

pub(super) mod front;
pub(super) mod place;
pub(super) mod turn;

/// An admitted position whose shared membership must be resolved at each use.
#[derive(Clone, Copy, Debug)]
pub struct Snapshot<O, P> {
    pub(in crate::depot::queue::round) owner: O,
    pub(in crate::depot::queue::round) position: round::Position,
    phase: marker::PhantomData<P>,
}

impl<O, P> Snapshot<O, P> {
    pub(in crate::depot::queue::round) fn new(owner: O, position: round::Position) -> Self {
        Self {
            owner,
            position,
            phase: marker::PhantomData,
        }
    }

    pub const fn index(&self) -> usize {
        self.position.index.get()
    }

    pub const fn lane(&self) -> usize {
        self.position.lane.get()
    }
}

impl<'a, T, const G: usize, const N: usize, P> Snapshot<&'a round::Lanes<T, G, N>, P> {
    fn resolve<const FAIR: bool>(
        &self,
    ) -> Option<round::Location<&'a round::Lanes<T, G, N>, phase::Head>> {
        // SAFETY: the snapshot retains this lane's admission to its original owner.
        let lane = unsafe { self.owner.join(self.position.lane) };
        if lane.entries.first() != Some(self.position.index) {
            return None;
        }
        let head = round::Location::new(self.owner, self.position);
        // SAFETY: the admitted FIFO front proves its owner's lane and group remain nonempty.
        (!FAIR || unsafe { lane.active.first_live() } == self.position.lane).then_some(head)
    }

    pub(in crate::depot::queue::round) fn filter<const FAIR: bool>(
        &self,
        test: impl FnOnce(round::Location<&'a round::Lanes<T, G, N>, phase::Head>) -> bool,
    ) -> Option<round::Location<&'a round::Lanes<T, G, N>, phase::Head>> {
        if !test(self.resolve::<FAIR>()?) {
            return None;
        }
        self.resolve::<FAIR>()
    }
}