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use std::marker;

use super::{local, Shared};

/// An RAII-style guard for pinning the current epoch.
///
/// A guard must be acquired before most operations on an `Atomic` pointer. On
/// destruction, it unpins the epoch.
#[must_use]
#[derive(Debug)]
pub struct Guard {
    _marker: marker::PhantomData<*mut ()>, // !Send and !Sync
}

/// Pin the current epoch.
///
/// Threads generally pin before interacting with a lock-free data
/// structure. Pinning requires a full memory barrier, so is somewhat
/// expensive. It is rentrant -- you can safely acquire nested guards, and only
/// the first guard requires a barrier. Thus, in cases where you expect to
/// perform several lock-free operations in quick succession, you may consider
/// pinning around the entire set of operations.
pub fn pin() -> Guard {
    local::with_participant(|p| {
        p.enter();

        let g = Guard {
            _marker: marker::PhantomData,
        };

        if p.should_gc() {
            p.try_collect(&g);
        }

        g
    })
}

impl Guard {
    /// Assert that the value is no longer reachable from a lock-free data
    /// structure and should be collected when sufficient epochs have passed.
    pub unsafe fn unlinked<T>(&self, val: Shared<T>) {
        local::with_participant(|p| p.reclaim(val.as_raw()))
    }

    /// Move the thread-local garbage into the global set of garbage.
    pub fn migrate_garbage(&self) {
        local::with_participant(|p| p.migrate_garbage())
    }
}

impl Drop for Guard {
    fn drop(&mut self) {
        local::with_participant(|p| p.exit());
    }
}