dashmap 5.1.0

Blazing fast concurrent HashMap for Rust.
Documentation 
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use crate::HashMap;
use core::hash::{BuildHasher, Hash};
use core::ops::{Deref, DerefMut};
use parking_lot::{RwLockReadGuard, RwLockWriteGuard};
use std::collections::hash_map::RandomState;

pub struct Ref<'a, K, V, S = RandomState> {
    _guard: RwLockReadGuard<'a, HashMap<K, V, S>>,
    k: *const K,
    v: *const V,
}

unsafe impl<'a, K: Eq + Hash + Send, V: Send, S: BuildHasher> Send for Ref<'a, K, V, S> {}

unsafe impl<'a, K: Eq + Hash + Send + Sync, V: Send + Sync, S: BuildHasher> Sync
    for Ref<'a, K, V, S>
{
}

impl<'a, K: Eq + Hash, V, S: BuildHasher> Ref<'a, K, V, S> {
    pub(crate) unsafe fn new(
        guard: RwLockReadGuard<'a, HashMap<K, V, S>>,
        k: *const K,
        v: *const V,
    ) -> Self {
        Self {
            _guard: guard,
            k,
            v,
        }
    }

    pub fn key(&self) -> &K {
        self.pair().0
    }

    pub fn value(&self) -> &V {
        self.pair().1
    }

    pub fn pair(&self) -> (&K, &V) {
        unsafe { (&*self.k, &*self.v) }
    }
}

impl<'a, K: Eq + Hash, V, S: BuildHasher> Deref for Ref<'a, K, V, S> {
    type Target = V;

    fn deref(&self) -> &V {
        self.value()
    }
}

pub struct RefMut<'a, K, V, S = RandomState> {
    guard: RwLockWriteGuard<'a, HashMap<K, V, S>>,
    k: *const K,
    v: *mut V,
}

unsafe impl<'a, K: Eq + Hash + Send, V: Send, S: BuildHasher> Send for RefMut<'a, K, V, S> {}

unsafe impl<'a, K: Eq + Hash + Send + Sync, V: Send + Sync, S: BuildHasher> Sync
    for RefMut<'a, K, V, S>
{
}

impl<'a, K: Eq + Hash, V, S: BuildHasher> RefMut<'a, K, V, S> {
    pub(crate) unsafe fn new(
        guard: RwLockWriteGuard<'a, HashMap<K, V, S>>,
        k: *const K,
        v: *mut V,
    ) -> Self {
        Self { guard, k, v }
    }

    pub fn key(&self) -> &K {
        self.pair().0
    }

    pub fn value(&self) -> &V {
        self.pair().1
    }

    pub fn value_mut(&mut self) -> &mut V {
        self.pair_mut().1
    }

    pub fn pair(&self) -> (&K, &V) {
        unsafe { (&*self.k, &*self.v) }
    }

    pub fn pair_mut(&mut self) -> (&K, &mut V) {
        unsafe { (&*self.k, &mut *self.v) }
    }

    pub fn downgrade(self) -> Ref<'a, K, V, S> {
        unsafe {
            Ref::new(
                parking_lot::RwLockWriteGuard::downgrade(self.guard),
                self.k,
                self.v,
            )
        }
    }
}

impl<'a, K: Eq + Hash, V, S: BuildHasher> Deref for RefMut<'a, K, V, S> {
    type Target = V;

    fn deref(&self) -> &V {
        self.value()
    }
}

impl<'a, K: Eq + Hash, V, S: BuildHasher> DerefMut for RefMut<'a, K, V, S> {
    fn deref_mut(&mut self) -> &mut V {
        self.value_mut()
    }
}