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use core::cell::UnsafeCell;
use core::ops::{Deref, DerefMut};
use core::sync::atomic::{AtomicBool, Ordering::*};

/// By the use of Spinlocks, it is possible to serialize blocks of code
/// that might run parallel on multiple CPU cores.
///
/// Synchronization is implemented using a lock variable. Once a thread enters
/// the critical area, it sets the lock variable (to a non-zero value); when
/// this thread leaves the critical area, it resets the lock variable to zero.
/// When trying to enter an already locked critical area, the trying thread
/// actively waits until the critical area is free again.
#[derive(Debug, Default)]
pub struct Spin<T> {
    lock: AtomicBool,
    /// Wrap the mutable data inside an unsafe cell, so that the compiler doesn't optimize it away
    value: UnsafeCell<T>,
}

unsafe impl<T> Sync for Spin<T> {}
unsafe impl<T> Send for Spin<T> {}

impl<T> Spin<T> {
    /// Create a new spin lock, protecting `value`
    pub const fn new(value: T) -> Self {
        Self {
            lock: AtomicBool::new(false),
            value: UnsafeCell::new(value),
        }
    }
    /// Locks the lock and returns the protected data.
    ///
    /// The data is wrapped in a guard, that unlocks the lock if it goes out of scope.
    #[must_use]
    pub fn lock(&self) -> SpinGuard<T> {
        while let Err(_) = self
            .lock
            .compare_exchange_weak(false, true, Acquire, Relaxed)
        {
            // pausing the cpu
            core::hint::spin_loop();
        }
        SpinGuard { lock: self }
    }
    /// If you really want to unlock it. This is not protected by race conditions!
    pub unsafe fn unlock(&self) {
        self.lock.store(false, Release);
    }
    /// If you really want to access the contents. This is not protected by race conditions!
    #[allow(clippy::mut_from_ref)]
    pub unsafe fn raw(&self) -> &mut T {
        &mut *self.value.get()
    }
}

/// Accessor for the locks data, that unlocks if it goes out of scope.
pub struct SpinGuard<'a, T> {
    lock: &'a Spin<T>,
}
impl<'a, T> Deref for SpinGuard<'a, T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        unsafe { self.lock.raw() }
    }
}
impl<'a, T> DerefMut for SpinGuard<'a, T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        unsafe { self.lock.raw() }
    }
}
impl<'a, T> Drop for SpinGuard<'a, T> {
    fn drop(&mut self) {
        unsafe { self.lock.unlock() };
    }
}