From 03d0cdb7145042ce51c9eb98bfd081a4f10fe8e1 Mon Sep 17 00:00:00 2001
From: Mica White <botahamec@outlook.com>
Date: Fri, 8 Mar 2024 12:18:41 -0500
Subject: Keyable

---
 src/key.rs | 118 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 118 insertions(+)
 create mode 100644 src/key.rs

(limited to 'src/key.rs')

diff --git a/src/key.rs b/src/key.rs
new file mode 100644
index 0000000..92f3b99
--- /dev/null
+++ b/src/key.rs
@@ -0,0 +1,118 @@
+use std::fmt::{self, Debug};
+use std::marker::PhantomData;
+use std::sync::atomic::{AtomicBool, Ordering};
+
+use once_cell::sync::Lazy;
+use thread_local::ThreadLocal;
+
+use self::sealed::Sealed;
+
+mod sealed {
+	use super::ThreadKey;
+	pub trait Sealed {}
+	impl Sealed for ThreadKey {}
+	impl Sealed for &mut ThreadKey {}
+}
+
+static KEY: Lazy<ThreadLocal<AtomicLock>> = Lazy::new(ThreadLocal::new);
+
+/// The key for the current thread.
+///
+/// Only one of these exist per thread. To get the current thread's key, call
+/// [`ThreadKey::lock`]. If the `ThreadKey` is dropped, it can be reobtained.
+pub type ThreadKey = Key<'static>;
+
+/// A dumb lock that's just a wrapper for an [`AtomicBool`].
+#[derive(Debug, Default)]
+pub struct AtomicLock {
+	is_locked: AtomicBool,
+}
+
+pub struct Key<'a> {
+	phantom: PhantomData<*const ()>, // implement !Send and !Sync
+	lock: &'a AtomicLock,
+}
+
+/// Allows the type to be used as a key for a lock
+///
+/// # Safety
+///
+/// Only one value which implements this trait may be allowed to exist at a
+/// time. Creating a new `Keyable` value requires making any other `Keyable`
+/// values invalid.
+pub unsafe trait Keyable: Sealed {}
+unsafe impl Keyable for ThreadKey {}
+unsafe impl Keyable for &mut ThreadKey {}
+
+impl Debug for ThreadKey {
+	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+		write!(f, "ThreadKey")
+	}
+}
+
+impl<'a> Drop for Key<'a> {
+	fn drop(&mut self) {
+		unsafe { self.lock.force_unlock() }
+	}
+}
+
+impl ThreadKey {
+	/// Get the current thread's `ThreadKey`, if it's not already taken.
+	///
+	/// The first time this is called, it will successfully return a
+	/// `ThreadKey`. However, future calls to this function will return
+	/// [`None`], unless the key is dropped or unlocked first.
+	#[must_use]
+	pub fn lock() -> Option<Self> {
+		KEY.get_or_default().try_lock()
+	}
+
+	/// Unlocks the `ThreadKey`.
+	///
+	/// After this method is called, a call to [`ThreadKey::lock`] will return
+	/// this `ThreadKey`.
+	pub fn unlock(key: Self) {
+		drop(key);
+	}
+}
+
+impl AtomicLock {
+	/// Create a new unlocked `AtomicLock`.
+	#[must_use]
+	pub const fn new() -> Self {
+		Self {
+			is_locked: AtomicBool::new(false),
+		}
+	}
+
+	/// Checks whether this `Lock` is currently locked.
+	pub fn is_locked(&self) -> bool {
+		self.is_locked.load(Ordering::Relaxed)
+	}
+
+	/// Attempt to lock the `AtomicLock`.
+	///
+	/// If the lock is already locked, then this'll return false. If it is
+	/// unlocked, it'll return true. If the lock is currently unlocked, then it
+	/// will be locked after this function is called.
+	///
+	/// This is not a fair lock. It is not recommended to call this function
+	/// repeatedly in a loop.
+	pub fn try_lock(&self) -> Option<Key> {
+		// safety: we just acquired the lock
+		(!self.is_locked.swap(true, Ordering::Acquire)).then_some(Key {
+			phantom: PhantomData,
+			lock: self,
+		})
+	}
+
+	/// Forcibly unlocks the `AtomicLock`.
+	///
+	/// # Safety
+	///
+	/// This should only be called if the key to the lock has been "lost". That
+	/// means the program no longer has a reference to the key.
+	pub unsafe fn force_unlock(&self) {
+		self.is_locked.store(false, Ordering::Release);
+	}
+}
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