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use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
// This structure represents a lazily initialized static usize value. Useful
// when it is preferable to just rerun initialization instead of locking.
// Both unsync_init and sync_init will invoke an init() function until it
// succeeds, then return the cached value for future calls.
//
// Both methods support init() "failing". If the init() method returns UNINIT,
// that value will be returned as normal, but will not be cached.
//
// Users should only depend on the _value_ returned by init() functions.
// Specifically, for the following init() function:
// fn init() -> usize {
// a();
// let v = b();
// c();
// v
// }
// the effects of c() or writes to shared memory will not necessarily be
// observed and additional synchronization methods with be needed.
pub(crate) struct LazyUsize(AtomicUsize);
impl LazyUsize {
pub const fn new() -> Self {
Self(AtomicUsize::new(Self::UNINIT))
}
// The initialization is not completed.
pub const UNINIT: usize = usize::max_value();
// Runs the init() function at least once, returning the value of some run
// of init(). Multiple callers can run their init() functions in parallel.
// init() should always return the same value, if it succeeds.
pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize {
// Relaxed ordering is fine, as we only have a single atomic variable.
let mut val = self.0.load(Relaxed);
if val == Self::UNINIT {
val = init();
self.0.store(val, Relaxed);
}
val
}
}
// Identical to LazyUsize except with bool instead of usize.
pub(crate) struct LazyBool(LazyUsize);
impl LazyBool {
pub const fn new() -> Self {
Self(LazyUsize::new())
}
pub fn unsync_init(&self, init: impl FnOnce() -> bool) -> bool {
self.0.unsync_init(|| init() as usize) != 0
}
}
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