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.

// unsync_init will invoke an init() function until it succeeds, then return the

// cached value for future calls.

//

// unsync_init supports 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 may 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 most 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

    }

}