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    // Copyright 2019 Developers of the Rand project.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};

    // This structure represents a laziliy initialized static usize value. Useful
    // when it is perferable 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 syncronization methods with be needed.
    pub 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();
        // The initialization is currently running.
        pub const ACTIVE: usize = usize::max_value() - 1;

        // 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
        }

        // Synchronously runs the init() function. Only one caller will have their
        // init() function running at a time, and exactly one successful call will
        // be run. init() returning UNINIT or ACTIVE will be considered a failure,
        // and future calls to sync_init will rerun their init() function.
        pub fn sync_init(&self, init: impl FnOnce() -> usize, mut wait: impl FnMut()) -> usize {
            // Common and fast path with no contention. Don't wast time on CAS.
            match self.0.load(Relaxed) {
                Self::UNINIT | Self::ACTIVE => {}
                val => return val,
            }
            // Relaxed ordering is fine, as we only have a single atomic variable.
            loop {
                match self.0.compare_and_swap(Self::UNINIT, Self::ACTIVE, Relaxed) {
                    Self::UNINIT => {
                        let val = init();
                        self.0.store(
                            match val {
                                Self::UNINIT | Self::ACTIVE => Self::UNINIT,
                                val => val,
                            },
                            Relaxed,
                        );
                        return val;
                    }
                    Self::ACTIVE => wait(),
                    val => return val,
                }
            }
        }
    }

    // Identical to LazyUsize except with bool instead of usize.
    pub 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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