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    // Copyright 2016 Amanieu d'Antras
    //
    // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
    // http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
    // http://opensource.org/licenses/MIT>, at your option. This file may not be
    // copied, modified, or distributed except according to those terms.

    use crate::util::UncheckedOptionExt;
    use core::{
        fmt, mem,
        sync::atomic::{fence, AtomicU8, Ordering},
    };
    use parking_lot_core::{self, SpinWait, DEFAULT_PARK_TOKEN, DEFAULT_UNPARK_TOKEN};

    const DONE_BIT: u8 = 1;
    const POISON_BIT: u8 = 2;
    const LOCKED_BIT: u8 = 4;
    const PARKED_BIT: u8 = 8;

    /// Current state of a `Once`.
    #[derive(Copy, Clone, Eq, PartialEq, Debug)]
    pub enum OnceState {
        /// A closure has not been executed yet
        New,

        /// A closure was executed but panicked.
        Poisoned,

        /// A thread is currently executing a closure.
        InProgress,

        /// A closure has completed successfully.
        Done,
    }

    impl OnceState {
        /// Returns whether the associated `Once` has been poisoned.
        ///
        /// Once an initialization routine for a `Once` has panicked it will forever
        /// indicate to future forced initialization routines that it is poisoned.
        #[inline]
        pub fn poisoned(self) -> bool {
            match self {
                OnceState::Poisoned => true,
                _ => false,
            }
        }

        /// Returns whether the associated `Once` has successfully executed a
        /// closure.
        #[inline]
        pub fn done(self) -> bool {
            match self {
                OnceState::Done => true,
                _ => false,
            }
        }
    }

    /// A synchronization primitive which can be used to run a one-time
    /// initialization. Useful for one-time initialization for globals, FFI or
    /// related functionality.
    ///
    /// # Differences from the standard library `Once`
    ///
    /// - Only requires 1 byte of space, instead of 1 word.
    /// - Not required to be `'static`.
    /// - Relaxed memory barriers in the fast path, which can significantly improve
    ///   performance on some architectures.
    /// - Efficient handling of micro-contention using adaptive spinning.
    ///
    /// # Examples
    ///
    /// ```
    /// use parking_lot::Once;
    ///
    /// static START: Once = Once::new();
    ///
    /// START.call_once(|| {
    ///     // run initialization here
    /// });
    /// ```
    pub struct Once(AtomicU8);

    impl Once {
        /// Creates a new `Once` value.
        #[inline]
        pub const fn new() -> Once {
            Once(AtomicU8::new(0))
        }

        /// Returns the current state of this `Once`.
        #[inline]
        pub fn state(&self) -> OnceState {
            let state = self.0.load(Ordering::Acquire);
            if state & DONE_BIT != 0 {
                OnceState::Done
            } else if state & LOCKED_BIT != 0 {
                OnceState::InProgress
            } else if state & POISON_BIT != 0 {
                OnceState::Poisoned
            } else {
                OnceState::New
            }
        }

        /// Performs an initialization routine once and only once. The given closure
        /// will be executed if this is the first time `call_once` has been called,
        /// and otherwise the routine will *not* be invoked.
        ///
        /// This method will block the calling thread if another initialization
        /// routine is currently running.
        ///
        /// When this function returns, it is guaranteed that some initialization
        /// has run and completed (it may not be the closure specified). It is also
        /// guaranteed that any memory writes performed by the executed closure can
        /// be reliably observed by other threads at this point (there is a
        /// happens-before relation between the closure and code executing after the
        /// return).
        ///
        /// # Examples
        ///
        /// ```
        /// use parking_lot::Once;
        ///
        /// static mut VAL: usize = 0;
        /// static INIT: Once = Once::new();
        ///
        /// // Accessing a `static mut` is unsafe much of the time, but if we do so
        /// // in a synchronized fashion (e.g. write once or read all) then we're
        /// // good to go!
        /// //
        /// // This function will only call `expensive_computation` once, and will
        /// // otherwise always return the value returned from the first invocation.
        /// fn get_cached_val() -> usize {
        ///     unsafe {
        ///         INIT.call_once(|| {
        ///             VAL = expensive_computation();
        ///         });
        ///         VAL
        ///     }
        /// }
        ///
        /// fn expensive_computation() -> usize {
        ///     // ...
        /// # 2
        /// }
        /// ```
        ///
        /// # Panics
        ///
        /// The closure `f` will only be executed once if this is called
        /// concurrently amongst many threads. If that closure panics, however, then
        /// it will *poison* this `Once` instance, causing all future invocations of
        /// `call_once` to also panic.
        #[inline]
        pub fn call_once<F>(&self, f: F)
        where
            F: FnOnce(),
        {
            if self.0.load(Ordering::Acquire) == DONE_BIT {
                return;
            }

            let mut f = Some(f);
            self.call_once_slow(false, &mut |_| unsafe { f.take().unchecked_unwrap()() });
        }

        /// Performs the same function as `call_once` except ignores poisoning.
        ///
        /// If this `Once` has been poisoned (some initialization panicked) then
        /// this function will continue to attempt to call initialization functions
        /// until one of them doesn't panic.
        ///
        /// The closure `f` is yielded a structure which can be used to query the
        /// state of this `Once` (whether initialization has previously panicked or
        /// not).
        #[inline]
        pub fn call_once_force<F>(&self, f: F)
        where
            F: FnOnce(OnceState),
        {
            if self.0.load(Ordering::Acquire) == DONE_BIT {
                return;
            }

            let mut f = Some(f);
            self.call_once_slow(true, &mut |state| unsafe {
                f.take().unchecked_unwrap()(state)
            });
        }

        // This is a non-generic function to reduce the monomorphization cost of
        // using `call_once` (this isn't exactly a trivial or small implementation).
        //
        // Additionally, this is tagged with `#[cold]` as it should indeed be cold
        // and it helps let LLVM know that calls to this function should be off the
        // fast path. Essentially, this should help generate more straight line code
        // in LLVM.
        //
        // Finally, this takes an `FnMut` instead of a `FnOnce` because there's
        // currently no way to take an `FnOnce` and call it via virtual dispatch
        // without some allocation overhead.
        #[cold]
        fn call_once_slow(&self, ignore_poison: bool, f: &mut dyn FnMut(OnceState)) {
            let mut spinwait = SpinWait::new();
            let mut state = self.0.load(Ordering::Relaxed);
            loop {
                // If another thread called the closure, we're done
                if state & DONE_BIT != 0 {
                    // An acquire fence is needed here since we didn't load the
                    // state with Ordering::Acquire.
                    fence(Ordering::Acquire);
                    return;
                }

                // If the state has been poisoned and we aren't forcing, then panic
                if state & POISON_BIT != 0 && !ignore_poison {
                    // Need the fence here as well for the same reason
                    fence(Ordering::Acquire);
                    panic!("Once instance has previously been poisoned");
                }

                // Grab the lock if it isn't locked, even if there is a queue on it.
                // We also clear the poison bit since we are going to try running
                // the closure again.
                if state & LOCKED_BIT == 0 {
                    match self.0.compare_exchange_weak(
                        state,
                        (state | LOCKED_BIT) & !POISON_BIT,
                        Ordering::Acquire,
                        Ordering::Relaxed,
                    ) {
                        Ok(_) => break,
                        Err(x) => state = x,
                    }
                    continue;
                }

                // If there is no queue, try spinning a few times
                if state & PARKED_BIT == 0 && spinwait.spin() {
                    state = self.0.load(Ordering::Relaxed);
                    continue;
                }

                // Set the parked bit
                if state & PARKED_BIT == 0 {
                    if let Err(x) = self.0.compare_exchange_weak(
                        state,
                        state | PARKED_BIT,
                        Ordering::Relaxed,
                        Ordering::Relaxed,
                    ) {
                        state = x;
                        continue;
                    }
                }

                // Park our thread until we are woken up by the thread that owns the
                // lock.
                unsafe {
                    let addr = self as *const _ as usize;
                    let validate = || self.0.load(Ordering::Relaxed) == LOCKED_BIT | PARKED_BIT;
                    let before_sleep = || {};
                    let timed_out = |_, _| unreachable!();
                    parking_lot_core::park(
                        addr,
                        validate,
                        before_sleep,
                        timed_out,
                        DEFAULT_PARK_TOKEN,
                        None,
                    );
                }

                // Loop back and check if the done bit was set
                spinwait.reset();
                state = self.0.load(Ordering::Relaxed);
            }

            struct PanicGuard<'a>(&'a Once);
            impl<'a> Drop for PanicGuard<'a> {
                fn drop(&mut self) {
                    // Mark the state as poisoned, unlock it and unpark all threads.
                    let once = self.0;
                    let state = once.0.swap(POISON_BIT, Ordering::Release);
                    if state & PARKED_BIT != 0 {
                        unsafe {
                            let addr = once as *const _ as usize;
                            parking_lot_core::unpark_all(addr, DEFAULT_UNPARK_TOKEN);
                        }
                    }
                }
            }

            // At this point we have the lock, so run the closure. Make sure we
            // properly clean up if the closure panicks.
            let guard = PanicGuard(self);
            let once_state = if state & POISON_BIT != 0 {
                OnceState::Poisoned
            } else {
                OnceState::New
            };
            f(once_state);
            mem::forget(guard);

            // Now unlock the state, set the done bit and unpark all threads
            let state = self.0.swap(DONE_BIT, Ordering::Release);
            if state & PARKED_BIT != 0 {
                unsafe {
                    let addr = self as *const _ as usize;
                    parking_lot_core::unpark_all(addr, DEFAULT_UNPARK_TOKEN);
                }
            }
        }
    }

    impl Default for Once {
        #[inline]
        fn default() -> Once {
            Once::new()
        }
    }

    impl fmt::Debug for Once {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Once")
                .field("state", &self.state())
                .finish()
        }
    }

    #[cfg(test)]
    mod tests {
        use crate::Once;
        use std::panic;
        use std::sync::mpsc::channel;
        use std::thread;

        #[test]
        fn smoke_once() {
            static O: Once = Once::new();
            let mut a = 0;
            O.call_once(|| a += 1);
            assert_eq!(a, 1);
            O.call_once(|| a += 1);
            assert_eq!(a, 1);
        }

        #[test]
        fn stampede_once() {
            static O: Once = Once::new();
            static mut RUN: bool = false;

            let (tx, rx) = channel();
            for _ in 0..10 {
                let tx = tx.clone();
                thread::spawn(move || {
                    for _ in 0..4 {
                        thread::yield_now()
                    }
                    unsafe {
                        O.call_once(|| {
                            assert!(!RUN);
                            RUN = true;
                        });
                        assert!(RUN);
                    }
                    tx.send(()).unwrap();
                });
            }

            unsafe {
                O.call_once(|| {
                    assert!(!RUN);
                    RUN = true;
                });
                assert!(RUN);
            }

            for _ in 0..10 {
                rx.recv().unwrap();
            }
        }

        #[test]
        fn poison_bad() {
            static O: Once = Once::new();

            // poison the once
            let t = panic::catch_unwind(|| {
                O.call_once(|| panic!());
            });
            assert!(t.is_err());

            // poisoning propagates
            let t = panic::catch_unwind(|| {
                O.call_once(|| {});
            });
            assert!(t.is_err());

            // we can subvert poisoning, however
            let mut called = false;
            O.call_once_force(|p| {
                called = true;
                assert!(p.poisoned())
            });
            assert!(called);

            // once any success happens, we stop propagating the poison
            O.call_once(|| {});
        }

        #[test]
        fn wait_for_force_to_finish() {
            static O: Once = Once::new();

            // poison the once
            let t = panic::catch_unwind(|| {
                O.call_once(|| panic!());
            });
            assert!(t.is_err());

            // make sure someone's waiting inside the once via a force
            let (tx1, rx1) = channel();
            let (tx2, rx2) = channel();
            let t1 = thread::spawn(move || {
                O.call_once_force(|p| {
                    assert!(p.poisoned());
                    tx1.send(()).unwrap();
                    rx2.recv().unwrap();
                });
            });

            rx1.recv().unwrap();

            // put another waiter on the once
            let t2 = thread::spawn(|| {
                let mut called = false;
                O.call_once(|| {
                    called = true;
                });
                assert!(!called);
            });

            tx2.send(()).unwrap();

            assert!(t1.join().is_ok());
            assert!(t2.join().is_ok());
        }

        #[test]
        fn test_once_debug() {
            static O: Once = Once::new();

            assert_eq!(format!("{:?}", O), "Once { state: New }");
        }
    }