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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::{deadlock, util};
    use core::{
        sync::atomic::{AtomicU8, Ordering},
        time::Duration,
    };
    use instant::Instant;
    use lock_api::RawMutex as RawMutex_;
    use parking_lot_core::{self, ParkResult, SpinWait, UnparkResult, UnparkToken, DEFAULT_PARK_TOKEN};

    // UnparkToken used to indicate that that the target thread should attempt to
    // lock the mutex again as soon as it is unparked.
    pub(crate) const TOKEN_NORMAL: UnparkToken = UnparkToken(0);

    // UnparkToken used to indicate that the mutex is being handed off to the target
    // thread directly without unlocking it.
    pub(crate) const TOKEN_HANDOFF: UnparkToken = UnparkToken(1);

    /// This bit is set in the `state` of a `RawMutex` when that mutex is locked by some thread.
    const LOCKED_BIT: u8 = 0b01;
    /// This bit is set in the `state` of a `RawMutex` just before parking a thread. A thread is being
    /// parked if it wants to lock the mutex, but it is currently being held by some other thread.
    const PARKED_BIT: u8 = 0b10;

    /// Raw mutex type backed by the parking lot.
    pub struct RawMutex {
        /// This atomic integer holds the current state of the mutex instance. Only the two lowest bits
        /// are used. See `LOCKED_BIT` and `PARKED_BIT` for the bitmask for these bits.
        ///
        /// # State table:
        ///
        /// PARKED_BIT | LOCKED_BIT | Description
        ///     0      |     0      | The mutex is not locked, nor is anyone waiting for it.
        /// -----------+------------+------------------------------------------------------------------
        ///     0      |     1      | The mutex is locked by exactly one thread. No other thread is
        ///            |            | waiting for it.
        /// -----------+------------+------------------------------------------------------------------
        ///     1      |     0      | The mutex is not locked. One or more thread is parked or about to
        ///            |            | park. At least one of the parked threads are just about to be
        ///            |            | unparked, or a thread heading for parking might abort the park.
        /// -----------+------------+------------------------------------------------------------------
        ///     1      |     1      | The mutex is locked by exactly one thread. One or more thread is
        ///            |            | parked or about to park, waiting for the lock to become available.
        ///            |            | In this state, PARKED_BIT is only ever cleared when a bucket lock
        ///            |            | is held (i.e. in a parking_lot_core callback). This ensures that
        ///            |            | we never end up in a situation where there are parked threads but
        ///            |            | PARKED_BIT is not set (which would result in those threads
        ///            |            | potentially never getting woken up).
        state: AtomicU8,
    }

    unsafe impl lock_api::RawMutex for RawMutex {
        const INIT: RawMutex = RawMutex {
            state: AtomicU8::new(0),
        };

        type GuardMarker = crate::GuardMarker;

        #[inline]
        fn lock(&self) {
            if self
                .state
                .compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
                .is_err()
            {
                self.lock_slow(None);
            }
            unsafe { deadlock::acquire_resource(self as *const _ as usize) };
        }

        #[inline]
        fn try_lock(&self) -> bool {
            let mut state = self.state.load(Ordering::Relaxed);
            loop {
                if state & LOCKED_BIT != 0 {
                    return false;
                }
                match self.state.compare_exchange_weak(
                    state,
                    state | LOCKED_BIT,
                    Ordering::Acquire,
                    Ordering::Relaxed,
                ) {
                    Ok(_) => {
                        unsafe { deadlock::acquire_resource(self as *const _ as usize) };
                        return true;
                    }
                    Err(x) => state = x,
                }
            }
        }

        #[inline]
        unsafe fn unlock(&self) {
            deadlock::release_resource(self as *const _ as usize);
            if self
                .state
                .compare_exchange(LOCKED_BIT, 0, Ordering::Release, Ordering::Relaxed)
                .is_ok()
            {
                return;
            }
            self.unlock_slow(false);
        }

        #[inline]
        fn is_locked(&self) -> bool {
            let state = self.state.load(Ordering::Relaxed);
            state & LOCKED_BIT != 0
        }
    }

    unsafe impl lock_api::RawMutexFair for RawMutex {
        #[inline]
        unsafe fn unlock_fair(&self) {
            deadlock::release_resource(self as *const _ as usize);
            if self
                .state
                .compare_exchange(LOCKED_BIT, 0, Ordering::Release, Ordering::Relaxed)
                .is_ok()
            {
                return;
            }
            self.unlock_slow(true);
        }

        #[inline]
        unsafe fn bump(&self) {
            if self.state.load(Ordering::Relaxed) & PARKED_BIT != 0 {
                self.bump_slow();
            }
        }
    }

    unsafe impl lock_api::RawMutexTimed for RawMutex {
        type Duration = Duration;
        type Instant = Instant;

        #[inline]
        fn try_lock_until(&self, timeout: Instant) -> bool {
            let result = if self
                .state
                .compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
                .is_ok()
            {
                true
            } else {
                self.lock_slow(Some(timeout))
            };
            if result {
                unsafe { deadlock::acquire_resource(self as *const _ as usize) };
            }
            result
        }

        #[inline]
        fn try_lock_for(&self, timeout: Duration) -> bool {
            let result = if self
                .state
                .compare_exchange_weak(0, LOCKED_BIT, Ordering::Acquire, Ordering::Relaxed)
                .is_ok()
            {
                true
            } else {
                self.lock_slow(util::to_deadline(timeout))
            };
            if result {
                unsafe { deadlock::acquire_resource(self as *const _ as usize) };
            }
            result
        }
    }

    impl RawMutex {
        // Used by Condvar when requeuing threads to us, must be called while
        // holding the queue lock.
        #[inline]
        pub(crate) fn mark_parked_if_locked(&self) -> bool {
            let mut state = self.state.load(Ordering::Relaxed);
            loop {
                if state & LOCKED_BIT == 0 {
                    return false;
                }
                match self.state.compare_exchange_weak(
                    state,
                    state | PARKED_BIT,
                    Ordering::Relaxed,
                    Ordering::Relaxed,
                ) {
                    Ok(_) => return true,
                    Err(x) => state = x,
                }
            }
        }

        // Used by Condvar when requeuing threads to us, must be called while
        // holding the queue lock.
        #[inline]
        pub(crate) fn mark_parked(&self) {
            self.state.fetch_or(PARKED_BIT, Ordering::Relaxed);
        }

        #[cold]
        fn lock_slow(&self, timeout: Option<Instant>) -> bool {
            let mut spinwait = SpinWait::new();
            let mut state = self.state.load(Ordering::Relaxed);
            loop {
                // Grab the lock if it isn't locked, even if there is a queue on it
                if state & LOCKED_BIT == 0 {
                    match self.state.compare_exchange_weak(
                        state,
                        state | LOCKED_BIT,
                        Ordering::Acquire,
                        Ordering::Relaxed,
                    ) {
                        Ok(_) => return true,
                        Err(x) => state = x,
                    }
                    continue;
                }

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

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

                // Park our thread until we are woken up by an unlock
                let addr = self as *const _ as usize;
                let validate = || self.state.load(Ordering::Relaxed) == LOCKED_BIT | PARKED_BIT;
                let before_sleep = || {};
                let timed_out = |_, was_last_thread| {
                    // Clear the parked bit if we were the last parked thread
                    if was_last_thread {
                        self.state.fetch_and(!PARKED_BIT, Ordering::Relaxed);
                    }
                };
                // SAFETY:
                //   * `addr` is an address we control.
                //   * `validate`/`timed_out` does not panic or call into any function of `parking_lot`.
                //   * `before_sleep` does not call `park`, nor does it panic.
                match unsafe {
                    parking_lot_core::park(
                        addr,
                        validate,
                        before_sleep,
                        timed_out,
                        DEFAULT_PARK_TOKEN,
                        timeout,
                    )
                } {
                    // The thread that unparked us passed the lock on to us
                    // directly without unlocking it.
                    ParkResult::Unparked(TOKEN_HANDOFF) => return true,

                    // We were unparked normally, try acquiring the lock again
                    ParkResult::Unparked(_) => (),

                    // The validation function failed, try locking again
                    ParkResult::Invalid => (),

                    // Timeout expired
                    ParkResult::TimedOut => return false,
                }

                // Loop back and try locking again
                spinwait.reset();
                state = self.state.load(Ordering::Relaxed);
            }
        }

        #[cold]
        fn unlock_slow(&self, force_fair: bool) {
            // Unpark one thread and leave the parked bit set if there might
            // still be parked threads on this address.
            let addr = self as *const _ as usize;
            let callback = |result: UnparkResult| {
                // If we are using a fair unlock then we should keep the
                // mutex locked and hand it off to the unparked thread.
                if result.unparked_threads != 0 && (force_fair || result.be_fair) {
                    // Clear the parked bit if there are no more parked
                    // threads.
                    if !result.have_more_threads {
                        self.state.store(LOCKED_BIT, Ordering::Relaxed);
                    }
                    return TOKEN_HANDOFF;
                }

                // Clear the locked bit, and the parked bit as well if there
                // are no more parked threads.
                if result.have_more_threads {
                    self.state.store(PARKED_BIT, Ordering::Release);
                } else {
                    self.state.store(0, Ordering::Release);
                }
                TOKEN_NORMAL
            };
            // SAFETY:
            //   * `addr` is an address we control.
            //   * `callback` does not panic or call into any function of `parking_lot`.
            unsafe {
                parking_lot_core::unpark_one(addr, callback);
            }
        }

        #[cold]
        fn bump_slow(&self) {
            unsafe { deadlock::release_resource(self as *const _ as usize) };
            self.unlock_slow(true);
            self.lock();
        }
    }