//use futures_core::future::{FusedFuture, Future};
use futures::future::{FusedFuture, Future};

//use futures_core::task::{Context, Poll, Waker};
use futures::task::{Context, Poll, Waker};

use slab::Slab;
use std::cell::UnsafeCell;
use std::marker::PhantomData;
use std::ops::{Deref, DerefMut};
use std::pin::Pin;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::Mutex as StdMutex;
use std::{fmt, mem};

/// A futures-aware mutex.
pub struct Mutex<T: ?Sized> {
    state: AtomicUsize,
    waiters: StdMutex<Slab<Waiter>>,
    read_waiters: StdMutex<Slab<Waiter>>,
    value: UnsafeCell<T>,
}

impl<T: ?Sized> fmt::Debug for Mutex<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let s = State(self.state.load(Ordering::SeqCst));
        f.debug_struct("Mutex")
            .field("is_locked", &s.is_locked())
            .field("has_waiters", &s.has_waiters())
            .field("has_read_waiters", &s.has_read_waiters())
            .field("is_corrupted", &s.is_corrupted())
            .field("read_lock_count", &s.read_lock_count())
            .finish()
    }
}

impl<T> From<T> for Mutex<T> {
    fn from(t: T) -> Self {
        Self::new(t)
    }
}

impl<T: Default> Default for Mutex<T> {
    fn default() -> Mutex<T> {
        Mutex::new(Default::default())
    }
}

pub enum Waiter {
    Waiting(Waker),
    Woken,
}

impl Waiter {
    fn register(&mut self, waker: &Waker) {
        match self {
            Waiter::Waiting(w) if waker.will_wake(w) => {}
            _ => *self = Waiter::Waiting(waker.clone()),
        }
    }

    fn wake(&mut self) {
        match mem::replace(self, Waiter::Woken) {
            Waiter::Waiting(waker) => waker.wake(),
            Waiter::Woken => {}
        }
    }
}

macro_rules! bitflag {
    ($name:ident, $bit:expr) => {
        const fn $name(&self) -> bool {
            self.0 & $bit != 0
        }
    };
    ($name:ident, $flag:ident, $bit:expr) => {
        const $flag: usize = $bit;
        bitflag!($name, Self::$flag);
    }
}
struct State(usize);
impl State {
    // set if it's write locked
    bitflag!(is_locked, IS_LOCKED, 1 << 31);
    // set if there are write waiters
    bitflag!(has_waiters, HAS_WAITERS, 1 << 30);
    // set if there are read waiters
    bitflag!(has_read_waiters, HAS_READ_WAITERS, 1 << 29);

    // the bits which are valid for read lock count (supports 16777216 readers)
    const MAX_READ_LOCKS: usize = (1 << 24) - 1;

    // bits which should always be zero (for future use)
    bitflag!(is_corrupted, (1 << 29) - 1 - Self::MAX_READ_LOCKS);

    const fn read_lock_count(&self) -> usize {
        self.0 & Self::MAX_READ_LOCKS
    }
}

impl<T> Mutex<T> {
    /// Creates a new futures-aware mutex.
    pub fn new(t: T) -> Mutex<T> {
        Mutex {
            state: AtomicUsize::new(0),
            waiters: StdMutex::new(Slab::new()),
            read_waiters: StdMutex::new(Slab::new()),
            value: UnsafeCell::new(t),
        }
    }

    /// Consumes this mutex, returning the underlying data.
    ///
    /// # Examples
    ///
    /// ```
    /// use futures::lock::Mutex;
    ///
    /// let mutex = Mutex::new(0);
    /// assert_eq!(mutex.into_inner(), 0);
    /// ```
    pub fn into_inner(self) -> T {
        self.value.into_inner()
    }
}

pub struct Lock<'a, RW: ReadWrite, T: ?Sized> {
    pub mutex: &'a Mutex<T>,
    _rw: PhantomData<RW>,
}

impl<'a, RW: ReadWrite, T: ?Sized> Lock<'a, RW, T> {
    /// Attempt to acquire the lock immediately.
    ///
    /// If the lock is currently held, this will return `None`.
    pub fn try_lock(&self) -> Option<MutexGuard<'a, RW, T>> {
        if RW::IS_WRITE {
            let old_state = State(
                self.mutex
                    .state
                    .fetch_or(State::IS_LOCKED, Ordering::Acquire),
            );
            if old_state.is_locked() {
                return None;
            }
        } else {
            let old_state = State(self.mutex.state.fetch_add(1, Ordering::Acquire));
            if old_state.read_lock_count() == State::MAX_READ_LOCKS {
                panic!("Overflowed the number of read locks");
            }
            if old_state.is_locked() {
                self.mutex.state.fetch_sub(1, Ordering::Relaxed);
                return None;
            }
        }
        Some(MutexGuard {
            mutex: self.mutex,
            _rw: PhantomData,
        })
    }

    /// Acquire the lock asynchronously.
    ///
    /// This method returns a future that will resolve once the lock has been
    /// successfully acquired.
    pub fn lock(&self) -> MutexLockFuture<'a, RW, T> {
        MutexLockFuture {
            mutex: Some(self.mutex),
            wait_key: WAIT_KEY_NONE,
            _rw: PhantomData,
        }
    }
}

impl<'a, T: ?Sized> Mutex<T> {
    pub const fn read(&'a self) -> Lock<'a, Read, T> {
        Lock {
            mutex: self,
            _rw: PhantomData,
        }
    }
    pub const fn write(&'a self) -> Lock<'a, Write, T> {
        Lock {
            mutex: self,
            _rw: PhantomData,
        }
    }
    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the `Mutex` mutably, no actual locking needs to
    /// take place -- the mutable borrow statically guarantees no locks exist.
    ///
    /// # Examples
    ///
    /// ```
    /// # futures::executor::block_on(async {
    /// use futures::lock::Mutex;
    ///
    /// let mut mutex = Mutex::new(0);
    /// *mutex.get_mut() = 10;
    /// assert_eq!(*mutex.lock().await, 10);
    /// # });
    /// ```
    pub fn get_mut(&mut self) -> &mut T {
        // We know statically that there are no other references to `self`, so
        // there's no need to lock the inner mutex.
        unsafe { &mut *self.value.get() }
    }

    fn remove_waker<RW:ReadWrite>(&self, wait_key: usize, wake_another: bool) {
        if wait_key == WAIT_KEY_NONE {
            return;
        }
        let mut waiters = RW::waiters(self).lock().unwrap();
        match waiters.remove(wait_key) {
            Waiter::Waiting(_) => {}
            Waiter::Woken => {
                // We were awoken, but then dropped before we could
                // wake up to acquire the lock. Wake up another
                // waiter.
                if RW::IS_WRITE && wake_another {
                    if let Some((_i, waiter)) = waiters.iter_mut().next() {
                        waiter.wake();
                    }
                }
            }
        }
        if waiters.is_empty() {
            self.state.fetch_and(!State::HAS_WAITERS, Ordering::Relaxed); // released by mutex unlock
            // If we're in read mode, wake_another is true, AND we're the last one
            // Lets wake a write lock
            if !RW::IS_WRITE && wake_another {
                let mut write_waiters = self.waiters.lock().unwrap();
                if let Some((_i, waiter)) = write_waiters.iter_mut().next() {
                    waiter.wake();
                }
            }
        }
    }
}

pub trait ReadWrite: Sized + Unpin {
    const IS_WRITE: bool;
    const STR: &'static str;
    const HAS_WAITERS_FLAG: usize;
    fn lock<'a, T: ?Sized>(m: &'a Mutex<T>) -> Lock<'a, Self, T>;
    fn waiters<'a, T: ?Sized>(m: &Mutex<T>) -> &StdMutex<Slab<Waiter>>;
}
pub struct Write;
pub struct Read;
impl ReadWrite for Write {
    const IS_WRITE: bool = true;
    const STR: &'static str = "Write";
    const HAS_WAITERS_FLAG: usize = State::HAS_WAITERS;
    fn lock<'a, T: ?Sized>(m: &'a Mutex<T>) -> Lock<'a, Self, T> {
        m.write()
    }
    fn waiters<'a, T: ?Sized>(m: &Mutex<T>) -> &StdMutex<Slab<Waiter>> {
        &m.waiters
    }
}
impl ReadWrite for Read {
    const IS_WRITE: bool = true;
    const STR: &'static str = "Read";
    const HAS_WAITERS_FLAG: usize = State::HAS_READ_WAITERS;
    fn lock<'a, T: ?Sized>(m: &'a Mutex<T>) -> Lock<'a, Self, T> {
        m.read()
    }
    fn waiters<'a, T: ?Sized>(m: &Mutex<T>) -> &StdMutex<Slab<Waiter>> {
        &m.read_waiters
    }
}

// Sentinel for when no slot in the `Slab` has been dedicated to this object.
const WAIT_KEY_NONE: usize = usize::max_value();

/// A future which resolves when the target mutex has been successfully acquired.
pub struct MutexLockFuture<'a, RW: ReadWrite, T: ?Sized> {
    // `None` indicates that the mutex was successfully acquired.
    mutex: Option<&'a Mutex<T>>,
    wait_key: usize,
    _rw: PhantomData<RW>,
}

impl<RW: ReadWrite, T: ?Sized> fmt::Debug for MutexLockFuture<'_, RW, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct(&format!("MutexLockFuture<{}>", RW::STR)[..])
            .field("was_acquired", &self.mutex.is_none())
            .field("mutex", &self.mutex)
            .field(
                "wait_key",
                &(if self.wait_key == WAIT_KEY_NONE {
                    None
                } else {
                    Some(self.wait_key)
                }),
            )
            .finish()
    }
}

impl<'a, RW: ReadWrite, T: ?Sized> FusedFuture for MutexLockFuture<'a, RW, T> {
    fn is_terminated(&self) -> bool {
        self.mutex.is_none()
    }
}
impl<'a, RW: ReadWrite, T: ?Sized> Future for MutexLockFuture<'a, RW, T> {
    type Output = MutexGuard<'a, RW, T>;
    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let mutex = self.mutex.expect("polled MutexLockFuture after completion");

        let lock = RW::lock(mutex);
        if let Some(locked) = lock.try_lock() {
            mutex.remove_waker::<RW>(self.wait_key, false);
            self.mutex = None;
            return Poll::Ready(locked);
        }

        {
            let mut waiters = RW::waiters(mutex).lock().unwrap();
            if self.wait_key == WAIT_KEY_NONE {
                self.wait_key = waiters.insert(Waiter::Waiting(cx.waker().clone()));
                if waiters.len() == 1 {
                    mutex
                        .state
                        .fetch_or(RW::HAS_WAITERS_FLAG, Ordering::Relaxed); // released by mutex unlock
                }
            } else {
                waiters[self.wait_key].register(cx.waker());
            }
        }

        // Ensure that we haven't raced `MutexGuard::drop`'s unlock path by
        // attempting to acquire the lock again.
        if let Some(locked) = lock.try_lock() {
            mutex.remove_waker::<RW>(self.wait_key, false);
            self.mutex = None;
            return Poll::Ready(locked);
        }

        Poll::Pending
    }
}

impl<RW: ReadWrite, T: ?Sized> Drop for MutexLockFuture<'_, RW, T> {
    fn drop(&mut self) {
        if let Some(mutex) = self.mutex {
            // This future was dropped before it acquired the mutex.
            //
            // Remove ourselves from the map, waking up another waiter if we
            // had been awoken to acquire the lock.
            mutex.remove_waker::<RW>(self.wait_key, true);
        }
    }
}

/// An RAII guard returned by the `lock` and `try_lock` methods.
/// When this structure is dropped (falls out of scope), the lock will be
/// unlocked.
pub struct MutexGuard<'a, RW: ReadWrite, T: ?Sized> {
    mutex: &'a Mutex<T>,
    _rw: PhantomData<RW>,
}

impl<RW: ReadWrite, T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, RW, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct(&format!("MutexGuard<{}>", RW::STR)[..])
            .field("value", &*self)
            .field("mutex", &self.mutex)
            .finish()
    }
}

impl<RW: ReadWrite, T: ?Sized> Drop for MutexGuard<'_, RW, T> {
    fn drop(&mut self) {
        if RW::IS_WRITE {
            let old_state = State(
                self.mutex
                    .state
                    .fetch_and(!State::IS_LOCKED, Ordering::AcqRel),
            );
            if old_state.has_waiters() {
                // Wake one write waiter
                let mut waiters = self.mutex.waiters.lock().unwrap();
                if let Some((_i, waiter)) = waiters.iter_mut().next() {
                    waiter.wake();
                }
            } else if old_state.has_read_waiters() {
                // Wake all read waiters
                let mut waiters = self.mutex.waiters.lock().unwrap();
                for w in waiters.iter_mut() {
                    w.1.wake()
                }
            }
        } else {
            let old_state = State(self.mutex.state.fetch_sub(1, Ordering::AcqRel));
            if old_state.read_lock_count() > 1 {
                // We're not the last read lock holder
            } else if old_state.has_waiters() {
                // Wake one write waiter
                let mut waiters = self.mutex.waiters.lock().unwrap();
                if let Some((_i, waiter)) = waiters.iter_mut().next() {
                    waiter.wake();
                }
            } else if old_state.has_read_waiters() {
                panic!("Race condition, read waiters were queued while the read lock was held")
            }
        }
    }
}

impl<RW: ReadWrite, T: ?Sized> Deref for MutexGuard<'_, RW, T> {
    type Target = T;
    fn deref(&self) -> &T {
        unsafe { &*self.mutex.value.get() }
    }
}
impl<T: ?Sized> DerefMut for MutexGuard<'_, Write, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.mutex.value.get() }
    }
}

// Mutexes can be moved freely between threads and acquired on any thread so long
// as the inner value can be safely sent between threads.
unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}

// It's safe to switch which thread the acquire is being attempted on so long as
// `T` can be accessed on that thread.
unsafe impl<RW: ReadWrite, T: ?Sized + Send> Send for MutexLockFuture<'_, RW, T> {}
// doesn't have any interesting `&self` methods (only Debug)
unsafe impl<RW: ReadWrite, T: ?Sized> Sync for MutexLockFuture<'_, RW, T> {}

// Safe to send since we don't track any thread-specific details-- the inner
// lock is essentially spinlock-equivalent (attempt to flip an atomic bool)
unsafe impl<RW: ReadWrite, T: ?Sized + Send> Send for MutexGuard<'_, RW, T> {}
unsafe impl<RW: ReadWrite, T: ?Sized + Sync> Sync for MutexGuard<'_, RW, T> {}