use std::boxed::Box;
use std::cell::UnsafeCell;
use std::mem;
use std::ops::{Deref, DerefMut};
use std::sync::Arc;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::SeqCst;

use {Async, Future, Poll};
use task::{self, Task};

/// A type of futures-powered synchronization primitive which is a mutex between
/// two possible owners.
///
/// This primitive is not as generic as a full-blown mutex but is sufficient for
/// many use cases where there are only two possible owners of a resource. The
/// implementation of `BiLock` can be more optimized for just the two possible
/// owners.
///
/// Note that it's possible to use this lock through a poll-style interface with
/// the `poll_lock` method but you can also use it as a future with the `lock`
/// method that consumes a `BiLock` and returns a future that will resolve when
/// it's locked.
///
/// A `BiLock` is typically used for "split" operations where data which serves
/// two purposes wants to be split into two to be worked with separately. For
/// example a TCP stream could be both a reader and a writer or a framing layer
/// could be both a stream and a sink for messages. A `BiLock` enables splitting
/// these two and then using each independently in a futures-powered fashion.
#[derive(Debug)]
pub struct BiLock<T> {
    inner: Arc<Inner<T>>,
}

#[derive(Debug)]
struct Inner<T> {
    state: AtomicUsize,
    inner: UnsafeCell<T>,
}

unsafe impl<T: Send> Send for Inner<T> {}
unsafe impl<T: Send> Sync for Inner<T> {}

impl<T> BiLock<T> {
    /// Creates a new `BiLock` protecting the provided data.
    ///
    /// Two handles to the lock are returned, and these are the only two handles
    /// that will ever be available to the lock. These can then be sent to separate
    /// tasks to be managed there.
    pub fn new(t: T) -> (BiLock<T>, BiLock<T>) {
        let inner = Arc::new(Inner {
            state: AtomicUsize::new(0),
            inner: UnsafeCell::new(t),
        });

        (BiLock { inner: inner.clone() }, BiLock { inner: inner })
    }

    /// Attempt to acquire this lock, returning `NotReady` if it can't be
    /// acquired.
    ///
    /// This function will acquire the lock in a nonblocking fashion, returning
    /// immediately if the lock is already held. If the lock is successfully
    /// acquired then `Async::Ready` is returned with a value that represents
    /// the locked value (and can be used to access the protected data). The
    /// lock is unlocked when the returned `BiLockGuard` is dropped.
    ///
    /// If the lock is already held then this function will return
    /// `Async::NotReady`. In this case the current task will also be scheduled
    /// to receive a notification when the lock would otherwise become
    /// available.
    ///
    /// # Panics
    ///
    /// This function will panic if called outside the context of a future's
    /// task.
    pub fn poll_lock(&self) -> Async<BiLockGuard<T>> {
        loop {
            match self.inner.state.swap(1, SeqCst) {
                // Woohoo, we grabbed the lock!
                0 => return Async::Ready(BiLockGuard { inner: self }),

                // Oops, someone else has locked the lock
                1 => {}

                // A task was previously blocked on this lock, likely our task,
                // so we need to update that task.
                n => unsafe {
                    drop(Box::from_raw(n as *mut Task));
                }
            }

            let me = Box::new(task::park());
            let me = Box::into_raw(me) as usize;

            match self.inner.state.compare_exchange(1, me, SeqCst, SeqCst) {
                // The lock is still locked, but we've now parked ourselves, so
                // just report that we're scheduled to receive a notification.
                Ok(_) => return Async::NotReady,

                // Oops, looks like the lock was unlocked after our swap above
                // and before the compare_exchange. Deallocate what we just
                // allocated and go through the loop again.
                Err(0) => unsafe {
                    drop(Box::from_raw(me as *mut Task));
                },

                // The top of this loop set the previous state to 1, so if we
                // failed the CAS above then it's because the previous value was
                // *not* zero or one. This indicates that a task was blocked,
                // but we're trying to acquire the lock and there's only one
                // other reference of the lock, so it should be impossible for
                // that task to ever block itself.
                Err(n) => panic!("invalid state: {}", n),
            }
        }
    }

    /// Perform a "blocking lock" of this lock, consuming this lock handle and
    /// returning a future to the acquired lock.
    ///
    /// This function consumes the `BiLock<T>` and returns a sentinel future,
    /// `BiLockAcquire<T>`. The returned future will resolve to
    /// `BiLockAcquired<T>` which represents a locked lock similarly to
    /// `BiLockGuard<T>`.
    ///
    /// Note that the returned future will never resolve to an error.
    pub fn lock(self) -> BiLockAcquire<T> {
        BiLockAcquire {
            inner: self,
        }
    }

    fn unlock(&self) {
        match self.inner.state.swap(0, SeqCst) {
            // we've locked the lock, shouldn't be possible for us to see an
            // unlocked lock.
            0 => panic!("invalid unlocked state"),

            // Ok, no one else tried to get the lock, we're done.
            1 => {}

            // Another task has parked themselves on this lock, let's wake them
            // up as its now their turn.
            n => unsafe {
                Box::from_raw(n as *mut Task).unpark();
            }
        }
    }
}

impl<T> Drop for Inner<T> {
    fn drop(&mut self) {
        assert_eq!(self.state.load(SeqCst), 0);
    }
}

/// Returned RAII guard from the `poll_lock` method.
///
/// This structure acts as a sentinel to the data in the `BiLock<T>` itself,
/// implementing `Deref` and `DerefMut` to `T`. When dropped, the lock will be
/// unlocked.
#[derive(Debug)]
pub struct BiLockGuard<'a, T: 'a> {
    inner: &'a BiLock<T>,
}

impl<'a, T> Deref for BiLockGuard<'a, T> {
    type Target = T;
    fn deref(&self) -> &T {
        unsafe { &*self.inner.inner.inner.get() }
    }
}

impl<'a, T> DerefMut for BiLockGuard<'a, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.inner.inner.inner.get() }
    }
}

impl<'a, T> Drop for BiLockGuard<'a, T> {
    fn drop(&mut self) {
        self.inner.unlock();
    }
}

/// Future returned by `BiLock::lock` which will resolve when the lock is
/// acquired.
#[derive(Debug)]
pub struct BiLockAcquire<T> {
    inner: BiLock<T>,
}

impl<T> Future for BiLockAcquire<T> {
    type Item = BiLockAcquired<T>;
    type Error = ();

    fn poll(&mut self) -> Poll<BiLockAcquired<T>, ()> {
        match self.inner.poll_lock() {
            Async::Ready(r) => {
                mem::forget(r);
                Ok(BiLockAcquired {
                    inner: BiLock { inner: self.inner.inner.clone() },
                }.into())
            }
            Async::NotReady => Ok(Async::NotReady),
        }
    }
}

/// Resolved value of the `BiLockAcquire<T>` future.
///
/// This value, like `BiLockGuard<T>`, is a sentinel to the value `T` through
/// implementations of `Deref` and `DerefMut`. When dropped will unlock the
/// lock, and the original unlocked `BiLock<T>` can be recovered through the
/// `unlock` method.
#[derive(Debug)]
pub struct BiLockAcquired<T> {
    inner: BiLock<T>,
}

impl<T> BiLockAcquired<T> {
    /// Recovers the original `BiLock<T>`, unlocking this lock.
    pub fn unlock(self) -> BiLock<T> {
        // note that unlocked is implemented in `Drop`, so we don't do anything
        // here other than creating a new handle to return.
        BiLock { inner: self.inner.inner.clone() }
    }
}

impl<T> Deref for BiLockAcquired<T> {
    type Target = T;
    fn deref(&self) -> &T {
        unsafe { &*self.inner.inner.inner.get() }
    }
}

impl<T> DerefMut for BiLockAcquired<T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.inner.inner.inner.get() }
    }
}

impl<T> Drop for BiLockAcquired<T> {
    fn drop(&mut self) {
        self.inner.unlock();
    }
}