use crate::sync::batch_semaphore::{AcquireError, Semaphore};
use std::cell::UnsafeCell;
use std::ops;

#[cfg(not(loom))]
const MAX_READS: usize = 32;

#[cfg(loom)]
const MAX_READS: usize = 10;

/// An asynchronous reader-writer lock
///
/// This type of lock allows a number of readers or at most one writer at any
/// point in time. The write portion of this lock typically allows modification
/// of the underlying data (exclusive access) and the read portion of this lock
/// typically allows for read-only access (shared access).
///
/// In comparison, a [`Mutex`] does not distinguish between readers or writers
/// that acquire the lock, therefore causing any tasks waiting for the lock to
/// become available to yield. An `RwLock` will allow any number of readers to
/// acquire the lock as long as a writer is not holding the lock.
///
/// The priority policy of Tokio's read-write lock is _fair_ (or
/// [_write-preferring_]), in order to ensure that readers cannot starve
/// writers. Fairness is ensured using a first-in, first-out queue for the tasks
/// awaiting the lock; if a task that wishes to acquire the write lock is at the
/// head of the queue, read locks will not be given out until the write lock has
/// been released. This is in contrast to the Rust standard library's
/// `std::sync::RwLock`, where the priority policy is dependent on the
/// operating system's implementation.
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies [`Send`] to be shared across threads. The RAII guards
/// returned from the locking methods implement [`Deref`](trait@std::ops::Deref)
/// (and [`DerefMut`](trait@std::ops::DerefMut)
/// for the `write` methods) to allow access to the content of the lock.
///
/// # Examples
///
/// ```
/// use tokio::sync::RwLock;
///
/// #[tokio::main]
/// async fn main() {
///     let lock = RwLock::new(5);
///
///     // many reader locks can be held at once
///     {
///         let r1 = lock.read().await;
///         let r2 = lock.read().await;
///         assert_eq!(*r1, 5);
///         assert_eq!(*r2, 5);
///     } // read locks are dropped at this point
///
///     // only one write lock may be held, however
///     {
///         let mut w = lock.write().await;
///         *w += 1;
///         assert_eq!(*w, 6);
///     } // write lock is dropped here
/// }
/// ```
///
/// [`Mutex`]: struct@super::Mutex
/// [`RwLock`]: struct@RwLock
/// [`RwLockReadGuard`]: struct@RwLockReadGuard
/// [`RwLockWriteGuard`]: struct@RwLockWriteGuard
/// [`Send`]: trait@std::marker::Send
/// [_write-preferring_]: https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock#Priority_policies
#[derive(Debug)]
pub struct RwLock<T: ?Sized> {
    //semaphore to coordinate read and write access to T
    s: Semaphore,

    //inner data T
    c: UnsafeCell<T>,
}

/// RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read`] method on
/// [`RwLock`].
///
/// [`read`]: method@RwLock::read
#[derive(Debug)]
pub struct RwLockReadGuard<'a, T: ?Sized> {
    permit: ReleasingPermit<'a, T>,
    lock: &'a RwLock<T>,
}

/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
///
/// This structure is created by the [`write`] and method
/// on [`RwLock`].
///
/// [`write`]: method@RwLock::write
/// [`RwLock`]: struct@RwLock
#[derive(Debug)]
pub struct RwLockWriteGuard<'a, T: ?Sized> {
    permit: ReleasingPermit<'a, T>,
    lock: &'a RwLock<T>,
}

// Wrapper arround Permit that releases on Drop
#[derive(Debug)]
struct ReleasingPermit<'a, T: ?Sized> {
    num_permits: u16,
    lock: &'a RwLock<T>,
}

impl<'a, T: ?Sized> ReleasingPermit<'a, T> {
    async fn acquire(
        lock: &'a RwLock<T>,
        num_permits: u16,
    ) -> Result<ReleasingPermit<'a, T>, AcquireError> {
        lock.s.acquire(num_permits.into()).await?;
        Ok(Self { num_permits, lock })
    }
}

impl<T: ?Sized> Drop for ReleasingPermit<'_, T> {
    fn drop(&mut self) {
        self.lock.s.release(self.num_permits as usize);
    }
}

#[test]
#[cfg(not(loom))]
fn bounds() {
    fn check_send<T: Send>() {}
    fn check_sync<T: Sync>() {}
    fn check_unpin<T: Unpin>() {}
    // This has to take a value, since the async fn's return type is unnameable.
    fn check_send_sync_val<T: Send + Sync>(_t: T) {}

    check_send::<RwLock<u32>>();
    check_sync::<RwLock<u32>>();
    check_unpin::<RwLock<u32>>();

    check_sync::<RwLockReadGuard<'_, u32>>();
    check_unpin::<RwLockReadGuard<'_, u32>>();

    check_sync::<RwLockWriteGuard<'_, u32>>();
    check_unpin::<RwLockWriteGuard<'_, u32>>();

    let rwlock = RwLock::new(0);
    check_send_sync_val(rwlock.read());
    check_send_sync_val(rwlock.write());
}

// As long as T: Send + Sync, it's fine to send and share RwLock<T> between threads.
// If T were not Send, sending and sharing a RwLock<T> would be bad, since you can access T through
// RwLock<T>.
unsafe impl<T> Send for RwLock<T> where T: ?Sized + Send {}
unsafe impl<T> Sync for RwLock<T> where T: ?Sized + Send + Sync {}
unsafe impl<T> Sync for RwLockReadGuard<'_, T> where T: ?Sized + Send + Sync {}
unsafe impl<T> Sync for RwLockWriteGuard<'_, T> where T: ?Sized + Send + Sync {}

impl<T: ?Sized> RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::RwLock;
    ///
    /// let lock = RwLock::new(5);
    /// ```
    pub fn new(value: T) -> RwLock<T>
    where
        T: Sized,
    {
        RwLock {
            c: UnsafeCell::new(value),
            s: Semaphore::new(MAX_READS),
        }
    }

    /// Locks this rwlock with shared read access, causing the current task
    /// to yield until the lock has been acquired.
    ///
    /// The calling task will yield until there are no more writers which
    /// hold the lock. There may be other readers currently inside the lock when
    /// this method returns.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::Arc;
    /// use tokio::sync::RwLock;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///     let lock = Arc::new(RwLock::new(1));
    ///     let c_lock = lock.clone();
    ///
    ///     let n = lock.read().await;
    ///     assert_eq!(*n, 1);
    ///
    ///     tokio::spawn(async move {
    ///         // While main has an active read lock, we acquire one too.
    ///         let r = c_lock.read().await;
    ///         assert_eq!(*r, 1);
    ///     }).await.expect("The spawned task has paniced");
    ///
    ///     // Drop the guard after the spawned task finishes.
    ///     drop(n);
    ///}
    /// ```
    pub async fn read(&self) -> RwLockReadGuard<'_, T> {
        let permit = ReleasingPermit::acquire(self, 1).await.unwrap_or_else(|_| {
            // The semaphore was closed. but, we never explicitly close it, and we have a
            // handle to it through the Arc, which means that this can never happen.
            unreachable!()
        });
        RwLockReadGuard { lock: self, permit }
    }

    /// Locks this rwlock with exclusive write access, causing the current task
    /// to yield until the lock has been acquired.
    ///
    /// This function will not return while other writers or other readers
    /// currently have access to the lock.
    ///
    /// Returns an RAII guard which will drop the write access of this rwlock
    /// when dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::sync::RwLock;
    ///
    /// #[tokio::main]
    /// async fn main() {
    ///   let lock = RwLock::new(1);
    ///
    ///   let mut n = lock.write().await;
    ///   *n = 2;
    ///}
    /// ```
    pub async fn write(&self) -> RwLockWriteGuard<'_, T> {
        let permit = ReleasingPermit::acquire(self, MAX_READS as u16)
            .await
            .unwrap_or_else(|_| {
                // The semaphore was closed. but, we never explicitly close it, and we have a
                // handle to it through the Arc, which means that this can never happen.
                unreachable!()
            });

        RwLockWriteGuard { lock: self, permit }
    }

    /// Consumes the lock, returning the underlying data.
    pub fn into_inner(self) -> T
    where
        T: Sized,
    {
        self.c.into_inner()
    }
}

impl<T: ?Sized> ops::Deref for RwLockReadGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.lock.c.get() }
    }
}

impl<T: ?Sized> ops::Deref for RwLockWriteGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.lock.c.get() }
    }
}

impl<T: ?Sized> ops::DerefMut for RwLockWriteGuard<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.lock.c.get() }
    }
}

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

impl<T> Default for RwLock<T>
where
    T: Default,
{
    fn default() -> Self {
        Self::new(T::default())
    }
}