use core::{
    cell::UnsafeCell,
    fmt,
    ops::{Deref, DerefMut},
    sync::atomic::{AtomicUsize, Ordering},
};

const WRITE_LOCK: usize = usize::max_value();
const NO_LOCK: usize = usize::min_value();

/// A 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`](crate::sync::Mutex) does not distinguish between
/// readers or writers that acquire the lock, therefore blocking any threads
/// waiting for the lock to become available. An `RwLock` will allow any number
/// of readers to acquire the lock as long as a writer is not holding the lock.
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies [`Send`] to be shared across threads and
/// [`Sync`] to allow concurrent access through readers. The RAII guards
/// returned from the locking methods implement [`Deref`] (and [`DerefMut`] for
/// the `write` methods) to allow access to the content of the lock.
pub struct RwLock<T: ?Sized> {
    state: AtomicUsize,
    data: UnsafeCell<T>,
}

/// RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`try_read`](RwLock::try_read) method on
/// [`RwLock`].
#[must_use = "if unused the RwLock will immediately unlock"]
pub struct RwLockReadGuard<'a, T: ?Sized> {
    rw_lock: &'a RwLock<T>,
}

/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
///
/// This structure is created by the [`try_write`](RwLock::try_write) method on
/// [`RwLock`].
#[must_use = "if unused the RwLock will immediately unlock"]
pub struct RwLockWriteGuard<'a, T: ?Sized> {
    rw_lock: &'a RwLock<T>,
}

unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}

impl<T: ?Sized> !Send for RwLockReadGuard<'_, T> {}
unsafe impl<T: ?Sized + Sync> Sync for RwLockReadGuard<'_, T> {}

impl<T: ?Sized> !Send for RwLockWriteGuard<'_, T> {}
unsafe impl<T: ?Sized + Sync> Sync for RwLockWriteGuard<'_, T> {}

impl<T> RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    ///
    /// # Examples
    ///
    /// ```
    /// use drone_core::sync::RwLock;
    ///
    /// let lock = RwLock::new(5);
    /// ```
    #[inline]
    pub const fn new(data: T) -> Self {
        Self {
            state: AtomicUsize::new(NO_LOCK),
            data: UnsafeCell::new(data),
        }
    }

    /// Consumes this `RwLock`, returning the underlying data.
    ///
    /// # Examples
    ///
    /// ```
    /// use drone_core::sync::RwLock;
    ///
    /// let lock = RwLock::new(String::new());
    /// {
    ///     let mut s = lock.try_write().unwrap();
    ///     *s = "modified".to_owned();
    /// }
    /// assert_eq!(lock.into_inner(), "modified");
    /// ```
    #[inline]
    pub fn into_inner(self) -> T {
        self.data.into_inner()
    }
}

impl<T: ?Sized> RwLock<T> {
    /// Attempts to acquire this rwlock with shared read access.
    ///
    /// If the access could not be granted at this time, then `None` is
    /// returned. Otherwise, an RAII guard is returned which will release the
    /// shared access when it is dropped.
    ///
    /// This function does not provide any guarantees with respect to the
    /// ordering of whether contentious readers or writers will acquire the
    /// lock first.
    ///
    /// # Examples
    ///
    /// ```
    /// use drone_core::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// match lock.try_read() {
    ///     Some(n) => assert_eq!(*n, 1),
    ///     None => unreachable!(),
    /// };
    /// ```
    #[inline]
    pub fn try_read(&self) -> Option<RwLockReadGuard<'_, T>> {
        loop {
            let current = self.state.load(Ordering::Relaxed);
            if current >= WRITE_LOCK - 1 {
                break None;
            }
            if self
                .state
                .compare_and_swap(current, current + 1, Ordering::Acquire)
                == current
            {
                break Some(RwLockReadGuard { rw_lock: self });
            }
        }
    }

    /// Attempts to lock this rwlock with exclusive write access.
    ///
    /// If the lock could not be acquired at this time, then `None` is returned.
    /// Otherwise, an RAII guard is returned which will release the lock when it
    /// is dropped.
    ///
    /// This function does not provide any guarantees with respect to the
    /// ordering of whether contentious readers or writers will acquire the
    /// lock first.
    ///
    /// # Examples
    ///
    /// ```
    /// use drone_core::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// let n = lock.try_read().unwrap();
    /// assert_eq!(*n, 1);
    ///
    /// assert!(lock.try_write().is_none());
    /// ```
    #[inline]
    pub fn try_write(&self) -> Option<RwLockWriteGuard<'_, T>> {
        if self
            .state
            .compare_and_swap(NO_LOCK, WRITE_LOCK, Ordering::Acquire)
            == NO_LOCK
        {
            Some(RwLockWriteGuard { rw_lock: self })
        } else {
            None
        }
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the `RwLock` mutably, no actual locking needs to
    /// take place -- the mutable borrow statically guarantees no locks exist.
    ///
    /// # Examples
    ///
    /// ```
    /// use drone_core::sync::RwLock;
    ///
    /// let mut lock = RwLock::new(0);
    /// *lock.get_mut() = 10;
    /// assert_eq!(*lock.try_read().unwrap(), 10);
    /// ```
    #[inline]
    pub fn get_mut(&mut self) -> &mut T {
        unsafe { &mut *self.data.get() }
    }
}

impl<T> From<T> for RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked. This is
    /// equivalent to [`RwLock::new`].
    fn from(data: T) -> Self {
        RwLock::new(data)
    }
}

impl<T: ?Sized + Default> Default for RwLock<T> {
    /// Creates a new `RwLock<T>`, with the `Default` value for T.
    #[inline]
    fn default() -> Self {
        Self::new(Default::default())
    }
}

impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(guard) = self.try_read() {
            f.debug_struct("RwLock").field("data", &&*guard).finish()
        } else {
            struct LockedPlaceholder;
            impl fmt::Debug for LockedPlaceholder {
                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                    f.write_str("<locked>")
                }
            }
            f.debug_struct("RwLock")
                .field("data", &LockedPlaceholder)
                .finish()
        }
    }
}

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

    #[inline]
    fn deref(&self) -> &T {
        unsafe { &*self.rw_lock.data.get() }
    }
}

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

    #[inline]
    fn deref(&self) -> &T {
        unsafe { &*self.rw_lock.data.get() }
    }
}

impl<T: ?Sized> DerefMut for RwLockWriteGuard<'_, T> {
    #[inline]
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.rw_lock.data.get() }
    }
}

impl<T: ?Sized> Drop for RwLockReadGuard<'_, T> {
    #[inline]
    fn drop(&mut self) {
        self.rw_lock.state.fetch_sub(1, Ordering::Release);
    }
}

impl<T: ?Sized> Drop for RwLockWriteGuard<'_, T> {
    #[inline]
    fn drop(&mut self) {
        self.rw_lock.state.store(NO_LOCK, Ordering::Release);
    }
}

impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockReadGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RwLockReadGuard")
            .field("rw_lock", &self.rw_lock)
            .finish()
    }
}

impl<T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLockWriteGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RwLockWriteGuard")
            .field("rw_lock", &self.rw_lock)
            .finish()
    }
}

impl<T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[cfg(test)]
mod tests {
    use crate::sync::RwLock;
    use std::sync::{
        atomic::{AtomicUsize, Ordering},
        Arc,
    };

    #[derive(Eq, PartialEq, Debug)]
    struct NonCopy(i32);

    #[test]
    fn rwlock_unsized() {
        let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
        {
            let b = &mut *rw.try_write().unwrap();
            b[0] = 4;
            b[2] = 5;
        }
        let comp: &[i32] = &[4, 2, 5];
        assert_eq!(&*rw.try_read().unwrap(), comp);
    }

    #[test]
    fn rwlock_try_write() {
        let lock = RwLock::new(0isize);
        let read_guard = lock.try_read().unwrap();

        let write_result = lock.try_write();
        match write_result {
            None => (),
            Some(_) => assert!(
                false,
                "try_write should not succeed while read_guard is in scope"
            ),
        }

        drop(read_guard);
    }

    #[test]
    fn into_inner() {
        let m = RwLock::new(NonCopy(10));
        assert_eq!(m.into_inner(), NonCopy(10));
    }

    #[test]
    fn into_inner_drop() {
        struct Foo(Arc<AtomicUsize>);
        impl Drop for Foo {
            fn drop(&mut self) {
                self.0.fetch_add(1, Ordering::SeqCst);
            }
        }
        let num_drops = Arc::new(AtomicUsize::new(0));
        let m = RwLock::new(Foo(num_drops.clone()));
        assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        {
            let _inner = m.into_inner();
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        }
        assert_eq!(num_drops.load(Ordering::SeqCst), 1);
    }

    #[test]
    fn get_mut() {
        let mut m = RwLock::new(NonCopy(10));
        *m.get_mut() = NonCopy(20);
        assert_eq!(m.into_inner(), NonCopy(20));
    }
}