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

/// A mutual exclusion primitive useful for protecting shared data.
///
/// The mutex can be statically initialized or created via a [`Mutex::new`]
/// constructor. Each mutex has a type parameter which represents the data that
/// it is protecting. The data can only be accessed through the RAII guard
/// returned from [`Mutex::try_lock`], which guarantees that the data is only
/// ever accessed when the mutex is locked.
pub struct Mutex<T: ?Sized> {
    state: AtomicBool,
    data: UnsafeCell<T>,
}

/// An RAII implementation of a "scoped lock" of a mutex. When this structure is
/// dropped (falls out of scope), the lock will be unlocked.
///
/// The data protected by the mutex can be accessed through this guard via its
/// [`Deref`] and [`DerefMut`] implementations.
///
/// This structure is created by the [`try_lock`](Mutex::try_lock) method on
/// [`Mutex`].
#[must_use = "if unused the Mutex will immediately unlock"]
pub struct MutexGuard<'a, T: ?Sized> {
    mutex: &'a Mutex<T>,
}

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

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

impl<T> Mutex<T> {
    /// Creates a new mutex in an unlocked state ready for use.
    ///
    /// # Examples
    ///
    /// ```
    /// use drone_core::sync::Mutex;
    ///
    /// let mutex = Mutex::new(0);
    /// ```
    #[inline]
    pub const fn new(data: T) -> Self {
        Self {
            state: AtomicBool::new(false),
            data: UnsafeCell::new(data),
        }
    }

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

impl<T: ?Sized> Mutex<T> {
    /// Attempts to acquire this lock.
    ///
    /// If the lock could not be acquired at this time, then `None` is returned.
    /// Otherwise, an RAII guard is returned. The lock will be unlocked when the
    /// guard is dropped.
    ///
    /// # Examples
    ///
    /// ```
    /// use drone_core::sync::Mutex;
    /// use std::{sync::Arc, thread};
    ///
    /// let mutex = Arc::new(Mutex::new(0));
    /// let c_mutex = Arc::clone(&mutex);
    ///
    /// thread::spawn(move || {
    ///     let mut lock = c_mutex.try_lock();
    ///     if let Some(ref mut mutex) = lock {
    ///         **mutex = 10;
    ///     } else {
    ///         println!("try_lock failed");
    ///     }
    /// })
    /// .join()
    /// .expect("thread::spawn failed");
    /// assert_eq!(*mutex.try_lock().unwrap(), 10);
    /// ```
    #[inline]
    pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
        if self.state.swap(true, Ordering::Acquire) {
            None
        } else {
            Some(MutexGuard { mutex: self })
        }
    }

    /// 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
    ///
    /// ```
    /// use drone_core::sync::Mutex;
    ///
    /// let mut mutex = Mutex::new(0);
    /// *mutex.get_mut() = 10;
    /// assert_eq!(*mutex.try_lock().unwrap(), 10);
    /// ```
    #[inline]
    pub fn get_mut(&mut self) -> &mut T {
        unsafe { &mut *self.data.get() }
    }
}

impl<T> From<T> for Mutex<T> {
    /// Creates a new mutex in an unlocked state ready for use. This is
    /// equivalent to [`Mutex::new`].
    #[inline]
    fn from(data: T) -> Self {
        Self::new(data)
    }
}

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

impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(guard) = self.try_lock() {
            f.debug_struct("Mutex").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("Mutex")
                .field("data", &LockedPlaceholder)
                .finish()
        }
    }
}

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

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

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

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

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

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

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

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

    #[test]
    fn try_lock() {
        let m = Mutex::new(());
        *m.try_lock().unwrap() = ();
    }

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

    #[test]
    fn mutex_arc_nested() {
        let arc = Arc::new(Mutex::new(1));
        let arc2 = Arc::new(Mutex::new(arc));
        let (tx, rx) = channel();
        let _t = thread::spawn(move || {
            let lock = arc2.try_lock().unwrap();
            let lock2 = lock.try_lock().unwrap();
            assert_eq!(*lock2, 1);
            tx.send(()).unwrap();
        });
        rx.recv().unwrap();
    }

    #[test]
    fn mutex_unsized() {
        let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
        {
            let b = &mut *mutex.try_lock().unwrap();
            b[0] = 4;
            b[2] = 5;
        }
        let comp: &[i32] = &[4, 2, 5];
        assert_eq!(&*mutex.try_lock().unwrap(), comp);
    }
}