use alloc::boxed::Box;
use core::borrow;
use core::cmp::Ordering;
use core::convert::From;
use core::ffi::c_void;
use core::fmt;
use core::hash::{Hash, Hasher};
use core::marker::PhantomData;
use core::mem;
use core::mem::ManuallyDrop;
use core::ops::Deref;
use core::ptr;
use core::sync::atomic;
use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
use core::{isize, usize};
#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};
#[cfg(feature = "stable_deref_trait")]
use stable_deref_trait::{CloneStableDeref, StableDeref};

use super::{ArcBorrow, OffsetArc, abort};

/// A soft limit on the amount of references that may be made to an `Arc`.
///
/// Going above this limit will abort your program (although not
/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
const MAX_REFCOUNT: usize = (isize::MAX) as usize;

/// The object allocated by an Arc<T>
#[repr(C)]
pub(crate) struct ArcInner<T: ?Sized> {
    pub(crate) count: atomic::AtomicUsize,
    pub(crate) data: T,
}

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

/// An atomically reference counted shared pointer
///
/// See the documentation for [`Arc`] in the standard library. Unlike the
/// standard library `Arc`, this `Arc` does not support weak reference counting.
///
/// [`Arc`]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html
#[repr(transparent)]
pub struct Arc<T: ?Sized> {
    pub(crate) p: ptr::NonNull<ArcInner<T>>,
    pub(crate) phantom: PhantomData<T>,
}

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

impl<T> Arc<T> {
    /// Construct an `Arc<T>`
    #[inline]
    pub fn new(data: T) -> Self {
        let ptr = Box::into_raw(Box::new(ArcInner {
            count: atomic::AtomicUsize::new(1),
            data,
        }));

        unsafe {
            Arc {
                p: ptr::NonNull::new_unchecked(ptr),
                phantom: PhantomData,
            }
        }
    }

    /// Convert the Arc<T> to a raw pointer, suitable for use across FFI
    ///
    /// Note: This returns a pointer to the data T, which is offset in the allocation.
    ///
    /// It is recommended to use OffsetArc for this.
    #[inline]
    pub fn into_raw(this: Self) -> *const T {
        let ptr = unsafe { &((*this.ptr()).data) as *const _ };
        mem::forget(this);
        ptr
    }

    /// Reconstruct the Arc<T> from a raw pointer obtained from into_raw()
    ///
    /// Note: This raw pointer will be offset in the allocation and must be preceded
    /// by the atomic count.
    ///
    /// It is recommended to use OffsetArc for this
    #[inline]
    pub unsafe fn from_raw(ptr: *const T) -> Self {
        // To find the corresponding pointer to the `ArcInner` we need
        // to subtract the offset of the `data` field from the pointer.
        let ptr = (ptr as *const u8).sub(offset_of!(ArcInner<T>, data));
        Arc {
            p: ptr::NonNull::new_unchecked(ptr as *mut ArcInner<T>),
            phantom: PhantomData,
        }
    }

    /// Produce a pointer to the data that can be converted back
    /// to an Arc. This is basically an `&Arc<T>`, without the extra indirection.
    /// It has the benefits of an `&T` but also knows about the underlying refcount
    /// and can be converted into more `Arc<T>`s if necessary.
    #[inline]
    pub fn borrow_arc<'a>(&'a self) -> ArcBorrow<'a, T> {
        ArcBorrow(&**self)
    }

    /// Temporarily converts |self| into a bonafide OffsetArc and exposes it to the
    /// provided callback. The refcount is not modified.
    #[inline(always)]
    pub fn with_raw_offset_arc<F, U>(&self, f: F) -> U
    where
        F: FnOnce(&OffsetArc<T>) -> U,
    {
        // Synthesize transient Arc, which never touches the refcount of the ArcInner.
        let transient = unsafe { ManuallyDrop::new(Arc::into_raw_offset(ptr::read(self))) };

        // Expose the transient Arc to the callback, which may clone it if it wants.
        let result = f(&transient);

        // Forget the transient Arc to leave the refcount untouched.
        mem::forget(transient);

        // Forward the result.
        result
    }

    /// Returns the address on the heap of the Arc itself -- not the T within it -- for memory
    /// reporting.
    pub fn heap_ptr(&self) -> *const c_void {
        self.p.as_ptr() as *const ArcInner<T> as *const c_void
    }

    /// Converts an `Arc` into a `OffsetArc`. This consumes the `Arc`, so the refcount
    /// is not modified.
    #[inline]
    pub fn into_raw_offset(a: Self) -> OffsetArc<T> {
        unsafe {
            OffsetArc {
                ptr: ptr::NonNull::new_unchecked(Arc::into_raw(a) as *mut T),
                phantom: PhantomData,
            }
        }
    }

    /// Converts a `OffsetArc` into an `Arc`. This consumes the `OffsetArc`, so the refcount
    /// is not modified.
    #[inline]
    pub fn from_raw_offset(a: OffsetArc<T>) -> Self {
        let ptr = a.ptr.as_ptr();
        mem::forget(a);
        unsafe { Arc::from_raw(ptr) }
    }
}

impl<T: ?Sized> Arc<T> {
    #[inline]
    fn inner(&self) -> &ArcInner<T> {
        // This unsafety is ok because while this arc is alive we're guaranteed
        // that the inner pointer is valid. Furthermore, we know that the
        // `ArcInner` structure itself is `Sync` because the inner data is
        // `Sync` as well, so we're ok loaning out an immutable pointer to these
        // contents.
        unsafe { &*self.ptr() }
    }

    // Non-inlined part of `drop`. Just invokes the destructor.
    #[inline(never)]
    unsafe fn drop_slow(&mut self) {
        let _ = Box::from_raw(self.ptr());
    }

    /// Test pointer equality between the two Arcs, i.e. they must be the _same_
    /// allocation
    #[inline]
    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
        this.ptr() == other.ptr()
    }

    pub(crate) fn ptr(&self) -> *mut ArcInner<T> {
        self.p.as_ptr()
    }
}

impl<T: ?Sized> Clone for Arc<T> {
    #[inline]
    fn clone(&self) -> Self {
        // Using a relaxed ordering is alright here, as knowledge of the
        // original reference prevents other threads from erroneously deleting
        // the object.
        //
        // As explained in the [Boost documentation][1], Increasing the
        // reference counter can always be done with memory_order_relaxed: New
        // references to an object can only be formed from an existing
        // reference, and passing an existing reference from one thread to
        // another must already provide any required synchronization.
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        let old_size = self.inner().count.fetch_add(1, Relaxed);

        // However we need to guard against massive refcounts in case someone
        // is `mem::forget`ing Arcs. If we don't do this the count can overflow
        // and users will use-after free. We racily saturate to `isize::MAX` on
        // the assumption that there aren't ~2 billion threads incrementing
        // the reference count at once. This branch will never be taken in
        // any realistic program.
        //
        // We abort because such a program is incredibly degenerate, and we
        // don't care to support it.
        if old_size > MAX_REFCOUNT {
            abort();
        }

        unsafe {
            Arc {
                p: ptr::NonNull::new_unchecked(self.ptr()),
                phantom: PhantomData,
            }
        }
    }
}

impl<T: ?Sized> Deref for Arc<T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        &self.inner().data
    }
}

impl<T: Clone> Arc<T> {
    /// Makes a mutable reference to the `Arc`, cloning if necessary
    ///
    /// This is functionally equivalent to [`Arc::make_mut`][mm] from the standard library.
    ///
    /// If this `Arc` is uniquely owned, `make_mut()` will provide a mutable
    /// reference to the contents. If not, `make_mut()` will create a _new_ `Arc`
    /// with a copy of the contents, update `this` to point to it, and provide
    /// a mutable reference to its contents.
    ///
    /// This is useful for implementing copy-on-write schemes where you wish to
    /// avoid copying things if your `Arc` is not shared.
    ///
    /// [mm]: https://doc.rust-lang.org/stable/std/sync/struct.Arc.html#method.make_mut
    #[inline]
    pub fn make_mut(this: &mut Self) -> &mut T {
        if !this.is_unique() {
            // Another pointer exists; clone
            *this = Arc::new((**this).clone());
        }

        unsafe {
            // This unsafety is ok because we're guaranteed that the pointer
            // returned is the *only* pointer that will ever be returned to T. Our
            // reference count is guaranteed to be 1 at this point, and we required
            // the Arc itself to be `mut`, so we're returning the only possible
            // reference to the inner data.
            &mut (*this.ptr()).data
        }
    }
}

impl<T: ?Sized> Arc<T> {
    /// Provides mutable access to the contents _if_ the `Arc` is uniquely owned.
    #[inline]
    pub fn get_mut(this: &mut Self) -> Option<&mut T> {
        if this.is_unique() {
            unsafe {
                // See make_mut() for documentation of the threadsafety here.
                Some(&mut (*this.ptr()).data)
            }
        } else {
            None
        }
    }

    /// Whether or not the `Arc` is uniquely owned (is the refcount 1?).
    pub fn is_unique(&self) -> bool {
        // See the extensive discussion in [1] for why this needs to be Acquire.
        //
        // [1] https://github.com/servo/servo/issues/21186
        self.inner().count.load(Acquire) == 1
    }
}

impl<T: ?Sized> Drop for Arc<T> {
    #[inline]
    fn drop(&mut self) {
        // Because `fetch_sub` is already atomic, we do not need to synchronize
        // with other threads unless we are going to delete the object.
        if self.inner().count.fetch_sub(1, Release) != 1 {
            return;
        }

        // FIXME(bholley): Use the updated comment when [2] is merged.
        //
        // This load is needed to prevent reordering of use of the data and
        // deletion of the data.  Because it is marked `Release`, the decreasing
        // of the reference count synchronizes with this `Acquire` load. This
        // means that use of the data happens before decreasing the reference
        // count, which happens before this load, which happens before the
        // deletion of the data.
        //
        // As explained in the [Boost documentation][1],
        //
        // > It is important to enforce any possible access to the object in one
        // > thread (through an existing reference) to *happen before* deleting
        // > the object in a different thread. This is achieved by a "release"
        // > operation after dropping a reference (any access to the object
        // > through this reference must obviously happened before), and an
        // > "acquire" operation before deleting the object.
        //
        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
        // [2]: https://github.com/rust-lang/rust/pull/41714
        self.inner().count.load(Acquire);

        unsafe {
            self.drop_slow();
        }
    }
}

impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
    fn eq(&self, other: &Arc<T>) -> bool {
        Self::ptr_eq(self, other) || *(*self) == *(*other)
    }

    fn ne(&self, other: &Arc<T>) -> bool {
        !Self::ptr_eq(self, other) && *(*self) != *(*other)
    }
}

impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
    fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
        (**self).partial_cmp(&**other)
    }

    fn lt(&self, other: &Arc<T>) -> bool {
        *(*self) < *(*other)
    }

    fn le(&self, other: &Arc<T>) -> bool {
        *(*self) <= *(*other)
    }

    fn gt(&self, other: &Arc<T>) -> bool {
        *(*self) > *(*other)
    }

    fn ge(&self, other: &Arc<T>) -> bool {
        *(*self) >= *(*other)
    }
}

impl<T: ?Sized + Ord> Ord for Arc<T> {
    fn cmp(&self, other: &Arc<T>) -> Ordering {
        (**self).cmp(&**other)
    }
}

impl<T: ?Sized + Eq> Eq for Arc<T> {}

impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Display::fmt(&**self, f)
    }
}

impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Debug::fmt(&**self, f)
    }
}

impl<T: ?Sized> fmt::Pointer for Arc<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Pointer::fmt(&self.ptr(), f)
    }
}

impl<T: Default> Default for Arc<T> {
    #[inline]
    fn default() -> Arc<T> {
        Arc::new(Default::default())
    }
}

impl<T: ?Sized + Hash> Hash for Arc<T> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        (**self).hash(state)
    }
}

impl<T> From<T> for Arc<T> {
    #[inline]
    fn from(t: T) -> Self {
        Arc::new(t)
    }
}

impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
    #[inline]
    fn borrow(&self) -> &T {
        &**self
    }
}

impl<T: ?Sized> AsRef<T> for Arc<T> {
    #[inline]
    fn as_ref(&self) -> &T {
        &**self
    }
}

#[cfg(feature = "stable_deref_trait")]
unsafe impl<T: ?Sized> StableDeref for Arc<T> {}
#[cfg(feature = "stable_deref_trait")]
unsafe impl<T: ?Sized> CloneStableDeref for Arc<T> {}

#[cfg(feature = "serde")]
impl<'de, T: Deserialize<'de>> Deserialize<'de> for Arc<T> {
    fn deserialize<D>(deserializer: D) -> Result<Arc<T>, D::Error>
    where
        D: ::serde::de::Deserializer<'de>,
    {
        T::deserialize(deserializer).map(Arc::new)
    }
}

#[cfg(feature = "serde")]
impl<T: Serialize> Serialize for Arc<T> {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: ::serde::ser::Serializer,
    {
        (**self).serialize(serializer)
    }
}