use std::borrow::Borrow;
use std::cell::Cell;
use std::hash::Hash;
use std::hash::Hasher;
use std::marker::PhantomData;
use std::mem::forget;
use std::mem::transmute;
use std::ops::Deref;
use std::ptr::NonNull;

use libc::c_void;

use crate::support::Opaque;
use crate::Data;
use crate::HandleScope;
use crate::Isolate;
use crate::IsolateHandle;

extern "C" {
  fn v8__Local__New(isolate: *mut Isolate, other: *const Data) -> *const Data;
  fn v8__Global__New(isolate: *mut Isolate, data: *const Data) -> *const Data;
  fn v8__Global__NewWeak(
    isolate: *mut Isolate,
    data: *const Data,
    parameter: *const c_void,
    callback: extern "C" fn(*const WeakCallbackInfo),
  ) -> *const Data;
  fn v8__Global__Reset(data: *const Data);
  fn v8__WeakCallbackInfo__GetIsolate(
    this: *const WeakCallbackInfo,
  ) -> *mut Isolate;
  fn v8__WeakCallbackInfo__GetParameter(
    this: *const WeakCallbackInfo,
  ) -> *mut c_void;
  fn v8__WeakCallbackInfo__SetSecondPassCallback(
    this: *const WeakCallbackInfo,
    callback: extern "C" fn(*const WeakCallbackInfo),
  );
}

/// An object reference managed by the v8 garbage collector.
///
/// All objects returned from v8 have to be tracked by the garbage
/// collector so that it knows that the objects are still alive.  Also,
/// because the garbage collector may move objects, it is unsafe to
/// point directly to an object.  Instead, all objects are stored in
/// handles which are known by the garbage collector and updated
/// whenever an object moves.  Handles should always be passed by value
/// (except in cases like out-parameters) and they should never be
/// allocated on the heap.
///
/// There are two types of handles: local and persistent handles.
///
/// Local handles are light-weight and transient and typically used in
/// local operations.  They are managed by HandleScopes. That means that a
/// HandleScope must exist on the stack when they are created and that they are
/// only valid inside of the `HandleScope` active during their creation.
/// For passing a local handle to an outer `HandleScope`, an
/// `EscapableHandleScope` and its `Escape()` method must be used.
///
/// Persistent handles can be used when storing objects across several
/// independent operations and have to be explicitly deallocated when they're no
/// longer used.
///
/// It is safe to extract the object stored in the handle by
/// dereferencing the handle (for instance, to extract the *Object from
/// a Local<Object>); the value will still be governed by a handle
/// behind the scenes and the same rules apply to these values as to
/// their handles.
///
/// Note: Local handles in Rusty V8 differ from the V8 C++ API in that they are
/// never empty. In situations where empty handles are needed, use
/// Option<Local>.
#[repr(C)]
#[derive(Debug)]
pub struct Local<'s, T>(NonNull<T>, PhantomData<&'s ()>);

impl<'s, T> Local<'s, T> {
  /// Construct a new Local from an existing Handle.
  pub fn new(
    scope: &mut HandleScope<'s, ()>,
    handle: impl Handle<Data = T>,
  ) -> Self {
    let HandleInfo { data, host } = handle.get_handle_info();
    host.assert_match_isolate(scope);
    unsafe {
      scope.cast_local(|sd| {
        v8__Local__New(sd.get_isolate_ptr(), data.cast().as_ptr()) as *const T
      })
    }
    .unwrap()
  }

  /// Create a local handle by downcasting from one of its super types.
  /// This function is unsafe because the cast is unchecked.
  pub unsafe fn cast<A>(other: Local<'s, A>) -> Self
  where
    Local<'s, A>: From<Self>,
  {
    transmute(other)
  }

  pub(crate) unsafe fn from_raw(ptr: *const T) -> Option<Self> {
    NonNull::new(ptr as *mut _).map(|nn| Self::from_non_null(nn))
  }

  pub(crate) unsafe fn from_non_null(nn: NonNull<T>) -> Self {
    Self(nn, PhantomData)
  }

  pub(crate) fn as_non_null(self) -> NonNull<T> {
    self.0
  }

  pub(crate) fn slice_into_raw(slice: &[Self]) -> &[*const T] {
    unsafe { &*(slice as *const [Self] as *const [*const T]) }
  }
}

impl<'s, T> Copy for Local<'s, T> {}

impl<'s, T> Clone for Local<'s, T> {
  fn clone(&self) -> Self {
    *self
  }
}

impl<'s, T> Deref for Local<'s, T> {
  type Target = T;
  fn deref(&self) -> &T {
    unsafe { self.0.as_ref() }
  }
}

/// An object reference that is independent of any handle scope. Where
/// a Local handle only lives as long as the HandleScope in which it was
/// allocated, a global handle remains valid until it is explicitly
/// disposed using reset().
///
/// A global handle contains a reference to a storage cell within
/// the V8 engine which holds an object value and which is updated by
/// the garbage collector whenever the object is moved.
#[derive(Debug)]
pub struct Global<T> {
  data: NonNull<T>,
  isolate_handle: IsolateHandle,
}

impl<T> Global<T> {
  /// Construct a new Global from an existing Handle.
  pub fn new(isolate: &mut Isolate, handle: impl Handle<Data = T>) -> Self {
    let HandleInfo { data, host } = handle.get_handle_info();
    host.assert_match_isolate(isolate);
    unsafe { Self::new_raw(isolate, data) }
  }

  /// Implementation helper function that contains the code that can be shared
  /// between `Global::new()` and `Global::clone()`.
  unsafe fn new_raw(isolate: *mut Isolate, data: NonNull<T>) -> Self {
    let data = data.cast().as_ptr();
    let data = v8__Global__New(isolate, data) as *const T;
    let data = NonNull::new_unchecked(data as *mut _);
    let isolate_handle = (*isolate).thread_safe_handle();
    Self {
      data,
      isolate_handle,
    }
  }

  /// Consume this `Global` and return the underlying raw pointer.
  ///
  /// The returned raw pointer must be converted back into a `Global` by using
  /// [`Global::from_raw`], otherwise the V8 value referenced by this global
  /// handle will be pinned on the V8 heap permanently and never get garbage
  /// collected.
  pub fn into_raw(self) -> NonNull<T> {
    let data = self.data;
    forget(self);
    data
  }

  /// Converts a raw pointer created with [`Global::into_raw()`] back to its
  /// original `Global`.
  pub unsafe fn from_raw(isolate: &mut Isolate, data: NonNull<T>) -> Self {
    let isolate_handle = isolate.thread_safe_handle();
    Self {
      data,
      isolate_handle,
    }
  }

  pub fn open<'a>(&'a self, scope: &mut Isolate) -> &'a T {
    Handle::open(self, scope)
  }
}

impl<T> Clone for Global<T> {
  fn clone(&self) -> Self {
    let HandleInfo { data, host } = self.get_handle_info();
    unsafe { Self::new_raw(host.get_isolate().as_mut(), data) }
  }
}

impl<T> Drop for Global<T> {
  fn drop(&mut self) {
    unsafe {
      if self.isolate_handle.get_isolate_ptr().is_null() {
        // This `Global` handle is associated with an `Isolate` that has already
        // been disposed.
      } else {
        // Destroy the storage cell that contains the contents of this Global.
        v8__Global__Reset(self.data.cast().as_ptr())
      }
    }
  }
}

/// An implementation of [`Handle`] that can be constructed unsafely from a
/// reference.
pub(crate) struct UnsafeRefHandle<'a, T> {
  reference: &'a T,
  isolate_handle: IsolateHandle,
}
impl<'a, T> UnsafeRefHandle<'a, T> {
  /// Constructs an `UnsafeRefHandle`.
  ///
  /// # Safety
  ///
  /// `reference` must be derived from a [`Local`] or [`Global`] handle, and its
  /// lifetime must not outlive that handle. Furthermore, `isolate` must be the
  /// isolate associated with the handle (for [`Local`], the current isolate;
  /// for [`Global`], the isolate you would pass to the [`Global::open()`]
  /// method).
  pub unsafe fn new(reference: &'a T, isolate: &mut Isolate) -> Self {
    UnsafeRefHandle {
      reference,
      isolate_handle: isolate.thread_safe_handle(),
    }
  }
}

pub trait Handle: Sized {
  type Data;

  #[doc(hidden)]
  fn get_handle_info(&self) -> HandleInfo<Self::Data>;

  /// Returns a reference to the V8 heap object that this handle represents.
  /// The handle does not get cloned, nor is it converted to a `Local` handle.
  ///
  /// # Panics
  ///
  /// This function panics in the following situations:
  /// - The handle is not hosted by the specified Isolate.
  /// - The Isolate that hosts this handle has been disposed.
  fn open<'a>(&'a self, isolate: &mut Isolate) -> &'a Self::Data {
    let HandleInfo { data, host } = self.get_handle_info();
    host.assert_match_isolate(isolate);
    unsafe { &*data.as_ptr() }
  }

  /// Reads the inner value contained in this handle, _without_ verifying that
  /// the this handle is hosted by the currently active `Isolate`.
  ///
  /// # Safety
  ///
  /// Using a V8 heap object with another `Isolate` than the `Isolate` that
  /// hosts it is not permitted under any circumstance. Doing so leads to
  /// undefined behavior, likely a crash.
  ///
  /// # Panics
  ///
  /// This function panics if the `Isolate` that hosts the handle has been
  /// disposed.
  unsafe fn get_unchecked(&self) -> &Self::Data {
    let HandleInfo { data, host } = self.get_handle_info();
    if let HandleHost::DisposedIsolate = host {
      panic!("attempt to access Handle hosted by disposed Isolate");
    }
    &*data.as_ptr()
  }
}

impl<'s, T> Handle for Local<'s, T> {
  type Data = T;
  fn get_handle_info(&self) -> HandleInfo<T> {
    HandleInfo::new(self.as_non_null(), HandleHost::Scope)
  }
}

impl<'a, 's: 'a, T> Handle for &'a Local<'s, T> {
  type Data = T;
  fn get_handle_info(&self) -> HandleInfo<T> {
    HandleInfo::new(self.as_non_null(), HandleHost::Scope)
  }
}

impl<T> Handle for Global<T> {
  type Data = T;
  fn get_handle_info(&self) -> HandleInfo<T> {
    HandleInfo::new(self.data, (&self.isolate_handle).into())
  }
}

impl<'a, T> Handle for &'a Global<T> {
  type Data = T;
  fn get_handle_info(&self) -> HandleInfo<T> {
    HandleInfo::new(self.data, (&self.isolate_handle).into())
  }
}

impl<'a, T> Handle for UnsafeRefHandle<'a, T> {
  type Data = T;
  fn get_handle_info(&self) -> HandleInfo<T> {
    HandleInfo::new(
      NonNull::from(self.reference),
      (&self.isolate_handle).into(),
    )
  }
}

impl<'a, T> Handle for &'a UnsafeRefHandle<'_, T> {
  type Data = T;
  fn get_handle_info(&self) -> HandleInfo<T> {
    HandleInfo::new(
      NonNull::from(self.reference),
      (&self.isolate_handle).into(),
    )
  }
}

impl<'s, T> Borrow<T> for Local<'s, T> {
  fn borrow(&self) -> &T {
    &**self
  }
}

impl<T> Borrow<T> for Global<T> {
  fn borrow(&self) -> &T {
    let HandleInfo { data, host } = self.get_handle_info();
    if let HandleHost::DisposedIsolate = host {
      panic!("attempt to access Handle hosted by disposed Isolate");
    }
    unsafe { &*data.as_ptr() }
  }
}

impl<'s, T> Eq for Local<'s, T> where T: Eq {}
impl<T> Eq for Global<T> where T: Eq {}

impl<'s, T: Hash> Hash for Local<'s, T> {
  fn hash<H: Hasher>(&self, state: &mut H) {
    (&**self).hash(state)
  }
}

impl<T: Hash> Hash for Global<T> {
  fn hash<H: Hasher>(&self, state: &mut H) {
    unsafe {
      if self.isolate_handle.get_isolate_ptr().is_null() {
        panic!("can't hash Global after its host Isolate has been disposed");
      }
      self.data.as_ref().hash(state);
    }
  }
}

impl<'s, T, Rhs: Handle> PartialEq<Rhs> for Local<'s, T>
where
  T: PartialEq<Rhs::Data>,
{
  fn eq(&self, other: &Rhs) -> bool {
    let i1 = self.get_handle_info();
    let i2 = other.get_handle_info();
    i1.host.match_host(i2.host, None)
      && unsafe { i1.data.as_ref() == i2.data.as_ref() }
  }
}

impl<'s, T, Rhs: Handle> PartialEq<Rhs> for Global<T>
where
  T: PartialEq<Rhs::Data>,
{
  fn eq(&self, other: &Rhs) -> bool {
    let i1 = self.get_handle_info();
    let i2 = other.get_handle_info();
    i1.host.match_host(i2.host, None)
      && unsafe { i1.data.as_ref() == i2.data.as_ref() }
  }
}

#[derive(Copy, Debug, Clone)]
pub struct HandleInfo<T> {
  data: NonNull<T>,
  host: HandleHost,
}

impl<T> HandleInfo<T> {
  fn new(data: NonNull<T>, host: HandleHost) -> Self {
    Self { data, host }
  }
}

#[derive(Copy, Debug, Clone)]
enum HandleHost {
  // Note: the `HandleHost::Scope` variant does not indicate that the handle
  // it applies to is not associated with an `Isolate`. It only means that
  // the handle is a `Local` handle that was unable to provide a pointer to
  // the `Isolate` that hosts it (the handle) and the currently entered
  // scope.
  Scope,
  Isolate(NonNull<Isolate>),
  DisposedIsolate,
}

impl From<&'_ mut Isolate> for HandleHost {
  fn from(isolate: &'_ mut Isolate) -> Self {
    Self::Isolate(NonNull::from(isolate))
  }
}

impl From<&'_ IsolateHandle> for HandleHost {
  fn from(isolate_handle: &IsolateHandle) -> Self {
    NonNull::new(unsafe { isolate_handle.get_isolate_ptr() })
      .map(Self::Isolate)
      .unwrap_or(Self::DisposedIsolate)
  }
}

impl HandleHost {
  /// Compares two `HandleHost` values, returning `true` if they refer to the
  /// same `Isolate`, or `false` if they refer to different isolates.
  ///
  /// If the caller knows which `Isolate` the currently entered scope (if any)
  /// belongs to, it should pass on this information via the second argument
  /// (`scope_isolate_opt`).
  ///
  /// # Panics
  ///
  /// This function panics if one of the `HandleHost` values refers to an
  /// `Isolate` that has been disposed.
  ///
  /// # Safety / Bugs
  ///
  /// The current implementation is a bit too forgiving. If it cannot decide
  /// whether two hosts refer to the same `Isolate`, it just returns `true`.
  /// Note that this can only happen when the caller does _not_ provide a value
  /// for the `scope_isolate_opt` argument.
  fn match_host(
    self,
    other: Self,
    scope_isolate_opt: Option<&mut Isolate>,
  ) -> bool {
    let scope_isolate_opt_nn = scope_isolate_opt.map(NonNull::from);
    match (self, other, scope_isolate_opt_nn) {
      (Self::Scope, Self::Scope, _) => true,
      (Self::Isolate(ile1), Self::Isolate(ile2), _) => ile1 == ile2,
      (Self::Scope, Self::Isolate(ile1), Some(ile2)) => ile1 == ile2,
      (Self::Isolate(ile1), Self::Scope, Some(ile2)) => ile1 == ile2,
      // TODO(pisciaureus): If the caller didn't provide a `scope_isolate_opt`
      // value that works, we can't do a meaningful check. So all we do for now
      // is pretend the Isolates match and hope for the best. This eventually
      // needs to be tightened up.
      (Self::Scope, Self::Isolate(_), _) => true,
      (Self::Isolate(_), Self::Scope, _) => true,
      // Handles hosted in an Isolate that has been disposed aren't good for
      // anything, even if a pair of handles used to to be hosted in the same
      // now-disposed solate.
      (Self::DisposedIsolate, ..) | (_, Self::DisposedIsolate, _) => {
        panic!("attempt to access Handle hosted by disposed Isolate")
      }
    }
  }

  fn assert_match_host(self, other: Self, scope_opt: Option<&mut Isolate>) {
    assert!(
      self.match_host(other, scope_opt),
      "attempt to use Handle in an Isolate that is not its host"
    )
  }

  #[allow(dead_code)]
  fn match_isolate(self, isolate: &mut Isolate) -> bool {
    self.match_host(isolate.into(), Some(isolate))
  }

  fn assert_match_isolate(self, isolate: &mut Isolate) {
    self.assert_match_host(isolate.into(), Some(isolate))
  }

  fn get_isolate(self) -> NonNull<Isolate> {
    match self {
      Self::Scope => panic!("host Isolate for Handle not available"),
      Self::Isolate(ile) => ile,
      Self::DisposedIsolate => panic!("attempt to access disposed Isolate"),
    }
  }

  #[allow(dead_code)]
  fn get_isolate_handle(self) -> IsolateHandle {
    unsafe { self.get_isolate().as_ref() }.thread_safe_handle()
  }
}

/// An object reference that does not prevent garbage collection for the object,
/// and which allows installing finalization callbacks which will be called
/// after the object has been GC'd.
///
/// Note that finalization callbacks are tied to the lifetime of a `Weak<T>`,
/// and will not be called after the `Weak<T>` is dropped.
///
/// # `Clone`
///
/// Since finalization callbacks are specific to a `Weak<T>` instance, cloning
/// will create a new object reference without a finalizer, as if created by
/// [`Self::new`]. You can use [`Self::clone_with_finalizer`] to attach a
/// finalization callback to the clone.
#[derive(Debug)]
pub struct Weak<T> {
  data: Option<Box<WeakData<T>>>,
  isolate_handle: IsolateHandle,
}

impl<T> Weak<T> {
  pub fn new(isolate: &mut Isolate, handle: impl Handle<Data = T>) -> Self {
    let HandleInfo { data, host } = handle.get_handle_info();
    host.assert_match_isolate(isolate);
    Self::new_raw(isolate, data, None)
  }

  /// Create a weak handle with a finalization callback installed.
  ///
  /// There is no guarantee as to *when* the finalization callback will be
  /// invoked. However, unlike the C++ API, this API guarantees that when an
  /// isolate is destroyed, any finalizers that haven't been called yet will be
  /// run, unless a [`Global`] reference is keeping the object alive. Other than
  /// that, there is still no guarantee as to when the finalizers will be
  /// called.
  ///
  /// The callback does not have access to the inner value, because it has
  /// already been collected by the time it runs.
  pub fn with_finalizer(
    isolate: &mut Isolate,
    handle: impl Handle<Data = T>,
    finalizer: Box<dyn FnOnce(&mut Isolate)>,
  ) -> Self {
    let HandleInfo { data, host } = handle.get_handle_info();
    host.assert_match_isolate(isolate);
    let finalizer_id = isolate.get_finalizer_map_mut().add(finalizer);
    Self::new_raw(isolate, data, Some(finalizer_id))
  }

  fn new_raw(
    isolate: *mut Isolate,
    data: NonNull<T>,
    finalizer_id: Option<FinalizerId>,
  ) -> Self {
    let weak_data = Box::new(WeakData {
      pointer: Default::default(),
      finalizer_id,
      weak_dropped: Cell::new(false),
    });
    let data = data.cast().as_ptr();
    let data = unsafe {
      v8__Global__NewWeak(
        isolate,
        data,
        weak_data.deref() as *const _ as *const c_void,
        Self::first_pass_callback,
      )
    };
    weak_data
      .pointer
      .set(Some(unsafe { NonNull::new_unchecked(data as *mut _) }));
    Self {
      data: Some(weak_data),
      isolate_handle: unsafe { (*isolate).thread_safe_handle() },
    }
  }

  /// Creates a new empty handle, identical to one for an object that has
  /// already been GC'd.
  pub fn empty(isolate: &mut Isolate) -> Self {
    Weak {
      data: None,
      isolate_handle: isolate.thread_safe_handle(),
    }
  }

  /// Clones this handle and installs a finalizer callback on the clone, as if
  /// by calling [`Self::with_finalizer`].
  ///
  /// Note that if this handle is empty (its value has already been GC'd), the
  /// finalization callback will never run.
  pub fn clone_with_finalizer(
    &self,
    finalizer: Box<dyn FnOnce(&mut Isolate)>,
  ) -> Self {
    self.clone_raw(Some(finalizer))
  }

  fn clone_raw(
    &self,
    finalizer: Option<Box<dyn FnOnce(&mut Isolate)>>,
  ) -> Self {
    if let Some(data) = self.get_pointer() {
      // SAFETY: We're in the isolate's thread, because Weak<T> isn't Send or
      // Sync.
      let isolate_ptr = unsafe { self.isolate_handle.get_isolate_ptr() };
      if isolate_ptr.is_null() {
        unreachable!("Isolate was dropped but weak handle wasn't reset.");
      }

      let finalizer_id = if let Some(finalizer) = finalizer {
        let isolate = unsafe { &mut *isolate_ptr };
        Some(isolate.get_finalizer_map_mut().add(finalizer))
      } else {
        None
      };
      Self::new_raw(isolate_ptr, data, finalizer_id)
    } else {
      Weak {
        data: None,
        isolate_handle: self.isolate_handle.clone(),
      }
    }
  }

  /// Converts an optional raw pointer created with [`Weak::into_raw()`] back to
  /// its original `Weak`.
  ///
  /// This method is called with `Some`, the pointer is invalidated and it
  /// cannot be used with this method again. Additionally, it is unsound to call
  /// this method with an isolate other than that in which the original `Weak`
  /// was created.
  pub unsafe fn from_raw(
    isolate: &mut Isolate,
    data: Option<NonNull<WeakData<T>>>,
  ) -> Self {
    Weak {
      data: data.map(|raw| Box::from_raw(raw.cast().as_ptr())),
      isolate_handle: isolate.thread_safe_handle(),
    }
  }

  /// Consume this `Weak` handle and return the underlying raw pointer, or
  /// `None` if the value has been GC'd.
  ///
  /// The return value can be converted back into a `Weak` by using
  /// [`Weak::from_raw`]. Note that `Weak` allocates some memory, and if this
  /// method returns `Some`, the pointer must be converted back into a `Weak`
  /// for it to be freed.
  ///
  /// Note that this method might return `Some` even after the V8 value has been
  /// GC'd.
  pub fn into_raw(mut self) -> Option<NonNull<WeakData<T>>> {
    if let Some(data) = self.data.take() {
      let has_finalizer = if let Some(finalizer_id) = data.finalizer_id {
        // SAFETY: We're in the isolate's thread because Weak isn't Send or Sync
        let isolate_ptr = unsafe { self.isolate_handle.get_isolate_ptr() };
        if isolate_ptr.is_null() {
          // Disposed isolates have no finalizers.
          false
        } else {
          let isolate = unsafe { &mut *isolate_ptr };
          isolate.get_finalizer_map().map.contains_key(&finalizer_id)
        }
      } else {
        false
      };

      if data.pointer.get().is_none() && !has_finalizer {
        // If the pointer is None and we're not waiting for the second pass,
        // drop the box and return None.
        None
      } else {
        assert!(!data.weak_dropped.get());
        Some(unsafe { NonNull::new_unchecked(Box::into_raw(data)) })
      }
    } else {
      None
    }
  }

  fn get_pointer(&self) -> Option<NonNull<T>> {
    if let Some(data) = &self.data {
      // It seems like when the isolate is dropped, even the first pass callback
      // might not be called.
      if unsafe { self.isolate_handle.get_isolate_ptr() }.is_null() {
        None
      } else {
        data.pointer.get()
      }
    } else {
      None
    }
  }

  pub fn is_empty(&self) -> bool {
    self.get_pointer().is_none()
  }

  pub fn to_global(&self, isolate: &mut Isolate) -> Option<Global<T>> {
    if let Some(data) = self.get_pointer() {
      let handle_host: HandleHost = (&self.isolate_handle).into();
      handle_host.assert_match_isolate(isolate);
      Some(unsafe { Global::new_raw(isolate, data) })
    } else {
      None
    }
  }

  pub fn to_local<'s>(
    &self,
    scope: &mut HandleScope<'s, ()>,
  ) -> Option<Local<'s, T>> {
    if let Some(data) = self.get_pointer() {
      let handle_host: HandleHost = (&self.isolate_handle).into();
      handle_host.assert_match_isolate(scope);
      let local = unsafe {
        scope.cast_local(|sd| {
          v8__Local__New(sd.get_isolate_ptr(), data.cast().as_ptr()) as *const T
        })
      };
      Some(local.unwrap())
    } else {
      None
    }
  }

  // Finalization callbacks.

  extern "C" fn first_pass_callback(wci: *const WeakCallbackInfo) {
    // SAFETY: If this callback is called, then the weak handle hasn't been
    // reset, which means the `Weak` instance which owns the pinned box that the
    // parameter points to hasn't been dropped.
    let weak_data = unsafe {
      let ptr = v8__WeakCallbackInfo__GetParameter(wci);
      &*(ptr as *mut WeakData<T>)
    };

    let data = weak_data.pointer.take().unwrap();
    unsafe {
      v8__Global__Reset(data.cast().as_ptr());
    }

    // Only set the second pass callback if there could be a finalizer.
    if weak_data.finalizer_id.is_some() {
      unsafe {
        v8__WeakCallbackInfo__SetSecondPassCallback(
          wci,
          Self::second_pass_callback,
        )
      };
    }
  }

  extern "C" fn second_pass_callback(wci: *const WeakCallbackInfo) {
    // SAFETY: This callback is guaranteed by V8 to be called in the isolate's
    // thread before the isolate is disposed.
    let isolate = unsafe { &mut *v8__WeakCallbackInfo__GetIsolate(wci) };

    // SAFETY: This callback might be called well after the first pass callback,
    // which means the corresponding Weak might have been dropped. In Weak's
    // Drop impl we make sure that if the second pass callback hasn't yet run, the
    // Box<WeakData<T>> is leaked, so it will still be alive by the time this
    // callback is called.
    let weak_data = unsafe {
      let ptr = v8__WeakCallbackInfo__GetParameter(wci);
      &*(ptr as *mut WeakData<T>)
    };
    let finalizer: Option<Box<dyn FnOnce(&mut Isolate)>> = {
      let finalizer_id = weak_data.finalizer_id.unwrap();
      isolate.get_finalizer_map_mut().map.remove(&finalizer_id)
    };

    if weak_data.weak_dropped.get() {
      // SAFETY: If weak_dropped is true, the corresponding Weak has been dropped,
      // so it's safe to take ownership of the Box<WeakData<T>> and drop it.
      let _ = unsafe {
        Box::from_raw(weak_data as *const WeakData<T> as *mut WeakData<T>)
      };
    }

    if let Some(finalizer) = finalizer {
      finalizer(isolate);
    }
  }
}

impl<T> Clone for Weak<T> {
  fn clone(&self) -> Self {
    self.clone_raw(None)
  }
}

impl<T> Drop for Weak<T> {
  fn drop(&mut self) {
    // Returns whether the finalizer existed.
    let remove_finalizer = |finalizer_id: Option<FinalizerId>| -> bool {
      if let Some(finalizer_id) = finalizer_id {
        // SAFETY: We're in the isolate's thread because `Weak` isn't Send or Sync.
        let isolate_ptr = unsafe { self.isolate_handle.get_isolate_ptr() };
        if !isolate_ptr.is_null() {
          let isolate = unsafe { &mut *isolate_ptr };
          let finalizer =
            isolate.get_finalizer_map_mut().map.remove(&finalizer_id);
          return finalizer.is_some();
        }
      }
      false
    };

    if let Some(data) = self.get_pointer() {
      // If the pointer is not None, the first pass callback hasn't been
      // called yet, and resetting will prevent it from being called.
      unsafe { v8__Global__Reset(data.cast().as_ptr()) };
      remove_finalizer(self.data.as_ref().unwrap().finalizer_id);
    } else if let Some(weak_data) = self.data.take() {
      // The second pass callback removes the finalizer, so if there is one,
      // the second pass hasn't yet run, and WeakData will have to be alive.
      // In that case we leak the WeakData but remove the finalizer.
      if remove_finalizer(weak_data.finalizer_id) {
        weak_data.weak_dropped.set(true);
        Box::leak(weak_data);
      }
    }
  }
}

impl<T> Eq for Weak<T> where T: Eq {}

impl<T, Rhs: Handle> PartialEq<Rhs> for Weak<T>
where
  T: PartialEq<Rhs::Data>,
{
  fn eq(&self, other: &Rhs) -> bool {
    let HandleInfo {
      data: other_data,
      host: other_host,
    } = other.get_handle_info();
    let self_host: HandleHost = (&self.isolate_handle).into();
    if !self_host.match_host(other_host, None) {
      false
    } else if let Some(self_data) = self.get_pointer() {
      unsafe { self_data.as_ref() == other_data.as_ref() }
    } else {
      false
    }
  }
}

impl<T, T2> PartialEq<Weak<T2>> for Weak<T>
where
  T: PartialEq<T2>,
{
  fn eq(&self, other: &Weak<T2>) -> bool {
    let self_host: HandleHost = (&self.isolate_handle).into();
    let other_host: HandleHost = (&other.isolate_handle).into();
    if !self_host.match_host(other_host, None) {
      return false;
    }
    match (self.get_pointer(), other.get_pointer()) {
      (Some(self_data), Some(other_data)) => unsafe {
        self_data.as_ref() == other_data.as_ref()
      },
      (None, None) => true,
      _ => false,
    }
  }
}

/// The inner mechanism behind [`Weak`] and finalizations.
///
/// This struct is heap-allocated and will not move until it's dropped, so it
/// can be accessed by the finalization callbacks by creating a shared reference
/// from a pointer. The fields are wrapped in [`Cell`] so they are modifiable by
/// both the [`Weak`] and the finalization callbacks.
pub struct WeakData<T> {
  pointer: Cell<Option<NonNull<T>>>,
  finalizer_id: Option<FinalizerId>,
  weak_dropped: Cell<bool>,
}

impl<T> std::fmt::Debug for WeakData<T> {
  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
    f.debug_struct("WeakData")
      .field("pointer", &self.pointer)
      .finish_non_exhaustive()
  }
}

#[repr(C)]
struct WeakCallbackInfo(Opaque);

type FinalizerId = usize;

#[derive(Default)]
pub(crate) struct FinalizerMap {
  map: std::collections::HashMap<FinalizerId, Box<dyn FnOnce(&mut Isolate)>>,
  next_id: FinalizerId,
}

impl FinalizerMap {
  pub(crate) fn add(
    &mut self,
    finalizer: Box<dyn FnOnce(&mut Isolate)>,
  ) -> FinalizerId {
    let id = self.next_id;
    // TODO: Overflow.
    self.next_id += 1;
    self.map.insert(id, finalizer);
    id
  }

  pub(crate) fn is_empty(&self) -> bool {
    self.map.is_empty()
  }
}