/*! Allocating resource ids, and tracking the resources they refer to.

The `wgpu_core` API uses identifiers of type [`Id<R>`] to refer to
resources of type `R`. For example, [`id::DeviceId`] is an alias for
`Id<Device<Empty>>`, and [`id::BufferId`] is an alias for
`Id<Buffer<Empty>>`. `Id` implements `Copy`, `Hash`, `Eq`, `Ord`, and
of course `Debug`.

Each `Id` contains not only an index for the resource it denotes but
also a [`Backend`] indicating which `wgpu` backend it belongs to. You
can use the [`gfx_select`] macro to dynamically dispatch on an id's
backend to a function specialized at compile time for a specific
backend. See that macro's documentation for details.

`Id`s also incorporate a generation number, for additional validation.

The resources to which identifiers refer are freed explicitly.
Attempting to use an identifier for a resource that has been freed
elicits an error result.

## Assigning ids to resources

The users of `wgpu_core` generally want resource ids to be assigned
in one of two ways:

- Users like `wgpu` want `wgpu_core` to assign ids to resources itself.
  For example, `wgpu` expects to call `Global::device_create_buffer`
  and have the return value indicate the newly created buffer's id.

- Users like `player` and Firefox want to allocate ids themselves, and
  pass `Global::device_create_buffer` and friends the id to assign the
  new resource.

To accommodate either pattern, `wgpu_core` methods that create
resources all expect an `id_in` argument that the caller can use to
specify the id, and they all return the id used. For example, the
declaration of `Global::device_create_buffer` looks like this:

```ignore
impl<G: GlobalIdentityHandlerFactory> Global<G> {
    /* ... */
    pub fn device_create_buffer<A: HalApi>(
        &self,
        device_id: id::DeviceId,
        desc: &resource::BufferDescriptor,
        id_in: Input<G, id::BufferId>,
    ) -> (id::BufferId, Option<resource::CreateBufferError>) {
        /* ... */
    }
    /* ... */
}
```

Users that want to assign resource ids themselves pass in the id they
want as the `id_in` argument, whereas users that want `wgpu_core`
itself to choose ids always pass `()`. In either case, the id
ultimately assigned is returned as the first element of the tuple.

Producing true identifiers from `id_in` values is the job of an
[`IdentityHandler`] implementation, which has an associated type
[`Input`] saying what type of `id_in` values it accepts, and a
[`process`] method that turns such values into true identifiers of
type `I`. There are two kinds of `IdentityHandler`s:

- Users that want `wgpu_core` to assign ids generally use
  [`IdentityManager`] ([wrapped in a mutex]). Its `Input` type is
  `()`, and it tracks assigned ids and generation numbers as
  necessary. (This is what `wgpu` does.)

- Users that want to assign ids themselves use an `IdentityHandler`
  whose `Input` type is `I` itself, and whose `process` method simply
  passes the `id_in` argument through unchanged. For example, the
  `player` crate uses an `IdentityPassThrough` type whose `process`
  method simply adjusts the id's backend (since recordings can be
  replayed on a different backend than the one they were created on)
  but passes the rest of the id's content through unchanged.

Because an `IdentityHandler<I>` can only create ids for a single
resource type `I`, constructing a [`Global`] entails constructing a
separate `IdentityHandler<I>` for each resource type `I` that the
`Global` will manage: an `IdentityHandler<DeviceId>`, an
`IdentityHandler<TextureId>`, and so on.

The [`Global::new`] function could simply take a large collection of
`IdentityHandler<I>` implementations as arguments, but that would be
ungainly. Instead, `Global::new` expects a `factory` argument that
implements the [`GlobalIdentityHandlerFactory`] trait, which extends
[`IdentityHandlerFactory<I>`] for each resource id type `I`. This
trait, in turn, has a `spawn` method that constructs an
`IdentityHandler<I>` for the `Global` to use.

What this means is that the types of resource creation functions'
`id_in` arguments depend on the `Global`'s `G` type parameter. A
`Global<G>`'s `IdentityHandler<I>` implementation is:

```ignore
<G as IdentityHandlerFactory<I>>::Filter
```

where `Filter` is an associated type of the `IdentityHandlerFactory` trait.
Thus, its `id_in` type is:

```ignore
<<G as IdentityHandlerFactory<I>>::Filter as IdentityHandler<I>>::Input
```

The [`Input<G, I>`] type is an alias for this construction.

## Id allocation and streaming

Perhaps surprisingly, allowing users to assign resource ids themselves
enables major performance improvements in some applications.

The `wgpu_core` API is designed for use by Firefox's [WebGPU]
implementation. For security, web content and GPU use must be kept
segregated in separate processes, with all interaction between them
mediated by an inter-process communication protocol. As web content uses
the WebGPU API, the content process sends messages to the GPU process,
which interacts with the platform's GPU APIs on content's behalf,
occasionally sending results back.

In a classic Rust API, a resource allocation function takes parameters
describing the resource to create, and if creation succeeds, it returns
the resource id in a `Result::Ok` value. However, this design is a poor
fit for the split-process design described above: content must wait for
the reply to its buffer-creation message (say) before it can know which
id it can use in the next message that uses that buffer. On a common
usage pattern, the classic Rust design imposes the latency of a full
cross-process round trip.

We can avoid incurring these round-trip latencies simply by letting the
content process assign resource ids itself. With this approach, content
can choose an id for the new buffer, send a message to create the
buffer, and then immediately send the next message operating on that
buffer, since it already knows its id. Allowing content and GPU process
activity to be pipelined greatly improves throughput.

To help propagate errors correctly in this style of usage, when resource
creation fails, the id supplied for that resource is marked to indicate
as much, allowing subsequent operations using that id to be properly
flagged as errors as well.

[`gfx_select`]: crate::gfx_select
[`Input`]: IdentityHandler::Input
[`process`]: IdentityHandler::process
[`Id<R>`]: crate::id::Id
[wrapped in a mutex]: trait.IdentityHandler.html#impl-IdentityHandler%3CI%3E-for-Mutex%3CIdentityManager%3E
[WebGPU]: https://www.w3.org/TR/webgpu/

*/

use crate::{
    binding_model::{BindGroup, BindGroupLayout, PipelineLayout},
    command::{CommandBuffer, RenderBundle},
    device::Device,
    id,
    instance::{Adapter, HalSurface, Instance, Surface},
    pipeline::{ComputePipeline, RenderPipeline, ShaderModule},
    resource::{Buffer, QuerySet, Sampler, StagingBuffer, Texture, TextureClearMode, TextureView},
    Epoch, Index,
};

use parking_lot::{Mutex, RwLock, RwLockReadGuard, RwLockWriteGuard};
use wgt::Backend;

#[cfg(debug_assertions)]
use std::cell::Cell;
use std::{fmt::Debug, marker::PhantomData, mem, ops};

/// A simple structure to allocate [`Id`] identifiers.
///
/// Calling [`alloc`] returns a fresh, never-before-seen id. Calling [`free`]
/// marks an id as dead; it will never be returned again by `alloc`.
///
/// Use `IdentityManager::default` to construct new instances.
///
/// `IdentityManager` returns `Id`s whose index values are suitable for use as
/// indices into a `Storage<T>` that holds those ids' referents:
///
/// - Every live id has a distinct index value. Each live id's index selects a
///   distinct element in the vector.
///
/// - `IdentityManager` prefers low index numbers. If you size your vector to
///   accommodate the indices produced here, the vector's length will reflect
///   the highwater mark of actual occupancy.
///
/// - `IdentityManager` reuses the index values of freed ids before returning
///   ids with new index values. Freed vector entries get reused.
///
/// See the module-level documentation for an overview of how this
/// fits together.
///
/// [`Id`]: crate::id::Id
/// [`Backend`]: wgt::Backend;
/// [`alloc`]: IdentityManager::alloc
/// [`free`]: IdentityManager::free
#[derive(Debug, Default)]
pub struct IdentityManager {
    /// Available index values. If empty, then `epochs.len()` is the next index
    /// to allocate.
    free: Vec<Index>,

    /// The next or currently-live epoch value associated with each `Id` index.
    ///
    /// If there is a live id with index `i`, then `epochs[i]` is its epoch; any
    /// id with the same index but an older epoch is dead.
    ///
    /// If index `i` is currently unused, `epochs[i]` is the epoch to use in its
    /// next `Id`.
    epochs: Vec<Epoch>,
}

impl IdentityManager {
    /// Allocate a fresh, never-before-seen id with the given `backend`.
    ///
    /// The backend is incorporated into the id, so that ids allocated with
    /// different `backend` values are always distinct.
    pub fn alloc<I: id::TypedId>(&mut self, backend: Backend) -> I {
        match self.free.pop() {
            Some(index) => I::zip(index, self.epochs[index as usize], backend),
            None => {
                let epoch = 1;
                let id = I::zip(self.epochs.len() as Index, epoch, backend);
                self.epochs.push(epoch);
                id
            }
        }
    }

    /// Free `id`. It will never be returned from `alloc` again.
    pub fn free<I: id::TypedId + Debug>(&mut self, id: I) {
        let (index, epoch, _backend) = id.unzip();
        let pe = &mut self.epochs[index as usize];
        assert_eq!(*pe, epoch);
        // If the epoch reaches EOL, the index doesn't go
        // into the free list, will never be reused again.
        if epoch < id::EPOCH_MASK {
            *pe = epoch + 1;
            self.free.push(index);
        }
    }
}

/// An entry in a `Storage::map` table.
#[derive(Debug)]
enum Element<T> {
    /// There are no live ids with this index.
    Vacant,

    /// There is one live id with this index, allocated at the given
    /// epoch.
    Occupied(T, Epoch),

    /// Like `Occupied`, but an error occurred when creating the
    /// resource.
    ///
    /// The given `String` is the resource's descriptor label.
    Error(Epoch, String),
}

#[derive(Clone, Debug, Default)]
pub struct StorageReport {
    pub num_occupied: usize,
    pub num_vacant: usize,
    pub num_error: usize,
    pub element_size: usize,
}

impl StorageReport {
    pub fn is_empty(&self) -> bool {
        self.num_occupied + self.num_vacant + self.num_error == 0
    }
}

#[derive(Clone, Debug)]
pub(crate) struct InvalidId;

/// A table of `T` values indexed by the id type `I`.
///
/// The table is represented as a vector indexed by the ids' index
/// values, so you should use an id allocator like `IdentityManager`
/// that keeps the index values dense and close to zero.
#[derive(Debug)]
pub struct Storage<T, I: id::TypedId> {
    map: Vec<Element<T>>,
    kind: &'static str,
    _phantom: PhantomData<I>,
}

impl<T, I: id::TypedId> ops::Index<id::Valid<I>> for Storage<T, I> {
    type Output = T;
    fn index(&self, id: id::Valid<I>) -> &T {
        self.get(id.0).unwrap()
    }
}

impl<T, I: id::TypedId> ops::IndexMut<id::Valid<I>> for Storage<T, I> {
    fn index_mut(&mut self, id: id::Valid<I>) -> &mut T {
        self.get_mut(id.0).unwrap()
    }
}

impl<T, I: id::TypedId> Storage<T, I> {
    pub(crate) fn contains(&self, id: I) -> bool {
        let (index, epoch, _) = id.unzip();
        match self.map.get(index as usize) {
            Some(&Element::Vacant) => false,
            Some(&Element::Occupied(_, storage_epoch) | &Element::Error(storage_epoch, _)) => {
                storage_epoch == epoch
            }
            None => false,
        }
    }

    /// Attempts to get a reference to an item behind a potentially invalid ID.
    ///
    /// Returns [`None`] if there is an epoch mismatch, or the entry is empty.
    ///
    /// This function is primarily intended for the `as_hal` family of functions
    /// where you may need to fallibly get a object backed by an id that could
    /// be in a different hub.
    pub(crate) fn try_get(&self, id: I) -> Result<Option<&T>, InvalidId> {
        let (index, epoch, _) = id.unzip();
        let (result, storage_epoch) = match self.map.get(index as usize) {
            Some(&Element::Occupied(ref v, epoch)) => (Ok(Some(v)), epoch),
            Some(&Element::Vacant) => return Ok(None),
            Some(&Element::Error(epoch, ..)) => (Err(InvalidId), epoch),
            None => return Err(InvalidId),
        };
        assert_eq!(
            epoch, storage_epoch,
            "{}[{}] is no longer alive",
            self.kind, index
        );
        result
    }

    /// Get a reference to an item behind a potentially invalid ID.
    /// Panics if there is an epoch mismatch, or the entry is empty.
    pub(crate) fn get(&self, id: I) -> Result<&T, InvalidId> {
        let (index, epoch, _) = id.unzip();
        let (result, storage_epoch) = match self.map.get(index as usize) {
            Some(&Element::Occupied(ref v, epoch)) => (Ok(v), epoch),
            Some(&Element::Vacant) => panic!("{}[{}] does not exist", self.kind, index),
            Some(&Element::Error(epoch, ..)) => (Err(InvalidId), epoch),
            None => return Err(InvalidId),
        };
        assert_eq!(
            epoch, storage_epoch,
            "{}[{}] is no longer alive",
            self.kind, index
        );
        result
    }

    /// Get a mutable reference to an item behind a potentially invalid ID.
    /// Panics if there is an epoch mismatch, or the entry is empty.
    pub(crate) fn get_mut(&mut self, id: I) -> Result<&mut T, InvalidId> {
        let (index, epoch, _) = id.unzip();
        let (result, storage_epoch) = match self.map.get_mut(index as usize) {
            Some(&mut Element::Occupied(ref mut v, epoch)) => (Ok(v), epoch),
            Some(&mut Element::Vacant) | None => panic!("{}[{}] does not exist", self.kind, index),
            Some(&mut Element::Error(epoch, ..)) => (Err(InvalidId), epoch),
        };
        assert_eq!(
            epoch, storage_epoch,
            "{}[{}] is no longer alive",
            self.kind, index
        );
        result
    }

    pub(crate) unsafe fn get_unchecked(&self, id: u32) -> &T {
        match self.map[id as usize] {
            Element::Occupied(ref v, _) => v,
            Element::Vacant => panic!("{}[{}] does not exist", self.kind, id),
            Element::Error(_, _) => panic!(""),
        }
    }

    pub(crate) fn label_for_invalid_id(&self, id: I) -> &str {
        let (index, _, _) = id.unzip();
        match self.map.get(index as usize) {
            Some(&Element::Error(_, ref label)) => label,
            _ => "",
        }
    }

    fn insert_impl(&mut self, index: usize, element: Element<T>) {
        if index >= self.map.len() {
            self.map.resize_with(index + 1, || Element::Vacant);
        }
        match std::mem::replace(&mut self.map[index], element) {
            Element::Vacant => {}
            _ => panic!("Index {index:?} is already occupied"),
        }
    }

    pub(crate) fn insert(&mut self, id: I, value: T) {
        let (index, epoch, _) = id.unzip();
        self.insert_impl(index as usize, Element::Occupied(value, epoch))
    }

    pub(crate) fn insert_error(&mut self, id: I, label: &str) {
        let (index, epoch, _) = id.unzip();
        self.insert_impl(index as usize, Element::Error(epoch, label.to_string()))
    }

    pub(crate) fn force_replace(&mut self, id: I, value: T) {
        let (index, epoch, _) = id.unzip();
        self.map[index as usize] = Element::Occupied(value, epoch);
    }

    pub(crate) fn remove(&mut self, id: I) -> Option<T> {
        let (index, epoch, _) = id.unzip();
        match std::mem::replace(&mut self.map[index as usize], Element::Vacant) {
            Element::Occupied(value, storage_epoch) => {
                assert_eq!(epoch, storage_epoch);
                Some(value)
            }
            Element::Error(..) => None,
            Element::Vacant => panic!("Cannot remove a vacant resource"),
        }
    }

    // Prevents panic on out of range access, allows Vacant elements.
    pub(crate) fn _try_remove(&mut self, id: I) -> Option<T> {
        let (index, epoch, _) = id.unzip();
        if index as usize >= self.map.len() {
            None
        } else if let Element::Occupied(value, storage_epoch) =
            std::mem::replace(&mut self.map[index as usize], Element::Vacant)
        {
            assert_eq!(epoch, storage_epoch);
            Some(value)
        } else {
            None
        }
    }

    pub(crate) fn iter(&self, backend: Backend) -> impl Iterator<Item = (I, &T)> {
        self.map
            .iter()
            .enumerate()
            .filter_map(move |(index, x)| match *x {
                Element::Occupied(ref value, storage_epoch) => {
                    Some((I::zip(index as Index, storage_epoch, backend), value))
                }
                _ => None,
            })
    }

    pub(crate) fn len(&self) -> usize {
        self.map.len()
    }

    fn generate_report(&self) -> StorageReport {
        let mut report = StorageReport {
            element_size: mem::size_of::<T>(),
            ..Default::default()
        };
        for element in self.map.iter() {
            match *element {
                Element::Occupied(..) => report.num_occupied += 1,
                Element::Vacant => report.num_vacant += 1,
                Element::Error(..) => report.num_error += 1,
            }
        }
        report
    }
}

/// Type system for enforcing the lock order on [`Hub`] fields.
///
/// If type `A` implements `Access<B>`, that means we are allowed to
/// proceed with locking resource `B` after we lock `A`.
///
/// The implementations of `Access` basically describe the edges in an
/// acyclic directed graph of lock transitions. As long as it doesn't have
/// cycles, any number of threads can acquire locks along paths through
/// the graph without deadlock. That is, if you look at each thread's
/// lock acquisitions as steps along a path in the graph, then because
/// there are no cycles in the graph, there must always be some thread
/// that is able to acquire its next lock, or that is about to release
/// a lock. (Assume that no thread just sits on its locks forever.)
///
/// Locks must be acquired in the following order:
///
/// - [`Adapter`]
/// - [`Device`]
/// - [`CommandBuffer`]
/// - [`RenderBundle`]
/// - [`PipelineLayout`]
/// - [`BindGroupLayout`]
/// - [`BindGroup`]
/// - [`ComputePipeline`]
/// - [`RenderPipeline`]
/// - [`ShaderModule`]
/// - [`Buffer`]
/// - [`StagingBuffer`]
/// - [`Texture`]
/// - [`TextureView`]
/// - [`Sampler`]
/// - [`QuerySet`]
///
/// That is, you may only acquire a new lock on a `Hub` field if it
/// appears in the list after all the other fields you're already
/// holding locks for. When you are holding no locks, you can start
/// anywhere.
///
/// It's fine to add more `Access` implementations as needed, as long
/// as you do not introduce a cycle. In other words, as long as there
/// is some ordering you can put the resource types in that respects
/// the extant `Access` implementations, that's fine.
///
/// See the documentation for [`Hub`] for more details.
pub trait Access<A> {}

pub enum Root {}

// These impls are arranged so that the target types (that is, the `T`
// in `Access<T>`) appear in locking order.
//
// TODO: establish an order instead of declaring all the pairs.
impl Access<Instance> for Root {}
impl Access<Surface> for Root {}
impl Access<Surface> for Instance {}
impl<A: HalApi> Access<Adapter<A>> for Root {}
impl<A: HalApi> Access<Adapter<A>> for Surface {}
impl<A: HalApi> Access<Device<A>> for Root {}
impl<A: HalApi> Access<Device<A>> for Surface {}
impl<A: HalApi> Access<Device<A>> for Adapter<A> {}
impl<A: HalApi> Access<CommandBuffer<A>> for Root {}
impl<A: HalApi> Access<CommandBuffer<A>> for Device<A> {}
impl<A: HalApi> Access<RenderBundle<A>> for Device<A> {}
impl<A: HalApi> Access<RenderBundle<A>> for CommandBuffer<A> {}
impl<A: HalApi> Access<PipelineLayout<A>> for Root {}
impl<A: HalApi> Access<PipelineLayout<A>> for Device<A> {}
impl<A: HalApi> Access<PipelineLayout<A>> for RenderBundle<A> {}
impl<A: HalApi> Access<BindGroupLayout<A>> for Root {}
impl<A: HalApi> Access<BindGroupLayout<A>> for Device<A> {}
impl<A: HalApi> Access<BindGroupLayout<A>> for PipelineLayout<A> {}
impl<A: HalApi> Access<BindGroup<A>> for Root {}
impl<A: HalApi> Access<BindGroup<A>> for Device<A> {}
impl<A: HalApi> Access<BindGroup<A>> for BindGroupLayout<A> {}
impl<A: HalApi> Access<BindGroup<A>> for PipelineLayout<A> {}
impl<A: HalApi> Access<BindGroup<A>> for CommandBuffer<A> {}
impl<A: HalApi> Access<ComputePipeline<A>> for Device<A> {}
impl<A: HalApi> Access<ComputePipeline<A>> for BindGroup<A> {}
impl<A: HalApi> Access<RenderPipeline<A>> for Device<A> {}
impl<A: HalApi> Access<RenderPipeline<A>> for BindGroup<A> {}
impl<A: HalApi> Access<RenderPipeline<A>> for ComputePipeline<A> {}
impl<A: HalApi> Access<ShaderModule<A>> for Device<A> {}
impl<A: HalApi> Access<ShaderModule<A>> for BindGroupLayout<A> {}
impl<A: HalApi> Access<Buffer<A>> for Root {}
impl<A: HalApi> Access<Buffer<A>> for Device<A> {}
impl<A: HalApi> Access<Buffer<A>> for BindGroupLayout<A> {}
impl<A: HalApi> Access<Buffer<A>> for BindGroup<A> {}
impl<A: HalApi> Access<Buffer<A>> for CommandBuffer<A> {}
impl<A: HalApi> Access<Buffer<A>> for ComputePipeline<A> {}
impl<A: HalApi> Access<Buffer<A>> for RenderPipeline<A> {}
impl<A: HalApi> Access<Buffer<A>> for QuerySet<A> {}
impl<A: HalApi> Access<StagingBuffer<A>> for Device<A> {}
impl<A: HalApi> Access<Texture<A>> for Root {}
impl<A: HalApi> Access<Texture<A>> for Device<A> {}
impl<A: HalApi> Access<Texture<A>> for Buffer<A> {}
impl<A: HalApi> Access<TextureView<A>> for Root {}
impl<A: HalApi> Access<TextureView<A>> for Device<A> {}
impl<A: HalApi> Access<TextureView<A>> for Texture<A> {}
impl<A: HalApi> Access<Sampler<A>> for Root {}
impl<A: HalApi> Access<Sampler<A>> for Device<A> {}
impl<A: HalApi> Access<Sampler<A>> for TextureView<A> {}
impl<A: HalApi> Access<QuerySet<A>> for Root {}
impl<A: HalApi> Access<QuerySet<A>> for Device<A> {}
impl<A: HalApi> Access<QuerySet<A>> for CommandBuffer<A> {}
impl<A: HalApi> Access<QuerySet<A>> for RenderPipeline<A> {}
impl<A: HalApi> Access<QuerySet<A>> for ComputePipeline<A> {}
impl<A: HalApi> Access<QuerySet<A>> for Sampler<A> {}

#[cfg(debug_assertions)]
thread_local! {
    /// Per-thread state checking `Token<Root>` creation in debug builds.
    ///
    /// This is the number of `Token` values alive on the current
    /// thread. Since `Token` creation respects the [`Access`] graph,
    /// there can never be more tokens alive than there are fields of
    /// [`Hub`], so a `u8` is plenty.
    static ACTIVE_TOKEN: Cell<u8> = Cell::new(0);
}

/// A zero-size permission token to lock some fields of [`Hub`].
///
/// Access to a `Token<T>` grants permission to lock any field of
/// [`Hub`] following the one of type [`Registry<T, ...>`], where
/// "following" is as defined by the [`Access`] implementations.
///
/// Calling [`Token::root()`] returns a `Token<Root>`, which grants
/// permission to lock any field. Dynamic checks ensure that each
/// thread has at most one `Token<Root>` live at a time, in debug
/// builds.
///
/// The locking methods on `Registry<T, ...>` take a `&'t mut
/// Token<A>`, and return a fresh `Token<'t, T>` and a lock guard with
/// lifetime `'t`, so the caller cannot access their `Token<A>` again
/// until they have dropped both the `Token<T>` and the lock guard.
///
/// Tokens are `!Send`, so one thread can't send its permissions to
/// another.
pub(crate) struct Token<'a, T: 'a> {
    // The `*const` makes us `!Send` and `!Sync`.
    level: PhantomData<&'a *const T>,
}

impl<'a, T> Token<'a, T> {
    /// Return a new token for a locked field.
    ///
    /// This should only be used by `Registry` locking methods.
    fn new() -> Self {
        #[cfg(debug_assertions)]
        ACTIVE_TOKEN.with(|active| {
            let old = active.get();
            assert_ne!(old, 0, "Root token was dropped");
            active.set(old + 1);
        });
        Self { level: PhantomData }
    }
}

impl Token<'static, Root> {
    /// Return a `Token<Root>`, granting permission to lock any [`Hub`] field.
    ///
    /// Debug builds check dynamically that each thread has at most
    /// one root token at a time.
    pub fn root() -> Self {
        #[cfg(debug_assertions)]
        ACTIVE_TOKEN.with(|active| {
            assert_eq!(0, active.replace(1), "Root token is already active");
        });

        Self { level: PhantomData }
    }
}

impl<'a, T> Drop for Token<'a, T> {
    fn drop(&mut self) {
        #[cfg(debug_assertions)]
        ACTIVE_TOKEN.with(|active| {
            let old = active.get();
            active.set(old - 1);
        });
    }
}

/// A type that can build true ids from proto-ids, and free true ids.
///
/// For some implementations, the true id is based on the proto-id.
/// The caller is responsible for providing well-allocated proto-ids.
///
/// For other implementations, the proto-id carries no information
/// (it's `()`, say), and this `IdentityHandler` type takes care of
/// allocating a fresh true id.
///
/// See the module-level documentation for details.
pub trait IdentityHandler<I>: Debug {
    /// The type of proto-id consumed by this filter, to produce a true id.
    type Input: Clone + Debug;

    /// Given a proto-id value `id`, return a true id for `backend`.
    fn process(&self, id: Self::Input, backend: Backend) -> I;

    /// Free the true id `id`.
    fn free(&self, id: I);
}

impl<I: id::TypedId + Debug> IdentityHandler<I> for Mutex<IdentityManager> {
    type Input = ();
    fn process(&self, _id: Self::Input, backend: Backend) -> I {
        self.lock().alloc(backend)
    }
    fn free(&self, id: I) {
        self.lock().free(id)
    }
}

/// A type that can produce [`IdentityHandler`] filters for ids of type `I`.
///
/// See the module-level documentation for details.
pub trait IdentityHandlerFactory<I> {
    /// The type of filter this factory constructs.
    ///
    /// "Filter" and "handler" seem to both mean the same thing here:
    /// something that can produce true ids from proto-ids.
    type Filter: IdentityHandler<I>;

    /// Create an [`IdentityHandler<I>`] implementation that can
    /// transform proto-ids into ids of type `I`.
    ///
    /// [`IdentityHandler<I>`]: IdentityHandler
    fn spawn(&self) -> Self::Filter;
}

/// A global identity handler factory based on [`IdentityManager`].
///
/// Each of this type's `IdentityHandlerFactory<I>::spawn` methods
/// returns a `Mutex<IdentityManager<I>>`, which allocates fresh `I`
/// ids itself, and takes `()` as its proto-id type.
#[derive(Debug)]
pub struct IdentityManagerFactory;

impl<I: id::TypedId + Debug> IdentityHandlerFactory<I> for IdentityManagerFactory {
    type Filter = Mutex<IdentityManager>;
    fn spawn(&self) -> Self::Filter {
        Mutex::new(IdentityManager::default())
    }
}

/// A factory that can build [`IdentityHandler`]s for all resource
/// types.
pub trait GlobalIdentityHandlerFactory:
    IdentityHandlerFactory<id::AdapterId>
    + IdentityHandlerFactory<id::DeviceId>
    + IdentityHandlerFactory<id::PipelineLayoutId>
    + IdentityHandlerFactory<id::ShaderModuleId>
    + IdentityHandlerFactory<id::BindGroupLayoutId>
    + IdentityHandlerFactory<id::BindGroupId>
    + IdentityHandlerFactory<id::CommandBufferId>
    + IdentityHandlerFactory<id::RenderBundleId>
    + IdentityHandlerFactory<id::RenderPipelineId>
    + IdentityHandlerFactory<id::ComputePipelineId>
    + IdentityHandlerFactory<id::QuerySetId>
    + IdentityHandlerFactory<id::BufferId>
    + IdentityHandlerFactory<id::StagingBufferId>
    + IdentityHandlerFactory<id::TextureId>
    + IdentityHandlerFactory<id::TextureViewId>
    + IdentityHandlerFactory<id::SamplerId>
    + IdentityHandlerFactory<id::SurfaceId>
{
}

impl GlobalIdentityHandlerFactory for IdentityManagerFactory {}

pub type Input<G, I> = <<G as IdentityHandlerFactory<I>>::Filter as IdentityHandler<I>>::Input;

pub trait Resource {
    const TYPE: &'static str;
    fn life_guard(&self) -> &crate::LifeGuard;
    fn label(&self) -> &str {
        #[cfg(debug_assertions)]
        return &self.life_guard().label;
        #[cfg(not(debug_assertions))]
        return "";
    }
}

#[derive(Debug)]
pub struct Registry<T: Resource, I: id::TypedId, F: IdentityHandlerFactory<I>> {
    identity: F::Filter,
    data: RwLock<Storage<T, I>>,
    backend: Backend,
}

impl<T: Resource, I: id::TypedId, F: IdentityHandlerFactory<I>> Registry<T, I, F> {
    fn new(backend: Backend, factory: &F) -> Self {
        Self {
            identity: factory.spawn(),
            data: RwLock::new(Storage {
                map: Vec::new(),
                kind: T::TYPE,
                _phantom: PhantomData,
            }),
            backend,
        }
    }

    fn without_backend(factory: &F, kind: &'static str) -> Self {
        Self {
            identity: factory.spawn(),
            data: RwLock::new(Storage {
                map: Vec::new(),
                kind,
                _phantom: PhantomData,
            }),
            backend: Backend::Empty,
        }
    }
}

#[must_use]
pub(crate) struct FutureId<'a, I: id::TypedId, T> {
    id: I,
    data: &'a RwLock<Storage<T, I>>,
}

impl<I: id::TypedId + Copy, T> FutureId<'_, I, T> {
    #[cfg(feature = "trace")]
    pub fn id(&self) -> I {
        self.id
    }

    pub fn into_id(self) -> I {
        self.id
    }

    pub fn assign<'a, A: Access<T>>(self, value: T, _: &'a mut Token<A>) -> id::Valid<I> {
        self.data.write().insert(self.id, value);
        id::Valid(self.id)
    }

    pub fn assign_error<'a, A: Access<T>>(self, label: &str, _: &'a mut Token<A>) -> I {
        self.data.write().insert_error(self.id, label);
        self.id
    }
}

impl<T: Resource, I: id::TypedId + Copy, F: IdentityHandlerFactory<I>> Registry<T, I, F> {
    pub(crate) fn prepare(
        &self,
        id_in: <F::Filter as IdentityHandler<I>>::Input,
    ) -> FutureId<I, T> {
        FutureId {
            id: self.identity.process(id_in, self.backend),
            data: &self.data,
        }
    }

    /// Acquire read access to this `Registry`'s contents.
    ///
    /// The caller must present a mutable reference to a `Token<A>`,
    /// for some type `A` that comes before this `Registry`'s resource
    /// type `T` in the lock ordering. A `Token<Root>` grants
    /// permission to lock any field; see [`Token::root`].
    ///
    /// Once the read lock is acquired, return a new `Token<T>`, along
    /// with a read guard for this `Registry`'s [`Storage`], which can
    /// be indexed by id to get at the actual resources.
    ///
    /// The borrow checker ensures that the caller cannot again access
    /// its `Token<A>` until it has dropped both the guard and the
    /// `Token<T>`.
    ///
    /// See the [`Hub`] type for more details on locking.
    pub(crate) fn read<'a, A: Access<T>>(
        &'a self,
        _token: &'a mut Token<A>,
    ) -> (RwLockReadGuard<'a, Storage<T, I>>, Token<'a, T>) {
        (self.data.read(), Token::new())
    }

    /// Acquire write access to this `Registry`'s contents.
    ///
    /// The caller must present a mutable reference to a `Token<A>`,
    /// for some type `A` that comes before this `Registry`'s resource
    /// type `T` in the lock ordering. A `Token<Root>` grants
    /// permission to lock any field; see [`Token::root`].
    ///
    /// Once the lock is acquired, return a new `Token<T>`, along with
    /// a write guard for this `Registry`'s [`Storage`], which can be
    /// indexed by id to get at the actual resources.
    ///
    /// The borrow checker ensures that the caller cannot again access
    /// its `Token<A>` until it has dropped both the guard and the
    /// `Token<T>`.
    ///
    /// See the [`Hub`] type for more details on locking.
    pub(crate) fn write<'a, A: Access<T>>(
        &'a self,
        _token: &'a mut Token<A>,
    ) -> (RwLockWriteGuard<'a, Storage<T, I>>, Token<'a, T>) {
        (self.data.write(), Token::new())
    }

    /// Unregister the resource at `id`.
    ///
    /// The caller must prove that it already holds a write lock for
    /// this `Registry` by passing a mutable reference to this
    /// `Registry`'s storage, obtained from the write guard returned
    /// by a previous call to [`write`], as the `guard` parameter.
    pub fn unregister_locked(&self, id: I, guard: &mut Storage<T, I>) -> Option<T> {
        let value = guard.remove(id);
        //Note: careful about the order here!
        self.identity.free(id);
        //Returning None is legal if it's an error ID
        value
    }

    /// Unregister the resource at `id` and return its value, if any.
    ///
    /// The caller must present a mutable reference to a `Token<A>`,
    /// for some type `A` that comes before this `Registry`'s resource
    /// type `T` in the lock ordering.
    ///
    /// This returns a `Token<T>`, but it's almost useless, because it
    /// doesn't return a lock guard to go with it: its only effect is
    /// to make the token you passed to this function inaccessible.
    /// However, the `Token<T>` can be used to satisfy some functions'
    /// bureacratic expectations that you will have one available.
    ///
    /// The borrow checker ensures that the caller cannot again access
    /// its `Token<A>` until it has dropped both the guard and the
    /// `Token<T>`.
    ///
    /// See the [`Hub`] type for more details on locking.
    pub(crate) fn unregister<'a, A: Access<T>>(
        &self,
        id: I,
        _token: &'a mut Token<A>,
    ) -> (Option<T>, Token<'a, T>) {
        let value = self.data.write().remove(id);
        //Note: careful about the order here!
        self.identity.free(id);
        //Returning None is legal if it's an error ID
        (value, Token::new())
    }

    pub fn label_for_resource(&self, id: I) -> String {
        let guard = self.data.read();

        let type_name = guard.kind;
        match guard.get(id) {
            Ok(res) => {
                let label = res.label();
                if label.is_empty() {
                    format!("<{}-{:?}>", type_name, id.unzip())
                } else {
                    label.to_string()
                }
            }
            Err(_) => format!(
                "<Invalid-{} label={}>",
                type_name,
                guard.label_for_invalid_id(id)
            ),
        }
    }
}

#[derive(Debug)]
pub struct HubReport {
    pub adapters: StorageReport,
    pub devices: StorageReport,
    pub pipeline_layouts: StorageReport,
    pub shader_modules: StorageReport,
    pub bind_group_layouts: StorageReport,
    pub bind_groups: StorageReport,
    pub command_buffers: StorageReport,
    pub render_bundles: StorageReport,
    pub render_pipelines: StorageReport,
    pub compute_pipelines: StorageReport,
    pub query_sets: StorageReport,
    pub buffers: StorageReport,
    pub textures: StorageReport,
    pub texture_views: StorageReport,
    pub samplers: StorageReport,
}

impl HubReport {
    pub fn is_empty(&self) -> bool {
        self.adapters.is_empty()
    }
}

#[allow(rustdoc::private_intra_doc_links)]
/// All the resources for a particular backend in a [`Global`].
///
/// To obtain `global`'s `Hub` for some [`HalApi`] backend type `A`,
/// call [`A::hub(global)`].
///
/// ## Locking
///
/// Each field in `Hub` is a [`Registry`] holding all the values of a
/// particular type of resource, all protected by a single [`RwLock`].
/// So for example, to access any [`Buffer`], you must acquire a read
/// lock on the `Hub`s entire [`buffers`] registry. The lock guard
/// gives you access to the `Registry`'s [`Storage`], which you can
/// then index with the buffer's id. (Yes, this design causes
/// contention; see [#2272].)
///
/// But most `wgpu` operations require access to several different
/// kinds of resource, so you often need to hold locks on several
/// different fields of your [`Hub`] simultaneously. To avoid
/// deadlock, there is an ordering imposed on the fields, and you may
/// only acquire new locks on fields that come *after* all those you
/// are already holding locks on, in this ordering. (The ordering is
/// described in the documentation for the [`Access`] trait.)
///
/// We use Rust's type system to statically check that `wgpu_core` can
/// only ever acquire locks in the correct order:
///
/// - A value of type [`Token<T>`] represents proof that the owner
///   only holds locks on the `Hub` fields holding resources of type
///   `T` or earlier in the lock ordering. A special value of type
///   `Token<Root>`, obtained by calling [`Token::root`], represents
///   proof that no `Hub` field locks are held.
///
/// - To lock the `Hub` field holding resources of type `T`, you must
///   call its [`read`] or [`write`] methods. These require you to
///   pass in a `&mut Token<A>`, for some `A` that implements
///   [`Access<T>`]. This implementation exists only if `T` follows `A`
///   in the field ordering, which statically ensures that you are
///   indeed allowed to lock this new `Hub` field.
///
/// - The locking methods return both an [`RwLock`] guard that you can
///   use to access the field's resources, and a new `Token<T>` value.
///   These both borrow from the lifetime of your `Token<A>`, so since
///   you passed that by mutable reference, you cannot access it again
///   until you drop the new token and lock guard.
///
/// Because a thread only ever has access to the `Token<T>` for the
/// last resource type `T` it holds a lock for, and the `Access` trait
/// implementations only permit acquiring locks for types `U` that
/// follow `T` in the lock ordering, it is statically impossible for a
/// program to violate the locking order.
///
/// This does assume that threads cannot call `Token<Root>` when they
/// already hold locks (dynamically enforced in debug builds) and that
/// threads cannot send their `Token`s to other threads (enforced by
/// making `Token` neither `Send` nor `Sync`).
///
/// [`A::hub(global)`]: HalApi::hub
/// [`RwLock`]: parking_lot::RwLock
/// [`buffers`]: Hub::buffers
/// [`read`]: Registry::read
/// [`write`]: Registry::write
/// [`Token<T>`]: Token
/// [`Access<T>`]: Access
/// [#2272]: https://github.com/gfx-rs/wgpu/pull/2272
pub struct Hub<A: HalApi, F: GlobalIdentityHandlerFactory> {
    pub adapters: Registry<Adapter<A>, id::AdapterId, F>,
    pub devices: Registry<Device<A>, id::DeviceId, F>,
    pub pipeline_layouts: Registry<PipelineLayout<A>, id::PipelineLayoutId, F>,
    pub shader_modules: Registry<ShaderModule<A>, id::ShaderModuleId, F>,
    pub bind_group_layouts: Registry<BindGroupLayout<A>, id::BindGroupLayoutId, F>,
    pub bind_groups: Registry<BindGroup<A>, id::BindGroupId, F>,
    pub command_buffers: Registry<CommandBuffer<A>, id::CommandBufferId, F>,
    pub render_bundles: Registry<RenderBundle<A>, id::RenderBundleId, F>,
    pub render_pipelines: Registry<RenderPipeline<A>, id::RenderPipelineId, F>,
    pub compute_pipelines: Registry<ComputePipeline<A>, id::ComputePipelineId, F>,
    pub query_sets: Registry<QuerySet<A>, id::QuerySetId, F>,
    pub buffers: Registry<Buffer<A>, id::BufferId, F>,
    pub staging_buffers: Registry<StagingBuffer<A>, id::StagingBufferId, F>,
    pub textures: Registry<Texture<A>, id::TextureId, F>,
    pub texture_views: Registry<TextureView<A>, id::TextureViewId, F>,
    pub samplers: Registry<Sampler<A>, id::SamplerId, F>,
}

impl<A: HalApi, F: GlobalIdentityHandlerFactory> Hub<A, F> {
    fn new(factory: &F) -> Self {
        Self {
            adapters: Registry::new(A::VARIANT, factory),
            devices: Registry::new(A::VARIANT, factory),
            pipeline_layouts: Registry::new(A::VARIANT, factory),
            shader_modules: Registry::new(A::VARIANT, factory),
            bind_group_layouts: Registry::new(A::VARIANT, factory),
            bind_groups: Registry::new(A::VARIANT, factory),
            command_buffers: Registry::new(A::VARIANT, factory),
            render_bundles: Registry::new(A::VARIANT, factory),
            render_pipelines: Registry::new(A::VARIANT, factory),
            compute_pipelines: Registry::new(A::VARIANT, factory),
            query_sets: Registry::new(A::VARIANT, factory),
            buffers: Registry::new(A::VARIANT, factory),
            staging_buffers: Registry::new(A::VARIANT, factory),
            textures: Registry::new(A::VARIANT, factory),
            texture_views: Registry::new(A::VARIANT, factory),
            samplers: Registry::new(A::VARIANT, factory),
        }
    }

    //TODO: instead of having a hacky `with_adapters` parameter,
    // we should have `clear_device(device_id)` that specifically destroys
    // everything related to a logical device.
    fn clear(&self, surface_guard: &mut Storage<Surface, id::SurfaceId>, with_adapters: bool) {
        use crate::resource::TextureInner;
        use hal::{Device as _, Surface as _};

        let mut devices = self.devices.data.write();
        for element in devices.map.iter_mut() {
            if let Element::Occupied(ref mut device, _) = *element {
                device.prepare_to_die();
            }
        }

        // destroy command buffers first, since otherwise DX12 isn't happy
        for element in self.command_buffers.data.write().map.drain(..) {
            if let Element::Occupied(command_buffer, _) = element {
                let device = &devices[command_buffer.device_id.value];
                device.destroy_command_buffer(command_buffer);
            }
        }

        for element in self.samplers.data.write().map.drain(..) {
            if let Element::Occupied(sampler, _) = element {
                unsafe {
                    devices[sampler.device_id.value]
                        .raw
                        .destroy_sampler(sampler.raw);
                }
            }
        }

        for element in self.texture_views.data.write().map.drain(..) {
            if let Element::Occupied(texture_view, _) = element {
                let device = &devices[texture_view.device_id.value];
                unsafe {
                    device.raw.destroy_texture_view(texture_view.raw);
                }
            }
        }

        for element in self.textures.data.write().map.drain(..) {
            if let Element::Occupied(texture, _) = element {
                let device = &devices[texture.device_id.value];
                if let TextureInner::Native { raw: Some(raw) } = texture.inner {
                    unsafe {
                        device.raw.destroy_texture(raw);
                    }
                }
                if let TextureClearMode::RenderPass { clear_views, .. } = texture.clear_mode {
                    for view in clear_views {
                        unsafe {
                            device.raw.destroy_texture_view(view);
                        }
                    }
                }
            }
        }
        for element in self.buffers.data.write().map.drain(..) {
            if let Element::Occupied(buffer, _) = element {
                //TODO: unmap if needed
                devices[buffer.device_id.value].destroy_buffer(buffer);
            }
        }
        for element in self.bind_groups.data.write().map.drain(..) {
            if let Element::Occupied(bind_group, _) = element {
                let device = &devices[bind_group.device_id.value];
                unsafe {
                    device.raw.destroy_bind_group(bind_group.raw);
                }
            }
        }

        for element in self.shader_modules.data.write().map.drain(..) {
            if let Element::Occupied(module, _) = element {
                let device = &devices[module.device_id.value];
                unsafe {
                    device.raw.destroy_shader_module(module.raw);
                }
            }
        }
        for element in self.bind_group_layouts.data.write().map.drain(..) {
            if let Element::Occupied(bgl, _) = element {
                let device = &devices[bgl.device_id.value];
                unsafe {
                    device.raw.destroy_bind_group_layout(bgl.raw);
                }
            }
        }
        for element in self.pipeline_layouts.data.write().map.drain(..) {
            if let Element::Occupied(pipeline_layout, _) = element {
                let device = &devices[pipeline_layout.device_id.value];
                unsafe {
                    device.raw.destroy_pipeline_layout(pipeline_layout.raw);
                }
            }
        }
        for element in self.compute_pipelines.data.write().map.drain(..) {
            if let Element::Occupied(pipeline, _) = element {
                let device = &devices[pipeline.device_id.value];
                unsafe {
                    device.raw.destroy_compute_pipeline(pipeline.raw);
                }
            }
        }
        for element in self.render_pipelines.data.write().map.drain(..) {
            if let Element::Occupied(pipeline, _) = element {
                let device = &devices[pipeline.device_id.value];
                unsafe {
                    device.raw.destroy_render_pipeline(pipeline.raw);
                }
            }
        }

        for element in surface_guard.map.iter_mut() {
            if let Element::Occupied(ref mut surface, _epoch) = *element {
                if surface
                    .presentation
                    .as_ref()
                    .map_or(wgt::Backend::Empty, |p| p.backend())
                    != A::VARIANT
                {
                    continue;
                }
                if let Some(present) = surface.presentation.take() {
                    let device = &devices[present.device_id.value];
                    let suf = A::get_surface_mut(surface);
                    unsafe {
                        suf.unwrap().raw.unconfigure(&device.raw);
                        //TODO: we could destroy the surface here
                    }
                }
            }
        }

        for element in self.query_sets.data.write().map.drain(..) {
            if let Element::Occupied(query_set, _) = element {
                let device = &devices[query_set.device_id.value];
                unsafe {
                    device.raw.destroy_query_set(query_set.raw);
                }
            }
        }

        for element in devices.map.drain(..) {
            if let Element::Occupied(device, _) = element {
                device.dispose();
            }
        }

        if with_adapters {
            drop(devices);
            self.adapters.data.write().map.clear();
        }
    }

    pub(crate) fn surface_unconfigure(
        &self,
        device_id: id::Valid<id::DeviceId>,
        surface: &mut HalSurface<A>,
    ) {
        use hal::Surface as _;

        let devices = self.devices.data.read();
        let device = &devices[device_id];
        unsafe {
            surface.raw.unconfigure(&device.raw);
        }
    }

    pub fn generate_report(&self) -> HubReport {
        HubReport {
            adapters: self.adapters.data.read().generate_report(),
            devices: self.devices.data.read().generate_report(),
            pipeline_layouts: self.pipeline_layouts.data.read().generate_report(),
            shader_modules: self.shader_modules.data.read().generate_report(),
            bind_group_layouts: self.bind_group_layouts.data.read().generate_report(),
            bind_groups: self.bind_groups.data.read().generate_report(),
            command_buffers: self.command_buffers.data.read().generate_report(),
            render_bundles: self.render_bundles.data.read().generate_report(),
            render_pipelines: self.render_pipelines.data.read().generate_report(),
            compute_pipelines: self.compute_pipelines.data.read().generate_report(),
            query_sets: self.query_sets.data.read().generate_report(),
            buffers: self.buffers.data.read().generate_report(),
            textures: self.textures.data.read().generate_report(),
            texture_views: self.texture_views.data.read().generate_report(),
            samplers: self.samplers.data.read().generate_report(),
        }
    }
}

pub struct Hubs<F: GlobalIdentityHandlerFactory> {
    #[cfg(all(feature = "vulkan", not(target_arch = "wasm32")))]
    vulkan: Hub<hal::api::Vulkan, F>,
    #[cfg(all(feature = "metal", any(target_os = "macos", target_os = "ios")))]
    metal: Hub<hal::api::Metal, F>,
    #[cfg(all(feature = "dx12", windows))]
    dx12: Hub<hal::api::Dx12, F>,
    #[cfg(all(feature = "dx11", windows))]
    dx11: Hub<hal::api::Dx11, F>,
    #[cfg(feature = "gles")]
    gl: Hub<hal::api::Gles, F>,
}

impl<F: GlobalIdentityHandlerFactory> Hubs<F> {
    fn new(factory: &F) -> Self {
        Self {
            #[cfg(all(feature = "vulkan", not(target_arch = "wasm32")))]
            vulkan: Hub::new(factory),
            #[cfg(all(feature = "metal", any(target_os = "macos", target_os = "ios")))]
            metal: Hub::new(factory),
            #[cfg(all(feature = "dx12", windows))]
            dx12: Hub::new(factory),
            #[cfg(all(feature = "dx11", windows))]
            dx11: Hub::new(factory),
            #[cfg(feature = "gles")]
            gl: Hub::new(factory),
        }
    }
}

#[derive(Debug)]
pub struct GlobalReport {
    pub surfaces: StorageReport,
    #[cfg(all(feature = "vulkan", not(target_arch = "wasm32")))]
    pub vulkan: Option<HubReport>,
    #[cfg(all(feature = "metal", any(target_os = "macos", target_os = "ios")))]
    pub metal: Option<HubReport>,
    #[cfg(all(feature = "dx12", windows))]
    pub dx12: Option<HubReport>,
    #[cfg(all(feature = "dx11", windows))]
    pub dx11: Option<HubReport>,
    #[cfg(feature = "gles")]
    pub gl: Option<HubReport>,
}

pub struct Global<G: GlobalIdentityHandlerFactory> {
    pub instance: Instance,
    pub surfaces: Registry<Surface, id::SurfaceId, G>,
    hubs: Hubs<G>,
}

impl<G: GlobalIdentityHandlerFactory> Global<G> {
    pub fn new(name: &str, factory: G, instance_desc: wgt::InstanceDescriptor) -> Self {
        profiling::scope!("Global::new");
        Self {
            instance: Instance::new(name, instance_desc),
            surfaces: Registry::without_backend(&factory, "Surface"),
            hubs: Hubs::new(&factory),
        }
    }

    /// # Safety
    ///
    /// Refer to the creation of wgpu-hal Instance for every backend.
    pub unsafe fn from_hal_instance<A: HalApi>(
        name: &str,
        factory: G,
        hal_instance: A::Instance,
    ) -> Self {
        profiling::scope!("Global::new");
        Self {
            instance: A::create_instance_from_hal(name, hal_instance),
            surfaces: Registry::without_backend(&factory, "Surface"),
            hubs: Hubs::new(&factory),
        }
    }

    /// # Safety
    ///
    /// - The raw instance handle returned must not be manually destroyed.
    pub unsafe fn instance_as_hal<A: HalApi>(&self) -> Option<&A::Instance> {
        A::instance_as_hal(&self.instance)
    }

    /// # Safety
    ///
    /// - The raw handles obtained from the Instance must not be manually destroyed
    pub unsafe fn from_instance(factory: G, instance: Instance) -> Self {
        profiling::scope!("Global::new");
        Self {
            instance,
            surfaces: Registry::without_backend(&factory, "Surface"),
            hubs: Hubs::new(&factory),
        }
    }

    pub fn clear_backend<A: HalApi>(&self, _dummy: ()) {
        let mut surface_guard = self.surfaces.data.write();
        let hub = A::hub(self);
        // this is used for tests, which keep the adapter
        hub.clear(&mut surface_guard, false);
    }

    pub fn generate_report(&self) -> GlobalReport {
        GlobalReport {
            surfaces: self.surfaces.data.read().generate_report(),
            #[cfg(all(feature = "vulkan", not(target_arch = "wasm32")))]
            vulkan: if self.instance.vulkan.is_some() {
                Some(self.hubs.vulkan.generate_report())
            } else {
                None
            },
            #[cfg(all(feature = "metal", any(target_os = "macos", target_os = "ios")))]
            metal: if self.instance.metal.is_some() {
                Some(self.hubs.metal.generate_report())
            } else {
                None
            },
            #[cfg(all(feature = "dx12", windows))]
            dx12: if self.instance.dx12.is_some() {
                Some(self.hubs.dx12.generate_report())
            } else {
                None
            },
            #[cfg(all(feature = "dx11", windows))]
            dx11: if self.instance.dx11.is_some() {
                Some(self.hubs.dx11.generate_report())
            } else {
                None
            },
            #[cfg(feature = "gles")]
            gl: if self.instance.gl.is_some() {
                Some(self.hubs.gl.generate_report())
            } else {
                None
            },
        }
    }
}

impl<G: GlobalIdentityHandlerFactory> Drop for Global<G> {
    fn drop(&mut self) {
        profiling::scope!("Global::drop");
        log::info!("Dropping Global");
        let mut surface_guard = self.surfaces.data.write();

        // destroy hubs before the instance gets dropped
        #[cfg(all(feature = "vulkan", not(target_arch = "wasm32")))]
        {
            self.hubs.vulkan.clear(&mut surface_guard, true);
        }
        #[cfg(all(feature = "metal", any(target_os = "macos", target_os = "ios")))]
        {
            self.hubs.metal.clear(&mut surface_guard, true);
        }
        #[cfg(all(feature = "dx12", windows))]
        {
            self.hubs.dx12.clear(&mut surface_guard, true);
        }
        #[cfg(all(feature = "dx11", windows))]
        {
            self.hubs.dx11.clear(&mut surface_guard, true);
        }
        #[cfg(feature = "gles")]
        {
            self.hubs.gl.clear(&mut surface_guard, true);
        }

        // destroy surfaces
        for element in surface_guard.map.drain(..) {
            if let Element::Occupied(surface, _) = element {
                self.instance.destroy_surface(surface);
            }
        }
    }
}

pub trait HalApi: hal::Api {
    const VARIANT: Backend;
    fn create_instance_from_hal(name: &str, hal_instance: Self::Instance) -> Instance;
    fn instance_as_hal(instance: &Instance) -> Option<&Self::Instance>;
    fn hub<G: GlobalIdentityHandlerFactory>(global: &Global<G>) -> &Hub<Self, G>;
    fn get_surface(surface: &Surface) -> Option<&HalSurface<Self>>;
    fn get_surface_mut(surface: &mut Surface) -> Option<&mut HalSurface<Self>>;
}

impl HalApi for hal::api::Empty {
    const VARIANT: Backend = Backend::Empty;
    fn create_instance_from_hal(_: &str, _: Self::Instance) -> Instance {
        unimplemented!("called empty api")
    }
    fn instance_as_hal(_: &Instance) -> Option<&Self::Instance> {
        unimplemented!("called empty api")
    }
    fn hub<G: GlobalIdentityHandlerFactory>(_: &Global<G>) -> &Hub<Self, G> {
        unimplemented!("called empty api")
    }
    fn get_surface(_: &Surface) -> Option<&HalSurface<Self>> {
        unimplemented!("called empty api")
    }
    fn get_surface_mut(_: &mut Surface) -> Option<&mut HalSurface<Self>> {
        unimplemented!("called empty api")
    }
}

#[cfg(all(feature = "vulkan", not(target_arch = "wasm32")))]
impl HalApi for hal::api::Vulkan {
    const VARIANT: Backend = Backend::Vulkan;
    fn create_instance_from_hal(name: &str, hal_instance: Self::Instance) -> Instance {
        Instance {
            name: name.to_owned(),
            vulkan: Some(hal_instance),
            ..Default::default()
        }
    }
    fn instance_as_hal(instance: &Instance) -> Option<&Self::Instance> {
        instance.vulkan.as_ref()
    }
    fn hub<G: GlobalIdentityHandlerFactory>(global: &Global<G>) -> &Hub<Self, G> {
        &global.hubs.vulkan
    }
    fn get_surface(surface: &Surface) -> Option<&HalSurface<Self>> {
        surface.vulkan.as_ref()
    }
    fn get_surface_mut(surface: &mut Surface) -> Option<&mut HalSurface<Self>> {
        surface.vulkan.as_mut()
    }
}

#[cfg(all(feature = "metal", any(target_os = "macos", target_os = "ios")))]
impl HalApi for hal::api::Metal {
    const VARIANT: Backend = Backend::Metal;
    fn create_instance_from_hal(name: &str, hal_instance: Self::Instance) -> Instance {
        Instance {
            name: name.to_owned(),
            metal: Some(hal_instance),
            ..Default::default()
        }
    }
    fn instance_as_hal(instance: &Instance) -> Option<&Self::Instance> {
        instance.metal.as_ref()
    }
    fn hub<G: GlobalIdentityHandlerFactory>(global: &Global<G>) -> &Hub<Self, G> {
        &global.hubs.metal
    }
    fn get_surface(surface: &Surface) -> Option<&HalSurface<Self>> {
        surface.metal.as_ref()
    }
    fn get_surface_mut(surface: &mut Surface) -> Option<&mut HalSurface<Self>> {
        surface.metal.as_mut()
    }
}

#[cfg(all(feature = "dx12", windows))]
impl HalApi for hal::api::Dx12 {
    const VARIANT: Backend = Backend::Dx12;
    fn create_instance_from_hal(name: &str, hal_instance: Self::Instance) -> Instance {
        Instance {
            name: name.to_owned(),
            dx12: Some(hal_instance),
            ..Default::default()
        }
    }
    fn instance_as_hal(instance: &Instance) -> Option<&Self::Instance> {
        instance.dx12.as_ref()
    }
    fn hub<G: GlobalIdentityHandlerFactory>(global: &Global<G>) -> &Hub<Self, G> {
        &global.hubs.dx12
    }
    fn get_surface(surface: &Surface) -> Option<&HalSurface<Self>> {
        surface.dx12.as_ref()
    }
    fn get_surface_mut(surface: &mut Surface) -> Option<&mut HalSurface<Self>> {
        surface.dx12.as_mut()
    }
}

#[cfg(all(feature = "dx11", windows))]
impl HalApi for hal::api::Dx11 {
    const VARIANT: Backend = Backend::Dx11;
    fn create_instance_from_hal(name: &str, hal_instance: Self::Instance) -> Instance {
        Instance {
            name: name.to_owned(),
            dx11: Some(hal_instance),
            ..Default::default()
        }
    }
    fn instance_as_hal(instance: &Instance) -> Option<&Self::Instance> {
        instance.dx11.as_ref()
    }
    fn hub<G: GlobalIdentityHandlerFactory>(global: &Global<G>) -> &Hub<Self, G> {
        &global.hubs.dx11
    }
    fn get_surface(surface: &Surface) -> Option<&HalSurface<Self>> {
        surface.dx11.as_ref()
    }
    fn get_surface_mut(surface: &mut Surface) -> Option<&mut HalSurface<Self>> {
        surface.dx11.as_mut()
    }
}

#[cfg(feature = "gles")]
impl HalApi for hal::api::Gles {
    const VARIANT: Backend = Backend::Gl;
    fn create_instance_from_hal(name: &str, hal_instance: Self::Instance) -> Instance {
        #[allow(clippy::needless_update)]
        Instance {
            name: name.to_owned(),
            gl: Some(hal_instance),
            ..Default::default()
        }
    }
    fn instance_as_hal(instance: &Instance) -> Option<&Self::Instance> {
        instance.gl.as_ref()
    }
    fn hub<G: GlobalIdentityHandlerFactory>(global: &Global<G>) -> &Hub<Self, G> {
        &global.hubs.gl
    }
    fn get_surface(surface: &Surface) -> Option<&HalSurface<Self>> {
        surface.gl.as_ref()
    }
    fn get_surface_mut(surface: &mut Surface) -> Option<&mut HalSurface<Self>> {
        surface.gl.as_mut()
    }
}

#[cfg(test)]
fn _test_send_sync(global: &Global<IdentityManagerFactory>) {
    fn test_internal<T: Send + Sync>(_: T) {}
    test_internal(global)
}

#[test]
fn test_epoch_end_of_life() {
    use id::TypedId as _;
    let mut man = IdentityManager::default();
    man.epochs.push(id::EPOCH_MASK);
    man.free.push(0);
    let id1 = man.alloc::<id::BufferId>(Backend::Empty);
    assert_eq!(id1.unzip().0, 0);
    man.free(id1);
    let id2 = man.alloc::<id::BufferId>(Backend::Empty);
    // confirm that the index 0 is no longer re-used
    assert_eq!(id2.unzip().0, 1);
}