use super::dirstate_map::DirstateMap;

use stable_deref_trait::StableDeref;

use std::ops::Deref;

/// Keep a `DirstateMap<'on_disk>` next to the `on_disk` buffer that it

/// borrows.

///

/// This is similar to [`OwningRef`] which is more limited because it

/// represents exactly one `&T` reference next to the value it borrows, as

/// opposed to a struct that may contain an arbitrary number of references in

/// arbitrarily-nested data structures.

///

/// [`OwningRef`]: https://docs.rs/owning_ref/0.4.1/owning_ref/struct.OwningRef.html

pub struct OwningDirstateMap {

    /// Owned handle to a bytes buffer with a stable address.

    ///

    /// See <https://docs.rs/owning_ref/0.4.1/owning_ref/trait.StableAddress.html>.

    on_disk: Box<dyn Deref<Target = [u8]> + Send>,

    /// Pointer for `Box<DirstateMap<'on_disk>>`, typed-erased because the

    /// language cannot represent a lifetime referencing a sibling field.

    /// This is not quite a self-referencial struct (moving this struct is not

    /// a problem as it doesn’t change the address of the bytes buffer owned

    /// by `on_disk`) but touches similar borrow-checker limitations.

    ptr: *mut (),

}

impl OwningDirstateMap {

    pub fn new_empty<OnDisk>(on_disk: OnDisk) -> Self

    where

        OnDisk: Deref<Target = [u8]> + StableDeref + Send + 'static,

    {

        let on_disk = Box::new(on_disk);

        let bytes: &'_ [u8] = &on_disk;

        let map = DirstateMap::empty(bytes);

        // Like in `bytes` above, this `'_` lifetime parameter borrows from

        // the bytes buffer owned by `on_disk`.

        let ptr: *mut DirstateMap<'_> = Box::into_raw(Box::new(map));

        // Erase the pointed type entirely in order to erase the lifetime.

        let ptr: *mut () = ptr.cast();

        Self { on_disk, ptr }

    }

    pub fn get_pair_mut<'a>(

        &'a mut self,

    ) -> (&'a [u8], &'a mut DirstateMap<'a>) {

        // SAFETY: We cast the type-erased pointer back to the same type it had

        // in `new`, except with a different lifetime parameter. This time we

        // connect the lifetime to that of `self`. This cast is valid because

        // `self` owns the same `on_disk` whose buffer `DirstateMap`

        // references. That buffer has a stable memory address because our

        // `Self::new_empty` counstructor requires `StableDeref`.

        let ptr: *mut DirstateMap<'a> = self.ptr.cast();

        // SAFETY: we dereference that pointer, connecting the lifetime of the

        // new `&mut` to that of `self`. This is valid because the

        // raw pointer is to a boxed value, and `self` owns that box.

        (&self.on_disk, unsafe { &mut *ptr })

    }

    pub fn get_map_mut<'a>(&'a mut self) -> &'a mut DirstateMap<'a> {

        self.get_pair_mut().1

    }

    pub fn get_map<'a>(&'a self) -> &'a DirstateMap<'a> {

        // SAFETY: same reasoning as in `get_pair_mut` above.

        let ptr: *mut DirstateMap<'a> = self.ptr.cast();

        unsafe { &*ptr }

    }

    pub fn on_disk<'a>(&'a self) -> &'a [u8] {

        &self.on_disk

    }

}

impl Drop for OwningDirstateMap {

    fn drop(&mut self) {

        // Silence a "field is never read" warning, and demonstrate that this

        // value is still alive.

        let _: &Box<dyn Deref<Target = [u8]> + Send> = &self.on_disk;

        // SAFETY: this cast is the same as in `get_mut`, and is valid for the

        // same reason. `self.on_disk` still exists at this point, drop glue

        // will drop it implicitly after this `drop` method returns.

        let ptr: *mut DirstateMap<'_> = self.ptr.cast();

        // SAFETY: `Box::from_raw` takes ownership of the box away from `self`.

        // This is fine because drop glue does nothing for `*mut ()` and we’re

        // in `drop`, so `get` and `get_mut` cannot be called again.

        unsafe { drop(Box::from_raw(ptr)) }

    }

}

fn _static_assert_is_send<T: Send>() {}

fn _static_assert_fields_are_send() {

    _static_assert_is_send::<Box<DirstateMap<'_>>>();

}

// SAFETY: we don’t get this impl implicitly because `*mut (): !Send` because

// thread-safety of raw pointers is unknown in the general case. However this

// particular raw pointer represents a `Box<DirstateMap<'on_disk>>` that we

// own. Since that `Box` is `Send` as shown in above, it is sound to mark

// this struct as `Send` too.

unsafe impl Send for OwningDirstateMap {}