use std::marker::PhantomData;
use std::mem::ManuallyDrop;
use std::ops::Deref;
use std::sync::Arc;
use std::task::{RawWaker, RawWakerVTable, Waker};

/// Simplified waking interface based on Arcs
pub(crate) trait Wake: Send + Sync {
    /// Wake by value
    fn wake(self: Arc<Self>);

    /// Wake by reference
    fn wake_by_ref(arc_self: &Arc<Self>);
}

/// A `Waker` that is only valid for a given lifetime.
#[derive(Debug)]
pub(crate) struct WakerRef<'a> {
    waker: ManuallyDrop<Waker>,
    _p: PhantomData<&'a ()>,
}

impl Deref for WakerRef<'_> {
    type Target = Waker;

    fn deref(&self) -> &Waker {
        &self.waker
    }
}

/// Creates a reference to a `Waker` from a reference to `Arc<impl Wake>`.
pub(crate) fn waker_ref<W: Wake>(wake: &Arc<W>) -> WakerRef<'_> {
    let ptr = &**wake as *const _ as *const ();

    let waker = unsafe { Waker::from_raw(RawWaker::new(ptr, waker_vtable::<W>())) };

    WakerRef {
        waker: ManuallyDrop::new(waker),
        _p: PhantomData,
    }
}

fn waker_vtable<W: Wake>() -> &'static RawWakerVTable {
    &RawWakerVTable::new(
        clone_arc_raw::<W>,
        wake_arc_raw::<W>,
        wake_by_ref_arc_raw::<W>,
        drop_arc_raw::<W>,
    )
}

unsafe fn inc_ref_count<T: Wake>(data: *const ()) {
    // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
    let arc = ManuallyDrop::new(Arc::<T>::from_raw(data as *const T));

    // Now increase refcount, but don't drop new refcount either
    let arc_clone: ManuallyDrop<_> = arc.clone();

    // Drop explicitly to avoid clippy warnings
    drop(arc);
    drop(arc_clone);
}

unsafe fn clone_arc_raw<T: Wake>(data: *const ()) -> RawWaker {
    inc_ref_count::<T>(data);
    RawWaker::new(data, waker_vtable::<T>())
}

unsafe fn wake_arc_raw<T: Wake>(data: *const ()) {
    let arc: Arc<T> = Arc::from_raw(data as *const T);
    Wake::wake(arc);
}

// used by `waker_ref`
unsafe fn wake_by_ref_arc_raw<T: Wake>(data: *const ()) {
    // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
    let arc = ManuallyDrop::new(Arc::<T>::from_raw(data as *const T));
    Wake::wake_by_ref(&arc);
}

unsafe fn drop_arc_raw<T: Wake>(data: *const ()) {
    drop(Arc::<T>::from_raw(data as *const T))
}
