// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
use core::pin::Pin;
use core::task::Context;
use core::task::Poll;
use futures::Future;
use std::marker::PhantomData;
use tokio::runtime::Handle;
use tokio::runtime::RuntimeFlavor;

/// Equivalent to [`tokio::task::JoinHandle`].
#[repr(transparent)]
pub struct JoinHandle<R> {
  handle: tokio::task::JoinHandle<MaskResultAsSend<R>>,
  _r: PhantomData<R>,
}

impl<R> JoinHandle<R> {
  /// Equivalent to [`tokio::task::JoinHandle::abort`].
  pub fn abort(&self) {
    self.handle.abort()
  }
}

impl<R> Future for JoinHandle<R> {
  type Output = Result<R, tokio::task::JoinError>;

  fn poll(
    self: Pin<&mut Self>,
    cx: &mut std::task::Context<'_>,
  ) -> std::task::Poll<Self::Output> {
    // SAFETY: We are sure that handle is valid here
    unsafe {
      let me: &mut Self = Pin::into_inner_unchecked(self);
      let handle = Pin::new_unchecked(&mut me.handle);
      match handle.poll(cx) {
        Poll::Pending => Poll::Pending,
        Poll::Ready(Ok(r)) => Poll::Ready(Ok(r.into_inner())),
        Poll::Ready(Err(e)) => Poll::Ready(Err(e)),
      }
    }
  }
}

/// Equivalent to [`tokio::task::spawn`], but does not require the future to be [`Send`]. Must only be
/// used on a [`RuntimeFlavor::CurrentThread`] executor, though this is only checked when running with
/// debug assertions.
#[inline(always)]
pub fn spawn<F: Future<Output = R> + 'static, R: 'static>(
  f: F,
) -> JoinHandle<R> {
  debug_assert!(
    Handle::current().runtime_flavor() == RuntimeFlavor::CurrentThread
  );
  // SAFETY: we know this is a current-thread executor
  let future = unsafe { MaskFutureAsSend::new(f) };
  JoinHandle {
    handle: tokio::task::spawn(future),
    _r: Default::default(),
  }
}

/// Equivalent to [`tokio::task::spawn_blocking`]. Currently a thin wrapper around the tokio API, but this
/// may change in the future.
#[inline(always)]
pub fn spawn_blocking<
  F: (FnOnce() -> R) + Send + 'static,
  R: Send + 'static,
>(
  f: F,
) -> JoinHandle<R> {
  let handle = tokio::task::spawn_blocking(|| MaskResultAsSend { result: f() });
  JoinHandle {
    handle,
    _r: Default::default(),
  }
}

#[repr(transparent)]
#[doc(hidden)]
pub struct MaskResultAsSend<R> {
  result: R,
}

/// SAFETY: We ensure that Send bounds are only faked when tokio is running on a current-thread executor
unsafe impl<R> Send for MaskResultAsSend<R> {}

impl<R> MaskResultAsSend<R> {
  #[inline(always)]
  pub fn into_inner(self) -> R {
    self.result
  }
}

#[repr(transparent)]
pub struct MaskFutureAsSend<F> {
  future: F,
}

impl<F> MaskFutureAsSend<F> {
  /// Mark a non-`Send` future as `Send`. This is a trick to be able to use
  /// `tokio::spawn()` (which requires `Send` futures) in a current thread
  /// runtime.
  ///
  /// # Safety
  ///
  /// You must ensure that the future is actually used on the same
  /// thread, ie. always use current thread runtime flavor from Tokio.
  #[inline(always)]
  pub unsafe fn new(future: F) -> Self {
    Self { future }
  }
}

// SAFETY: we are cheating here - this struct is NOT really Send,
// but we need to mark it Send so that we can use `spawn()` in Tokio.
unsafe impl<F> Send for MaskFutureAsSend<F> {}

impl<F: Future> Future for MaskFutureAsSend<F> {
  type Output = MaskResultAsSend<F::Output>;

  fn poll(
    self: Pin<&mut Self>,
    cx: &mut Context<'_>,
  ) -> Poll<MaskResultAsSend<F::Output>> {
    // SAFETY: We are sure that future is valid here
    unsafe {
      let me: &mut MaskFutureAsSend<F> = Pin::into_inner_unchecked(self);
      let future = Pin::new_unchecked(&mut me.future);
      match future.poll(cx) {
        Poll::Pending => Poll::Pending,
        Poll::Ready(result) => Poll::Ready(MaskResultAsSend { result }),
      }
    }
  }
}