1 files changed, 230 insertions, 0 deletions

diff --git a/embassy-futures/src/select.rs b/embassy-futures/src/select.rs
new file mode 100644
index 000000000..8cecb7fa0
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+++ b/embassy-futures/src/select.rs
@@ -0,0 +1,230 @@
+use core::future::Future;
+use core::pin::Pin;
+use core::task::{Context, Poll};
+
+/// Result for [`select`].
+#[derive(Debug, Clone)]
+pub enum Either<A, B> {
+    /// First future finished first.
+    First(A),
+    /// Second future finished first.
+    Second(B),
+}
+
+/// Wait for one of two futures to complete.
+///
+/// This function returns a new future which polls all the futures.
+/// When one of them completes, it will complete with its result value.
+///
+/// The other future is dropped.
+pub fn select<A, B>(a: A, b: B) -> Select<A, B>
+where
+    A: Future,
+    B: Future,
+{
+    Select { a, b }
+}
+
+/// Future for the [`select`] function.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Select<A, B> {
+    a: A,
+    b: B,
+}
+
+impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
+
+impl<A, B> Future for Select<A, B>
+where
+    A: Future,
+    B: Future,
+{
+    type Output = Either<A::Output, B::Output>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = unsafe { self.get_unchecked_mut() };
+        let a = unsafe { Pin::new_unchecked(&mut this.a) };
+        let b = unsafe { Pin::new_unchecked(&mut this.b) };
+        if let Poll::Ready(x) = a.poll(cx) {
+            return Poll::Ready(Either::First(x));
+        }
+        if let Poll::Ready(x) = b.poll(cx) {
+            return Poll::Ready(Either::Second(x));
+        }
+        Poll::Pending
+    }
+}
+
+// ====================================================================
+
+/// Result for [`select3`].
+#[derive(Debug, Clone)]
+pub enum Either3<A, B, C> {
+    /// First future finished first.
+    First(A),
+    /// Second future finished first.
+    Second(B),
+    /// Third future finished first.
+    Third(C),
+}
+
+/// Same as [`select`], but with more futures.
+pub fn select3<A, B, C>(a: A, b: B, c: C) -> Select3<A, B, C>
+where
+    A: Future,
+    B: Future,
+    C: Future,
+{
+    Select3 { a, b, c }
+}
+
+/// Future for the [`select3`] function.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Select3<A, B, C> {
+    a: A,
+    b: B,
+    c: C,
+}
+
+impl<A, B, C> Future for Select3<A, B, C>
+where
+    A: Future,
+    B: Future,
+    C: Future,
+{
+    type Output = Either3<A::Output, B::Output, C::Output>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = unsafe { self.get_unchecked_mut() };
+        let a = unsafe { Pin::new_unchecked(&mut this.a) };
+        let b = unsafe { Pin::new_unchecked(&mut this.b) };
+        let c = unsafe { Pin::new_unchecked(&mut this.c) };
+        if let Poll::Ready(x) = a.poll(cx) {
+            return Poll::Ready(Either3::First(x));
+        }
+        if let Poll::Ready(x) = b.poll(cx) {
+            return Poll::Ready(Either3::Second(x));
+        }
+        if let Poll::Ready(x) = c.poll(cx) {
+            return Poll::Ready(Either3::Third(x));
+        }
+        Poll::Pending
+    }
+}
+
+// ====================================================================
+
+/// Result for [`select4`].
+#[derive(Debug, Clone)]
+pub enum Either4<A, B, C, D> {
+    /// First future finished first.
+    First(A),
+    /// Second future finished first.
+    Second(B),
+    /// Third future finished first.
+    Third(C),
+    /// Fourth future finished first.
+    Fourth(D),
+}
+
+/// Same as [`select`], but with more futures.
+pub fn select4<A, B, C, D>(a: A, b: B, c: C, d: D) -> Select4<A, B, C, D>
+where
+    A: Future,
+    B: Future,
+    C: Future,
+    D: Future,
+{
+    Select4 { a, b, c, d }
+}
+
+/// Future for the [`select4`] function.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct Select4<A, B, C, D> {
+    a: A,
+    b: B,
+    c: C,
+    d: D,
+}
+
+impl<A, B, C, D> Future for Select4<A, B, C, D>
+where
+    A: Future,
+    B: Future,
+    C: Future,
+    D: Future,
+{
+    type Output = Either4<A::Output, B::Output, C::Output, D::Output>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = unsafe { self.get_unchecked_mut() };
+        let a = unsafe { Pin::new_unchecked(&mut this.a) };
+        let b = unsafe { Pin::new_unchecked(&mut this.b) };
+        let c = unsafe { Pin::new_unchecked(&mut this.c) };
+        let d = unsafe { Pin::new_unchecked(&mut this.d) };
+        if let Poll::Ready(x) = a.poll(cx) {
+            return Poll::Ready(Either4::First(x));
+        }
+        if let Poll::Ready(x) = b.poll(cx) {
+            return Poll::Ready(Either4::Second(x));
+        }
+        if let Poll::Ready(x) = c.poll(cx) {
+            return Poll::Ready(Either4::Third(x));
+        }
+        if let Poll::Ready(x) = d.poll(cx) {
+            return Poll::Ready(Either4::Fourth(x));
+        }
+        Poll::Pending
+    }
+}
+
+// ====================================================================
+
+/// Future for the [`select_all`] function.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+pub struct SelectAll<Fut, const N: usize> {
+    inner: [Fut; N],
+}
+
+/// Creates a new future which will select over a list of futures.
+///
+/// The returned future will wait for any future within `iter` to be ready. Upon
+/// completion the item resolved will be returned, along with the index of the
+/// future that was ready.
+///
+/// # Panics
+///
+/// This function will panic if the array specified contains no items.
+pub fn select_all<Fut: Future, const N: usize>(arr: [Fut; N]) -> SelectAll<Fut, N> {
+    assert!(N > 0);
+    SelectAll { inner: arr }
+}
+
+impl<Fut: Future, const N: usize> Future for SelectAll<Fut, N> {
+    type Output = (Fut::Output, usize);
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        // Safety: Since `self` is pinned, `inner` cannot move. Since `inner` cannot move,
+        // its elements also cannot move. Therefore it is safe to access `inner` and pin
+        // references to the contained futures.
+        let item = unsafe {
+            self.get_unchecked_mut()
+                .inner
+                .iter_mut()
+                .enumerate()
+                .find_map(|(i, f)| match Pin::new_unchecked(f).poll(cx) {
+                    Poll::Pending => None,
+                    Poll::Ready(e) => Some((i, e)),
+                })
+        };
+
+        match item {
+            Some((idx, res)) => Poll::Ready((res, idx)),
+            None => Poll::Pending,
+        }
+    }
+}