Initial commit

Author: Stjepan Glavina

.gitignore

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+/target
+Cargo.lock

Cargo.toml

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+[package]
+name = "futures-lite"
+version = "0.1.1"
+authors = ["Stjepan Glavina <stjepang@gmail.com>"]
+edition = "2018"
+description = "A lightweight subset of the futures crate"
+license = "Apache-2.0 OR MIT"
+
+[features]
+default = ["std"]
+std = []
+
+[dependencies]
+futures-core = "0.3.5"
+futures-io = { version = "0.3.5", default-features = false, features = ["std"] }

src/lib.rs

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+use std::fmt;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+pub use futures_core::{Future, Stream};
+pub use futures_io::{AsyncRead, AsyncSeek, AsyncWrite};
+
+#[macro_export]
+macro_rules! ready {
+    ($e:expr $(,)?) => {
+        match $e {
+            std::task::Poll::Ready(t) => t,
+            std::task::Poll::Pending => return std::task::Poll::Pending,
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! pin {
+    ($($x:ident),* $(,)?) => {
+        $(
+            let mut $x = $x;
+
+            #[allow(unused_mut)]
+            let mut $x = unsafe {
+                std::pin::Pin::new_unchecked(&mut $x)
+            };
+        )*
+    }
+}
+
+pub mod future {
+    use super::*;
+
+    /// Future for the [`poll_fn`] function.
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct PollFn<F> {
+        f: F,
+    }
+
+    impl<F> Unpin for PollFn<F> {}
+
+    /// Creates a new future wrapping around a function returning [`Poll`].
+    ///
+    /// Polling the returned future delegates to the wrapped function.
+    pub fn poll_fn<T, F>(f: F) -> PollFn<F>
+    where
+        F: FnMut(&mut Context<'_>) -> Poll<T>,
+    {
+        PollFn { f }
+    }
+
+    impl<F> fmt::Debug for PollFn<F> {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            f.debug_struct("PollFn").finish()
+        }
+    }
+
+    impl<T, F> Future for PollFn<F>
+    where
+        F: FnMut(&mut Context<'_>) -> Poll<T>,
+    {
+        type Output = T;
+
+        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+            (&mut self.f)(cx)
+        }
+    }
+}
+
+pub mod stream {
+    use super::*;
+
+    /// Creates a `Stream` from a seed and a closure returning a `Future`.
+    ///
+    /// This function is the dual for the `Stream::fold()` adapter: while
+    /// `Stream::fold()` reduces a `Stream` to one single value, `unfold()` creates a
+    /// `Stream` from a seed value.
+    ///
+    /// `unfold()` will call the provided closure with the provided seed, then wait
+    /// for the returned `Future` to complete with `(a, b)`. It will then yield the
+    /// value `a`, and use `b` as the next internal state.
+    ///
+    /// If the closure returns `None` instead of `Some(Future)`, then the `unfold()`
+    /// will stop producing items and return `Poll::Ready(None)` in future
+    /// calls to `poll()`.
+    ///
+    /// This function can typically be used when wanting to go from the "world of
+    /// futures" to the "world of streams": the provided closure can build a
+    /// `Future` using other library functions working on futures, and `unfold()`
+    /// will turn it into a `Stream` by repeating the operation.
+    pub fn unfold<T, F, Fut, Item>(init: T, f: F) -> Unfold<T, F, Fut>
+    where
+        F: FnMut(T) -> Fut,
+        Fut: Future<Output = Option<(Item, T)>>,
+    {
+        Unfold {
+            f,
+            state: Some(init),
+            fut: None,
+        }
+    }
+
+    /// Stream for the [`unfold`] function.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Unfold<T, F, Fut> {
+        f: F,
+        state: Option<T>,
+        fut: Option<Fut>,
+    }
+
+    impl<T, F, Fut> fmt::Debug for Unfold<T, F, Fut>
+    where
+        T: fmt::Debug,
+        Fut: fmt::Debug,
+    {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            f.debug_struct("Unfold")
+                .field("state", &self.state)
+                .field("fut", &self.fut)
+                .finish()
+        }
+    }
+
+    impl<T, F, Fut, Item> Stream for Unfold<T, F, Fut>
+    where
+        F: FnMut(T) -> Fut,
+        Fut: Future<Output = Option<(Item, T)>>,
+    {
+        type Item = Item;
+
+        fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+            let mut this = unsafe { self.get_unchecked_mut() };
+
+            if let Some(state) = this.state.take() {
+                this.fut = Some((this.f)(state));
+            }
+
+            let fut = unsafe {
+                Pin::new_unchecked(
+                    this.fut
+                        .as_mut()
+                        .expect("Unfold must not be polled after it returned `Poll::Ready(None)`"),
+                )
+            };
+            let step = futures_core::ready!(fut.poll(cx));
+            this.fut = None;
+
+            if let Some((item, next_state)) = step {
+                this.state = Some(next_state);
+                Poll::Ready(Some(item))
+            } else {
+                Poll::Ready(None)
+            }
+        }
+    }
+}