Merge #226

226: adds stream::step_by combinator r=stjepang a=montekki

Ref: #129 
Stdlib: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.step_by

Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>

Author: bors[bot]

src/stream/stream/mod.rs

@@ -37,6 +37,7 @@ mod nth;
 mod scan;
 mod skip;
 mod skip_while;
+mod step_by;
 mod take;
 mod zip;
 
@@ -46,6 +47,7 @@ pub use inspect::Inspect;
 pub use scan::Scan;
 pub use skip::Skip;
 pub use skip_while::SkipWhile;
+pub use step_by::StepBy;
 pub use take::Take;
 pub use zip::Zip;
 
@@ -232,6 +234,40 @@ pub trait Stream {
         }
     }
 
+    /// Creates a stream that yields each `step`th element.
+    ///
+    /// # Panics
+    ///
+    /// This method will panic if the given step is `0`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// # fn main() { async_std::task::block_on(async {
+    /// #
+    /// use async_std::prelude::*;
+    /// use std::collections::VecDeque;
+    ///
+    /// let s: VecDeque<_> = vec![0u8, 1, 2, 3, 4].into_iter().collect();
+    /// let mut stepped = s.step_by(2);
+    ///
+    /// assert_eq!(stepped.next().await, Some(0));
+    /// assert_eq!(stepped.next().await, Some(2));
+    /// assert_eq!(stepped.next().await, Some(4));
+    /// assert_eq!(stepped.next().await, None);
+    ///
+    /// #
+    /// # }) }
+    /// ```
+    fn step_by(self, step: usize) -> StepBy<Self>
+    where
+        Self: Sized,
+    {
+        StepBy::new(self, step)
+    }
+
     /// Creates a stream that gives the current element's count as well as the next value.
     ///
     /// # Overflow behaviour.

src/stream/stream/step_by.rs

@@ -0,0 +1,50 @@
+use std::pin::Pin;
+
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+/// A stream that steps a given amount of elements of another stream.
+#[derive(Debug)]
+pub struct StepBy<S> {
+    stream: S,
+    step: usize,
+    i: usize,
+}
+
+impl<S> StepBy<S> {
+    pin_utils::unsafe_pinned!(stream: S);
+    pin_utils::unsafe_unpinned!(step: usize);
+    pin_utils::unsafe_unpinned!(i: usize);
+
+    pub(crate) fn new(stream: S, step: usize) -> Self {
+        StepBy {
+            stream,
+            step: step.checked_sub(1).unwrap(),
+            i: 0,
+        }
+    }
+}
+
+impl<S> Stream for StepBy<S>
+where
+    S: Stream,
+{
+    type Item = S::Item;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        loop {
+            let next = futures_core::ready!(self.as_mut().stream().poll_next(cx));
+
+            match next {
+                Some(v) => match self.i {
+                    0 => {
+                        *self.as_mut().i() = self.step;
+                        return Poll::Ready(Some(v));
+                    }
+                    _ => *self.as_mut().i() -= 1,
+                },
+                None => return Poll::Ready(None),
+            }
+        }
+    }
+}