Add iterable::Reverse. (#41)

Author: mmaker

src/iterable/mod.rs

@@ -4,11 +4,15 @@
 //! Essentially, it provides a set of handy utilities as a wrapper around
 //! iterators.
 
+mod rev;
+pub use rev::Reverse;
+
 /// The trait [`Iterable`] represents a streamable object that can produce
 /// an arbitrary number of streams of length [`Iterable::len`](Iterable::len).
 ///
 /// An Iterable is pretty much like an [`IntoIterator`] that can be copied over
-/// and over, and has an hint of the length.  Copies are meant to be shared across threads safely.
+/// and over, and has an hint of the length.  Copies are meant to be shared
+/// across threads safely.
 ///
 /// # Examples
 ///
@@ -41,8 +45,8 @@
 /// [`Borrow`](std::borrow::Borrow) in order to avoid copying the contents of
 /// the iterator..
 ///
-/// The `Iter` associated type has a lifetime that is independent from that of the
-/// [`Iterable`] object. This means that implicitly a copy of the relevant
+/// The `Iter` associated type has a lifetime that is independent from that of
+/// the [`Iterable`] object. This means that implicitly a copy of the relevant
 /// contents of the object will happen whenever
 /// [`Iterable::iter`](crate::iterable::Iterable::iter) is called. This might
 /// change in the future as associated type constructors

src/iterable/rev.rs

@@ -0,0 +1,44 @@
+use super::Iterable;
+use crate::iter::Rev;
+
+/// Stream that goes over an `[ExactSizeIterator]` in reverse order.
+///
+/// This stream allows to switch fast from little endian ordering used in
+/// time-efficient algorithms, e.g. in slices `&[T]` into big endia ordering
+/// (used in space-efficient algorithms.
+///
+/// # Examples
+/// ```
+/// use ark_std::iterable::{Iterable, Reverse};
+///
+/// let le_v = &[1, 2, 3];
+/// let be_v = Reverse(le_v);
+/// let mut be_v_iter = be_v.iter();
+/// assert_eq!(be_v_iter.next(), Some(&3));
+/// assert_eq!(be_v_iter.next(), Some(&2));
+/// assert_eq!(be_v_iter.next(), Some(&1));
+/// ```
+#[derive(Clone, Copy)]
+pub struct Reverse<I>(pub I)
+where
+    I: Iterable,
+    I::Iter: DoubleEndedIterator;
+
+impl<I> Iterable for Reverse<I>
+where
+    I: Iterable,
+    I::Iter: DoubleEndedIterator,
+{
+    type Item = I::Item;
+    type Iter = Rev<I::Iter>;
+
+    #[inline]
+    fn iter(&self) -> Self::Iter {
+        self.0.iter().rev()
+    }
+
+    #[inline]
+    fn len(&self) -> usize {
+        self.0.len()
+    }
+}