From d7d4e670ed53883672d8c4458226d91dcc731569 Mon Sep 17 00:00:00 2001
From: Armin Ronacher <armin.ronacher@active-4.com>
Date: Wed, 22 Mar 2017 00:01:37 +0100
Subject: [PATCH] Added core::cmp::Reverse for sort_by_key reverse sorting

---
 src/libcore/cmp.rs | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/src/libcore/cmp.rs b/src/libcore/cmp.rs
index cb39796eecd7..d87615ad9a2c 100644
--- a/src/libcore/cmp.rs
+++ b/src/libcore/cmp.rs
@@ -322,6 +322,40 @@ impl Ordering {
     }
 }
 
+/// A helper struct for reverse ordering.
+///
+/// This struct is a helper to be used with functions like `Vec::sort_by_key` and
+/// can be used to reverse order a part of a key.
+///
+/// Example usage:
+///
+/// ```
+/// use std::cmp::Reverse;
+///
+/// let mut v = vec![1, 2, 3, 4, 5, 6];
+/// v.sort_by_key(|&num| (num >= 3, Reverse(num)));
+/// assert_eq!(v, vec![3, 2, 1, 6, 5, 4]);
+/// ```
+#[derive(PartialEq, Eq, Debug)]
+#[stable(feature = "rust1", since = "1.8.0")]
+pub struct Reverse<T: Ord + PartialOrd + Eq + PartialEq>(pub T);
+
+#[stable(feature = "rust1", since = "1.8.0")]
+impl<T: Ord + PartialOrd + Eq + PartialEq> PartialOrd for Reverse<T> {
+    #[inline]
+    fn partial_cmp(&self, other: &Reverse<T>) -> Option<Ordering> {
+        other.0.partial_cmp(&self.0)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.8.0")]
+impl<T: Ord + PartialOrd + Eq + PartialEq> Ord for Reverse<T> {
+    #[inline]
+    fn cmp(&self, other: &Reverse<T>) -> Ordering {
+        other.0.cmp(&self.0)
+    }
+}
+
 /// Trait for types that form a [total order](https://en.wikipedia.org/wiki/Total_order).
 ///
 /// An order is a total order if it is (for all `a`, `b` and `c`):