From 26d4c8f01c07fdb3b0c0354dd9b509c955a87e9c Mon Sep 17 00:00:00 2001
From: Adrian Friedli <adi@koalatux.ch>
Date: Sat, 8 Jun 2019 22:30:45 +0200
Subject: [PATCH 1/2] implement nth_back for Range

---
 src/libcore/iter/range.rs | 13 +++++++++++++
 src/libcore/tests/iter.rs | 17 +++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/src/libcore/iter/range.rs b/src/libcore/iter/range.rs
index 6bbf776fb8f17..e171108a146a6 100644
--- a/src/libcore/iter/range.rs
+++ b/src/libcore/iter/range.rs
@@ -281,6 +281,19 @@ impl<A: Step> DoubleEndedIterator for ops::Range<A> {
             None
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<A> {
+        if let Some(minus_n) = self.end.sub_usize(n) {
+            if minus_n > self.start {
+                self.end = minus_n.sub_one();
+                return Some(self.end.clone())
+            }
+        }
+
+        self.end = self.start.clone();
+        None
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]
diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index bedb9e756129c..171a33695bc6a 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1657,6 +1657,23 @@ fn test_range_nth() {
     assert_eq!(r, 20..20);
 }
 
+#[test]
+fn test_range_nth_back() {
+    assert_eq!((10..15).nth_back(0), Some(14));
+    assert_eq!((10..15).nth_back(1), Some(13));
+    assert_eq!((10..15).nth_back(4), Some(10));
+    assert_eq!((10..15).nth_back(5), None);
+    assert_eq!((-120..80_i8).nth_back(199), Some(-120));
+
+    let mut r = 10..20;
+    assert_eq!(r.nth_back(2), Some(17));
+    assert_eq!(r, 10..17);
+    assert_eq!(r.nth_back(2), Some(14));
+    assert_eq!(r, 10..14);
+    assert_eq!(r.nth_back(10), None);
+    assert_eq!(r, 10..10);
+}
+
 #[test]
 fn test_range_from_nth() {
     assert_eq!((10..).nth(0), Some(10));

From 8590074a01364c2263f6e3c3c42e4137e2f77b65 Mon Sep 17 00:00:00 2001
From: Adrian Friedli <adi@koalatux.ch>
Date: Sun, 9 Jun 2019 22:45:11 +0200
Subject: [PATCH 2/2] implement nth_back for RangeInclusive

---
 src/libcore/iter/range.rs | 28 ++++++++++++++++++++++++++++
 src/libcore/tests/iter.rs | 20 ++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/src/libcore/iter/range.rs b/src/libcore/iter/range.rs
index e171108a146a6..efda3b263cc97 100644
--- a/src/libcore/iter/range.rs
+++ b/src/libcore/iter/range.rs
@@ -451,6 +451,34 @@ impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> {
         })
     }
 
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<A> {
+        self.compute_is_empty();
+        if self.is_empty.unwrap_or_default() {
+            return None;
+        }
+
+        if let Some(minus_n) = self.end.sub_usize(n) {
+            use crate::cmp::Ordering::*;
+
+            match minus_n.partial_cmp(&self.start) {
+                Some(Greater) => {
+                    self.is_empty = Some(false);
+                    self.end = minus_n.sub_one();
+                    return Some(minus_n);
+                }
+                Some(Equal) => {
+                    self.is_empty = Some(true);
+                    return Some(minus_n);
+                }
+                _ => {}
+            }
+        }
+
+        self.is_empty = Some(true);
+        None
+    }
+
     #[inline]
     fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
         Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index 171a33695bc6a..020618ae7aeed 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1731,6 +1731,26 @@ fn test_range_inclusive_nth() {
     assert_eq!(ExactSizeIterator::is_empty(&r), true);
 }
 
+#[test]
+fn test_range_inclusive_nth_back() {
+    assert_eq!((10..=15).nth_back(0), Some(15));
+    assert_eq!((10..=15).nth_back(1), Some(14));
+    assert_eq!((10..=15).nth_back(5), Some(10));
+    assert_eq!((10..=15).nth_back(6), None);
+    assert_eq!((-120..=80_i8).nth_back(200), Some(-120));
+
+    let mut r = 10_u8..=20;
+    assert_eq!(r.nth_back(2), Some(18));
+    assert_eq!(r, 10..=17);
+    assert_eq!(r.nth_back(2), Some(15));
+    assert_eq!(r, 10..=14);
+    assert_eq!(r.is_empty(), false);
+    assert_eq!(ExactSizeIterator::is_empty(&r), false);
+    assert_eq!(r.nth_back(10), None);
+    assert_eq!(r.is_empty(), true);
+    assert_eq!(ExactSizeIterator::is_empty(&r), true);
+}
+
 #[test]
 fn test_range_step() {
     #![allow(deprecated)]