From 4d9b9e9038102ada95d1898d303038803d752f62 Mon Sep 17 00:00:00 2001
From: "@amit.chandra" <@amit.chandra>
Date: Fri, 26 Apr 2019 19:53:18 +0530
Subject: [PATCH 1/2] wip nth_back on chunks

Signed-off-by: wizAmit <amitforfriends_dns@yahoo.com>
---
 src/libcore/slice/mod.rs   | 16 ++++++++++++++++
 src/libcore/tests/slice.rs | 13 +++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index 8731f48675356..049d38c158ce5 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -4152,6 +4152,22 @@ impl<'a, T> DoubleEndedIterator for Chunks<'a, T> {
             Some(snd)
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) {
+        let (end, overflow) = self.v.len().overflowing_sub(n);
+        if end < self.v.len()  || overflow {
+            self.v = &[];
+            None
+        } else {
+            let start = match end.checked_sub(self.chunk_size) {
+                Some(sum) => cmp::min(self.v.len(), sum),
+                None => self.v.len(),
+            };
+            let nth = &self.v[start..end];
+            self.v = &self.v[end..];
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
diff --git a/src/libcore/tests/slice.rs b/src/libcore/tests/slice.rs
index acf6b03791f01..0233b96d3a72d 100644
--- a/src/libcore/tests/slice.rs
+++ b/src/libcore/tests/slice.rs
@@ -134,6 +134,19 @@ fn test_chunks_nth() {
     assert_eq!(c2.next(), None);
 }
 
+#[test]
+fn test_chunks_nth_back() {
+    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
+    let mut c = v.chunks(2);
+    assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next().unwrap(), &[4, 5]);
+
+    let v2: &[i32] = &[0, 1, 2, 3, 4];
+    let mut c2 = v2.chunks(3);
+    assert_eq!(c2.nth_back(1).unwrap(), &[0, 1]);
+    assert_eq!(c2.next(), None);
+}
+
 #[test]
 fn test_chunks_last() {
     let v: &[i32] = &[0, 1, 2, 3, 4, 5];

From 73ca8bc140d4b0935ecb0199ea65723ec6a6791f Mon Sep 17 00:00:00 2001
From: "@amit.chandra" <@amit.chandra>
Date: Wed, 1 May 2019 23:34:07 +0530
Subject: [PATCH 2/2] hopefully working nth_back on chunks

Signed-off-by: wizAmit <amitforfriends_dns@yahoo.com>
---
 src/libcore/slice/mod.rs   | 17 +++++++++--------
 src/libcore/tests/slice.rs |  9 ++++++++-
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index 049d38c158ce5..00dbe7104d1a4 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -4154,18 +4154,19 @@ impl<'a, T> DoubleEndedIterator for Chunks<'a, T> {
     }
 
     #[inline]
-    fn nth_back(&mut self, n: usize) {
-        let (end, overflow) = self.v.len().overflowing_sub(n);
-        if end < self.v.len()  || overflow {
-            self.v = &[];
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let (end, overflow) = self.v.len().overflowing_sub(n * self.chunk_size);
+        if overflow {
+            self.v = &mut [];
             None
         } else {
             let start = match end.checked_sub(self.chunk_size) {
-                Some(sum) => cmp::min(self.v.len(), sum),
-                None => self.v.len(),
+                Some(res) => cmp::min(self.v.len(), res),
+                None => 0,
             };
-            let nth = &self.v[start..end];
-            self.v = &self.v[end..];
+            let nth_back = &self.v[start..end];
+            self.v = &self.v[..start];
+            Some(nth_back)
         }
     }
 }
diff --git a/src/libcore/tests/slice.rs b/src/libcore/tests/slice.rs
index 0233b96d3a72d..edea405fad791 100644
--- a/src/libcore/tests/slice.rs
+++ b/src/libcore/tests/slice.rs
@@ -139,12 +139,19 @@ fn test_chunks_nth_back() {
     let v: &[i32] = &[0, 1, 2, 3, 4, 5];
     let mut c = v.chunks(2);
     assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
-    assert_eq!(c.next().unwrap(), &[4, 5]);
+    assert_eq!(c.next().unwrap(), &[0, 1]);
+    assert_eq!(c.next(), None);
 
     let v2: &[i32] = &[0, 1, 2, 3, 4];
     let mut c2 = v2.chunks(3);
     assert_eq!(c2.nth_back(1).unwrap(), &[0, 1]);
     assert_eq!(c2.next(), None);
+    assert_eq!(c2.next_back(), None);
+
+    let v3: &[i32] = &[0, 1, 2, 3, 4];
+    let mut c3 = v3.chunks(10);
+    assert_eq!(c3.nth_back(0).unwrap(), &[0, 1, 2, 3, 4]);
+    assert_eq!(c3.next(), None);
 }
 
 #[test]