From dc82626262605665bbcea8f24925442e48dc897c Mon Sep 17 00:00:00 2001
From: "@amit.chandra" <@amit.chandra>
Date: Fri, 26 Apr 2019 19:53:18 +0530
Subject: [PATCH] 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 d06d107d32a..54c5285bc49 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -4178,6 +4178,22 @@ fn next_back(&mut self) -> Option<&'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 acf6b03791f..0233b96d3a7 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];
-- 
2.44.0