hopefully working nth_back on chunks

Signed-off-by: wizAmit <amitforfriends_dns@yahoo.com>

Author: @amit.chandra

src/libcore/slice/mod.rs

@@ -4180,18 +4180,19 @@ 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)
         }
     }
 }

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]