Rollup merge of #60555 - timvermeulen:rchunks_nth_back, r=scottmcm

Implement nth_back for RChunks(Exact)(Mut)

Part of #54054.

These implementations may not be optimal because of the use of `self.len()`, but it's quite cheap and simplifies the code a lot.

There's quite some duplication going on here, I wouldn't mind cleaning this up later. A good next step would probably be to add private `split_off_up_to`/`split_off_from` helper methods for slices since their behavior is commonly useful throughout the `Chunks` types.

r? @scottmcm

src/libcore/slice/mod.rs

@@ -4649,6 +4649,23 @@ impl<'a, T> DoubleEndedIterator for RChunks<'a, T> {
             Some(fst)
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.len();
+        if n >= len {
+            self.v = &[];
+            None
+        } else {
+            // can't underflow because `n < len`
+            let offset_from_end = (len - 1 - n) * self.chunk_size;
+            let end = self.v.len() - offset_from_end;
+            let start = end.saturating_sub(self.chunk_size);
+            let nth_back = &self.v[start..end];
+            self.v = &self.v[end..];
+            Some(nth_back)
+        }
+    }
 }
 
 #[stable(feature = "rchunks", since = "1.31.0")]
@@ -4774,6 +4791,24 @@ impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T> {
             Some(head)
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.len();
+        if n >= len {
+            self.v = &mut [];
+            None
+        } else {
+            // can't underflow because `n < len`
+            let offset_from_end = (len - 1 - n) * self.chunk_size;
+            let end = self.v.len() - offset_from_end;
+            let start = end.saturating_sub(self.chunk_size);
+            let (tmp, tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
+            let (_, nth_back) = tmp.split_at_mut(start);
+            self.v = tail;
+            Some(nth_back)
+        }
+    }
 }
 
 #[stable(feature = "rchunks", since = "1.31.0")]
@@ -4898,6 +4933,24 @@ impl<'a, T> DoubleEndedIterator for RChunksExact<'a, T> {
             Some(fst)
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.len();
+        if n >= len {
+            self.v = &[];
+            None
+        } else {
+            // now that we know that `n` corresponds to a chunk,
+            // none of these operations can underflow/overflow
+            let offset = (len - n) * self.chunk_size;
+            let start = self.v.len() - offset;
+            let end = start + self.chunk_size;
+            let nth_back = &self.v[start..end];
+            self.v = &self.v[end..];
+            Some(nth_back)
+        }
+    }
 }
 
 #[stable(feature = "rchunks", since = "1.31.0")]
@@ -5016,6 +5069,25 @@ impl<'a, T> DoubleEndedIterator for RChunksExactMut<'a, T> {
             Some(head)
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.len();
+        if n >= len {
+            self.v = &mut [];
+            None
+        } else {
+            // now that we know that `n` corresponds to a chunk,
+            // none of these operations can underflow/overflow
+            let offset = (len - n) * self.chunk_size;
+            let start = self.v.len() - offset;
+            let end = start + self.chunk_size;
+            let (tmp, tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
+            let (_, nth_back) = tmp.split_at_mut(start);
+            self.v = tail;
+            Some(nth_back)
+        }
+    }
 }
 
 #[stable(feature = "rchunks", since = "1.31.0")]

src/libcore/tests/slice.rs

@@ -356,6 +356,19 @@ fn test_rchunks_nth() {
     assert_eq!(c2.next(), None);
 }
 
+#[test]
+fn test_rchunks_nth_back() {
+    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
+    let mut c = v.rchunks(2);
+    assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next_back().unwrap(), &[4, 5]);
+
+    let v2: &[i32] = &[0, 1, 2, 3, 4];
+    let mut c2 = v2.rchunks(3);
+    assert_eq!(c2.nth_back(1).unwrap(), &[2, 3, 4]);
+    assert_eq!(c2.next_back(), None);
+}
+
 #[test]
 fn test_rchunks_last() {
     let v: &[i32] = &[0, 1, 2, 3, 4, 5];
@@ -407,6 +420,19 @@ fn test_rchunks_mut_nth() {
     assert_eq!(c2.next(), None);
 }
 
+#[test]
+fn test_rchunks_mut_nth_back() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let mut c = v.rchunks_mut(2);
+    assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next_back().unwrap(), &[4, 5]);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let mut c2 = v2.rchunks_mut(3);
+    assert_eq!(c2.nth_back(1).unwrap(), &[2, 3, 4]);
+    assert_eq!(c2.next_back(), None);
+}
+
 #[test]
 fn test_rchunks_mut_last() {
     let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
@@ -460,6 +486,19 @@ fn test_rchunks_exact_nth() {
     assert_eq!(c2.next(), None);
 }
 
+#[test]
+fn test_rchunks_exact_nth_back() {
+    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
+    let mut c = v.rchunks_exact(2);
+    assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next_back().unwrap(), &[4, 5]);
+
+    let v2: &[i32] = &[0, 1, 2, 3, 4, 5, 6];
+    let mut c2 = v2.rchunks_exact(3);
+    assert_eq!(c2.nth_back(1).unwrap(), &[4, 5, 6]);
+    assert_eq!(c2.next(), None);
+}
+
 #[test]
 fn test_rchunks_exact_last() {
     let v: &[i32] = &[0, 1, 2, 3, 4, 5];
@@ -518,6 +557,19 @@ fn test_rchunks_exact_mut_nth() {
     assert_eq!(c2.next(), None);
 }
 
+#[test]
+fn test_rchunks_exact_mut_nth_back() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let mut c = v.rchunks_exact_mut(2);
+    assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next_back().unwrap(), &[4, 5]);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4, 5, 6];
+    let mut c2 = v2.rchunks_exact_mut(3);
+    assert_eq!(c2.nth_back(1).unwrap(), &[4, 5, 6]);
+    assert_eq!(c2.next(), None);
+}
+
 #[test]
 fn test_rchunks_exact_mut_last() {
     let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];