Auto merge of #56842 - scottmcm:vecdeque-rotate, r=alexcrichton

Add unstable VecDeque::rotate_{left|right}

Like the ones on slices, but more efficient because vecdeque is a circular buffer.

Issue that proposed this: #56686

~~:bomb: Please someone look very carefully at the `unsafe` in this!  The `wrap_copy` seems to be exactly what this method needs, and the `len` passed to it is never more than half the length of the deque, but I haven't managed to prove to myself that it's correct :bomb:~~ I think I proved that this code meets the requirement of the unsafe code it's calling; please double-check, of course.

Author: bors

src/liballoc/collections/vec_deque.rs

@@ -1927,6 +1927,118 @@ impl<T> VecDeque<T> {
             self.truncate(new_len);
         }
     }
+
+    /// Rotates the double-ended queue `mid` places to the left.
+    ///
+    /// Equivalently,
+    /// - Rotates item `mid` into the first position.
+    /// - Pops the first `mid` items and pushes them to the end.
+    /// - Rotates `len() - mid` places to the right.
+    ///
+    /// # Panics
+    ///
+    /// If `mid` is greater than `len()`.  Note that `mid == len()`
+    /// does _not_ panic and is a no-op rotation.
+    ///
+    /// # Complexity
+    ///
+    /// Takes `O(min(mid, len() - mid))` time and no extra space.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vecdeque_rotate)]
+    ///
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf: VecDeque<_> = (0..10).collect();
+    ///
+    /// buf.rotate_left(3);
+    /// assert_eq!(buf, [3, 4, 5, 6, 7, 8, 9, 0, 1, 2]);
+    ///
+    /// for i in 1..10 {
+    ///     assert_eq!(i * 3 % 10, buf[0]);
+    ///     buf.rotate_left(3);
+    /// }
+    /// assert_eq!(buf, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+    /// ```
+    #[unstable(feature = "vecdeque_rotate", issue = "56686")]
+    pub fn rotate_left(&mut self, mid: usize) {
+        assert!(mid <= self.len());
+        let k = self.len() - mid;
+        if mid <= k {
+            unsafe { self.rotate_left_inner(mid) }
+        } else {
+            unsafe { self.rotate_right_inner(k) }
+        }
+    }
+
+    /// Rotates the double-ended queue `k` places to the right.
+    ///
+    /// Equivalently,
+    /// - Rotates the first item into position `k`.
+    /// - Pops the last `k` items and pushes them to the front.
+    /// - Rotates `len() - k` places to the left.
+    ///
+    /// # Panics
+    ///
+    /// If `k` is greater than `len()`.  Note that `k == len()`
+    /// does _not_ panic and is a no-op rotation.
+    ///
+    /// # Complexity
+    ///
+    /// Takes `O(min(k, len() - k))` time and no extra space.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vecdeque_rotate)]
+    ///
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf: VecDeque<_> = (0..10).collect();
+    ///
+    /// buf.rotate_right(3);
+    /// assert_eq!(buf, [7, 8, 9, 0, 1, 2, 3, 4, 5, 6]);
+    ///
+    /// for i in 1..10 {
+    ///     assert_eq!(0, buf[i * 3 % 10]);
+    ///     buf.rotate_right(3);
+    /// }
+    /// assert_eq!(buf, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+    /// ```
+    #[unstable(feature = "vecdeque_rotate", issue = "56686")]
+    pub fn rotate_right(&mut self, k: usize) {
+        assert!(k <= self.len());
+        let mid = self.len() - k;
+        if k <= mid {
+            unsafe { self.rotate_right_inner(k) }
+        } else {
+            unsafe { self.rotate_left_inner(mid) }
+        }
+    }
+
+    // Safety: the following two methods require that the rotation amount
+    // be less than half the length of the deque.
+    //
+    // `wrap_copy` requres that `min(x, cap() - x) + copy_len <= cap()`,
+    // but than `min` is never more than half the capacity, regardless of x,
+    // so it's sound to call here because we're calling with something
+    // less than half the length, which is never above half the capacity.
+
+    unsafe fn rotate_left_inner(&mut self, mid: usize) {
+        debug_assert!(mid * 2 <= self.len());
+        self.wrap_copy(self.head, self.tail, mid);
+        self.head = self.wrap_add(self.head, mid);
+        self.tail = self.wrap_add(self.tail, mid);
+    }
+
+    unsafe fn rotate_right_inner(&mut self, k: usize) {
+        debug_assert!(k * 2 <= self.len());
+        self.head = self.wrap_sub(self.head, k);
+        self.tail = self.wrap_sub(self.tail, k);
+        self.wrap_copy(self.tail, self.head, k);
+    }
 }
 
 impl<T: Clone> VecDeque<T> {

src/liballoc/tests/lib.rs

@@ -13,11 +13,12 @@
 #![feature(drain_filter)]
 #![feature(exact_size_is_empty)]
 #![feature(pattern)]
+#![feature(repeat_generic_slice)]
 #![feature(slice_sort_by_cached_key)]
 #![feature(str_escape)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
-#![feature(repeat_generic_slice)]
+#![feature(vecdeque_rotate)]
 
 extern crate core;
 extern crate rand;

src/liballoc/tests/vec_deque.rs

@@ -1309,3 +1309,137 @@ fn test_try_reserve_exact() {
     }
 
 }
+
+#[test]
+fn test_rotate_nop() {
+    let mut v: VecDeque<_> = (0..10).collect();
+    assert_unchanged(&v);
+
+    v.rotate_left(0);
+    assert_unchanged(&v);
+
+    v.rotate_left(10);
+    assert_unchanged(&v);
+
+    v.rotate_right(0);
+    assert_unchanged(&v);
+
+    v.rotate_right(10);
+    assert_unchanged(&v);
+
+    v.rotate_left(3);
+    v.rotate_right(3);
+    assert_unchanged(&v);
+
+    v.rotate_right(3);
+    v.rotate_left(3);
+    assert_unchanged(&v);
+
+    v.rotate_left(6);
+    v.rotate_right(6);
+    assert_unchanged(&v);
+
+    v.rotate_right(6);
+    v.rotate_left(6);
+    assert_unchanged(&v);
+
+    v.rotate_left(3);
+    v.rotate_left(7);
+    assert_unchanged(&v);
+
+    v.rotate_right(4);
+    v.rotate_right(6);
+    assert_unchanged(&v);
+
+    v.rotate_left(1);
+    v.rotate_left(2);
+    v.rotate_left(3);
+    v.rotate_left(4);
+    assert_unchanged(&v);
+
+    v.rotate_right(1);
+    v.rotate_right(2);
+    v.rotate_right(3);
+    v.rotate_right(4);
+    assert_unchanged(&v);
+
+    fn assert_unchanged(v: &VecDeque<i32>) {
+        assert_eq!(v, &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+    }
+}
+
+#[test]
+fn test_rotate_left_parts() {
+    let mut v: VecDeque<_> = (1..=7).collect();
+    v.rotate_left(2);
+    assert_eq!(v.as_slices(), (&[3, 4, 5, 6, 7, 1][..], &[2][..]));
+    v.rotate_left(2);
+    assert_eq!(v.as_slices(), (&[5, 6, 7, 1][..], &[2, 3, 4][..]));
+    v.rotate_left(2);
+    assert_eq!(v.as_slices(), (&[7, 1][..], &[2, 3, 4, 5, 6][..]));
+    v.rotate_left(2);
+    assert_eq!(v.as_slices(), (&[2, 3, 4, 5, 6, 7, 1][..], &[][..]));
+    v.rotate_left(2);
+    assert_eq!(v.as_slices(), (&[4, 5, 6, 7, 1, 2][..], &[3][..]));
+    v.rotate_left(2);
+    assert_eq!(v.as_slices(), (&[6, 7, 1, 2][..], &[3, 4, 5][..]));
+    v.rotate_left(2);
+    assert_eq!(v.as_slices(), (&[1, 2][..], &[3, 4, 5, 6, 7][..]));
+}
+
+#[test]
+fn test_rotate_right_parts() {
+    let mut v: VecDeque<_> = (1..=7).collect();
+    v.rotate_right(2);
+    assert_eq!(v.as_slices(), (&[6, 7][..], &[1, 2, 3, 4, 5][..]));
+    v.rotate_right(2);
+    assert_eq!(v.as_slices(), (&[4, 5, 6, 7][..], &[1, 2, 3][..]));
+    v.rotate_right(2);
+    assert_eq!(v.as_slices(), (&[2, 3, 4, 5, 6, 7][..], &[1][..]));
+    v.rotate_right(2);
+    assert_eq!(v.as_slices(), (&[7, 1, 2, 3, 4, 5, 6][..], &[][..]));
+    v.rotate_right(2);
+    assert_eq!(v.as_slices(), (&[5, 6][..], &[7, 1, 2, 3, 4][..]));
+    v.rotate_right(2);
+    assert_eq!(v.as_slices(), (&[3, 4, 5, 6][..], &[7, 1, 2][..]));
+    v.rotate_right(2);
+    assert_eq!(v.as_slices(), (&[1, 2, 3, 4, 5, 6][..], &[7][..]));
+}
+
+#[test]
+fn test_rotate_left_random() {
+    let shifts = [
+        6, 1, 0, 11, 12, 1, 11, 7, 9, 3, 6, 1,
+        4, 0, 5, 1, 3, 1, 12, 8, 3, 1, 11, 11,
+        9, 4, 12, 3, 12, 9, 11, 1, 7, 9, 7, 2,
+    ];
+    let n = 12;
+    let mut v: VecDeque<_> = (0..n).collect();
+    let mut total_shift = 0;
+    for shift in shifts.iter().cloned() {
+        v.rotate_left(shift);
+        total_shift += shift;
+        for i in 0..n {
+            assert_eq!(v[i], (i + total_shift) % n);
+        }
+    }
+}
+
+#[test]
+fn test_rotate_right_random() {
+    let shifts = [
+        6, 1, 0, 11, 12, 1, 11, 7, 9, 3, 6, 1,
+        4, 0, 5, 1, 3, 1, 12, 8, 3, 1, 11, 11,
+        9, 4, 12, 3, 12, 9, 11, 1, 7, 9, 7, 2,
+    ];
+    let n = 12;
+    let mut v: VecDeque<_> = (0..n).collect();
+    let mut total_shift = 0;
+    for shift in shifts.iter().cloned() {
+        v.rotate_right(shift);
+        total_shift += shift;
+        for i in 0..n {
+            assert_eq!(v[(i + total_shift) % n], i);
+        }
+    }
+}