rotate.rs.html -- source
Description: Source to the Rust file `src/libcore/slice/rotate.rs`.
Keywords: rust, rustlang, rust-lang

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    use cmp;
    use mem::{self, MaybeUninit};
    use ptr;

    /// Rotation is much faster if it has access to a little bit of memory. This
    /// union provides a RawVec-like interface, but to a fixed-size stack buffer.
    #[allow(unions_with_drop_fields)]
    union RawArray<T> {
        /// Ensure this is appropriately aligned for T, and is big
        /// enough for two elements even if T is enormous.
        typed: [T; 2],
        /// For normally-sized types, especially things like u8, having more
        /// than 2 in the buffer is necessary for usefulness, so pad it out
        /// enough to be helpful, but not so big as to risk overflow.
        _extra: [usize; 32],
    }

    impl<T> RawArray<T> {
        fn cap() -> usize {
            if mem::size_of::<T>() == 0 {
                usize::max_value()
            } else {
                mem::size_of::<Self>() / mem::size_of::<T>()
            }
        }
    }

    /// Rotates the range `[mid-left, mid+right)` such that the element at `mid`
    /// becomes the first element.  Equivalently, rotates the range `left`
    /// elements to the left or `right` elements to the right.
    ///
    /// # Safety
    ///
    /// The specified range must be valid for reading and writing.
    ///
    /// # Algorithm
    ///
    /// For longer rotations, swap the left-most `delta = min(left, right)`
    /// elements with the right-most `delta` elements.  LLVM vectorizes this,
    /// which is profitable as we only reach this step for a "large enough"
    /// rotation.  Doing this puts `delta` elements on the larger side into the
    /// correct position, leaving a smaller rotate problem.  Demonstration:
    ///
    /// ```text
    /// [ 6 7 8 9 10 11 12 13 . 1 2 3 4 5 ]
    /// 1 2 3 4 5 [ 11 12 13 . 6 7 8 9 10 ]
    /// 1 2 3 4 5 [ 8 9 10 . 6 7 ] 11 12 13
    /// 1 2 3 4 5 6 7 [ 10 . 8 9 ] 11 12 13
    /// 1 2 3 4 5 6 7 [ 9 . 8 ] 10 11 12 13
    /// 1 2 3 4 5 6 7 8 [ . ] 9 10 11 12 13
    /// ```
    ///
    /// Once the rotation is small enough, copy some elements into a stack
    /// buffer, `memmove` the others, and move the ones back from the buffer.
    pub unsafe fn ptr_rotate<T>(mut left: usize, mid: *mut T, mut right: usize) {
        loop {
            let delta = cmp::min(left, right);
            if delta <= RawArray::<T>::cap() {
                // We will always hit this immediately for ZST.
                break;
            }

            ptr::swap_nonoverlapping(
                mid.sub(left),
                mid.add(right - delta),
                delta);

            if left <= right {
                right -= delta;
            } else {
                left -= delta;
            }
        }

        let mut rawarray = MaybeUninit::<RawArray<T>>::uninitialized();
        let buf = &mut (*rawarray.as_mut_ptr()).typed as *mut [T; 2] as *mut T;

        let dim = mid.sub(left).add(right);
        if left <= right {
            ptr::copy_nonoverlapping(mid.sub(left), buf, left);
            ptr::copy(mid, mid.sub(left), right);
            ptr::copy_nonoverlapping(buf, dim, left);
        }
        else {
            ptr::copy_nonoverlapping(mid, buf, right);
            ptr::copy(mid.sub(left), dim, left);
            ptr::copy_nonoverlapping(buf, mid.sub(left), right);
        }
    }

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