Rotate Array in Swift

Question

While exploring algorithms in Swift, couldn\'t find algorithm for array rotation in swift without using funcs shiftLeft / shiftRight.

C has

Answer 1

If anybody lands here after watching the Embracing Algorithms WWDC18 session by David Abrahams, here is one of the implementations of rotate from the swift/test/Prototypes/Algorithms.swift file.

extension MutableCollection where Self: BidirectionalCollection {
    /// Rotates the elements of the collection so that the element
    /// at `middle` ends up first.
    ///
    /// - Returns: The new index of the element that was first
    ///   pre-rotation.
    /// **- Complexity: O(*n*)**
    @discardableResult
    public mutating func rotate(shiftingToStart middle: Index) -> Index {
        self[..

This algorithms depends on reverseUntil(:) defined in the same file

extension MutableCollection where Self: BidirectionalCollection {

/// Reverses the elements of the collection, moving from each end until
/// `limit` is reached from either direction. The returned indices are the
/// start and end of the range of unreversed elements.
///
///     Input:
///     [a b c d e f g h i j k l m n o p]
///             ^
///           limit
///     Output:
///     [p o n m e f g h i j k l d c b a]
///             ^               ^
///             f               l
///
/// - Postcondition: For returned indices `(f, l)`:
///   `f == limit || l == limit`
@inline(__always)
@discardableResult
internal mutating func _reverseUntil(_ limit: Index) -> (Index, Index) {
    var f = startIndex
    var l = endIndex
    while f != limit && l != limit {
        formIndex(before: &l)
        swapAt(f, l)
        formIndex(after: &f)
    }
    return (f, l)
}
}