Adjustment of my answer on Stack Overflow for a new comment: https://stackoverflow.com/questions/31554670/shift-swift-array/44739098?noredirect=1#comment109386944_44739098

ArrayShift.swift

import Foundation

extension Array {
    
    func shifted(by shiftAmount: Int) -> Array<Element> {
        
        // 1
        guard self.count > 0, (shiftAmount % self.count) != 0 else { return self }
        
        // 2
        let moduloShiftAmount = shiftAmount % self.count
        let negativeShift = shiftAmount < 0
        let effectiveShiftAmount = negativeShift ? moduloShiftAmount + self.count : moduloShiftAmount
        
        // 3
        let shift: (Int) -> Int = { return $0 + effectiveShiftAmount >= self.count ? $0 + effectiveShiftAmount - self.count : $0 + effectiveShiftAmount }
        
        // 4
        return self.enumerated().sorted(by: { shift($0.offset) < shift($1.offset) }).map { $0.element }
        
    }
    
    mutating func shifting(by shiftAmount: Int) {
        
        // 1
        guard self.count > 0, (shiftAmount % self.count) != 0 else { return }
        
        // 2
        let moduloShiftAmount = shiftAmount % self.count
        let negativeShift = shiftAmount < 0
        let effectiveShiftAmount = negativeShift ? moduloShiftAmount + self.count : moduloShiftAmount
        
        // 3
        let copy = self
        let shift: (Int) -> Int = { return $0 + effectiveShiftAmount >= copy.count ? $0 + effectiveShiftAmount - copy.count : $0 + effectiveShiftAmount }
        
        // 4
        self = self.enumerated().sorted(by: { shift($0.offset) < shift($1.offset) }).map { $0.element }
        
    }
    
}

let colorArray = [
    UIColor.red,
    UIColor.orange,
    UIColor.yellow,
    UIColor.green,
    UIColor.blue
]

let shiftedColorArray = [
    UIColor.green,
    UIColor.blue,
    UIColor.red,
    UIColor.orange,
    UIColor.yellow
]

colorArray.shifted(by: 2) == shiftedColorArray // returns true

[1,2,3,4,5,6,7].shifted(by: -23) // returns [3,4,5,6,7,1,2]

var testArray = [1,2,3,4,5,6,7]
testArray.shifting(by: -23)