Algorithm rotating the contents of an array by the number of positions specified as a parameter.

ArrayRotation.swift

extension Array
{
    mutating func rotateBy(var positions: Int) {
        
        positions = positions < 0 ? (self.count - abs(positions) % self.count) : positions % self.count
        
        if self.count % 2 == 0 {
            rotateFrom(0, elements: self.count/2, stride: positions)
            rotateFrom(1, elements: self.count/2, stride: positions)
        }else{
            rotateFrom(0, elements: self.count, stride: positions)
        }
    }
    
    private mutating func rotateFrom(position: Int, elements: Int, stride: Int)
    {
        var counter = elements
        var index = position
        var toMove = self[index]
        while counter > 0
        {
            let toSave = self[(index+stride) % self.count]
            self[(index+stride) % self.count] = toMove
            toMove = toSave
            index = (index+stride) % self.count
            counter--
        }
    }
}