Shuffle and pair in group

PairByGroup.swift

extension Array {
    func chunked(by chunkSize: Int) -> [[Element]] {
        return stride(from: 0, to: self.count, by: chunkSize).map {
            Array(self[$0 ..< Swift.min($0 + chunkSize, self.count)])
        }
    }
}
extension Array {
    mutating func shuffle () {
        for i in (0..<self.count).reversed() {
            let ix1 = i
            let ix2 = Int(arc4random_uniform(UInt32(i+1)))
            (self[ix1], self[ix2]) = (self[ix2], self[ix1])
        }
    }
}