Fisher–Yates shuffle 與acr4andom_uniform() in swift 4

Fisher- Yates shuffle在運算隨機排列的通用algorithm 。

-- To shuffle an array a of n elements (indices 0..n-1):
for i from n−1 downto 1 do
     j ← random integer such that 0 ≤ j ≤ i
     exchange a[j] and a[i]

An equivalent version which shuffles the array in the opposite direction (from lowest index to highest) is:
-- To shuffle an array a of n elements (indices 0..n-1):
for i from 0 to n−2 do
     j ← random integer such that i ≤ j < n
     exchange a[i] and a[j]

運用extension 的方式 加入func shuffle()（注意：前方必須加mutating）
swift 4 程式碼：

 extension Array {

    /// shuffles contents of array

    mutating func shuffle(){

        guard count > 1 else {return}

        

        for i in indices.dropLast(){

            let diff = distance(from: i, to: endIndex)

            let j = index(i, offsetBy: diff.arc4random)

            swapAt(i,j)

        }

    }

}

    extension Int {

        var arc4random : Int {

            if self > 0 {

                return Int(arc4random_uniform(UInt32(self)))

            }else if self < 0 {

                return -Int(arc4random_uniform(UInt32(abs(self))))

            }else {

                return 0

            }

        }

    }

acr4random_uniform(max)
max需為UInt32，生成隨機數範圍 0..


加入extension後使用起來很簡單
shuffle 範例：
              var x = [1,2,3,4,5,6,7]
              x.shuffle()
acr4random 範例：
            let y = 30
                 y.arc4random