This is an implementation of the Fisher-Yates shuffling algorithm in Swift as modernized by Richard Durstenfeld.

Fisher-Yates Shuffle

//# Fisher-Yates Shuffle
//## Implemented in Swift
//This is an implementation of the Fisher-Yates shuffling algorithm as modernized by Richard Durstenfeld. His version shuffles the collection in place. This implementation is based on the psuedo-code used to describe the algorithm at [Wikipedia](https://en.m.wikipedia.org/wiki/Fisher–Yates_shuffle#The_modern_algorithm). Here, we extend the Array struct with a ```shuffle()``` function. With this extension, we can quickly and uniformly shuffly any array (no matter the elements) in O(n) time.
//    From Wikipedia/n
//>The Fisher–Yates shuffle is an algorithm for generating a random permutation of a finite set—in plain terms, the algorithm shuffles the set. The algorithm effectively puts all the elements into a hat; it continually determines the next element by randomly drawing an element from the hat until no elements remain. The algorithm produces an unbiased permutation: every permutation is equally likely. The modern version of the algorithm is efficient: it takes time proportional to the number of items being shuffled and shuffles them in place.
//
//>The Fisher–Yates shuffle is named after Ronald Fisher and Frank Yates, who first described it, and is also known as the Knuth shuffle after Donald Knuth. A variant of the Fisher–Yates shuffle, known as Sattolo's algorithm, may be used to generate random cyclic permutations of length n instead of random permutations.
//> - Wikipedia

import Foundation

extension Array {
    mutating func shuffle() {
        guard count > 1 else {return}
        for shufflingIndex in (1..<count).reversed() {
            let randomIndex = Int(arc4random_uniform(UInt32(shufflingIndex)))
            swap(&self[shufflingIndex], &self[randomIndex])
        }
    }
}

Fisher-Yates Shuffle

Implemented in Swift

This is an implementation of the Fisher-Yates shuffling algorithm as modernized by Richard Durstenfeld. This implementation is based on the psuedo-code used to describe the algorithm at Wikipedia. Here, we extend the Array struct with a shuffle() function. With this extension, we can quickly and uniformly shuffly any array (no matter the elements) in O(n) time.
From Wikipedia/n

The Fisher–Yates shuffle is an algorithm for generating a random permutation of a finite set—in plain terms, the algorithm shuffles the set. The algorithm effectively puts all the elements into a hat; it continually determines the next element by randomly drawing an element from the hat until no elements remain. The algorithm produces an unbiased permutation: every permutation is equally likely. The modern version of the algorithm is efficient: it takes time proportional to the number of items being shuffled and shuffles them in place.
The Fisher–Yates shuffle is named after Ronald Fisher and Frank Yates, who first described it, and is also known as the Knuth shuffle after Donald Knuth. A variant of the Fisher–Yates shuffle, known as Sattolo's algorithm, may be used to generate random cyclic permutations of length n instead of random permutations.

• Wikipedia