Extension to CollectionType and MutableCollectionType to shuffle all or part of the collection elements.

CollectionType+Shuffle.swift

//
//  Created by Khanh Le Do on 6/4/16.
//  Copyright © 2016 kloc. All rights reserved.
//

import Foundation

/// Original shuffle methods given on Stackoverflow:
/// http://stackoverflow.com/questions/24026510/how-do-i-shuffle-an-array-in-swift

/// Here I added two more methods for special use cases:
/// - CollectionType.shuffleK
/// - MutableCollectionType.shuffleKInPlace

public extension MutableCollectionType where Index == Int {
    /// Shuffle the elements of `self` in-place.
    public mutating func shuffleInPlace() {
        // empty and single-element collections don't shuffle
        if count < 2 { return }
        
        for i in 0..<count - 1 {
            let j = Int(arc4random_uniform(UInt32(count - i))) + i
            guard i != j else { continue }
            swap(&self[i], &self[j])
        }
    }
    
    /// Shuffle the first k elements of `self` in-place.
    public mutating func shuffleKInPlace(k: Int) {
        // empty and single-element collections don't shuffle
        if count < 2 { return }
        
        let finalCount = min(k, count)
        for i in 0..<finalCount - 1 {
            let j = Int(arc4random_uniform(UInt32(count - i))) + i
            guard i != j else { continue }
            swap(&self[i], &self[j])
        }
    }
}

public extension CollectionType {
    /// Return a copy of `self` with its elements shuffled
    public func shuffle() -> [Generator.Element] {
        var list = Array(self)
        list.shuffleInPlace()
        return list
    }
    
    /// Return a subset (k-sized) of 'self' with elements shuffled from the original set
    public func shuffleK(k: Int) -> [Generator.Element] {
        var list = Array(self)
        list.shuffleKInPlace(k)
        let retSlice = list[0...(k-1)]
        return Array(retSlice)
    }
}