Random.swift

import Foundation

struct Random {
    static func randomNumberUpTo(upTo: Int) -> Int {
        return Int(arc4random_uniform(UInt32(upTo)))
    }
    
    static func randomNumberInRange(range: Range<Int>) -> Int {
        let distance = range.endIndex - range.startIndex
        let random = randomNumberUpTo(distance)
        
        return range.startIndex + random
    }
}

extension Array where Element: Equatable {
    func randomElements(numberOfElements: Int) -> [Element]? {
        guard self.count >= numberOfElements else {
            return nil
        }
        
        var elements = [Element]()
        
        while elements.count < numberOfElements {
            let index = Random.randomNumberUpTo(self.count)
            let randomElement = self[index]
            
            if elements.contains(randomElement) {
                continue
            }
            
            elements.append(randomElement)
        }
        
        return elements
    }
}