collection.swift

extension CollectionType {
    /// Return a copy of `self` with its elements shuffled
    func shuffle() -> [Generator.Element] {
        var list = Array(self)
        list.shuffleInPlace()
        return list
    }
}

extension CollectionType where Index == Int {
    func sample(count:Int) -> ArraySlice<Generator.Element> {
        return shuffle().prefix(count)
    }
}

extension CollectionType where Index.Distance == Int {
    func sample() -> Generator.Element? {
        guard count < 1 else { return nil }
        let offset = Int(arc4random_uniform(UInt32(self.count)))
        let index = startIndex.advancedBy(offset)
        return self[index]
    }
}

extension MutableCollectionType where Index == Int {
    /// Shuffle the elements of `self` in-place.
    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])
        }
    }
}