Fisher-Yates shuffle as protocol extension on any random-access collection

shuffle.swift

// adapted from original, which was (c) 2015 Nate Cook, licensed under the MIT license
//
// Fisher-Yates shuffle as protocol extensions

import Darwin

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


func arc4random_uniform<In: _SignedIntegerType, Out: _SignedIntegerType>(upto: In) -> Out {
    precondition(upto < numericCast(UInt32.max))
    return numericCast(Darwin.arc4random_uniform(numericCast(upto.toIntMax())))
}

extension MutableCollectionType where Index: RandomAccessIndexType {
    /// Shuffle the elements of `self` in-place.
    mutating func shuffleInPlace() {
        precondition(count < numericCast(UInt32.max))
        // empty and single-element collections don't shuffle
        if count < 2 { return }
        
        for var i = 0 as Index.Distance;i != count;++i {
            let j: Index.Distance = arc4random_uniform(count - i) + i
            swap(&self[startIndex.advancedBy(i)], &self[startIndex.advancedBy(j)])
        }
        
    }
}

[1, 2, 3].shuffle()
// [2, 3, 1]

let fiveStrings = stride(from: 0, through: 100, by: 5).map(String.init).shuffle()
// ["20", "45", "70", "30", ...]

var numbers = [1, 2, 3, 4]
numbers.shuffleInPlace()
// [3, 2, 1, 4]

Need to add a check before the swap(...) to make sure i != j—that's a trapping error in Swift 2.0b6.