Fisher-Yates Shuffle for Swift

shuffle.swift

import Darwin // for arc4random_uniform

extension SequenceType {
    @warn_unused_result(mutable_variant="shuffleInPlace") func shuffle() -> [Generator.Element] {
        // this is more efficient than building an array and shuffling it
        // and it works on sequences too!
        var ary: [Generator.Element] = []
        ary.reserveCapacity(underestimateCount())
        for var x in self {
            let j = Int(arc4random_uniform(UInt32(ary.count+1)))
            if j != ary.count {
                swap(&x, &ary[j])
            }
            ary.append(x)
        }
        return ary
    }
}

extension MutableCollectionType where Index: RandomAccessIndexType {
    mutating func shuffleInPlace() {
        for i: Index in startIndex..<advance(endIndex, -2, startIndex) {
            let j: Index.Distance = numericCast(arc4random_uniform(numericCast(i.distanceTo(endIndex))))
            swap(&self[i], &self[i.advancedBy(j)])
        }
    }
}