permutation.swift

extension CollectionType {
    // Return a scrambled index generator
    func scrambledIndexGenerator() -> AnyGenerator<Self.Index> {
        var indices = Array(self.startIndex..<self.endIndex)
        return anyGenerator {
            if indices.count == 0 {return nil}
            let nextIndex = arc4random_uniform(UInt32(indices.count))
            let nextItem = indices.removeAtIndex(Int(nextIndex))
            return nextItem
        }
    }
    
    // Permutation generator
    func permutatedGenerator() -> PermutationGenerator<Self,AnySequence<Self.Index>> {
        return PermutationGenerator(elements: self, indices: AnySequence(self.scrambledIndexGenerator()))
    }
}