byePostfixDecrement.swift

extension CollectionType {
    
    /// create a lazy succession of randomly ordered indices
    ///
    /// - returns: random index generator
    func scrambledIndexGenerator() -> AnyGenerator<Self.Index> {
        
        /// The original indices
        var indexArray = Array(indices)
        
        /// The boundary separating available and consumed indices
        var endBoundary: Int = numericCast(count)
        
        /// Build a genetor
        return anyGenerator {
            
            /// Generator completes when indices are exhausted
            if endBoundary == 0 {return nil}
            
            /// Randomly select the next location in the
            /// array of indices to use and adjust the boundary
            let nextIndexIndex = Int(arc4random_uniform(UInt32(endBoundary)))
            
            /// Fetch the index
            let nextIndex = indexArray[Int(nextIndexIndex)]
            
            /// Adjust the boundary
            endBoundary -= 1
            
            /// Move the consumed index after the boundary
            if nextIndexIndex != endBoundary {
                swap(&indexArray[nextIndexIndex], &indexArray[endBoundary])
            }
            
            /// Return the index
            return nextIndex
        }
    }
}