slow copying

swift_slow_copying

extension Array {
    
    /** split in chunks with given chunk size */
    func chunks(chunksize:Int) -> [Array<T>] {
        var words:[[T]] = [[T]]()
        words.reserveCapacity(self.count / chunksize)
        
        for var idx = chunksize; idx <= self.count; idx = idx + chunksize {
            let word = Array(self[idx - chunksize..<idx]) // this is slow
            words.append(word) // slower than this
        }
        let reminder = Array(suffix(self, self.count % chunksize))
        if (reminder.count > 0) {
            words.append(reminder)
        }
        return words
    }
}

let message = [UInt8](count: 1024 * 1024, repeatedValue: 7)
//message.chunks(16)

rdar://20173825