let chunk1 = [0,1,2]
let chunk2 = [3,4,5]

let lock = NSLock()
var result = 0

var calculationOperations = [Operation]()
calculationOperations.append(BlockOperation(block: {
    // calculate chunk 1
    let res = 123
    print(Thread.current)
    lock.lock()
    result += res
    lock.unlock()
}))
calculationOperations.append(BlockOperation(block: {
    // calculate chunk 2
    let res = 123
    print(Thread.current)
    lock.lock()
    result += res
    lock.unlock()
}))


OperationQueue().addOperations(calculationOperations, waitUntilFinished: true)
print(result)