Concurrent access to elements of Array in Swift - arrays

Question

I try to write and read from Arrays (from different arrays) in Swift in concurrent way, and have exception in the runtime on write operation.
I have concurrent blur code in Objective-C++:
- (cpp_blur::Bitmap)blur {
Bitmap result(_source);
auto indexes = [[NSIndexSet alloc] initWithIndexesInRange:NSMakeRange(0, _source.size())];
int minIndex = 0;
int maxIndex = static_cast<int>(result.size()) - 1;
[indexes enumerateIndexesWithOptions:NSEnumerationConcurrent
usingBlock:[&](NSUInteger idx, BOOL* _Nonnull stop){
double sum = 0.0;
for (int i = static_cast<int>(idx) - static_cast<int>(self.radius); i <= idx + self.radius; i++) {
int index = clamp(i, minIndex, maxIndex);
sum += self->_source.at(index);
}
sum /= static_cast<double>(self.radius * 2 + 1);
result.at(idx) = sum;
}];
return result;
}
I try to write same code in Swift:
#objc public func blur() -> [Double] {
var result = source
let indexSet = NSIndexSet(indexesIn: NSMakeRange(0, source.count))
let from = -Int(self.blurRadius)
let to = Int(self.blurRadius)
let lastIndex = self.source.count - 1
indexSet.enumerate(options: .concurrent) { (index:Int, _) in
let sum = ((from + index)...(to + index)).reduce(0.0) { (partial:Double, nonClamped:Int) -> Double in
let index = self.clamp(nonClamped, min: 0, max: lastIndex)
let result = partial + self.source[index]
return result
}
let avg = sum / Double(2 * self.blurRadius + 1);
result[index] = avg //here I have crash in runtime
}
return result
}
If I add a lock around result[index] = avg, this exception will move away, but I will have performance issue.
How I can change values of elements in Swift Array without lock (In same time I change only different elements)?
Github repo.