[swift-users] dispatch concurrent map: is this right?

Karl razielim at gmail.com
Sun Oct 30 03:43:29 CDT 2016

> On 30 Oct 2016, at 09:15, Karl <raziel.im+swift-users at gmail.com> wrote:
> I had the need for a concurrent map recently. I had a part of a program which needed to read chunks of data and concurrently process them and assemble the results in an array. This isn’t necessarily as obvious as it sounds, because of arrays being value types. I came up with the following snippet which I’d like to check for correctness; it could also be helpful to others.
> Perhaps this is something Dispatch should provide out-of-the-box?
> - Karl

Ah one second, I was refactoring this and forgot to test it. Here’s the actual code:

extension DispatchQueue {

  static func concurrentMap<T>(iterations: Int, execute block: (Int) -> T) -> [T] {

    let __result = UnsafeMutableRawBufferPointer.allocate(count: iterations * MemoryLayout<T>.stride)
    defer { __result.deallocate() }
    let _result  = __result.baseAddress?.assumingMemoryBound(to: T.self)
    let result   = UnsafeMutableBufferPointer<T>(start: _result, count: iterations)
    concurrentPerform(iterations: iterations) { idx in
      result[idx] = block(idx)
    return Array(result)

extension Array {
  func concurrentMap<T>(execute block: (Element)->T) -> [T] {
    return DispatchQueue.concurrentMap(iterations: count) { block(self[$0]) }

Unfortunately I don’t think there’s a way to get an array to take over a +1 UnsafeMutableBufferPointer without copying.

- Karl
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