concurrentMap

gistfile1.txt

final class ThreadSafe<A> {
  private var _value: A
  private let queue = DispatchQueue(label: "ThreadSafe")
  init(_ value: A) {
    self._value = value
  }
  var value: A {
    return queue.sync { _value }
  }
  func atomically(_ transform: (inout A) -> ()) {
    queue.sync {
      transform(&self._value)
    }
  }
}

extension Array {
  func concurrentMap<B>(_ transform: @escaping (Element) -> B) -> [B] {
    let result = ThreadSafe(Array<B?>(repeating: nil, count: count))
    DispatchQueue.concurrentPerform(iterations: count) { idx in
      let element = self[idx]
      let transformed = transform(element)
      result.atomically {
        $0[idx] = transformed
      }
    }
    return result.value.map { $0! }
  }
}