Run a number of tasks in a SwiftNIO EventLoop

EventLoop+ParallelProcessing.swift

import NIO

extension EventLoop {
  
  public func process<Input, Output>(
    input: [Input],
    parallel: Int,
    process: @escaping (Input, EventLoop) -> EventLoopFuture<Output>)
    -> EventLoopFuture<[Result<Output, Error>]>
  {
    guard self.inEventLoop else {
      return self.flatSubmit { () in
        self.process(input: input, parallel: parallel, process: process)
      }
    }
    
    var slice   = input[...]
    var result  = [Result<Output, Error>?](repeating: nil, count: input.count)
    var running = 0
    
    var promise = self.makePromise(of: [Result<Output, Error>].self)
    
    func scheduleNextExecution() {
      guard running < parallel else {
        // no more space to schedule tasks
        return
      }
      
      let index = slice.startIndex
      guard let element = slice.popFirst() else {
        guard running == 0 else {
          // there are still tasks running
          return
        }

        return promise.succeed(result.map { $0! })
      }

      running += 1

      _ = process(element, self)
        .hop(to: self)
        .always { (res: Result<Output, Error>) in
          running -= 1
          result[index] = res
          scheduleNextExecution()
        }

      // try to schedule something new immediately
      scheduleNextExecution()
    }

    scheduleNextExecution()
    
    return promise.futureResult
  }
}