`roundRobin` function from D std.range in Swift.

roundRobin.swift

// [roundRobin](http://dlang.org/phobos/std_range.html#roundRobin)
public func roundRobin<S:SequenceType>(sequences:S ...)
  -> SequenceOf<S.Generator.Element>
{
  func isDone(gs:[S.Generator?]) -> Bool {
    for g in gs { if (g != nil) { return false } }
    return true
  }
 
  return SequenceOf {_ -> GeneratorOf<S.Generator.Element> in
    var generators:[S.Generator?] = sequences.map {$0.generate()}
    let count = generators.count
    var idx = 0
    
    return GeneratorOf {
      while !isDone(generators) {
        if let r = generators[idx]?.next() {
          idx = (idx + 1) % count
          return r
        }
        
        generators[idx] = nil
        idx = (idx + 1) % count
      }
      
      return nil
    }
  }
}

//roundRobin(
//  [0, 3, 6],
//  [1, 4, 7, 9],
//  [2, 5, 8]).array //=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

public func roundRobinShortest<S:SequenceType>(sequences:S ...)
  -> SequenceOf<S.Generator.Element>
{
  return SequenceOf {_ -> GeneratorOf<S.Generator.Element> in
    var generators:[S.Generator?] = sequences.map {$0.generate()}
    let count = generators.count
    var idx = 0
    
    return GeneratorOf {
      switch generators[idx]?.next() {
      case .None: return nil
      case .Some(let r):
        idx = (idx + 1) % count
        return r
      }
    }
  }
}

//roundRobinShortest(
//  [0, 3, 6],
//  [1, 4, 7, 9],
//  [2]).array //=>[0,1,2,3,4]