interspersed.swift

extension Sequence {
  func interspersed(with element: Element) -> [Element] {
    var result: [Element] = []
    var iterator = makeIterator()
    if let first = iterator.next() {
      result.append(first)
    }
    while let elementFromSelf = iterator.next() {
      result.append(element)
      result.append(elementFromSelf)
    }
    return result
  }
  
  func interspersed<S: Sequence>(with elements: S) -> [Element]
    where S.Element == Element
  {
    var result: [Element] = []
    var iterator = makeIterator()
    // Could optimize to only create this array if needed
    let interstitial = Array(elements)
    
    if let first = iterator.next() {
      result.append(first)
    }
    while let elementFromSelf = iterator.next() {
      result.append(contentsOf: interstitial)
      result.append(elementFromSelf)
    }
    return result
  }
  
  func alternating<S: Sequence>(with elements: S) -> [Element]
    where S.Element == Element
  {
    return zip(self, elements).reduce(into: []) { result, pair in
      result.append(pair.0)
      result.append(pair.1)
    }
  }
}

let numbers = [1, 2, 3, 4, 5]
numbers.interspersed(with: 0)         // [1, 0, 2, 0, 3, 0, 4, 0, 5]
numbers.interspersed(with: [10, 20])  // [1, 10, 20, 2, 10, 20, 3, 10, 20, 4, 10, 20, 5]
numbers.alternating(with: 100...200)  // [1, 100, 2, 101, 3, 102, 4, 103, 5, 104]