PeekingIterator.swift

public struct PeekingIterator<T : IteratorProtocol> : IteratorProtocol {
  fileprivate var _iterator: T
  public fileprivate(set) var buffered: T.Element?
  
  public init(_ iterator: T) {
    self._iterator = iterator
    self.buffered = _iterator.next()
  }
  
  public init<U: Sequence>(_ sequence: U) where U.Iterator == T {
    self.init(sequence.makeIterator())
  }
  
  public mutating func next() -> T.Element? {
    defer { buffered = _iterator.next() }
    return buffered
  }
}

let a = 1...10
var p = PeekingIterator(a)
p.buffered    // 1
p.next()      // 1
p.next()      // 2
p.next()      // 3

p.buffered    // 4