Swift sequences, generators etc

even_numbers_sequence.swift

class EvenNumbers: CollectionType {
  let startIndex : Int
  let endIndex : Int
  
  init(start: Int, end: Int) {
    self.startIndex = start
    self.endIndex = end
  }
  
  convenience init() {
    let start = 0
    let end = Int.max
    self.init(start: start, end: end)
  }
  
  func generate() -> AnyGenerator<Int> {
    var current = 0
    return anyGenerator {
      guard current <= self.endIndex else { return nil }
      
      let oldCurrent = current
      current += 1
      return self.nthEven(oldCurrent)
    }
  }
  
  subscript(i: Int) -> Int {
    assert(i >= 0)
    return nthEven(i)
  }
  
  private func nthEven(n: Int) -> Int {
    return n * 2
  }
}

let evens = EvenNumbers(start: 0, end: 6)

for x in evens {
  print(x)
}
// 0
// 2
// 4
// 6
// 8
// 10
// 12

evens[5] // 10
evens[-1] // assertion failed: : file <EXPR>, line 28

let evensGenerator = EvenNumbers().generate()

for i in 1...5 {
  print(evensGenerator.next())
}
// Optional(0)
// Optional(2)
// Optional(4)
// Optional(6)
// Optional(8)