String.swift

extension String {
  var length: Int {
  return count
}
subscript (i: Int) -> String {
  return self[i ..< i + 1]
}
func substring(fromIndex: Int) -> String {
  return self[min(fromIndex, length) ..< length]
}
func substring(toIndex: Int) -> String {
  return self[0 ..< max(0, toIndex)]
}
subscript (r: Range<Int>) -> String {
  let range = Range(uncheckedBounds: (lower: max(0, min(length, r.lowerBound)),
upper: min(length, max(0, r.upperBound))))
  let start = index(startIndex, offsetBy: range.lowerBound)
  let end = index(start, offsetBy: range.upperBound - range.lowerBound)
  return String(self[start ..< end])
  }
}