Easy substrings in Swift

String+substring.swift

public extension String {
  func substring(start: Int, length: Int) -> String {
    if start >= count {
      return ""
    }

    let startIndex = index(self.startIndex, offsetBy: start)
    let endIndex: Index
    if start + length > count {
      endIndex = index(self.startIndex, offsetBy: count)
    } else {
      endIndex = index(self.startIndex, offsetBy: start + length)
    }

    return "\(self[startIndex..<endIndex])"
  }
}