NSString+Surrogate.swift

import Foundation

extension NSString {
  func utf16IndexForIndex(index: Int) -> Int {
    var index = index
    var result = 0
    while result < index {
      if result + 1 < self.length &&
      CFStringIsSurrogateHighCharacter(self.characterAtIndex(result)) &&
      CFStringIsSurrogateLowCharacter(self.characterAtIndex(result + 1)) {
        index += 1
      }
      result += 1
    }
    return result
  }
  
  func indexForUTF16Index(index: Int) -> Int {
    var result = index
    (0..<min(self.length, index)).forEach { i in
      if i + 1 < self.length &&
      CFStringIsSurrogateHighCharacter(self.characterAtIndex(i)) &&
      CFStringIsSurrogateLowCharacter(self.characterAtIndex(i + 1)) {
        result -= 1
      }
    }
    return result
  }
}