/**
 * codePoint - an integer containing a Unicode code point
 * return - the number of bytes required to store the code point in UTF-8
 */
function utf8Len(codePoint) {
  if(codePoint >= 0xD800 && codePoint <= 0xDFFF)
    throw new Error("Illegal argument: "+codePoint);
  if(codePoint < 0) throw new Error("Illegal argument: "+codePoint);
  if(codePoint <= 0x7F) return 1;
  if(codePoint <= 0x7FF) return 2;
  if(codePoint <= 0xFFFF) return 3;
  if(codePoint <= 0x1FFFFF) return 4;
  if(codePoint <= 0x3FFFFFF) return 5;
  if(codePoint <= 0x7FFFFFFF) return 6;
  throw new Error("Illegal argument: "+codePoint);
}

function isHighSurrogate(codeUnit) {
  return codeUnit >= 0xD800 && codeUnit <= 0xDBFF;
}

function isLowSurrogate(codeUnit) {
  return codeUnit >= 0xDC00 && codeUnit <= 0xDFFF;
}

/**
 * Transforms UTF-16 surrogate pairs to a code point.
 * See RFC2781
 */
function toCodepoint(highCodeUnit, lowCodeUnit) {
  if(!isHighSurrogate(highCodeUnit)) throw new Error("Illegal argument: "+highCodeUnit);
  if(!isLowSurrogate(lowCodeUnit)) throw new Error("Illegal argument: "+lowCodeUnit);
  highCodeUnit = (0x3FF & highCodeUnit) << 10;
  var u = highCodeUnit | (0x3FF & lowCodeUnit);
  return u + 0x10000;
}

/**
 * Counts the length in bytes of a string when encoded as UTF-8.
 * str - a string
 * return - the length as an integer
 */
function utf8ByteCount(str) {
  var count = 0;
  for(var i=0; i<str.length; i++) {
    var ch = str.charCodeAt(i);
    if(isHighSurrogate(ch)) {
      var high = ch;
      var low = str.charCodeAt(++i);
      count += utf8Len(toCodepoint(high, low));
    } else {
      count += utf8Len(ch);
    }
  }
  return count;
}