/* ================================================================ */
ConversionResult ConvertUCS4toUTF8 ( UCS4** sourceStart, const UCS4* sourceEnd, 
                                     UTF8** targetStart, const UTF8* targetEnd)
{
  ConversionResult result = ok;
  register UCS4* source = *sourceStart;
  register UTF8* target = *targetStart;
  while (source < sourceEnd) {
    register UCS4 ch;
    register unsigned short bytesToWrite =  0 ;
    register const UCS4 byteMask = 0xBF;
    register const UCS4 byteMark = 0x80; 
    ch = *source++;
    if (ch >= kSurrogateHighStart && ch <= kSurrogateHighEnd
        && source < sourceEnd) {
      register UCS4 ch2 = *source;
      if (ch2 >= kSurrogateLowStart && ch2 <= kSurrogateLowEnd) {
        ch = ((ch - kSurrogateHighStart) << halfShift)
          + (ch2 - kSurrogateLowStart) + halfBase;
        ++source;
      };
    };
    if (ch < 0x80) {                            bytesToWrite =  1 ;
    } else if (ch < 0x800) {            bytesToWrite =  2 ;
    } else if (ch < 0x10000) {          bytesToWrite =  3 ;
    } else if (ch < 0x200000) {         bytesToWrite =  4 ;
    } else if (ch < 0x4000000) {        bytesToWrite =  5 ;
    } else if (ch <= kMaximumUCS4){     bytesToWrite =  6 ;
    } else {                                            bytesToWrite =  2 ;
    ch = kReplacementCharacter;
    }; /* I wish there were a smart way to avoid this conditional */
                
    target += bytesToWrite;
    if (target > targetEnd) {
      target -= bytesToWrite; result = targetExhausted; break;
    };
    switch (bytesToWrite) {     /* note: code falls through cases! */
      case  6 :   *--target = (ch | byteMark) & byteMask; ch >>=  6 ;
      case  5 :   *--target = (ch | byteMark) & byteMask; ch >>=  6 ;
      case  4 :   *--target = (ch | byteMark) & byteMask; ch >>=  6 ;
      case  3 :   *--target = (ch | byteMark) & byteMask; ch >>=  6 ;
      case  2 :   *--target = (ch | byteMark) & byteMask; ch >>=  6 ;
      case  1 :   *--target =  ch | firstByteMark[bytesToWrite];
    };
    target += bytesToWrite;
  };
  *sourceStart = source;
  *targetStart = target;
  return result;
};