code: protected final int getBMPOffset(char ch)
      {
          return (ch >= UTF16.LEAD_SURROGATE_MIN_VALUE
                  && ch <= UTF16.LEAD_SURROGATE_MAX_VALUE)
                  ? getRawOffset(LEAD_INDEX_OFFSET_, ch)
                  : getRawOffset(0, ch);
                  // using a getRawOffset(ch) makes no diff
      }
docs: Gets the offset to data which the BMP character points to
  Treats a lead surrogate as a normal code point.
  @param ch BMP character
  @return offset to data