month and day to constellation

constellation.js

/**
 * get constellation string 
 * @param p_m[number] month 1-12
 * @param p_d[day] 1-31
 * @return[string] constellation name
 **/
function constellation (p_m, p_d) {
  var _name = '魔羯水瓶双鱼牡羊金牛双子巨蟹狮子处女天秤天蝎射手魔羯';
  var _day = [20,19,21,20,21,22,23,23,23,24,23,22];
  var _sign = p_m * 2 - (p_d < _day[p_m - 1] ? 2 : 0);
  return _name.substring(_sign, _sign + 2) + '座';
}