fastDate.js

function fastDateFactory() {
    var DAYS_IN_YEAR = 365;
    var DAYS_IN_LEAP_YEAR = 366;
    var DAYS_IN_4_YEARS = DAYS_IN_LEAP_YEAR + DAYS_IN_YEAR * 3;
    var DAYS_IN_OTHER_FIRST_4_YEARS = DAYS_IN_YEAR * 4;
    var DAYS_IN_FIRST_100_YEARS = DAYS_IN_4_YEARS * 25;
    var DAYS_IN_NEXT_100_YEARS = DAYS_IN_OTHER_FIRST_4_YEARS + DAYS_IN_4_YEARS * 24;

    var DAYS_IN_400_YEARS = DAYS_IN_FIRST_100_YEARS + DAYS_IN_NEXT_100_YEARS * 3;

    var MONTH_DAYS_AGGREGATED_LEAP = [0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366];
    var MONTH_DAYS_AGGREGATED = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365];
    var FastDate;

    function stringToYMD(stringDate) {
      var dateParts;
      if (!(dateParts = /^(\d{4})-(\d{2})-(\d{2})$/.exec(stringDate))) {
        throw new Error('Date format invalid');
      }
      return {
        year: parseInt(dateParts[1], 10),
        month: parseInt(dateParts[2], 10),
        day: parseInt(dateParts[3], 10)
      };
    }

    function daysFromZeroPoint(ymd) {
      var yearRest = (ymd.year) % 400;
      /*jshint bitwise: false*/
      var leapYears = ((ymd.year - yearRest) / 400) * 97 +
        ((yearRest + 3) >> 2) - (yearRest > 300 ? 3 : yearRest > 200 ? 2 : yearRest > 100 ? 1 : 0);
      /*jshint bitwise: true*/
      return leapYears * DAYS_IN_LEAP_YEAR + (ymd.year - leapYears) * DAYS_IN_YEAR +
        (isLeap(ymd.year) ? MONTH_DAYS_AGGREGATED_LEAP[ymd.month - 1] : MONTH_DAYS_AGGREGATED[ymd.month - 1]) +
        ymd.day - 1;
    }

    function isLeap(year) {
      return (year % 4 === 0) && (year % 100 !== 0) || (year % 400 === 0);
    }

    FastDate = function(startDate) {
      if (this instanceof FastDate) {
        this.offsetFromZeroPoint = daysFromZeroPoint(stringToYMD(startDate));
      } else {
        return new FastDate(startDate);
      }
    };

    _.extend(
      FastDate.prototype,
      {
        stringToYMD: function(stringDate) {
          return stringToYMD(stringDate);
        },
        stringToFastDate: function(stringDate) {
          return daysFromZeroPoint(this.stringToYMD(stringDate)) - this.offsetFromZeroPoint;
        },
        fastDateToYMD: function(index) {
          var offset = index + this.offsetFromZeroPoint;
          var rest;
          var year;
          var month = 1;
          var inFirst100 = false;
          var inOtherFirst4 = false;
          var monthDays;
          rest = offset % DAYS_IN_400_YEARS;
          year = (offset - rest) / DAYS_IN_400_YEARS * 400;
          offset = rest;
          if (rest) {
            if (offset < DAYS_IN_FIRST_100_YEARS) {
              inFirst100 = true;
            } else {
              rest = (offset - DAYS_IN_FIRST_100_YEARS) % DAYS_IN_NEXT_100_YEARS;
              year += 100 + (offset - DAYS_IN_FIRST_100_YEARS - rest) / DAYS_IN_NEXT_100_YEARS * 100;
              offset = rest;
            }
            if (rest) {
              if (!inFirst100) {
                if (offset < DAYS_IN_OTHER_FIRST_4_YEARS) {
                  inOtherFirst4 = true;
                } else {
                  rest = (offset - DAYS_IN_OTHER_FIRST_4_YEARS) % DAYS_IN_4_YEARS;
                  year += 4 + (offset - DAYS_IN_OTHER_FIRST_4_YEARS - rest) / DAYS_IN_4_YEARS * 4;
                  offset = rest;
                }
              } else {
                rest = offset % DAYS_IN_4_YEARS;
                year += (offset - rest) / DAYS_IN_4_YEARS * 4;
                offset = rest;
              }
              if (rest) {
                if (!inOtherFirst4) {
                  if (offset >= DAYS_IN_LEAP_YEAR) {
                    rest = (offset - DAYS_IN_LEAP_YEAR) % DAYS_IN_YEAR;
                    year += 1 + (offset - DAYS_IN_LEAP_YEAR - rest) / DAYS_IN_YEAR;
                  }
                } else {
                  rest = offset % DAYS_IN_YEAR;
                  year += (offset - rest) / DAYS_IN_YEAR;
                }
              }
            }
          }
          monthDays = isLeap(year) ? MONTH_DAYS_AGGREGATED_LEAP : MONTH_DAYS_AGGREGATED;
          while (rest > monthDays[month]) {
            month++;
          }
          rest -= monthDays[month - 1] - 1;
          return {
            year: year,
            month: month,
            day: rest
          };
        },
        fastDateToSting: function(index) {
          var ymd = this.fastDateToYMD(index);
          return ('000' + ymd.year).substr(-4) + '-' + ('0' + ymd.month).substr(-2) + '-' + ('0' + ymd.day).substr(-2);
        },
        fastDateToDayOfWeek: function(index) {
          return (index + this.offsetFromZeroPoint + 6) % 7;
        },
        isFastDateWeekend: function(index) {
          return (index + this.offsetFromZeroPoint) % 7 < 2;
        }
      }
    );
    return FastDate;
  }