Calculate the iso week of year in dart

datetime_iso_week_of_year.dart

extension DateWeekExtensions on DateTime {
  int get isoWeekOfYear {
    // Get the monday of week 1
    final DateTime mondayWeek1 = _isoWeek1Monday();

    // If this date is before the first week of the year, it is the same week as the last week of the previous year.
    if (isBefore(mondayWeek1)) {
      return DateTime(year - 1, 12, 31).isoWeekOfYear;
    }

    final int ordinalWeek1Monday = mondayWeek1.ordinalDate();
    final int ordinal = ordinalDate();

    // Calculate number of days after monday week 1
    int diffInDays = ordinal - ordinalWeek1Monday;

    // If the monday occurs on the previous year, we'll need to add a year to the diff
    if (year > mondayWeek1.year) {
      diffInDays += 365;
      // If it's a leap year and the leap day is before the date checked, add an extra day
      if (isLeapYear && DateTime(year, 2, 29).isBefore(this)) {
        diffInDays += 1;
      }
    }

    // Divide the difference by 7 to get the number of weeks
    int week = (diffInDays ~/ 7) + 1;

    // The week wraps around to 1 if week 53 doesn't contain a thursday.
    // TODO: write more tests for this. Does this work with other dates than dec 31st?
    if (week == 53 && weekday < DateTime.thursday) {
      return 1;
    }
    return week;
  }

  /**
   * Calculates the ordinal date
   * The ordinal date is the number of days since December 31st the previous year.
   * January 1st has the ordinal date 1
   * December 31st has the ordinal date 365, or 366 in leap years
   */
  int ordinalDate() {
    final DAYS_IN_MONTH = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    final monthsBefore = DAYS_IN_MONTH.getRange(0, month - 1);

    int days = monthsBefore.length > 0
        ? monthsBefore.reduce((value, element) => value + element)
        : 0;

    if (month > 2 && isLeapYear) {
      days += 1;
    }

    return days + day;
  }

  /**
   * Check if this date is on a leap year
   */
  bool get isLeapYear {
    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
  }

  /**
   * Gets the date of the monday of ISO week 1 this year
   */
  DateTime _isoWeek1Monday() {
    final jan4 = DateTime(year, 1, 4); // Jan 4 is always in week 1

    return DateTime(
      jan4.year,
      jan4.month,
      jan4.day - jan4.weekday + 1,
    );
  }
}

datetime_iso_week_of_year_test.dart

import 'package:test/test.dart';

import './datetime_iso_week_of_year.dart';

void t(int year, List<int> expectations) {
  group("$year", () {
    test("January 1st", () {
      expect(DateTime(year, 1, 1).isoWeekOfYear, expectations[0]);
    });
    test("Last of February", () {
      expect(
        DateTime(year, 2, DateTime(year).isLeapYear ? 29 : 28).isoWeekOfYear,
        expectations[1],
      );
    });
    test("December 31st", () {
      expect(DateTime(year, 12, 31).isoWeekOfYear, expectations[2]);
    });
  });
}

void main() {
  group("week_of_year", () {
    t(2000, [52, 9, 52]);
    t(2001, [1, 9, 1]);
    t(2002, [1, 9, 1]);
    t(2003, [1, 9, 1]);
    t(2004, [1, 9, 53]);
    t(2005, [53, 9, 52]);
    t(2006, [52, 9, 52]);
    t(2007, [1, 9, 1]);
    t(2008, [1, 9, 1]);
    t(2009, [1, 9, 53]);
    t(2010, [53, 8, 52]);
    t(2011, [52, 9, 52]);
    t(2012, [52, 9, 1]);
    t(2013, [1, 9, 1]);
    t(2014, [1, 9, 1]);
    t(2015, [1, 9, 53]);
    t(2016, [53, 9, 52]);
    t(2017, [52, 9, 52]);
    t(2018, [1, 9, 1]);
    t(2019, [1, 9, 1]);
    t(2020, [1, 9, 53]);
  });
}