firstDayOffset static method

int firstDayOffset(int year, int month, MaterialLocalizations localizations)

Computes the offset from the first day of the week that the first day of the month falls on.

For example, September 1, 2017 falls on a Friday, which in the calendar localized for United States English appears as:

S M T W T F S
_ _ _ _ _ 1 2

The offset for the first day of the months is the number of leading blanks in the calendar, i.e. 5.

The same date localized for the Russian calendar has a different offset, because the first day of week is Monday rather than Sunday:

M T W T F S S
_ _ _ _ 1 2 3

So the offset is 4, rather than 5.

This code consolidates the following:

• DateTime.weekday provides a 1-based index into days of week, with 1 falling on Monday.
• MaterialLocalizations.firstDayOfWeekIndex provides a 0-based index into the MaterialLocalizations.narrowWeekdays list.
• MaterialLocalizations.narrowWeekdays list provides localized names of days of week, always starting with Sunday and ending with Saturday.

Implementation

static int firstDayOffset(
    int year, int month, MaterialLocalizations localizations) {
  // 0-based day of week for the month and year, with 0 representing Saturday.
  final int weekdayFromSaturday =
      (Jalali(year, month, 1).weekDay % 7); // Saturday is 0

  // 0-based start of week depending on the locale, with 0 representing Sunday.
  int firstDayOfWeekIndex = localizations.firstDayOfWeekIndex;

  // Adjust firstDayOfWeekIndex to be Saturday-based for comparison.
  firstDayOfWeekIndex =
      (firstDayOfWeekIndex - 6) % 7; // Saturday as 0, adjust accordingly

  // Compute the offset between the localized first day of the week and the first day of the month.
  return (weekdayFromSaturday - firstDayOfWeekIndex) % 7;
}