week_of_the_year.pl

#!/usr/bin/perl
use warnings;
use strict;

# it's the same as POSIX: "date +%V"

# Week of the Year
# INPUT: firstDayOfTheWeek = 0:sun, 1:mon , undef:sun
sub yweek(;$) {
    my $fdw = shift // 0;
    # current date => nth day of the year
    #   0     1     2     3     4     5      6      [7]0   8
    my (undef,undef,undef,undef,undef,undef, undef, $yday, undef) = localtime(time);
    # (1st day of the year) was (which day of the week)?
    #   0     1     2     3     4     5      [6]          7      8
    my (undef,undef,undef,undef,undef,undef, $first_yday, undef, undef) = localtime(time - 24*60*60*$yday);
    # if year starts w/ sunday and 1st day of the week is not sunday;
    my $addFirstSunDelta = ($first_yday == 0 && $fdw != 0) ? 7 : 0;
    my $weekNum = int(($yday + $first_yday - $fdw + $addFirstSunDelta) / 7)+1;

    return $weekNum;
}

# testing:
print yweek()   ."\n";
print yweek(0)  ."\n";
print yweek(1)  ."\n";

# special case: when a year starts with Sunday and the 1st day of the week is Monday;