package Weekday;

use strict;

use vars qw(@ISA @EXPORT_OK);

use Exporter;
@ISA = qw(Exporter);
@EXPORT_OK = qw(&day_of_week);

=head2 day_of_week($year, $month, $date)

Return the day of the week (0 == Sunday ... 6 == Saturday) for the
given date.  C<$year> should have four digits; C<$month> and C<$date>
both start at 1, not zero.

Algorithm is from the CRC Handbook of Mathematics, 30th Ed, page 738.

=cut

sub day_of_week {
  my ($year, $month, $date) = @_;

  $month -= 2;
  if ($month <= 0) {
    -- $year;
    $month += 12;
  }

  my $cent = int($year/100);
  $year %= 100;

  # print STDERR "year=$year, cent=$cent, date=$date, month=$month\n";

  my $dow = ($date
	     + int(2.6*$month - 0.2)
	     - 2*$cent
	     + $year
	     + int($year/4)
	     + int($cent/4));

  $dow += 7 while $dow < 0;

  $dow %= 7;

  return $dow;
}

1;