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	Rich Bowen
	CPAN ID: RBOW
	rbowen@rcbowen.com
	http://www.rcbowen.com

=head1 COPYRIGHT

Copyright (c) 2001 Rich Bowen. All rights reserved.
This program is free software; you can redistribute
it and/or modify it under the same terms as Perl itself.

The full text of the license can be found in the
LICENSE file included with this module.

=head1 SEE ALSO

 perl(1).
 Date::ICal
 Reefknot (http://reefknot.org/)
 Date::Easter
 Date::Passover
 Date::Leapyear

=cut

# }}}

# sub sundayletter {{{

sub letter { return sundayletters(@_) }

sub sundayletters {
    my $year = shift;
    
    my @letters = qw(A G F E D C B);

    my $p = parameter($year);

    my $q = int($year / 4); # divide by 4, discard remainder
    my $t = $year + $q + $p; # Add quotient to year, and add
                             # the parameter.
    my $d = $t % 7; # Remainder when divided by 7 gives the
                    # displacement from January 1 to the first Sunday
                    # of the year.

    my $letter;
    if (Date::Leapyear::isleap( $year )) {
        $letter = $letters[$d-1] . $letters[$d] ;
    } else {
        $letter = $letters[$d];
    }

    return $letter;
} #}}}

# sub parameter {{{

sub parameter {
# The "parameter" is a magic number that tracks how far the Gregorian
# calendar is from the Julian calendar. It has roughly to do with the
# fact that the Gregorian calendar observes leap year on the 4-century
# mark, and the Julian calendar does not.
    my $year = shift;

    my $S = int ($year / 100 );
    my $P = ( int( $S/4 ) - $S ) % 7;

    return $P;
} # }}}

1;