#!/usr/bin/perl
use 5.14.2;
use warnings;
use strict;
use autodie;
if(!defined($ARGV[0])){
    die "gimme a date DD/MM/YYYY!\n";
}

my $date = $ARGV[0];
my($day,$month,$year);
if(!($date =~ m#(?<day>\d\d)/(?<month>\d\d)/(?<year>\d\d\d\d)#)){
    print "$date did not match format DD/MM/YYYY\n";
    die;
}
else{
  $day = $+{day};
  $month = $+{month};
 $year = $+{year};
}
 my $isleap;
if( $year =~ m/\d\d00/){
    my $century = $year / 100;
    $isleap = $century % 4 == 0;
}
elsif($year =~ m/\d\d(?<decade>\d\d)/){
    $isleap = $+{decade} % 4 == 0;
}
if(!$isleap && $month == 02 && $day == 29){
    die "this ain't a leap year so $date is wrong!\n";
}
my $century_anchor_day = ((5 * (($year/100)%4))%7);
my $decade;
if($year =~ m/\d\d(?<decade>\d\d)/){
    $decade = $+{decade};
}
if($decade % 2 == 0){
    $decade /= 2;
}
else{
    $decade += 11;
    $decade /=2;
}
if($decade %2 == 0){
    $decade %= 7;
}
else{
    $decade += 11;
    $decade %= 7;
}
my $anchorday_drift = 7- $decade;
my @anchorday_week;
my $year_anchor_day = ($century_anchor_day+$anchorday_drift)%7;
given ($year_anchor_day){
    when(0){@anchorday_week = qw(tuesday wednesday thursday friday sat
+urday
    sunday monday );}
    when(1){@anchorday_week = qw(wednesday thursday friday saturday su
+nday
    monday tuesday);}
    when(2){@anchorday_week=qw(thursday friday saturday sunday monday 
+tuesday
    wednesday);}
    when(3){@anchorday_week=qw(friday saturday sunday monday tuesday w
+ednesday
    thursday);}
    when(4){@anchorday_week=qw(saturday sunday monday tuesday wednesda
+y
    thursday friday);}
    when(5){@anchorday_week=qw(sunday sunday monday tuesday wednesday 
+friday
    saturday);}
    when(6){@anchorday_week = qw(monday tuesday wednesday thursday fri
+day
    saturday sunday);}
}
my $doomsdates = {
        '01'=>3,
        '02'=>28,
        '03'=>0,
        '04'=>4,
        '05'=>9,
        '06'=>6,
        '07'=>11,
        '08'=>8,
        '09'=>5,
        '10'=>10,
        '11'=>7,
        '12'=>12,
};
if($isleap){
    $doomsdates->{'01'} = 4;
    $doomsdates->{'02'} = 29;
}
my $diff_to_doomsdates = $day - $doomsdates->{"$month"};
if($diff_to_doomsdates < 0){
    $diff_to_doomsdates *= -1;
}

my $nbdays = $diff_to_doomsdates % 7;
print "$ARGV[0] was a $anchorday_week[$nbdays]\n";</readmore>
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