#!/usr/bin/perl
use 5.14.2;
use strict;
use warnings;
use autodie;
sub isleap($){
    my $year = shift;
     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;
    }
    return $isleap;
}
sub result($){
    my $date = shift;
    my($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 $isleap = &isleap($year);
    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
+ saturday
        sunday monday );}
        when(1){@anchorday_week = qw(wednesday thursday friday saturda
+y sunday
        monday tuesday);}
        when(2){@anchorday_week=qw(thursday friday saturday sunday mon
+day tuesday
        wednesday);}
        when(3){@anchorday_week=qw(friday saturday sunday monday tuesd
+ay wednesday
        thursday);}
        when(4){@anchorday_week=qw(saturday sunday monday tuesday wedn
+esday
        thursday friday);}
        when(5){@anchorday_week=qw(sunday sunday monday tuesday wednes
+day friday
        saturday);}
        when(6){@anchorday_week = qw(monday tuesday wednesday thursday
+ friday
        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;
    return $anchorday_week[$nbdays];
}

sub format($){
    my $var = shift;
    my $isyear = shift;
    if($$var < 10 && !$isyear){
        $$var = '0'.$$var;
    }
    elsif($$var < 1000 && $isyear){
        while(!($$var =~ /\d\d\d\d/)){
            $$var = '0'.$$var;
        }
    }
}

my ($day,$month,$year);
$year = int(rand(3000));
$month = int(rand(11) +1);
given ($month){
    when(2){
        if(&isleap($year)){
            $day = int(rand(29)+1);
        }
        else{
            $day = int(rand(28)+1);
        }
    }
    default{
        if($month % 2 == 0 || $month == 8){
            $day = int(rand(31)+1);
        }
        else{
            $day = int(rand(30)+1);
        }
    }
}

&format(\$day,0);
&format(\$month,0);
&format(\$year,1);
my $date = $day.'/'.$month.'/'.$year;
print "what day of the week is $date?\n";
while(<>){
    my $day = $_;
    my $target = &result($date);
    if(!($day =~ m#$target#)){
        print "wrong, its ".&result($date)."\n";
        die;
    }
    else{
        print "you are right, its ".&result($date)."\n";
        die;
    }
}
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