# http://projecteuler.net/index.php?section=problems&id=11
# 
# In the 20x20 grid below, four numbers along a diagonal line have been marked in red.
# 
# The product of these numbers is 26 63 78 14 = 1788696.
# 
# What is the greatest product of four adjacent numbers in any direction 
# (up, down, left, right, or diagonally) in the 2020 grid?

use strict;
use warnings;
use Data::Dumper;

my $Grid    = 20;               # grid length, also height
my $Size    = $Grid * $Grid;    # items in the list
my $Product = 0;                # placeholder for highest product
                                # one item added for counting purposes
my @Items   = grep { length } 'EMPTY', split /\s+/, << '.';
        08 02 22 97 38 15 00 40 00 75 04 05 07 78 52 12 50 77 91 08
        49 49 99 40 17 81 18 57 60 87 17 40 98 43 69 48 04 56 62 00
        81 49 31 73 55 79 14 29 93 71 40 67 53 88 30 03 49 13 36 65
        52 70 95 23 04 60 11 42 69 24 68 56 01 32 56 71 37 02 36 91
        22 31 16 71 51 67 63 89 41 92 36 54 22 40 40 28 66 33 13 80
        24 47 32 60 99 03 45 02 44 75 33 53 78 36 84 20 35 17 12 50
        32 98 81 28 64 23 67 10 26 38 40 67 59 54 70 66 18 38 64 70
        67 26 20 68 02 62 12 20 95 63 94 39 63 08 40 91 66 49 94 21
        24 55 58 05 66 73 99 26 97 17 78 78 96 83 14 88 34 89 63 72
        21 36 23 09 75 00 76 44 20 45 35 14 00 61 33 97 34 31 33 95
        78 17 53 28 22 75 31 67 15 94 03 80 04 62 16 14 09 53 56 92
        16 39 05 42 96 35 31 47 55 58 88 24 00 17 54 24 36 29 85 57
        86 56 00 48 35 71 89 07 05 44 44 37 44 60 21 58 51 54 17 58
        19 80 81 68 05 94 47 69 28 73 92 13 86 52 17 77 04 89 55 40
        04 52 08 83 97 35 99 16 07 97 57 32 16 26 26 79 33 27 98 66
        88 36 68 87 57 62 20 72 03 46 33 67 46 55 12 32 63 93 53 69
        04 42 16 73 38 25 39 11 24 94 72 18 08 46 29 32 40 62 76 36
        20 69 36 41 72 30 23 88 34 62 99 69 82 67 59 85 74 04 36 16
        20 73 35 29 78 31 90 01 74 31 49 71 48 86 81 16 23 57 05 54
        01 70 54 71 83 51 54 69 16 92 33 48 61 43 52 01 89 19 67 48
.

my $i = 0;
while( ++$i < scalar @Items ) { 

    my @map = (
        ### condition                ### factors to multiple

        ### vertical down, can't be done on the last 3 rows
        ($i < ($Size - $Grid*3))    => [$i, $i+$Grid, $i+$Grid*2, $i+$Grid*3],
                                        "Vertical down    ",

        ### horizonal right, can't be done on the last 3 items in a row
        ((($i % $Grid) < ($Grid-3)) && ($i % $Grid))
                                    => [$i, $i+1, $i+2, $i+3],
                                        "Horizontal right ",

        ### right diag down, can't be done on the last 3 rows AND
        ### on the last 3 items in a row
        ($i < ($Size - $Grid*3)) && ($i % $Grid) < ($Grid-3) && ($i % $Grid),
                                    => [$i, $i+1+$Grid, $i+2+$Grid*2, $i+3+$Grid*3],
                                        "Right diag down  ",
        
        ### left diag down, can't be done on the last 3 rows AND
        ### on the first 3 items in a row
        ($i < ($Size - $Grid*3)) && ($i % $Grid) > 3  
                                    => [$i, $i-1+$Grid, $i-2+$Grid*2, $i-3+$Grid*3],
                                        "Left diag down   ",
    );
        
    my $j;
    while( @map ) {
        my $cond = shift @map;
        my $list = shift @map;
        my $diag = shift @map;

        $j++;
        
        print "[$i] Testing condition '$diag' [$j]...";

        if( $cond ) {
            my $val = 1;
            $val *= $Items[$_] for @$list;
    
            print "got: $val [@$list]\n";
    
            ### set it if is higher than the previous
            $Product = $val if $val > $Product;
        } else {
            print "condition failed\n";
        }        
    }
}        

print $Product;


__END__

                ### top left corner => right, down
    my @mut =   $pos == 1               ? ( $pos + 1, $Grid + 1 ) :   
                
                ### top right corner => left, down
                $pos == $Grid           ? ( $Grid - 1, $Grid + $Grid ) :   
                
                ### on the top bar => right, left, down
                $pos < $Grid            ? ( $pos - 1, $pos + 1, $pos + $Grid ) :
                
                ### bottom left corner  => up, right
                $pos == ($Size - $Grid) ? ( $pos - $Grid, $pos + 1 ) :
                
                ### bottom right corner => up, left
                $pos == $Size           ? ( $pos - $Grid, $pos - 1 ) :
                
                ### on the bottom bar => left, right, up
                $pos > ($Size - $Grid)  ? ( $pos - 1, $pos + 1, $pos - $Grid ) :
                
                ### on the left side => right, up, down
                $pos % $Grid == 1       ? ( $pos + 1, $pos - $Grid, $pos + $Grid ) :
                
                ### on the right side => left, up, down
                $pos % $Grid == 0       ? ( $pos - 1, $pos - $Grid, $pos + $Grid ) :

                ### normal square   => left, right, up, down
                ( $pos + 1, $pos - 1, $pos + $Grid, $pos - $Grid );