euler011.pl

#!/usr/bin/perl
################################################################################
# Euler 11
# Largest product in a grid
# Author: Eugene Kolo 2014
# Contact: www.eugenekolo.com

# In the 20×20 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 the same direction 
# (up, down, left, right, or diagonally) in the 20×20 grid?
################################################################################

use strict;

# Grid is 20x20
my @grid = (
    8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5, 7, 78, 52, 12, 50, 77, 91, 8,
    49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 4, 56, 62, 0,
    81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 3, 49, 13, 36, 65,
    52, 70, 95, 23, 4, 60, 11, 42, 69, 24, 68, 56, 1, 32, 56, 71, 37, 2, 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, 3, 45, 2, 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, 2, 62, 12, 20, 95, 63, 94, 39, 63, 8, 40, 91, 66, 49, 94, 21,
    24, 55, 58, 5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72,
    21, 36, 23, 9, 75, 0, 76, 44, 20, 45, 35, 14, 0, 61, 33, 97, 34, 31, 33, 95,
    78, 17, 53, 28, 22, 75, 31, 67, 15, 94, 3, 80, 4, 62, 16, 14, 9, 53, 56, 92,
    16, 39, 5, 42, 96, 35, 31, 47, 55, 58, 88, 24, 0, 17, 54, 24, 36, 29, 85, 57,
    86, 56, 0, 48, 35, 71, 89, 7, 5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58,
    19, 80, 81, 68, 5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77, 4, 89, 55, 40,
    4, 52, 8, 83, 97, 35, 99, 16, 7, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66,
    88, 36, 68, 87, 57, 62, 20, 72, 3, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69,
    4, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18, 8, 46, 29, 32, 40, 62, 76, 36,
    20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74, 4, 36, 16,
    20, 73, 35, 29, 78, 31, 90, 1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57, 5, 54,
    1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52, 1, 89, 19, 67, 48);

# Down, Up, Left, Right, DL, DR, UL, UR
my @dirFactors = (20, -20, -1, 1, 19, 21, -21, -19);
my $max = 0;
foreach my $gIndex (0..scalar(@grid)-1) {
    foreach my $dirFactor (@dirFactors) {
        my $product = 1;
        # Handle going right and out of bounds
        if ($dirFactor == 1 || $dirFactor == 21 || $dirFactor == -19) { 
            if ($gIndex % 20 == 7 || $gIndex % 20 == 8 || $gIndex % 20 == 9) { 
                next;
            }
        }
        # Handle going left and out of bounds
        if ($dirFactor == -1 || $dirFactor == 19 || $dirFactor == -21) { # Going Left
            if ($gIndex % 20 == 0 || $gIndex % 20 == 1 || $gIndex % 20 == 2) { # Close to edge
                next;
            }
        }
        # Handle going up or down and out of bounds
        if ($dirFactor == 20 || $dirFactor == -20) {
            if ($gIndex-60 <= 0 || $gIndex+60 >= 400) {
                next;
            }
        }
        # Multiply the 4 numbers going in the dir defined by $dirFactor
        foreach my $nthIndex (0..3) {
            $product = $product * $grid[$gIndex + $nthIndex * $dirFactor];
            if ($product > $max) {
                $max = $product;
            }
        }
    }
}
print "$max\n";