In the 20×20 grid below, four numbers along a diagonal line have been marked in red.

$$ \begin{array} 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 \end{array} $$

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?

Matrix manipulation is troublesome in standard Python so we'll use Numpy.


In [11]:
import numpy as np

A = np.array([[ 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]], np.int8)

First we'll create a function generatin all row combinations:


In [75]:
def row(A, count):
    n, m = A.shape
    for i in range(0,n):
        for j in range(0, m-count+1):
            yield A[i, j:j+count]

Get the max for the rows:


In [76]:
max(np.prod(s) for s in row(A, 4))


Out[76]:
48477312

and for the columns by transposing the matrix:


In [77]:
max(np.prod(s) for s in row(A.transpose(), 4))


Out[77]:
51267216

Then create a function to generate the main diagonals:


In [78]:
def diagonal(A, count):
    n, m = A.shape
    for i in range(0,n-count+1):
        for j in range(0, m-count+1):
            yield A[i:i+count, j:j+count].diagonal()

Calculate the max product:


In [79]:
max(np.prod(s) for s in diagonal(A, 4))


Out[79]:
40304286

for the inverse diagonal we just need to flip the matrix left to right and use the main diagonal:


In [80]:
max(np.prod(s) for s in diagonal(np.fliplr(A), 4))


Out[80]:
70600674