prob11.hs

import Timer

main = do
    time $ maximum $ concat $ map (quadprods) (rows ++ cols ++ ldiags ++ rdiags)

mfilter :: ((Int, Int) -> Int) -> Int -> [Int]
mfilter f n = map (snd) $ filter (\(idx, x) -> f idx == n) matrix

row n = mfilter (fst) n
col n = mfilter (snd) n
ldiag n = mfilter (\(r, c) -> c - r + size - 1) n
rdiag n = mfilter (\(r, c) -> r + c) n

quadprods (a:b:c:d:xs) = a * b * c * d : quadprods (b:c:d:xs)
quadprods _ = []

rows = map (row) [0..(size - 1)]
cols = map (col) [0..(size - 1)]
ldiags = map (ldiag) [0..2 * (size - 1)]
rdiags = map (rdiag) [0..2 * (size - 1)]

len = length nums
size = (floor.sqrt.fromIntegral) len

matrix = zip (map (flip divMod size) [0..len]) nums

nums :: [Int]
nums = map read (words
    "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")