Project Euler #11

11.hs

import Data.List.Split
import Control.Applicative
import Control.Monad

rows :: String -> [[Double]]
rows xs = (map read) <$> splitOn " " <$> lines xs

columns :: [[Double]] -> [[Double]]
columns rs = map (map head) $
    takeWhile (any (/= [])) $ 
    iterate (map tail) rs

extendDiags' acc ds [] = (reverse acc) ++ ds
extendDiags' acc (d:ds) (x:xs) = extendDiags' ((x:d):acc) ds xs

extendDiags = extendDiags' []
addDiags ds (c:col) = [c] : (extendDiags ds col)
diagonals' cs = foldl addDiags (map (:[]) (head cs)) (tail cs)

diagonals n k cs = 
    let ds = diagonals' cs
        nd = n-(k*2)+1
        in take nd $ drop (k-1) ds

checkLength n xs = n == (length $ take n xs)

-- group the lists into groups of k size
groups k xs =
    case checkLength k xs of
        True -> 
            let g = take k xs in
                g : (groups k $ tail xs)
        _    -> []

runsOf sideLength n matrix =
    let rs = rows matrix
        cs = columns rs
        lds = diagonals sideLength n cs
        rds = diagonals sideLength n (reverse cs)
        in join $ groups 4 <$> rs ++ cs ++ lds ++ rds

main = do
    matrix <- readFile "./grid"
    print $ maximum $ product <$> runsOf 40 4 matrix