Day 4, Part 1

Day4/puzzle1.hs

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+import Data.List (transpose, isPrefixOf)
+
+diagonals :: [String] -> [String]
+diagonals xs = diagonals' xs ++ diagonals' ((transpose . reverse) xs)
+               where diagonals' xs = transpose (zipWith drop [0..] xs)
+                                     ++ transpose (zipWith drop [1..] (transpose xs))
+
+countSubstrings :: String -> [String] -> Int
+countSubstrings word text = sum (map (countSubstrings' word) text) + sum (map (countSubstrings' word . reverse) text)
+                            + sum (map (countSubstrings' word) cols) + sum (map (countSubstrings' word . reverse) cols)
+                            + sum (map (countSubstrings' word) diags) + sum (map (countSubstrings' word . reverse) diags)
+                            where cols = transpose text
+                                  diags = diagonals text
+                                  countSubstrings' _ [] = 0
+                                  countSubstrings' word text@(_:rest) = if word `isPrefixOf` text
+                                                                               then 1 + countSubstrings' word rest
+                                                                               else countSubstrings' word rest
+    
+main = do
+    contents <- lines <$> readFile "day4.txt"
+    print $ countSubstrings "XMAS" contents