day 4: alternate solution

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

aoc2024.cabal

@@ -43,3 +43,9 @@ executable day4
   hs-source-dirs: src
   main-is:        Day4.hs
   build-depends:  libaoc
+
+executable day4alt
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day4Alt.hs
+  build-depends:  libaoc

lib/AoC.hs

@@ -1,6 +1,6 @@
 module AoC where
 
-import Data.List (transpose)
+import Data.List (tails, transpose)
 import Text.Parsec (ParseError)
 
 -- extract Right value after parsing
@@ -27,3 +27,7 @@ findSubstrings sub str = findSubstrings' sub str 0
     findSubstrings' sub str@(x : xs) idx
       | take (length sub) str == sub = idx : findSubstrings' sub xs (idx + 1)
       | otherwise = findSubstrings' sub xs (idx + 1)
+
+-- get all subarrays of size n
+subArrays :: Int -> [[a]] -> [[[a]]]
+subArrays n xss = [[take n t | t <- tails xs] | xs <- xss]

src/Day4Alt.hs

@@ -0,0 +1,36 @@
+module Main where
+
+import qualified AoC as A (diagonals, subArrays)
+import Data.List (isPrefixOf, tails, transpose)
+
+part1 :: [[Char]] -> Int
+part1 grid =
+  (sum . concatMap countXmas)
+    [ grid,
+      transpose grid,
+      A.diagonals grid,
+      A.diagonals $ map reverse grid
+    ]
+  where
+    countXmas :: [[Char]] -> [Int]
+    countXmas = map (length . filter ((||) <$> isPrefixOf "XMAS" <*> isPrefixOf "SAMX") . tails)
+
+part2 :: [[Char]] -> Int
+part2 grid =
+  let groups = concat $ A.subArrays 3 $ transpose $ A.subArrays 3 grid
+   in length $ filter check groups
+  where
+    check :: [[Char]] -> Bool
+    check
+      [ [a, _, b],
+        [_, 'A', _],
+        [c, _, d]
+        ] = elem [a, d] ["MS", "SM"] && elem [b, c] ["MS", "SM"]
+    check _ = False
+
+main :: IO ()
+main =
+  do
+    raw <- lines <$> readFile "./inputs/day4.in"
+    putStr "Part 1: " >> print (part1 raw)
+    putStr "Part 2: " >> print (part2 raw)