1 files changed, 21 insertions, 0 deletions
diff --git a/2024/4.hs b/2024/4.hs
new file mode 100644
index 0000000..913715f
--- /dev/null
+++ b/2024/4.hs
@@ -0,0 +1,21 @@
+import Data.List (tails, transpose)
+
+main :: IO ()
+main = do
+  let a `startsWith` b = take (length b) a == b
+  let searchIn word lns = length (filter (`startsWith` word) (tails lns)) + length (filter (`startsWith` word) (tails (reverse lns)))
+  let rows = id
+  let cols = transpose
+  let unshift = zipWith drop [0..]
+  let maindiag = concatMap (take 1) . unshift
+  let upperdiags = transpose . unshift
+  let diags1 mat = map maindiag (tails (tail mat)) ++ upperdiags mat
+  let diags2 = diags1 . map reverse
+  input <- lines <$> getContents
+  print $ sum (map (searchIn "XMAS") (concatMap ($ input) [rows, cols, diags1, diags2]))
+
+  let chunks n = filter ((== n) . length) . map (take n) . tails
+  let blocks n = concatMap (transpose . map (chunks n)) . chunks n
+  let rots blk = [blk, map reverse (transpose blk), reverse (map reverse blk), reverse (transpose blk)]
+  let isx_mas [[a,_,b],[_,c,_],[d,_,e]] = a == 'M' && b == 'M' && c == 'A' && d == 'S' && e == 'S'
+  print $ sum [fromEnum (isx_mas blk) | blk <- concatMap rots (blocks 3 input)]