1 files changed, 78 insertions, 0 deletions
diff --git a/day04/day04.hs b/day04/day04.hs
new file mode 100644
index 0000000..83ba829
--- /dev/null
+++ b/day04/day04.hs
@@ -0,0 +1,78 @@
+import Data.List
+
+type Pr = (Int, Int)
+
+(!?) :: [[a]] -> Pr -> Maybe a
+grid !? (row, col)
+  | row < 0                     = Nothing
+  | row >= length grid          = Nothing
+  | col < 0                     = Nothing
+  | col >= (length $ head grid) = Nothing
+  | otherwise                   = Just (grid !! row !! col)
+
+findChs1 :: Char -> String -> Int -> Int -> [Pr] -> [Pr]
+findChs1 _  []    _ _ acc = acc
+findChs1 ch (h:t) j i acc
+  | ch == h   = findChs1 ch t j (i + 1) (acc ++ [(j,i)])
+  | otherwise = findChs1 ch t j (i + 1) acc
+
+findChs :: Char -> [String] -> Int -> [Pr] -> [Pr]
+findChs _  []    _ acc = acc
+findChs ch (h:t) j acc = findChs ch t (j + 1) (acc ++ findChs1 ch h j 0 [])
+
+checkCh :: Char -> [String] -> Pr -> Pr -> [(Pr, Pr)]
+checkCh ch grid coo@(row, col) dir@(dr, dc)
+  | grid !? (row + dr, col + dc) == Just ch = [(coo, dir)]
+  | otherwise = []
+
+findMCorners :: [String] -> Pr -> [(Pr, Pr)]
+findMCorners grid coo =
+  let checkM = checkCh 'M' grid coo in
+    checkM (-1, -1) ++
+    checkM (-1,  1) ++
+    checkM ( 1, -1) ++
+    checkM ( 1,  1)
+
+findMNeighs :: [String] -> Pr -> [(Pr, Pr)]
+findMNeighs grid coo =
+  let checkM = checkCh 'M' grid coo in
+    findMCorners grid coo ++
+    checkM (-1,  0) ++
+    checkM ( 0, -1) ++
+    checkM ( 0,  1) ++
+    checkM ( 1,  0)
+
+testXmas :: [String] -> (Pr, Pr) -> Bool
+testXmas grid ((row, col), (dr, dc)) =
+  grid !? (row + 2 * dr, col + 2 * dc) == Just 'A' &&
+  grid !? (row + 3 * dr, col + 3 * dc) == Just 'S'
+
+testMas :: [String] -> (Pr, Pr) -> Bool
+testMas grid ((row, col), (dr, dc)) =
+  grid !? (row - dr, col - dc) == Just 'S'
+
+car :: (a, a) -> a
+car (a, _) = a
+
+findReps :: [Pr] -> [Pr] -> [Pr]
+findReps acc [] = acc
+findReps acc (h:t)
+  | h `elem` t = findReps (h:acc) t
+  | otherwise = findReps acc t
+
+part1 grid = 
+  length $ filter (testXmas grid) $
+    concat $ map (findMNeighs grid) $
+    findChs 'X' grid 0 []
+
+part2 grid =
+  length $ findReps [] $
+    map (car) $ filter (testMas grid) $
+    concat $ map (findMCorners grid) $
+    findChs 'A' grid 0 []
+
+main = do
+  file <- readFile "day04.input"
+  let lns = lines file
+  putStrLn ("Part 1: " ++ (show $ part1 lns))
+  putStrLn ("Part 2: " ++ (show $ part2 lns))