day 10

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

aoc2024.cabal

@@ -83,3 +83,9 @@ executable day9
   hs-source-dirs: src
   main-is:        Day9.hs
   build-depends:  containers
+
+executable day10
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day10.hs
+  build-depends:  containers

src/Day10.hs

@@ -0,0 +1,62 @@
+module Main where
+
+import Data.Char (digitToInt)
+import qualified Data.Map as M
+import qualified Data.Set as S
+
+type Coord = (Int, Int)
+
+type Grid = M.Map (Int, Int) Int
+
+data Dir = North | South | East | West deriving (Show)
+
+next :: Coord -> Dir -> Coord
+next (x, y) North = (x, y - 1)
+next (x, y) South = (x, y + 1)
+next (x, y) East = (x + 1, y)
+next (x, y) West = (x - 1, y)
+
+part1 :: Grid -> Int
+part1 grid = sum . map (S.size . go 0 S.empty) . M.keys . M.filter (== 0) $ grid
+  where
+    go :: Int -> S.Set Coord -> Coord -> S.Set Coord
+    go h s c
+      | h == 9 = S.insert c s
+      | otherwise =
+          S.unions
+            [ go h' s n
+              | d <- [North, South, East, West],
+                let n = next c d,
+                Just h' <- [M.lookup n grid],
+                h' == h + 1
+            ]
+
+part2 :: Grid -> Int
+part2 grid = sum . map (go 0) . M.keys . M.filter (== 0) $ grid
+  where
+    go :: Int -> Coord -> Int
+    go h c
+      | h == 9 = 1
+      | otherwise =
+          sum
+            [ go h' n
+              | d <- [North, South, East, West],
+                let n = next c d,
+                Just h' <- [M.lookup n grid],
+                h' == h + 1
+            ]
+
+main :: IO ()
+main =
+  do
+    raw <- lines <$> readFile "./inputs/day10.in"
+
+    let grid =
+          M.fromList
+            [ ((x, y), digitToInt ch)
+              | (y, row) <- zip [0 ..] raw,
+                (x, ch) <- zip [0 ..] row
+            ]
+
+    putStr "Part 1: " >> print (part1 grid)
+    putStr "Part 2: " >> print (part2 grid)