day 8

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

aoc2024.cabal

@@ -71,3 +71,9 @@ executable day7
   hs-source-dirs: src
   main-is:        Day7.hs
   build-depends:  libaoc
+
+executable day8
+  import:         common
+  hs-source-dirs: src
+  main-is:        Day8.hs
+  build-depends:  containers

src/Day8.hs

@@ -0,0 +1,44 @@
+module Main where
+
+import qualified Data.Set as S
+
+type Grid = [((Int, Int), Char)]
+
+part1 :: Int -> Int -> Grid -> Int
+part1 m n grid =
+  S.size . S.fromList $
+    [ (x, y)
+      | (c1@(x1, y1), ch1) <- grid,
+        (c2@(x2, y2), ch2) <- grid,
+        ch1 == ch2,
+        c1 /= c2,
+        let x = 2 * x1 - x2,
+        let y = 2 * y1 - y2,
+        x >= 0 && y >= 0 && x < n && y < m
+    ]
+
+part2 :: Int -> Int -> Grid -> Int
+part2 m n grid =
+  S.size . S.fromList $
+    [ (x, y)
+      | (c1@(x1, y1), ch1) <- grid,
+        (c2@(x2, y2), ch2) <- grid,
+        ch1 == ch2,
+        c1 /= c2,
+        let xs = takeWhile (\x -> x >= 0 && x < n) [x1, 2 * x1 - x2 ..],
+        let ys = takeWhile (\y -> y >= 0 && y < m) [y1, 2 * y1 - y2 ..],
+        (x, y) <- zip xs ys
+    ]
+
+main :: IO ()
+main =
+  do
+    raw <- lines <$> readFile "./inputs/day8.in"
+
+    let m = length raw
+        n = length $ head raw
+
+    let grid = [((x, y), ch) | (y, row) <- zip [0 ..] raw, (x, ch) <- zip [0 ..] row, ch /= '.']
+
+    putStr "Part 1: " >> print (part1 m n grid)
+    putStr "Part 2: " >> print (part2 m n grid)