Add Day 7, Puzzle 1 solution

adventofcode2025.cabal

@@ -19,6 +19,7 @@ executable adventofcode2025
   main-is:             Main.hs
   default-language:    Haskell2010
   build-depends:       base >= 4.7 && < 5
+                     , mtl
                      , split
   ghc-options:         -Wall
                        -Wcompat
@@ -36,3 +37,4 @@ executable adventofcode2025
     Day04
     Day05
     Day06
+    Day07

src/Day07.hs

@@ -0,0 +1,66 @@
+{-# OPTIONS_GHC -Wno-x-partial #-}
+
+module Day07
+  ( day07_1,
+  )
+where
+
+import Control.Monad.State
+import Data.List (elemIndex)
+import Data.Maybe (fromJust, isJust)
+
+type Coord = (Int, Int)
+
+type Grid = [String]
+
+parseInput :: IO [String]
+parseInput = do
+  lines <$> readFile "input/day7.txt"
+
+getStart :: Grid -> Int -> Coord
+getStart [] _ = (-1, -1)
+getStart (g : grid) row
+  | isJust start = (row, fromJust start)
+  | otherwise = getStart grid (row + 1)
+  where
+    start = elemIndex 'S' g
+
+isInside :: Coord -> Grid -> Bool
+isInside coord grid =
+  let w = length $ head grid
+      h = length grid
+   in fst coord >= 0 && fst coord < h && snd coord >= 0 && snd coord < w
+
+mark :: Coord -> State Grid ()
+mark coord = state $ \grid ->
+  ( (),
+    take (fst coord) grid ++ [take (snd coord) (grid !! fst coord) ++ ['x'] ++ drop (snd coord + 1) (grid !! fst coord)] ++ drop (fst coord + 1) grid
+  )
+
+beamSplitRecursive :: Coord -> State Grid ()
+beamSplitRecursive start = do
+  grid <- get
+  if not $ isInside start grid
+    then return ()
+    else
+      if grid !! fst start !! snd start == 'S' || grid !! fst start !! snd start == '.'
+        then beamSplitRecursive (fst start + 1, snd start)
+        else if grid !! fst start !! snd start == 'x' then return ()
+        else do
+          mark start
+          beamSplitRecursive (fst start, snd start - 1)
+          beamSplitRecursive (fst start, snd start + 1)
+
+beamSplit :: State Grid ()
+beamSplit = do
+  grid <- get
+  let start = getStart grid 0
+  beamSplitRecursive start
+
+day07_1 :: IO ()
+day07_1 = do
+  input <- parseInput
+  let grid = execState beamSplit input
+  putStrLn $
+      "Day 7, Puzzle 1 solution: "
+        ++ show (length $ concatMap (filter (== 'x')) grid)

src/Main.hs

@@ -6,6 +6,7 @@ import Day03 (day03_1, day03_2)
 import Day04 (day04_1, day04_2)
 import Day05 (day05_1, day05_2)
 import Day06 (day06_1, day06_2)
+import Day07 (day07_1)
 import System.Environment (getArgs)
 
 main :: IO ()
@@ -42,6 +43,9 @@ main = do
     "6" : _ -> do
       day06_1
       day06_2
+    "7" : "1" : _ -> day07_1
+    "7" : _ -> do
+      day07_1
     "all" : _ -> do
       day01_1
       day01_2
@@ -55,4 +59,5 @@ main = do
       day05_2
       day06_1
       day06_2
+      day07_1
     _ -> error "Not implemented"