Implement 2025 day 7 part 1 in Haskell

2025/day07/.gitignore

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+solve
+*.o
+*.hi

2025/day07/Makefile

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+.PHONY: all clean test
+
+all: solve
+
+test: solve
+	./solve sample.txt
+
+solve: solve.hs
+	ghc -dynamic -O -o $@ $^
+
+clean:
+	$(RM) solve
+	$(RM) *.o
+	$(RM) *.hi

2025/day07/sample.txt

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+.......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............

2025/day07/solve.hs

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+import Basement.Compat.IsList qualified as Set
+import Data.Array
+import Data.List
+import Data.Maybe
+import Data.Set
+import System.Environment
+import System.Exit
+
+main = getArgs >>= parse >>= (print . solve . lines)
+
+findStartingPoint line = fromJust (elemIndex 'S' line)
+
+solve lines = simulate (tail lines) 0 (Data.Set.singleton (findStartingPoint (head lines)))
+
+stringToArray :: [Char] -> Array Int Char
+stringToArray s = listArray (0, length s - 1) s
+
+checkHit :: Array Int Char -> Int -> [Int]
+checkHit line idx = case line ! idx of
+  '^' -> (idx - 1) : [idx + 1]
+  _ -> [idx]
+
+simulate :: [[Char]] -> Int -> Set Int -> Int
+simulate [] count active = count
+simulate lines count active =
+  let arr = stringToArray (head lines)
+      followUp = Data.Set.fromList $ concatMap (checkHit arr) (Set.toList active)
+      hits = length (Data.Set.filter (\i -> arr ! i == '^') active)
+      remainder = tail lines
+   in simulate remainder (hits + count) followUp
+
+parse ["-h"] = usage >> exitSuccess
+parse [] = usage >> die usageStr
+parse fs = concat `fmap` mapM readFile fs
+
+usageStr = "Usage: solve [-vh] [file ..]"
+
+usage = putStrLn usageStr