2025 day 7 part 2 in Haskell

2025/day07/.gitignore

@@ -0,0 +1,3 @@
+solve
+*.o
+*.hi

2025/day07/Makefile

@@ -0,0 +1,14 @@
+.PHONY: all clean test
+
+all: solve
+
+test: solve
+	./solve sample.txt
+
+solve: solve.hs
+	ghc -dynamic -O -g -o $@ $^
+
+clean:
+	$(RM) solve
+	$(RM) *.o
+	$(RM) *.hi

2025/day07/sample.txt

@@ -0,0 +1,16 @@
+.......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............

2025/day07/solve.hs

@@ -0,0 +1,67 @@
+import Data.Array
+import Data.List
+import Data.Maybe
+import Data.Set
+import System.Environment
+import System.Exit
+
+main = getArgs >>= parse >>= run
+
+stringToArray :: [Char] -> Array Int Char
+stringToArray s = listArray (0, length s - 1) s
+
+run :: String -> IO ()
+run input =
+  let gridLines = lines input
+      startingPoint = findStartingPoint (head gridLines)
+      initial = Data.Set.singleton startingPoint
+      arrLines = Data.List.map stringToArray (tail gridLines)
+   in do
+        print (part1 arrLines startingPoint)
+        print (part2 arrLines startingPoint)
+
+findStartingPoint :: [Char] -> Int
+findStartingPoint line = fromJust (elemIndex 'S' line)
+
+part1 arrLines startingPoint =
+  let initial = Data.Set.singleton startingPoint
+   in simulate arrLines 0 initial
+
+checkHit :: Array Int Char -> Int -> [Int]
+checkHit line idx = case line ! idx of
+  '^' -> (idx - 1) : [idx + 1]
+  _ -> [idx]
+
+simulate [] count active = count
+simulate lines count active =
+  let arr = head lines
+      followUp = Data.Set.fromList $ concatMap (checkHit arr) (Data.Set.toList active)
+      hits = length (Data.Set.filter (\i -> arr ! i == '^') active)
+      remainder = tail lines
+   in simulate remainder (hits + count) followUp
+
+part2 :: [Array Int Char] -> Int -> Int
+part2 arrLines startingPosition =
+  let n = length arrLines
+      arr = listArray (0, n - 1) arrLines
+      width = length $ head arrLines
+      compute i pos
+        | i >= n = 1
+        | otherwise =
+            let line = arr ! i
+             in case line ! pos of
+                  '^' -> memo ! (i + 1, pos - 1) + memo ! (i + 1, pos + 1)
+                  _ -> memo ! (i + 1, pos)
+      memo =
+        array
+          ((0, 0), (n, width - 1))
+          [((i, pos), compute i pos) | i <- [0 .. n], pos <- [0 .. width - 1]]
+   in memo ! (0, startingPosition)
+
+parse ["-h"] = usage >> exitSuccess
+parse [] = usage >> die usageStr
+parse fs = concat `fmap` mapM readFile fs
+
+usageStr = "Usage: solve [-vh] [file ..]"
+
+usage = putStrLn usageStr