2025 day 7 part 2 in Haskell

2025/day07/Makefile

@@ -6,7 +6,7 @@ test: solve
 	./solve sample.txt
 
 solve: solve.hs
-	ghc -dynamic -O -o $@ $^
+	ghc -dynamic -O -g -o $@ $^
 
 clean:
 	$(RM) solve

2025/day07/solve.hs

@@ -1,4 +1,3 @@
-import Basement.Compat.IsList qualified as Set
 import Data.Array
 import Data.List
 import Data.Maybe
@@ -6,29 +5,59 @@ import Data.Set
 import System.Environment
 import System.Exit
 
-main = getArgs >>= parse >>= (print . solve . lines)
+main = getArgs >>= parse >>= run
-
-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
 
+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 :: [[Char]] -> Int -> Set Int -> Int
 simulate [] count active = count
 simulate lines count active =
-  let arr = stringToArray (head lines)
+  let arr = head lines
-      followUp = Data.Set.fromList $ concatMap (checkHit arr) (Set.toList active)
+      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