aoc2025

commit eff0a03b28482b76bc2e3e7877ddfe3428529771
parent f52d5fc351e7563e8e75ef296e236679e1bef867
Author: Robert Russell <robert@rr3.xyz>
Date:   Sun,  7 Dec 2025 22:02:18 -0800

Solve 07 Part 2

Diffstat:
M
07/Main.hs
 | 
40
++++++++++++++++++++++++++--------------
M
lib/Common/Matrix.hs
 | 
11
++++++++++-

2 files changed, 36 insertions(+), 15 deletions(-)
diff --git a/07/Main.hs b/07/Main.hs
@@ -6,34 +6,47 @@ import Common.Matrix (Matrix)
 import Common.Matrix qualified as M
 import Common.Util ((.:))
 import Control.Applicative
-import Data.Bool
 import Data.ByteString qualified as BS
 import Data.Functor
+import Data.Maybe
 import Sparsec
 import System.Exit (die)
 
 --------------------------------------------------------------------------------
 -- Solution
 
-data State = Empty | Source | ColdSplitter | HotSplitter | Beam deriving (Eq, Show)
+data State = Source | Void Int | Splitter Int deriving (Eq, Show)
 
-belowPowered :: Cellular State Bool
-belowPowered = any (`elem` [Source, Beam]) <$> C.neighbor (-1, 0)
+strength :: State -> Int
+strength = \case
+    Source -> 1
+    Void n -> n
+    Splitter _ -> 0
 
-besideHotSplitter :: Cellular State Bool
-besideHotSplitter = fmap (elem (Just HotSplitter)) . traverse C.neighbor $ [(0, -1), (0, 1)]
+reflectance :: State -> Int
+reflectance = \case
+    Source -> 0
+    Void _ -> 0
+    Splitter n -> n
+
+topPower :: Cellular State Int
+topPower = maybe 0 strength <$> C.neighbor (-1, 0)
+
+sidePower :: Cellular State Int
+sidePower = sum . map (maybe 0 reflectance) <$> traverse C.neighbor [(0, -1), (0, 1)]
 
 tachyon :: Cellular State State
 tachyon =
     C.self >>= \case
-        Empty -> bool Empty Beam .: (||) <$> belowPowered <*> besideHotSplitter
         Source -> pure Source
-        ColdSplitter -> bool ColdSplitter HotSplitter <$> belowPowered
-        HotSplitter -> pure HotSplitter
-        Beam -> pure Beam
+        Void _ -> Void .: (+) <$> topPower <*> sidePower
+        Splitter _ -> Splitter <$> topPower
 
 part1 :: Matrix State -> Int
-part1 = M.count (== HotSplitter) . C.stabilize tachyon
+part1 = M.count ((> 0) . reflectance) . C.stabilize tachyon
+
+part2 :: Matrix State -> Int
+part2 m = sum . fmap strength . fromJust . M.row (M.nrows m - 1) . C.stabilize tachyon $ m
 
 --------------------------------------------------------------------------------
 -- Parsing
@@ -44,7 +57,7 @@ instance Utf8Error ParseError where
 type P a = Parse ParseError a
 
 pCell :: P State
-pCell = char '.' $> Empty <|> char 'S' $> Source <|> char '^' $> ColdSplitter
+pCell = char 'S' $> Source <|> char '.' $> Void 0 <|> char '^' $> Splitter 0
 
 pRow :: P [State]
 pRow = many pCell <* char '\n'
@@ -70,5 +83,4 @@ main :: IO ()
 main = do
     input <- getInput
     print . part1 $ input
-
--- print . part2 $ input
+    print . part2 $ input
diff --git a/lib/Common/Matrix.hs b/lib/Common/Matrix.hs
@@ -1,4 +1,4 @@
-module Common.Matrix (Matrix, nrows, ncols, generate, fromList, (!?), count) where
+module Common.Matrix (Matrix, nrows, ncols, generate, fromList, (!?), row, count) where
 
 import Common.Util (consensus)
 import Data.Vector (Vector)
@@ -35,5 +35,14 @@ Matrix nr nc vs !? (i, j) =
         then Just $ vs V.! (i * nc + j)
         else Nothing
 
+row :: Int -> Matrix a -> Maybe (Vector a)
+row i (Matrix nr nc vs) =
+    if 0 <= i && i < nr
+        then Just (V.slice (i * nc) nc vs)
+        else Nothing
+
+-- TODO (requires copy)
+-- col :: Int -> Matrix a -> Maybe (Vector a)
+
 count :: (a -> Bool) -> Matrix a -> Int
 count p = V.length . V.filter p . valuesRowMajor