day10 part one

aoc2024.cabal

@@ -29,6 +29,7 @@ library
       Data.Matrix
       Data.Point
       Day1
+      Day10
       Day2
       Day3
       Day4

input/day10.txt

@@ -0,0 +1,55 @@
+3265810567887610176543232112345687657894520112310124343
+2178923456998543289432103001236799548765011201459835432
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+9854001236765898543287306789230345101870892143489430143
+0123112345654387620196215276541236989896781002438321056
+1434876014567821212305454108910107858785432211320123987
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+1234232143010349898801201873297867873018962780369432132
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+9432165409677816676543294543014236765019410452129237326
+0121670318761965589430123678985345891054569567932106587
+7650181229650874594321210789076256702143678398867603496
+8542397834541123671010012698123105213432121210768712345
+9231456998432010789898763283434554396541030123459632434
+2110765107322989690176654196543676787612345014568541023
+2019894236011878521085545087212987988901456905873456911
+1238763245010965430294236789803101876216767876912367800
+0145632100123454340123107898714312345665898965101054321

src/Data/Matrix.hs

@@ -13,6 +13,7 @@ module Data.Matrix
     groupByWith,
     filter,
     isInBounds,
+    points,
   )
 where
 
@@ -56,6 +57,9 @@ lookup position (Matrix hmap) = Map.lookup position hmap
 lookupMultiple :: [Point] -> Matrix v -> [v]
 lookupMultiple positions matrix = mapMaybe (`lookup` matrix) positions
 
+points :: Matrix v -> [Point]
+points (Matrix hmap) = Map.keys hmap
+
 insert :: Point -> v -> Matrix v -> Matrix v
 insert position value (Matrix hmap) =
   Matrix $

src/Day10.hs

@@ -0,0 +1,43 @@
+module Day10 (partOne, partTwo) where
+
+import Data.Char (digitToInt)
+import Data.List (intersect)
+import Data.Matrix (Matrix)
+import qualified Data.Matrix as M
+import Data.Point (Point)
+
+partOne :: String -> Int
+partOne input = length $ filter (uncurry (isReachable matrix)) candidates
+  where
+    matrix = parseMatrix input
+    trailHeads = M.points $ M.filter (== 0) matrix
+    trailEnds = M.points $ M.filter (== 9) matrix
+    candidates = concatMap (zip trailHeads . repeat) trailEnds
+
+isReachable :: Matrix Int -> Point -> Point -> Bool
+isReachable matrix x y = go (getCandidates 1 x) [y] 1
+  where
+    go :: [Point] -> [Point] -> Int -> Bool
+    go _ _ 9 = False
+    go leftFront rightFront step =
+      let leftFront' = concatMap (getCandidates (1 + step)) leftFront
+          rightFront' = concatMap (getCandidates (9 - step)) rightFront
+       in not (null (leftFront `intersect` rightFront)) || go leftFront' rightFront' (step + 1)
+
+    getCandidates :: Int -> Point -> [Point]
+    getCandidates value (row, col) =
+      let positions = [(row + 1, col), (row - 1, col), (row, col + 1), (row, col - 1)]
+          values = map (`M.lookup` matrix) positions
+       in map fst
+            $ filter
+              ( \(_, mValue) -> case mValue of
+                  Nothing -> False
+                  Just v -> v == value
+              )
+            $ zip positions values
+
+partTwo :: String -> Int
+partTwo input = 0
+
+parseMatrix :: String -> Matrix Int
+parseMatrix = M.buildMatrix . map (map digitToInt) . lines