day 10

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

Lib.hs

@@ -1,6 +1,7 @@
-module Lib (readFile', split, count, tRead) where
+module Lib (readFile', tRead, buildGrid, split, count) where
 
 import Data.ByteString qualified as B (readFile)
+import Data.Map qualified as M
 import Data.Text (Text)
 import Data.Text.Encoding qualified as T (decodeUtf8)
 import Data.Text.Read qualified as TR
@@ -13,6 +14,10 @@ readFile' f = T.decodeUtf8 <$> B.readFile f
 tRead :: Text -> Int
 tRead = (\(Right (n, _)) -> n) . TR.signed TR.decimal
 
+-- Data.Map --
+buildGrid :: [[a]] -> M.Map (Int, Int) a
+buildGrid xs = M.fromList [((x, y), a) | (row, x) <- zip xs [0 ..], (a, y) <- zip row [0 ..]]
+
 -- Utility --
 split :: (Eq a) => a -> [a] -> [[a]]
 split delimiter list =

day10.hs

@@ -0,0 +1,48 @@
+import Data.List (find)
+import Data.Map qualified as M
+import Data.Maybe (catMaybes, isNothing, mapMaybe)
+import Data.Set qualified as S
+import Lib (buildGrid, readFile')
+
+data Direction = East | West | North | South deriving (Show, Eq)
+
+type Coord = (Int, Int)
+
+main :: IO ()
+main = do
+  grid <- buildGrid . lines <$> readFile "day10.in"
+  let [((sx, sy), _)] = M.toList $ M.filter (== 'S') grid
+  let path =
+        head $
+          mapMaybe
+            (uncurry $ travel grid [])
+            [ ((sx - 1, sy), North),
+              ((sx, sy + 1), East)
+            ]
+
+  putStr "Q1: "
+  print $ div (length path) 2
+
+  putStr "Q2: "
+  print $ pick (shoelace path) (length path)
+
+travel :: M.Map Coord Char -> [Coord] -> Coord -> Direction -> Maybe [Coord]
+travel grid a c@(x, y) d
+  | isNothing (M.lookup c grid) || cur == '.' = Nothing
+  | cur == 'S' = Just $ c : a
+  | (cur, d) `elem` [('L', West), ('|', North), ('J', East)] = travel grid (c : a) (x - 1, y) North
+  | (cur, d) `elem` [('F', West), ('|', South), ('7', East)] = travel grid (c : a) (x + 1, y) South
+  | (cur, d) `elem` [('L', South), ('-', East), ('F', North)] = travel grid (c : a) (x, y + 1) East
+  | (cur, d) `elem` [('J', South), ('-', West), ('7', North)] = travel grid (c : a) (x, y - 1) West
+  where
+    cur = grid M.! c
+
+-- https://en.wikipedia.org/wiki/Shoelace_formula
+-- this just uses trapezoids/triangles from consecutive points
+shoelace :: [Coord] -> Int
+shoelace xs = flip div 2 . abs . sum $ zipWith (\(x1, y1) (x2, y2) -> (x1 * y2) - (x2 * y1)) xs (tail $ cycle xs)
+
+-- https://en.wikipedia.org/wiki/Pick%27s_theorem
+-- area -> path length -> integers inside
+pick :: Int -> Int -> Int
+pick a b = (a + 1) - div b 2