From 26f16c9297ac937ad5ed66f2ba5011077ea0214b Mon Sep 17 00:00:00 2001
From: toastyandwarm <toby@prufrock.co.uk>
Date: Fri, 20 Dec 2024 18:43:56 +0000
Subject: [PATCH] day 20

---
 20/solution.hs | 109 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 20/solution.hs

diff --git a/20/solution.hs b/20/solution.hs
new file mode 100644
index 0000000..60b424f
--- /dev/null
+++ b/20/solution.hs
@@ -0,0 +1,109 @@
+import Data.List
+import Data.Array
+import Data.Maybe
+import Debug.Trace
+
+traceOut :: Show a => a -> a
+traceOut x = (traceShow x x)
+
+addFirst :: [[a]] -> a -> [[a]]
+addFirst [] x = [[x]]
+addFirst (y:ys) x = ([x] ++ y) : ys
+
+split :: [Char] -> String -> [String]
+split _ [] = []
+split delims (x:xs)
+  | elem x delims = [[]] ++ split delims xs
+  | otherwise     = addFirst (split delims xs) x
+
+--    x——>
+-- y
+-- |
+-- v
+
+data Square = Wall | Square {visited :: Bool, distance :: Maybe Int, start :: Bool, end :: Bool}
+
+instance Show Square where
+    show x = case x of
+        Wall -> "#"
+        Square {start=True} -> "S"
+        Square {end=True} -> "E"
+        Square {visited=True} -> "."
+        Square _ _ _ _ -> " "
+
+toSquare :: Char -> Maybe Square
+toSquare '#' = Just Wall
+toSquare '.' = Just Square {visited=False, distance=Nothing, start=False, end=False}
+toSquare 'S' = Just Square {visited=False, distance=Nothing, start=True,  end=False}
+toSquare 'E' = Just Square {visited=True,  distance=Just 0,  start=False, end=True}
+toSquare _ = Nothing
+
+--partial!
+splitInput :: String -> [[Char]]
+splitInput = split "\n"
+
+buildArray :: [[Char]] -> (Array (Int, Int) Square)
+buildArray grid = listArray ((0, 0), ((length grid)-1, (length (grid!!0))-1)) . concat . map (map (fromJust . toSquare)) $ grid
+
+inBounds :: ((Int, Int), (Int, Int)) -> (Int, Int) -> Bool
+inBounds ((a, b), (c, d)) (x, y) = (a <= x && x <= c && b <= y && y <= d)
+
+neighbours :: (Array (Int, Int) a) -> (Int, Int) -> [(Int, Int)]
+neighbours arr (x, y) = filter (inBounds (bounds arr)) [(x+1, y), (x, y+1), (x-1, y), (x, y-1)]
+
+doUpdate :: (Array (Int, Int) Square) -> [((Int, Int), Square)] -> (Array (Int, Int) Square)
+doUpdate arr [] = arr
+doUpdate arr ((update@(pos, square)):updates) = case distance (arr!pos) of
+    Nothing -> doUpdate (arr // [update]) updates
+    Just x -> case compare x (fromJust $ distance square) of
+        EQ -> doUpdate arr updates
+        GT -> doUpdate (arr // [update]) updates
+        LT -> doUpdate arr updates
+
+dijkstra :: (Array (Int, Int) Square) -> [(Int, Int)] -> (Array (Int, Int) Square)
+dijkstra arr buffer = if buffer == [] then arr else dijkstra (doUpdate arr updates) (nub $ map (\(x,_) -> x) updates)
+    where updates = concat $ map (\x -> concat $ map (\coords -> case (arr!coords) of
+            Wall -> []
+            Square {visited=True} -> []
+            y@(Square {}) -> [(coords, y {visited=True, distance=Just ((fromJust $ distance (arr!x)) + 1)})]) (neighbours arr x)) buffer
+
+findStart :: (Array (Int, Int) Square) -> (Int, Int)
+findStart = (\(a, _) -> a) . head . filter (\(_, b) -> case b of
+    Wall -> False
+    Square {start=s} -> s) . assocs
+findEnd :: (Array (Int, Int) Square) -> (Int, Int)
+findEnd = (\(a, _) -> a) . head . filter (\(_, b) -> case b of
+    Wall -> False
+    Square {end=e} -> e) . assocs
+
+--partial!
+solve1 :: (Array (Int, Int) Square) -> Int
+solve1 arr = length $ filter (>=100) $ concat $ map (\x -> map (\y -> (fromJust $ distance (dists!x)) - (fromJust $ distance (dists!y)) - 2) $ filter (\x -> isSquare (arr!x)) $ concat $ map (neighbours arr) $ neighbours arr x) $ map (\(i, e) -> i)$ filter (\(i, e) -> isSquare e) $ assocs arr
+    where dists = (dijkstra arr [findEnd arr])
+
+--partial!
+solve2 :: (Array (Int, Int) Square) -> Int
+solve2 arr = length $ filter (>=100) $ concat $ map (\x -> map (\y -> (fromJust $ distance (dists!x)) - (fromJust $ distance (dists!y)) - (taxicabDistance x y)) $ filter (\x -> isSquare (arr!x)) $ neighbourhood arr 20 x) $ map (\(i, e) -> i)$ filter (\(i, e) -> isSquare e) $ assocs arr
+    where dists = (dijkstra arr [findEnd arr])
+printGrid :: (Array (Int, Int) Square) -> String
+printGrid arr = concat $ [concat [show (arr!(x, y)) | y <- [0..140]] ++ "\n" | x <- [0..140]]
+
+isSquare :: Square -> Bool
+isSquare Wall = False
+isSquare Square {} = True
+
+taxicabDistance :: (Int, Int) -> (Int, Int) -> Int
+taxicabDistance (a, b) (c, d) = (abs(a-c)) + (abs(b-d))
+
+neighbourhood :: (Array (Int, Int) Square) -> Int -> (Int, Int) -> [(Int, Int)]
+neighbourhood arr r (x, y) = filter (inBounds (bounds arr)) $ map (\(a, b) -> (a+x, b+y)) $ filter (\(a, b) -> (abs a) + (abs b) <= r) [(a, b) | a <- [-r..r], b <- [-r..r]]
+
+main :: IO()
+main = do
+       fileinp <- readFile "input.txt"
+       let parsed = splitInput fileinp
+       let arr = buildArray parsed
+       let solved1 = solve1 arr
+       let solved2 = solve2 arr
+       print solved1
+       print solved2