this post was submitted on 18 Dec 2024
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Advent Of Code

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Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

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๐Ÿƒ - 2024 DAY 18 SOLUTIONS - ๐Ÿƒ (programming.dev)
submitted 1 week ago by CameronDev@programming.dev to c/advent_of_code@programming.dev
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Day 18: Ram Run

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[โ€“] gentooer@programming.dev 3 points 1 week ago* (last edited 1 week ago)

Haskell

Not really happy with performance, binary search would speed this up a bunch, takes about 1.3 seconds.

Update: Binary search got it to 960 ms.

Code

import Data.Maybe
import qualified Data.Set as S

type Coord = (Int, Int)

parse :: String -> [Coord]
parse = map (read . ('(' :) . (++ ")")) . takeWhile (not . null) . lines

shortest :: Coord -> [Coord] -> Maybe Int
shortest (x0, y0) corrupted' = go $ S.singleton (x0 - 1, y0 - 1)
    where
        corrupted = S.fromList corrupted'
        inside (x, y)
            | x < 0     = False
            | y < 0     = False
            | x0 <= x   = False
            | y0 <= y   = False
            | otherwise = True
        grow cs = S.filter inside $ S.unions $ cs :
            [ S.mapMonotonic (\(x, y) -> (x + dx, y + dy)) cs
            | (dx, dy) <- [(-1, 0), (0, -1), (0, 1), (1, 0)]
            ]
        go visited
            | (0, 0) `S.member` visited = Just 0
            | otherwise                 = case grow visited S.\\ corrupted of
                visited'
                    | S.size visited == S.size visited' -> Nothing
                    | otherwise                         -> succ <$> go visited'

main :: IO ()
main = do
    rs <- parse <$> getContents
    let size = (71, 71)
    print $ fromJust $ shortest size $ take 1024 rs
    putStrLn $ init $ tail $ show $ last $ zipWith const (reverse rs) $
        takeWhile (isNothing . shortest size) $ iterate init rs

Faster (binary search)

import Data.Maybe
import qualified Data.Set as S

type Coord = (Int, Int)

parse :: String -> [Coord]
parse = map (read . ('(' :) . (++ ")")) . takeWhile (not . null) . lines

shortest :: Coord -> [Coord] -> Maybe Int
shortest (x0, y0) corrupted' = go $ S.singleton (x0 - 1, y0 - 1)
    where
        corrupted = S.fromList corrupted'
        inside (x, y)
            | x < 0     = False
            | y < 0     = False
            | x0 <= x   = False
            | y0 <= y   = False
            | otherwise = True
        grow cs = S.filter inside $ S.unions $ cs :
            [ S.mapMonotonic (\(x, y) -> (x + dx, y + dy)) cs
            | (dx, dy) <- [(-1, 0), (0, -1), (0, 1), (1, 0)]
            ]
        go visited
            | (0, 0) `S.member` visited = Just 0
            | otherwise                 = case grow visited S.\\ corrupted of
                visited'
                    | S.size visited == S.size visited' -> Nothing
                    | otherwise                         -> succ <$> go visited'

solve2 :: Coord -> [Coord] -> Coord
solve2 r0 corrupted = go 0 $ length corrupted
    where
        go a z
            | succ a == z = corrupted !! a
            | otherwise   =
                let x = (a + z) `div` 2
                in  case shortest r0 $ take x corrupted of
                        Nothing -> go a x
                        Just _  -> go x z

main :: IO ()
main = do
    rs <- parse <$> getContents
    let size = (71, 71)
    print $ fromJust $ shortest size $ take 1024 rs
    putStrLn $ init $ tail $ show $ solve2 size rs