this post was submitted on 09 Dec 2024
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Advent Of Code

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๐Ÿ’ฟ - 2024 DAY 9 SOLUTIONS -๐Ÿ’ฟ (programming.dev)
submitted 4 weeks ago* (last edited 4 weeks ago) by [email protected] to c/[email protected]
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Day 9: Disk Fragmenter

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[โ€“] [email protected] 2 points 4 weeks ago* (last edited 4 weeks ago) (4 children)

Haskell

Not a lot of time to come up with a pretty solution today; sorry.

Ugly first solution

import Data.List
import Data.Maybe
import Data.Sequence (Seq)
import Data.Sequence qualified as Seq

readInput :: String -> Seq (Maybe Int, Int)
readInput =
  Seq.fromList
    . zip (intersperse Nothing $ map Just [0 ..])
    . (map (read . singleton) . head . lines)

expand :: Seq (Maybe Int, Int) -> [Maybe Int]
expand = concatMap (uncurry $ flip replicate)

compact :: Seq (Maybe Int, Int) -> Seq (Maybe Int, Int)
compact chunks =
  case Seq.spanr (isNothing . fst) chunks of
    (suffix, Seq.Empty) -> suffix
    (suffix, chunks' Seq.:|> file@(_, fileSize)) ->
      case Seq.breakl (\(id, size) -> isNothing id && size >= fileSize) chunks' of
        (_, Seq.Empty) -> compact chunks' Seq.>< file Seq.<| suffix
        (prefix, (Nothing, gapSize) Seq.:<| chunks'') ->
          compact $ prefix Seq.>< file Seq.<| (Nothing, gapSize - fileSize) Seq.<| chunks'' Seq.>< (Nothing, fileSize) Seq.<| suffix

part1, part2 :: Seq (Maybe Int, Int) -> Int
part1 input =
  let blocks = dropWhileEnd isNothing $ expand input
      files = catMaybes blocks
      space = length blocks - length files
      compacted = take (length files) $ fill blocks (reverse files)
   in sum $ zipWith (*) [0 ..] compacted
  where
    fill (Nothing : xs) (y : ys) = y : fill xs ys
    fill (Just x : xs) ys = x : fill xs ys
part2 = sum . zipWith (\i id -> maybe 0 (* i) id) [0 ..] . expand . compact

main = do
  input <- readInput <$> readFile "input09"
  print $ part1 input
  print $ part2 input

[โ€“] [email protected] 2 points 4 weeks ago* (last edited 4 weeks ago) (3 children)

Second attempt! I like this one much better.

Edit: down to 0.040 secs now!

import Control.Arrow
import Data.Either
import Data.List
import Data.Map (Map)
import Data.Map qualified as Map

type Layout = ([(Int, (Int, Int))], Map Int Int)

readInput :: String -> Layout
readInput =
  map (read . singleton) . head . lines
    >>> (scanl' (+) 0 >>= zip) -- list of (pos, len)
    >>> zipWith ($) (intersperse Right [Left . (id,) | id <- [0 ..]])
    >>> partitionEithers
    >>> filter ((> 0) . snd . snd) *** Map.filter (> 0) . Map.fromAscList

checksum :: Layout -> Int
checksum = sum . map (\(id, (pos, len)) -> id * len * (2 * pos + len - 1) `div` 2) . fst

compact :: (Int -> Int -> Bool) -> Layout -> Layout
compact select (files, spaces) = foldr moveFile ([], spaces) files
  where
    moveFile file@(fileId, (filePos, fileLen)) (files, spaces) =
      let candidates = Map.assocs $ fst . Map.split filePos $ spaces
       in case find (select fileLen . snd) candidates of
            Just (spacePos, spaceLen) ->
              let spaces' = Map.delete spacePos spaces
               in if spaceLen >= fileLen
                    then
                      ( (fileId, (spacePos, fileLen)) : files,
                        if spaceLen == fileLen
                          then spaces'
                          else Map.insert (spacePos + fileLen) (spaceLen - fileLen) spaces'
                      )
                    else
                      moveFile
                        (fileId, (filePos + spaceLen, fileLen - spaceLen))
                        ((fileId, (spacePos, spaceLen)) : files, spaces')
            Nothing -> (file : files, spaces)

main = do
  input <- readInput <$> readFile "input09"
  mapM_ (print . checksum . ($ input) . compact) [const $ const True, (<=)]
[โ€“] VegOwOtenks 2 points 4 weeks ago (1 children)

It will always be a wonder to me how you manage to do so much in so few lines, even your naive solution only takes a few seconds to run. ๐Ÿคฏ

[โ€“] [email protected] 1 points 4 weeks ago (1 children)

Aww, thank you <3

It's just practice, I guess? (The maths degree probably doesn't hurt either)

[โ€“] VegOwOtenks 2 points 4 weeks ago

Maths degree at least explains the choice of language

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