72 lines
3.4 KiB
Haskell
72 lines
3.4 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators heap vector
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-- very slow with runghc, use ghc -O3 -o second second.hs and get the result in seconds
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module Main (main) where
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import Control.Monad (void, when)
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import qualified Data.List as L
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import Data.Maybe
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import qualified Data.Vector as V
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import Data.Void (Void)
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import Text.Megaparsec
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import Text.Megaparsec.Char
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exampleExpectedOutput = 2858
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type Input = [Int]
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type Parser = Parsec Void String
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parseBlockSize :: Parser Int
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parseBlockSize = read . pure <$> digitChar
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parseInput' :: Parser Input
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parseInput' = some parseBlockSize <* eol <* eof
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parseInput :: String -> IO Input
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parseInput filename = do
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input <- readFile filename
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case runParser parseInput' filename input of
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Left bundle -> error $ errorBundlePretty bundle
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Right input' -> return input'
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type Disk = V.Vector (Maybe Int) -- Maybe fileId
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type File = (Int, Int, Int) -- fileId, fileIndex, fileSize
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type Free = (Int, Int) -- index, size
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compute :: Input -> Int
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compute input = V.sum $ V.imap checksum defragmentedDisk
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where
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checksum _ Nothing = 0
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checksum i (Just n) = i * n
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defragmentedDisk :: Disk
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(_, defragmentedDisk) = L.foldr defragment (frees, startingDisk) files
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startingDisk :: Disk
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startingDisk = V.replicate (sum input) Nothing
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(files, frees, _, _) = computeFileIndexAndSize input ([], [], 0, 0)
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computeFileIndexAndSize :: Input -> ([File], [Free], Int, Int) -> ([File], [Free], Int, Int)
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computeFileIndexAndSize [] acc = acc
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computeFileIndexAndSize (fileSize:[]) (files, frees, fileId, fileIndex) = (files ++ [(fileId, fileIndex, fileSize)], frees, 0, 0)
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computeFileIndexAndSize (fileSize:freeSize:fs) (files, frees, fileId, fileIndex) = computeFileIndexAndSize fs
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( files ++ [(fileId, fileIndex, fileSize)]
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, frees ++ [(fileIndex + fileSize, freeSize)]
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, fileId + 1
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, fileIndex + fileSize + freeSize )
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defragment :: File -> ([Free], Disk) -> ([Free], Disk)
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defragment (fileId, fileIndex, fileSize) (frees, disk) = (frees', disk V.// [(i, Just fileId)|i<-[fileIndex'..fileIndex'+fileSize-1]])
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where
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(fileIndex', frees') = findHole frees []
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findHole :: [Free] -> [Free] -> (Int, [Free])
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findHole [] _ = (fileIndex, frees)
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findHole (f@(freeIndex, freeSize):fs) acc | freeIndex > fileIndex = (fileIndex, frees)
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| freeSize == fileSize = (freeIndex, acc ++ fs)
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| freeSize > fileSize = (freeIndex, acc ++ ((freeIndex + fileSize, freeSize - fileSize):fs))
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| freeSize < fileSize = findHole fs (acc ++ [f])
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main :: IO ()
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main = do
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example <- parseInput "example"
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let exampleOutput = compute example
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when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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input <- parseInput "input"
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print $ compute input
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