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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import qualified Data.List as L
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 1928
type Input = [Int]
type Parser = Parsec Void String
parseBlockSize :: Parser Int
parseBlockSize = read . pure <$> digitChar
parseInput' :: Parser Input
parseInput' = some parseBlockSize <* eol <* eof
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> error $ errorBundlePretty bundle
Right input' -> return input'
compute :: Input -> Int
compute input = sum $ map (\(x, y) -> x * y) $ zip [0..] $ computeFile files' free []
where
files = [s | (i,s)<-zip [0..] input, even i]
files' = zip [0..] files
free = [s | (i,s)<-zip [0..] input, odd i]
computeFile :: [(Int, Int)] -> [Int] -> [Int] -> [Int]
computeFile ((fId, fSize):fs) free acc = computeFillFree fs free (acc ++ L.replicate fSize fId)
computeFillFree :: [(Int, Int)] -> [Int] -> [Int] -> [Int]
computeFillFree [] _ acc = acc
computeFillFree files (free:frees) acc | free == fSize = computeFile fs frees (acc ++ L.replicate fSize fId)
| free > fSize = computeFillFree fs (free-fSize:frees) (acc ++ L.replicate fSize fId)
| free < fSize = computeFile (fs++[(fId, fSize-free)]) frees (acc ++ L.replicate free fId)
where
(fId, fSize) = last files
fs = init files
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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