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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Applicative.Permutations
import Control.Monad (void, when)
import Data.Char qualified as C
import Data.Either
import Data.List qualified as L
import Data.Map qualified as M
import Data.Maybe
import Data.Set qualified as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 46
data Range = Range Int Int deriving Show -- start range
data Map = Map Int Range deriving Show -- destination range
data Mapping = Mapping String String [Map] deriving Show -- sourceStr destStr
data Input = Input [Range] [Mapping] deriving Show -- seeds mappings
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some digitChar <* many (char ' ')
parseRange :: Parser Range
parseRange = Range <$> parseNumber
<*> parseNumber
parseMap :: Parser Map
parseMap = Map <$> parseNumber
<*> parseRange <* eol
parseMapping :: Parser Mapping
parseMapping = Mapping <$> (some letterChar <* string "-to-")
<*> (some letterChar <* string " map:" <* eol)
<*> some parseMap <* optional eol
parseInput' :: Parser Input
parseInput' = Input <$> (string "seeds: " *> some parseRange <* some eol)
<*> some parseMapping <* 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'
step :: [Map] -> Range -> [Range]
step [] n = [n]
step (Map dest (Range src range):ms) r@(Range n l) | n+l<=src || n>=src+range = step ms r
| n>=src && n+l<=src+range = [Range (n-src+dest) l]
| n>=src = Range (n-src+dest) (range+src-n) : step ms (Range (src+range) (l-range-src+n))
| otherwise = Range dest (l-src+n) : step ms (Range n (src-n))
stepMapping :: [Range] -> Mapping -> [Range]
stepMapping ranges (Mapping _ _ ms) = concatMap (step ms) ranges
compute :: Input -> Int
compute (Input ranges mappings) = minimum $ map (\(Range start _) -> start) $ L.foldl' stepMapping ranges mappings
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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