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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import Data.List (foldl', isPrefixOf, transpose)
import Data.Map qualified as M
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 132
data Range = Range { field :: String
, ll :: Int
, lh :: Int
, hl :: Int
, hh :: Int
} deriving (Show)
type Ticket = [Int]
data Input = Input { ranges :: [Range]
, myTicket :: Ticket
, nearbyTickets :: [Ticket]
} deriving (Show)
type Parser = Parsec Void String
parseInt :: Parser Int
parseInt = do
n <- some digitChar
void $ optional (char ',')
return $ read n
parseRange :: Parser Range
parseRange = do
f <- some (letterChar <|> char ' ')
void $ string ": "
ll <- parseInt
void $ char '-'
lh <- parseInt
void $ string " or "
hl <- parseInt
void $ char '-'
hh <- parseInt
void $ char '\n'
return $ Range f ll lh hl hh
parseTicket :: Parser Ticket
parseTicket = some parseInt <* char '\n'
parseInput' :: Parser Input
parseInput' = do
r <- some parseRange
void $ string "\nyour ticket:\n"
t <- parseTicket
void $ string "\nnearby tickets:\n"
nts <- some parseTicket
void eof
return $ Input r t nts
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> die $ errorBundlePretty bundle
Right ops -> return ops
filterInvalid :: Input -> Input
filterInvalid (Input ranges ticket nearbyTickets) = Input ranges ticket (foldl' compute' [] nearbyTickets)
where
compute' :: [Ticket] -> Ticket -> [Ticket]
compute' acc nt
| and $ map isValid nt = nt : acc
| otherwise = acc
isValid :: Int -> Bool
isValid n = or $ map (isValid' n) ranges
isValid' :: Int -> Range -> Bool
isValid' n (Range{ll, lh, hl, hh}) = (n >= ll && n <= lh) || (n >= hl && n <= hh)
compute' :: Input -> Int
compute' Input{ranges, myTicket, nearbyTickets} = product . map (myTicket !!) . map snd . filter importantRange . snd $ iterate (potentialFieldIDsByRange, [])
where
importantRange :: (Range, Int) -> Bool
importantRange (Range{field}, _) = isPrefixOf "departure" field
nearbyTicketsByField = transpose nearbyTickets
iterate :: ([(Range, [Int])], [(Range, Int)]) -> ([(Range, [Int])], [(Range, Int)])
iterate ([], res) = ([], res)
iterate ((r, [i]):ps, res) = iterate (map (removeID i) ps, (r, i):res)
iterate (p:ps, res) = iterate (ps ++ [p], res)
removeID :: Int -> (Range, [Int]) -> (Range, [Int])
removeID i (r, l) = (r, filter (/= i) l)
potentialFieldIDsByRange :: [(Range, [Int])]
potentialFieldIDsByRange = zip ranges $ map (getMatchingFieldIDs myTicket nearbyTicketsByField) ranges
getMatchingFieldIDs :: Ticket -> [Ticket] -> Range -> [Int]
getMatchingFieldIDs [] [] _ = []
getMatchingFieldIDs (_:ts) (fields:fieldss) range
| and (map (match range) fields) = myId : getMatchingFieldIDs ts fieldss range
| otherwise = getMatchingFieldIDs ts fieldss range
where
myId :: Int
myId = (length myTicket) - (length ts) - 1
match :: Range -> Int -> Bool
match Range{ll, lh, hl, hh} n = (n >= ll && n <= lh) || (n >= hl && n <= hh)
compute :: Input -> Int
compute input = compute' $ filterInvalid input
main :: IO ()
main = do
example <- parseInput "example2"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
|
