advent-of-code/2020/day16/day16.hs

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{-# LANGUAGE NamedFieldPuns #-}
module Day16 where
import AoC
import Data.List
import Data.Maybe
import qualified Data.Set as S
import Data.Set (Set)
import Text.ParserCombinators.ReadP
import Text.Read.Lex
data Field = Field { name :: String
, valids :: Set Int
} deriving (Show)
type Ticket = [Int]
data Input = Input { fields :: [Field]
, myTicket :: Ticket
, otherTickets :: [Ticket]
} deriving (Show)
line :: ReadP a -> ReadP a
line p = p <* char '\n'
parseField :: ReadP Field
parseField = do
name <- many1 $ satisfy (/=':')
string ": "
ranges <- parseRange `sepBy` string " or "
let valids = S.fromList . concat . map (uncurry enumFromTo) $ ranges
return $ Field { name, valids }
where parseRange = do
n1 <- readDecP
char '-'
n2 <- readDecP
return (n1, n2)
parseTicket :: ReadP Ticket
parseTicket = readDecP `sepBy` char ','
parseInput :: ReadP Input
parseInput = do
fields <- many1 $ line parseField
line $ string ""
line $ string "your ticket:"
myTicket <- line parseTicket
line $ string ""
line $ string "nearby tickets:"
otherTickets <- many1 $ line parseTicket
return $ Input {fields, myTicket, otherTickets }
matchesAnyField :: [Field] -> Int -> Bool
matchesAnyField fields n = any ((n `S.member`) . valids) fields
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iterateUntilNothing :: (a -> Maybe a) -> a -> a
iterateUntilNothing f x = maybe x (iterateUntilNothing f) $ f x
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data Resolve a = Resolved a | Choice (Set a) deriving (Show)
resolve :: Ord a => [[a]] -> Maybe [a]
resolve = sequence . map fromResolved . iterateUntilNothing resolve' . map (Choice . S.fromList)
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where fromResolved (Resolved x) = Just x
fromResolved _ = Nothing
findSingletonChoice :: [Resolve a] -> Maybe (a, [Resolve a])
findSingletonChoice [] = Nothing
findSingletonChoice (x:xs) =
case x of
Choice s | S.size s == 1 -> let [r] = S.elems s
in Just (r, Resolved r : xs)
_ -> fmap (\(r, rest) -> (r, x:rest)) $ findSingletonChoice xs
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resolve' :: Ord a => [Resolve a] -> Maybe [Resolve a]
resolve' xs = do
(r, xs') <- findSingletonChoice xs
return $ map (dropChoice r) xs'
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where dropChoice x (Choice s) = Choice (S.delete x s)
dropChoice _ y = y
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part1 :: Input -> Int
part1 Input { fields, otherTickets } = sum . filter (not . matchesAnyField fields) . concat $ otherTickets
part2 :: Input -> Int
part2 Input { fields, myTicket, otherTickets } = product departureValues
where validTickets = filter (all $ matchesAnyField fields) otherTickets
columns = transpose validTickets
fieldNames = fromJust . resolve . map findPossibleFields $ columns
departureIndices = findIndices ("departure" `isPrefixOf`) fieldNames
departureValues = map (myTicket !!) departureIndices
findPossibleFields col = map name . filter (\field -> all (`S.member` valids field) col) $ fields
main = runAoC (fromJust . oneCompleteResult parseInput) part1 part2