Day 7: Bridge Repair
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Haskell
import Control.Arrow
import Data.Char
import Text.ParserCombinators.ReadP
numP = read <$> munch1 isDigit
parse = endBy ((,) <$> (numP <* string ": ") <*> sepBy numP (char ' ')) (char '\n')
valid n [m] = m == n
valid n (x : xs) = n > 0 && valid (n - x) xs || (n `mod` x) == 0 && valid (n `div` x) xs
part1 = sum . fmap fst . filter (uncurry valid . second reverse)
concatNum r = (+r) . (* 10 ^ digits r)
where
digits = succ . floor . logBase 10 . fromIntegral
allPossible [n] = [n]
allPossible (x:xs) = ((x+) <$> rest) ++ ((x*) <$> rest) ++ (concatNum x <$> rest)
where
rest = allPossible xs
part2 = sum . fmap fst . filter (uncurry elem . second (allPossible . reverse))
main = getContents >>= print . (part1 &&& part2) . fst . last . readP_to_S parse
2 points
