Day 3: Mull It Over
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4 points
Haskell
Oof, a parsing problem :/ This is some nasty-ass code. step is almost the State monad written out explicitly.
Solution
import Control.Monad
import Data.Either
import Data.List
import Text.Parsec
data Ins = Mul !Int !Int | Do | Dont
readInput :: String -> [Ins]
readInput = fromRight undefined . parse input ""
where
input = many ins <* many anyChar
ins =
choice . map try $
[ Mul <$> (string "mul(" *> arg) <*> (char ',' *> arg) <* char ')',
Do <$ string "do()",
Dont <$ string "don't()",
anyChar *> ins
]
arg = do
s <- many1 digit
guard $ length s <= 3
return $ read s
run f = snd . foldl' step (True, 0)
where
step (e, a) i =
case i of
Mul x y -> (e, if f e then a + x * y else a)
Do -> (True, a)
Dont -> (False, a)
main = do
input <- readInput <$> readFile "input03"
print $ run (const True) input
print $ run id input
2 points
Love to see you chewing through this parsing problem in Haskell, I didn’t dare use Parsec because I wasn’t confident enough.
Why did you decide to have a strict definition of Mul !Int !Int?
3 points
My guess is because a linter and/or HLS was suggesting it. I know HLS used to suggest making your fields strict in almost all cases. In this case I have a hunch that it slightly cuts down on memory usage because we use almost all Muls either way. So it does not need to keep the string it is parsed from in memory as part of the thunk.
But it probably makes a small/negligible difference here.
3 points
