41 lines
1.3 KiB
Haskell
41 lines
1.3 KiB
Haskell
module Puzzles.Day3 (puzzle) where
|
|
|
|
import Pre
|
|
|
|
import Data.List.NonEmpty qualified as NE
|
|
|
|
puzzle :: Puzzle
|
|
puzzle =
|
|
Puzzle
|
|
{ number = 3
|
|
, parser = const $ (Bank <$> some1 digit) `sepEndBy` newline
|
|
, parts =
|
|
( sum
|
|
. map (digitsToInt . fromMaybe (error "battery list too short") . maxBatteries 2)
|
|
)
|
|
/\ ( sum
|
|
. map (digitsToInt . fromMaybe (error "battery list too short") . maxBatteries 12)
|
|
)
|
|
/\ nil
|
|
, extraTests = mempty
|
|
}
|
|
|
|
newtype Bank = Bank (NonEmpty Battery)
|
|
deriving newtype (Eq, Ord, Show)
|
|
|
|
newtype Battery = Battery Word8
|
|
deriving newtype (Eq, Ord, Show, Num, Enum, Real, Integral)
|
|
|
|
-- maximal n-digit subsequence
|
|
-- returns `Nothing` if list isn't long enough (>= n)
|
|
maxBatteries :: Int -> Bank -> Maybe [Battery]
|
|
maxBatteries n0 (Bank bs0) = flip unfoldrM (n0, toList bs0) \case
|
|
(0, _) -> pure Nothing
|
|
(n, bs) -> do
|
|
(i, b) <- findMax <$> nonEmpty (dropEnd (n - 1) bs)
|
|
pure $ Just (b, (n - 1, drop (i + 1) bs))
|
|
|
|
-- returns the leftmost element in case of a tie
|
|
findMax :: (Ord a) => NonEmpty a -> (Int, a)
|
|
findMax = foldl1' (\m x -> if snd x > snd m then x else m) . NE.zip (0 :| [1 ..])
|