Advent of Code 2023 day 02

Day 2 was a day where a declarative approach to the problem paid off. Just writing down some definitions led to a neat solution.

Part 1

I started by writing down some things I know. A Game has an ID and a set of Revelations. A Revelation is a group of red, green, and blue reveals.

data Game = Game {getID :: Int, getRevelations :: [Revelation]} 
  deriving (Eq, Show)
data Revelation = Revelation Int Int Int deriving (Eq, Show)

A Revelation is compatibleWith a limit if all of the red, green, blue shown are less than the corresponding quantities in the limit. (This is the same as defining a partial order on revelations, but I don't need all of that here.) A game is possible (given a limit) if all its revelations are compatibleWith that limit.

compatibleWith :: Revelation -> Revelation -> Bool
compatibleWith (Revelation r0 g0 b0) (Revelation r1 g1 b1) = 
  (r0 <= r1) && (g0 <= g1) && (b0 <= b1)

possible :: Game -> Revelation -> Bool
possible game limit = 
  all (`compatibleWith` limit) $ getRevelations game

I can solve Part 1 by filtering the games that are possible, extracting the IDs, and adding them.

part1 :: [Game] -> Int
part1 games = sum $ fmap getID $ filter (`possible` limit) games
  where limit = Revelation 12 13 14

Part 2

This is all about combining revelations to find the minimum bounds on a game. Combining objects is what monoids are all about.

The bounds implied by two revelations are the maximums of each element. The bounds of no revelations is zero of each element.

instance Semigroup Revelation where
  (Revelation r0 g0 b0) <> (Revelation r1 g1 b1) = 
    Revelation (max r0 r1) (max g0 g1) (max b0 b1)

instance Monoid Revelation where
  mempty = Revelation 0 0 0

That gives me mconcat for free, and the bounds of a game is the combination of all the revelations.

required :: Game -> Revelation
required = mconcat . getRevelations

To solve part 2, I find the power of the requirements of each game, then add them up.

power :: Revelation -> Int
power (Revelation r g b) = r * g * b

part2 :: [Game] -> Int
part2 = sum . fmap (power . required)

Parsing

All the above needs Games and Revelations, which isn't what's in the input file. I decided to take a two-stage approach to handling the input.

1. Parse the input into a structure that follows that of the input.
2. Convert that structure into the one of Game and Revelation.

I could do it all in one, but I think this approach is easier to test and debug, keeps the code easier to debug, and allows for dual-use of the input if part 2 ended up doing something very different with the same input (as 2022 day 2 did).

Reading the input

I need new data structures to hold the results of parsing, so I define a ParsedGame, a Showing, a Cube, and the Colours.

data ParsedGame = ParsedGame Int [Showings] deriving (Eq, Show)
type Showings = [Cube]
data Cube = Cube Colour Int deriving (Eq, Show)
data Colour = Red | Green | Blue deriving (Eq, Show)

Parsing pretty much follows the pattern of the input file, using attoparsec. The only interesting part is the use of flip in cubeP to change the order of arguments.

gamesP = gameP `sepBy` endOfLine
gameP = ParsedGame <$> (("Game " *> decimal) <* ": ") <*> showingsP

showingsP = showingP `sepBy` "; "
showingP = cubeP `sepBy` ", "
cubeP = (flip Cube) <$> (decimal  <* " ") <*> colourP 

colourP = redP <|> greenP <|> blueP
redP = Red <$ "red"
greenP = Green <$ "green"
blueP = Blue <$ "blue"

Converting the parsed data

My overall approach here is to convert each Cube into a Revelation of only one colour, then merge those Revelations into the actual Revelation of this round of the Game.

Merging revelations again implies a monoid, but now I want to use a different combining operation. The idiomatic way to do that is to wrap the Revelation in a newtype and define my merging monoid on that newtype.

newtype Merging = Merging { getMerging :: Revelation } deriving (Eq, Show)

instance Semigroup Merging where
  (Merging (Revelation r0 g0 b0)) <> (Merging (Revelation r1 g1 b1)) = 
    Merging (Revelation (r0 + r1) (g0 + g1) (b0 + b1))

instance Monoid Merging where
  mempty = Merging (Revelation 0 0 0)

Revealing one Cube produces a Merging (Revelation ...), which I can merge with mconcat and extract the Revelation with getMerging.

engame :: ParsedGame -> Game
engame (ParsedGame n showings) = Game n (fmap revealify showings)

revealify :: Showings -> Revelation
revealify = getMerging . mconcat . (fmap reveal)

reveal :: Cube -> Merging
reveal (Cube Red   n) = Merging (Revelation n 0 0)
reveal (Cube Green n) = Merging (Revelation 0 n 0)
reveal (Cube Blue  n) = Merging (Revelation 0 0 n)

Code

You can get the code from my locally-hosted Git repo, or from Gitlab.