garnet/haskell/Puzzles/Day5.hs

module Puzzles.Day5 (puzzle) where

import Pre

puzzle :: Puzzle
puzzle =
    Puzzle
        { number = 5
        , parser = const do
            ranges <- (Range <$> decimal <* single '-' <*> decimal) `sepEndBy` newline
            void newline
            vals <- decimal `sepEndBy` newline
            pure (ranges, vals)
        , parts =
            ( \(ranges, vals) ->
                length
                    . filter (flip any ranges . isInRange)
                    $ vals
            )
                /\ ( sum
                        . map rangeLength
                        . foldr addInterval []
                        . sortOn (Down . (.lower))
                        . fst
                   )
                /\ nil
        , extraTests = mempty
        }

data Range = Range
    { lower :: Int
    , upper :: Int
    }
    deriving (Eq, Ord, Show)

rangeLength :: Range -> Int
rangeLength r = r.upper - r.lower + 1

isInRange :: Int -> Range -> Bool
isInRange n r = n >= r.lower && n <= r.upper

extend :: Int -> Range -> Range
extend upper r = r{upper = max r.upper upper}

addInterval :: Range -> [Range] -> [Range]
addInterval new = \case
    [] -> [new]
    (r : rs) ->
        if isInRange new.lower r
            then extend new.upper r : rs
            else new : r : rs