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    1 {-|
    2 Module:      Y2021.D07
    3 Description: Advent of Code 2021 Day 07 Solutions.
    4 License:     MIT
    5 Maintainer:  @tylerjl
    6 
    7 Solutions to the 2021 day 07 set of problems for <adventofcode.com>.
    8 -}
    9 module Y2021.D07 where
   10 
   11 import Data.Attoparsec.Text hiding (take)
   12 import Data.Either.Utils (fromRight)
   13 import Data.Text (Text)
   14 
   15 -- |Solve part A
   16 part7A :: Text -> Int
   17 part7A = solve7 id . parseCrabs
   18 
   19 -- |Solve part B
   20 part7B :: Text -> Int
   21 part7B = solve7 crabCost . parseCrabs
   22 
   23 -- |Function to accept a given `Int` and return the sum of an origin point to
   24 -- the given `Int`. Suitable for memoization... I think?
   25 crabCost :: Int -> Int
   26 crabCost n = (n * (n + 1)) `div` 2
   27 
   28 -- |Higher-order function to find the lowest fuel cost.
   29 solve7 :: (Ord a, Ord b, Enum b, Num a, Num b) => (b -> a) -> [b] -> a
   30 solve7 f crabs = minimum $ map costTo [0..maximum crabs]
   31   where costTo crab = sum [ (f . abs) (crab - dest) | dest <- crabs ]
   32 
   33 -- |Parse puzzle input into a list of `Int`s with faster attoparsec.
   34 parseCrabs :: Text -> [Int]
   35 parseCrabs = fromRight . parseOnly parser
   36   where parser = decimal `sepBy` char ',' <* endOfLine