2024 Day Eleven

DayEleven.hs

module DayEleven (partOne, partTwo, turnStone) where

import qualified Data.Map as DM


splitDownMiddle :: Int -> [Int]
splitDownMiddle toSplit = [leftSide, rightSide]
  where stringRepresentation = show toSplit
        newSize = length stringRepresentation `div` 2
        leftSide = read $ take newSize stringRepresentation
        rightSide = read $ drop newSize stringRepresentation

turnStone :: Int -> [Int]
turnStone 0 = [1]
turnStone n 
  | even $ length $ show n = splitDownMiddle n
  | otherwise = [2024 * n]


partOne :: [Int] -> [Int]
partOne inputStones = iterate takeTurn inputStones !! 25
  where takeTurn = concatMap turnStone

fastVersion :: Int -> Int -> Int
fastVersion = calculateCached 
  where cache = DM.fromList [((base, distance), sum (map (calculateCached (distance - 1)) (turnStone base))) | base <- [0..100], distance <- [0..75]]
        calculateCached :: Int -> Int -> Int
        calculateCached 0 _ = 1
        calculateCached newDistance stone = DM.findWithDefault fallback (stone, newDistance) cache
          where fallback = sum $ map (calculateCached (newDistance - 1)) $ turnStone stone


partTwo :: Int -> [Int] -> Int
partTwo howFar inputStones = sum $ map (fastVersion howFar) inputStones