adventofcode-2021/6/fishexploder.hs

import           Parsing (splitByString)

main :: IO ()
main = do
  input <- getContents
  let
    fish = parseFish input
  (putStrLn . show . solution1) fish
  (putStrLn . show . solution2) fish

solution1 :: [Int] -> Int
solution1 = sum . (simulateDays 79)

solution2 :: [Int] -> Int
solution2 = sum . (simulateDays 255)

parseFish :: String -> [Int]
parseFish = (map read) . (splitByString ",")

initBuffer :: [Int] -> [Int]
initBuffer initialFish = map occurrences [0..8]
  where
    occurrences x = length (filter (==x) initialFish)

simulateDays :: Int -> [Int] -> [Int]
simulateDays days initialFish = foldl tickDay (initBuffer initialFish) [0..days]
  where
    tickDay :: [Int] -> Int -> [Int]
    tickDay fish day = map fishMultiplier [0..length(fish)-1]
      where
        fishMultiplier :: Int -> Int
        fishMultiplier i
          | i == addedTo =
            (fish !! i) + (fish !! dayFish)
          | otherwise =
            fish !! i
            where
              dayFish = day `rem` 9
              addedTo = (dayFish + 7) `rem` 9

-- Tests

testInput = "3,4,3,1,2"
testParsed = parseFish testInput

test1 = solution1 (parseFish testInput) -- 5934
test2 = solution2 (parseFish testInput) -- 26984457539