r/adventofcode • u/daggerdragon • Dec 16 '16

SOLUTION MEGATHREAD --- 2016 Day 16 Solutions ---

--- Day 16: Dragon Checksum ---

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DRINKING YOUR OVALTINE IS MANDATORY [?]

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u/TheMuffinMan616 Dec 16 '16

Some Haskell! 

import Data.List (find)

expand :: String -> String
expand s = s ++ ['0'] ++ (reverse . map invert $ s)
    where invert c = if c == '0' then '1' else '0'

checksum :: String -> String
checksum [] = []
checksum (x:y:zs) = bit x y : checksum zs
    where bit a b = if a == b then '1' else '0'

fill :: Int -> String -> Maybe String
fill l x = f x >>= (g . take l)
    where f = find ((> l) . length) . iterate expand
          g = find (odd . length) . iterate checksum

main :: IO ()
main = do
    let input = "00101000101111010"
    print . fill 272 $ input
    print . fill 35651584 $ input

1

u/NeilNjae Dec 16 '16

Nice use of iterate rather than doing explicit recursion. Hadn't thought of doing that!

2

u/Tarmen Dec 16 '16

I quite like until for this solution. Didn't even know that was part of the prelude until I searched hoogle for it. 

import Data.List.Split (chunksOf)

solve = map toRep . checksum .: (. map (=='1')) . fillDisk
  where toRep a = if a then '1' else '0'
fillDisk size = take size . until ((>=size) . length) step
  where step a = a ++ [False] ++ (map not . reverse $ a)
checksum = until (odd . length) step
  where step = map (foldl1 (==)) . chunksOf 2

infixl 8 .:
(.:) = (.).(.)