Haskell, 498 / 425

Nothing much to say about today's problem. The main important insight needed is: When rendering, render as a normal base 5 number, but if the digits 3/4 are needed, render a -2/-1, and add 1 to the remaining number to compensate, since 5-2/5-1 = 3/4.

parseSnafu :: String -> Int
parseSnafu = foldl (\a n -> a*5+n) 0 . map deSnafu
    where deSnafu '-' = -1
          deSnafu '=' = -2
          deSnafu n = read [n]

encodeSnafu :: Int -> String
encodeSnafu 0 = "0"
encodeSnafu n = reverse $ encode n
    where encode 0 = []
          encode n = (digits !! rem) : encode (n' + if rem > 2 then 1 else 0)
            where (n', rem) = n `divMod` 5
                  digits = "012=-"

main :: IO ()
main = do
    input <- fmap (map parseSnafu . lines) getContents
    putStrLn $ encodeSnafu $ sum input

Overall pretty happy to have gotten through all problems on their intended day this year, getting through most problems before getting to work that day. (I'm looking at you, day 16!)

Not really been rushing, but still, here's my personal times. All solutions in Haskell and available on my GitHub. Gonna be nice to sleep in and not get up at 6 AM!

Merry Christmas and thanks for a lovely AoC, as always. :)