advent25/Main.hs

   1 -- Writeup at https://work.njae.me.uk/2022/12/01/advent-of-code-2022-day-1/
   2
   3 import AoC
   4 import Data.List
   5
   6
   7 main :: IO ()
   8 main =
   9   do  dataFileName <- getDataFileName
  10       numStrs <- readFile dataFileName
  11       let fuels = fmap readSnafu $ lines numStrs
  12       putStrLn $ showSnafu $ sum fuels
  13       -- print $ part1 fuels
  14
  15 readSnafu :: String -> Int
  16 readSnafu cs = foldl' go 0 cs
  17   where go acc c = acc * 5 + (snafuValue c)
  18
  19 snafuValue :: Char -> Int
  20 snafuValue '2' = 2
  21 snafuValue '1' = 1
  22 snafuValue '0' = 0
  23 snafuValue '-' = -1
  24 snafuValue '=' = -2
  25 snafuValue _ = error "Illegal digit in read"
  26
  27 showSnafu :: Int -> String
  28 showSnafu n = reverse $ packSnafu 0 $ toBase5R n
  29
  30 toBase5R :: Int -> [Int]
  31 toBase5R 0 = []
  32 toBase5R n = (r : (toBase5R k))
  33   where (k, r) = n `divMod` 5
  34
  35 packSnafu :: Int -> [Int] -> String
  36 packSnafu 0 [] = ""
  37 packSnafu carry [] = [snafuRep carry]
  38 packSnafu carry (d:ds)
  39   | d' <= 2 = ((snafuRep d') : (packSnafu 0 ds))
  40   | otherwise = ((snafuRep (d' - 5)) : (packSnafu 1 ds))
  41   where d' = d + carry
  42
  43 snafuRep :: Int -> Char
  44 snafuRep 2 = '2'
  45 snafuRep 1 = '1'
  46 snafuRep 0 = '0'
  47 snafuRep -1 = '-'
  48 snafuRep -2 = '='
  49 snafuRep _ = error "Illegal number in show"