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
  14 readSnafu :: String -> Int
  15 readSnafu cs = foldl' go 0 cs
  16   where go acc c = acc * 5 + (snafuValue c)
  17
  18 snafuValue :: Char -> Int
  19 snafuValue '2' = 2
  20 snafuValue '1' = 1
  21 snafuValue '0' = 0
  22 snafuValue '-' = -1
  23 snafuValue '=' = -2
  24 snafuValue _ = error "Illegal digit in read"
  25
  26 showSnafu :: Int -> String
  27 showSnafu = packSnafu . toBase5R
  28
  29 toBase5R :: Int -> [Int]
  30 toBase5R 0 = []
  31 toBase5R n = (r : (toBase5R k))
  32   where (k, r) = n `divMod` 5
  33
  34 packSnafu :: [Int] -> String
  35 packSnafu digits
  36   | carry == 0 = shown
  37   | otherwise = (snafuRep carry) : shown
  38   where (carry, shown) = foldl' packSnafuDigit (0, "") digits
  39
  40 packSnafuDigit :: (Int, String) -> Int -> (Int, String)
  41 packSnafuDigit (carry, acc) d
  42   | d' <= 2 = (0, (snafuRep d') : acc)
  43   | otherwise = (1, (snafuRep (d' - 5) : acc))
  44   where d' = d + carry
  45
  46 snafuRep :: Int -> Char
  47 snafuRep 2 = '2'
  48 snafuRep 1 = '1'
  49 snafuRep 0 = '0'
  50 snafuRep -1 = '-'
  51 snafuRep -2 = '='
  52 snafuRep _ = error "Illegal number in show"