src/advent10/advent10.hs

   1 import Data.List.Split (splitOn, chunksOf)
   2 import Data.Char (ord)
   3 import Data.Bits (xor)
   4 import Text.Printf (printf)
   5
   6 main :: IO ()
   7 main = do
   8         text <- readFile "data/advent10.txt"
   9         let ls = map read $ splitOn "," text
  10         print $ part1 ls
  11         putStrLn $ part2 text
  12
  13
  14 part1 :: [Int] -> Int
  15 part1 lengths = (tied!!0) * (tied!!1)
  16     where (tied, _, _) = foldl step ([0..255], 0, 0) lengths
  17
  18
  19 part2 :: String -> String
  20 part2 text = densify tied
  21     where lengths = p2lengths text
  22           (tied, _, _) = foldl step ([0..255], 0, 0) lengths
  23
  24 step :: ([Int], Int, Int) -> Int -> ([Int], Int, Int)
  25 step (original, start, skip) len = (replaced, start', skip + 1)
  26     where replaced = tie original start len
  27           start' = (start + len + skip) `mod` (length original)
  28
  29 tie :: [a] -> Int -> Int -> [a]
  30 tie original start len = replace original replacement start
  31     where replacement = reverse $ extract original start len
  32
  33 extract :: [a] -> Int -> Int -> [a]
  34 extract items from len = take len $ drop from $ items ++ items
  35
  36 replace :: [a] -> [a] -> Int -> [a]
  37 replace original replacement from = take (length original) (start ++ replacement ++ remainder)
  38     where excess = drop (length original - from) replacement
  39           stub = drop (length excess) original
  40           start = take from (excess ++ stub)
  41           remainder = drop (length $ start ++ replacement) original
  42
  43
  44 p2lengths :: String -> [Int]
  45 p2lengths text = take (length chunk * 64) $ cycle chunk
  46     where chunk = map ord text ++ [17, 31, 73, 47, 23]
  47
  48 densify :: [Int] -> String
  49 densify ns = concatMap (printf "%02x") codes
  50     where chunks = chunksOf 16 ns
  51           compress = foldl1 xor
  52           codes = map compress chunks