advent03/Main.hs

   1 import Data.List
   2 import Data.Char
   3
   4 main :: IO ()
   5 main =
   6   do  numStrs <- readFile "data/advent03.txt"
   7       let bits = map (map digitToInt) $ lines numStrs
   8       print $ part1 bits
   9       print $ part2 bits
  10
  11 part1 :: [[Int]] -> Int
  12 part1 allBits = gamma * epsilon
  13   where majorities = findMajorities allBits
  14         gamma = bitsToNum majorities
  15         epsilon = bitsToNum $ map not majorities
  16
  17 part2 :: [[Int]] -> Int
  18 part2 allBits = oxygenRating * coFilterRating
  19   where oxygenRating = bitsToNum $ oxygenFilter allBits 0
  20         coFilterRating = bitsToNum $ coFilter allBits 0
  21
  22 findMajorities :: [[Int]] -> [Bool]
  23 findMajorities allBits = map (>= threshold) columnSums
  24   where addBits = zipWith (+)
  25         columnSums = foldr1 addBits allBits
  26         (t, m) = (length allBits) `divMod` 2
  27         threshold = if m == 1 then t + 1 else t
  28
  29 bitsToNum :: [Bool] -> Int
  30 bitsToNum bits = foldl' includeBit 0 bits
  31   where includeBit n True  = n * 2 + 1
  32         includeBit n False = n * 2
  33
  34 oxygenFilter :: [[Int]] -> Int -> [Bool]
  35 oxygenFilter = partFilter id
  36
  37 coFilter :: [[Int]] -> Int -> [Bool]
  38 coFilter = partFilter not
  39
  40 partFilter :: (Bool -> Bool) -> [[Int]] -> Int -> [Bool]
  41 partFilter filterFunc allBits n
  42   | length allBits == 1 = majorities
  43   | otherwise = partFilter filterFunc filteredBits (n + 1)
  44   where majorities = findMajorities allBits
  45         filterValue = if (filterFunc $ majorities !! n) then 1 else 0
  46         filteredBits = filter (\bs -> bs !! n == filterValue) allBits