day 3

2021/day03/day03.hs

@@ -1,13 +1,28 @@
+import Data.Bits
+import Data.Char
 import Data.List
-import Control.Lens
-import Control.Arrow
 
 main :: IO ()
 main = do
   input <- lines <$> readFile "input"
-  let counts = map ((\[x,y]->x>y) . map length . group . sort) $ transpose input
-  let (gamma,epsilon) = boolToDec *** boolToDec $ (id &&& map not) counts
+  let rating = take (length $ head input) $ transpose (map tails input)
+  let [gamma, epsilon] = [toDec $ map (commonBit f) rating | f <- [(<=), (>)]]
   print $ gamma * epsilon
+  let part2 = [genRating x input [] | x <- [(<=), (>)]]
+  print $ product $ map toDec part2
 
-boolToDec :: [Bool] -> Int
-boolToDec = sum . imap (\x y->(2^x)*y) . reverse . map fromEnum
+commonBit :: (Int -> Int -> Bool) -> [String] -> Int
+commonBit f =
+  (\[x, y] -> fromEnum (x `f` y)) .
+  map (length) . group . sort . head . transpose
+
+genRating :: (Int -> Int -> Bool) -> [String] -> [Int] -> [Int]
+genRating _ (x:[]) acc = acc ++ (map digitToInt x)
+genRating f input acc =
+  genRating
+    f
+    (map tail $ filter (isPrefixOf (show $ commonBit f input)) input)
+    (acc ++ [commonBit f input])
+
+toDec :: [Int] -> Int
+toDec = sum . map (uncurry (*)) . zip (map bit [0 ..]) . reverse

2021/day03/testinput

@@ -0,0 +1,12 @@
+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010