day 4

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

day04.hs

@@ -0,0 +1,49 @@
+import Data.Either (rights)
+import Data.List (intersect)
+import Data.Text qualified as T
+import Lib (readFile')
+import Text.Parsec
+
+parser :: (Stream s m Char) => ParsecT s u m ([Int], [Int])
+parser =
+  do
+    card <- string "Card" *> many1 space *> many1 digit <* char ':'
+    many1 space
+    winning <- many digit `sepBy` many1 space
+    char '|'
+    many1 space
+    mine <- many digit `sepBy` many1 space
+
+    return (read <$> init winning, read <$> mine)
+
+main :: IO ()
+main = do
+  input <- T.lines <$> readFile' "day04.in"
+  let parsed = rights . map (parse parser "") $ input
+
+  putStr "Q1: "
+  print
+    . sum
+    . map
+      ( (\x -> if x < 0 then 0 else 2 ^ x)
+          . flip (-) 1
+          . length
+          . uncurry intersect
+      )
+    $ parsed
+
+  putStr "Q2: "
+  print
+    . sum
+    . consequence
+    . map
+      ( \(x, y) ->
+          length $
+            intersect x y
+      )
+    $ parsed
+  where
+    -- TODO: dont bruteforce (its slow)
+    consequence :: [Int] -> [Int]
+    consequence [] = []
+    consequence (x : xs) = 1 + sum (take x $ consequence xs) : consequence xs