day 5

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

src/Day5.hs

@@ -0,0 +1,50 @@
+module Main where
+
+import qualified AoC as A (extract)
+import Data.Bool (bool)
+import Data.List (sortBy, tails)
+import qualified Data.Set as S
+import Text.Parsec (char, digit, many1, newline, parse, sepBy1, sepEndBy1)
+import Text.Parsec.String (Parser)
+
+type Rule = (Int, Int)
+
+type Rules = S.Set Rule
+
+type Update = [Int]
+
+parseRules :: Parser (Rules, [Update])
+parseRules = do
+  rules <- S.fromList <$> parseRule `sepEndBy1` newline
+  newline
+  updates <- parseUpdate `sepEndBy1` newline
+  return (rules, updates)
+  where
+    parseRule :: Parser Rule
+    parseRule = (,) <$> (read <$> many1 digit <* char '|') <*> (read <$> many1 digit)
+    parseUpdate :: Parser Update
+    parseUpdate = (read <$> many1 digit) `sepBy1` char ','
+
+middle :: [a] -> a
+middle xs = xs !! (length xs `div` 2)
+
+isOrdered :: Rules -> Update -> Bool
+isOrdered rules (first : rest) = all (flip S.member rules . (,) first) rest
+isOrdered _ _ = False
+
+part1 :: Rules -> [Update] -> Int
+part1 rules = sum . map middle . filter (all (isOrdered rules) . init . tails)
+
+part2 :: Rules -> [Update] -> Int
+part2 rules =
+  sum
+    . map (middle . sortBy (\x -> bool GT LT . flip S.member rules . (,) x))
+    . filter (not . all (isOrdered rules) . init . tails)
+
+main :: IO ()
+main =
+  do
+    raw <- readFile "./inputs/day5.in"
+    let (rules, updates) = A.extract $ parse parseRules "" raw
+    putStr "Part 1: " >> print (part1 rules updates)
+    putStr "Part 2: " >> print (part2 rules updates)