[hs] Solve 2020_13 part 1

hs/src/Aoc/Y2020/D13.hs

@@ -0,0 +1,29 @@
+module Aoc.Y2020.D13
+  ( day
+  ) where
+
+import           Control.Monad
+import           Data.Function
+import           Data.List
+import           Data.Maybe
+
+import           Aoc.Day
+import           Aoc.Parse
+
+parser :: Parser (Int, [Int])
+parser = do
+  earliest <- decimal
+  void newline
+  buses <- sepBy ((Just <$> decimal) <|> (Nothing <$ char 'x')) (char ',')
+  void newline
+  pure (earliest, catMaybes buses)
+
+solver :: (Int, [Int]) -> IO ()
+solver (earliest, buses) = do
+  putStrLn ">> Part 1"
+  let busTimes = [(bus, earliest - mod earliest bus + bus) | bus <- buses]
+      (nextBus, nextBusTime) = minimumBy (compare `on` snd) busTimes
+  print $ nextBus * (nextBusTime - earliest)
+
+day :: Day
+day = dayParse parser solver