[hs] Solve 2020_21 part 1

hs/src/Aoc/Y2020.hs

@@ -23,6 +23,7 @@ import qualified Aoc.Y2020.D17 as D17
 import qualified Aoc.Y2020.D18 as D18
 import qualified Aoc.Y2020.D19 as D19
 import qualified Aoc.Y2020.D20 as D20
+import qualified Aoc.Y2020.D21 as D21
 
 year :: Year
 year = Year 2020
@@ -46,4 +47,5 @@ year = Year 2020
   , (18, D18.day)
   , (19, D19.day)
   , (20, D20.day)
+  , (21, D21.day)
   ]

hs/src/Aoc/Y2020/D21.hs

@@ -0,0 +1,35 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Aoc.Y2020.D21
+  ( day
+  ) where
+
+import           Control.Monad
+import           Data.Char
+
+import qualified Data.Set      as Set
+import qualified Data.Text     as T
+
+import           Aoc.Day
+import           Aoc.Parse
+
+parser :: Parser [(Set.Set T.Text, Set.Set T.Text)]
+parser = manyLines $ do
+  ingredients <- Set.fromList <$> many (lineWhile isAlpha <* string " ")
+  void $ string "(contains "
+  allergens <- Set.fromList <$> (lineWhile isAlpha `sepBy` string ", ")
+  void $ string ")"
+  pure (ingredients, allergens)
+
+solver :: [(Set.Set T.Text, Set.Set T.Text)] -> IO ()
+solver foods = do
+  putStrLn ">> Part 1"
+  let allFoods = Set.unions $ map fst foods
+      allAllergens = Set.toList $ Set.unions $ map snd foods
+      foodsByAllergen = map (\a -> (a, foldr1 Set.intersection $ map fst $ filter (Set.member a . snd) foods)) allAllergens
+      foodsWithAllergen = Set.unions $ map snd foodsByAllergen
+      foodsWithoutAllergen = allFoods Set.\\ foodsWithAllergen
+  print $ sum $ map (Set.size . Set.intersection foodsWithoutAllergen . fst) foods
+
+day :: Day
+day = dayParse parser solver