--- /dev/null
+-- Writeup at https://work.njae.me.uk/2022/12/03/advent-of-code-2022-day-3/
+
+import System.Environment
+import Data.Char
+import qualified Data.Set as S
+import Data.List
+import Data.List.Split
+
+type Contents = S.Set Char
+data Rucksack = Rucksack String String deriving (Show, Eq)
+
+main :: IO ()
+main =
+ do dataFileName <- getDataFileName
+ text <- readFile dataFileName
+ let rucksacks = fmap mkRucksack $ lines text
+ print $ part1 rucksacks
+ print $ part2 rucksacks
+
+getDataFileName :: IO String
+getDataFileName =
+ do args <- getArgs
+ progName <- getProgName
+ let baseDataName = if null args
+ then progName
+ else head args
+ let dataFileName = "data/" ++ baseDataName ++ ".txt"
+ return dataFileName
+
+part1 :: [Rucksack] -> Int
+part1 = sum . fmap (priority . commonItem)
+
+part2 :: [Rucksack] -> Int
+part2 rucksacks = sum $ fmap priority badges
+ where groups = chunksOf 3 rucksacks
+ badges = fmap badgeOf groups
+
+merge :: Rucksack -> Contents
+merge (Rucksack contents1 contents2) =
+ S.union (S.fromList contents1) (S.fromList contents2)
+
+badgeOf :: [Rucksack] -> Char
+badgeOf rucksacks = S.findMin $ intersections (fmap merge rucksacks)
+
+intersections :: [Contents] -> Contents
+intersections sets = foldl1' S.intersection sets
+
+
+mkRucksack :: String -> Rucksack
+mkRucksack contents = Rucksack (take n contents) (drop n contents)
+ where n = length contents `div` 2
+
+commonItem :: Rucksack -> Char
+commonItem (Rucksack contents1 contents2) = S.findMin (c1 `S.intersection` c2)
+ where c1 = S.fromList contents1
+ c2 = S.fromList contents2
+
+priority :: Char -> Int
+priority item
+ | item >= 'a' = ord item - ord 'a' + 1
+ | otherwise = ord item - ord 'A' + 27