refactor(day3): use a less performant but more readable solution

Author: IndecisionTree

Diff for: solutions/Days/Day03.hs

@@ -1,42 +1,30 @@
 module Days.Day03 (day03) where
 
 import AOC (Solution (..))
-import Control.Monad (join)
-import Data.Bifunctor (bimap)
 import Data.Char (isAsciiUpper, isAsciiLower)
+import Data.List (foldl1', intersect, nub)
 import Data.List.Split (chunksOf)
-import Data.IntMap.Strict qualified as M
 import Data.Text qualified as T
 
-type Counter = M.IntMap Int
-
 day03 :: Solution
 day03 = Solution parseInput part1 part2
 
-parseInput :: T.Text -> [T.Text]
-parseInput = T.lines
+parseInput :: T.Text -> [[Int]]
+parseInput = fmap (fmap priority . T.unpack) . T.lines
 
-part1 :: [T.Text] -> Int
-part1 = sum
-  . fmap (fst
-          . head
-          . M.toList
-          . uncurry M.intersection
-          . join bimap mkCounter
-          . (T.splitAt =<< ((`div` 2) . T.length)))
+part1 :: [[Int]] -> Int
+part1 = sum . concatMap (intersections . halve)
+  where
+    halve xs = chunksOf (length xs `div` 2) xs
 
-part2 :: [T.Text] -> Int
-part2 = sum
-  . fmap (fst . head . M.toList . foldr1 M.intersection)
-  . chunksOf 3
-  . fmap mkCounter
+part2 :: [[Int]] -> Int
+part2 = sum . concatMap intersections . chunksOf 3 
 
 priority :: Char -> Int
 priority ch
   | isAsciiLower ch = fromEnum ch - fromEnum 'a' + 1
   | isAsciiUpper ch = fromEnum ch - fromEnum 'A' + 27
   | otherwise = error $ show ch
 
-mkCounter :: T.Text -> Counter
-mkCounter =
-  M.fromListWith (+) . flip zip [1,1..] . fmap priority . T.unpack
+intersections :: Eq a => [[a]] -> [a]
+intersections = nub . foldl1' intersect