feat(day1): Implement part2

day1/Main.hs

@@ -3,6 +3,7 @@ module Main where
 import System.Environment (getArgs)
 
 import Data.List (sort)
+import Data.Function (on)
 
 import Text.Parsec (parse, many, char, endBy)
 import Text.ParserCombinators.Parsec (GenParser)
@@ -14,8 +15,11 @@ line = decimal >>= \l -> many (char ' ') >> decimal >>= \r -> return (l, r)
 locationIDs :: GenParser Char st [(Int, Int)]
 locationIDs = endBy line (char '\n')
 
-computeDistance :: [Int] -> [Int] -> Int
-computeDistance l r = sum (zipWith (curry (abs . uncurry (-))) l r)
+part1 :: [Int] -> [Int] -> Int
+part1 l r = sum (zipWith (curry (abs . uncurry (-))) l r)
+
+part2 :: [Int] -> [Int] -> Int
+part2 l r = (sum . map (\x -> sum ((map (const x) . filter (==x)) r))) l
 
 main :: IO ()
 main = do
@@ -24,6 +28,7 @@ main = do
     content <- readFile fp
     case parse locationIDs fp content of
         Left e -> putStrLn ("Input file is incorrectly formatted : " ++ show e)
-        Right ids -> let (l, r) = unzip ids
-                     in putStrLn ("Solution (Part 1) : "
-                         ++ show (computeDistance (sort l) (sort r)))
+        Right ids -> do
+            let (ls, rs) = uncurry ((,) `on` sort) (unzip ids)
+            putStrLn ("Solution (Part 1) : " ++ show (part1 ls rs))
+            putStrLn ("Solution (Part 2) : " ++ show (part2 ls rs))