Day05 yay

advent-of-code.cabal

@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: a31d69558fb7c391bf70ef12c046d4b4a10a2fd42e24f572cc4592799a825b1b
+-- hash: 7f535108a3ad03db9bb5b3d1976f1e0422bbe0eddd7da2483c1581fa3dd7c089
 
 name:           advent-of-code
 version:        0.0.0
@@ -31,6 +31,7 @@ executable advent-of-code
       Day02
       Day03
       Day04
+      Day05
       Paths_advent_of_code
   hs-source-dirs:
       src

src/Day05.hs

+module Day05 (part1, part2) where
+
+import Data.Char
+
+-- Reduces the input and returns the length.
+part1 :: [String] -> String
+part1 [input] = show $ length $ reduce input
+
+-- Reduces the input once with every character removed and returns the length of the shortest reduct.
+part2 :: [String] -> String
+part2 [input] = show $ minBySnd $ liftSnd length <$> liftSnd reduce <$> listRemoves input
+
+example :: [String]
+example = ["dabAcCaCBAcCcaDA"]
+
+
+-- Reduces a list of characters recursively.
+reduce :: [Char] -> [Char]
+reduce [] = []
+reduce (c:cs) = case reduce cs of
+    (d:ds) | c `react` d -> ds
+           | otherwise -> (c:d:ds)
+    [] -> [c]
+
+-- Checks whether two units can react, i.e. are the same character in different cases.
+react :: Char -> Char -> Bool
+react c1 c2 = c1 /= c2 && toLower c1 == toLower c2
+
+-- Returns a list which contains a pair for each character from 'a' to 'z' with the character and the given string with all occurences (ignoring case) of that character removed.
+listRemoves :: String -> [(Char, String)]
+listRemoves str = [(c, filter ((/=c) . toLower) str) | c <- ['a'..'z']]
+
+-- Lifts a function to a function working on the second component of a pair.
+liftSnd :: (a -> b) -> (c,a) -> (c,b)
+liftSnd f (c,a) = (c, f a)
+
+-- Gets the pair with the minimal value in the second component from a list.
+minBySnd :: (Ord b) => [(a,b)] -> (a,b)
+minBySnd [(a,b)] = (a,b)
+minBySnd ((a,b):ls)
+    | snd (minBySnd ls) < b = minBySnd ls
+    | otherwise = (a,b)

src/Main.hs

@@ -3,6 +3,7 @@ import Day01
 import Day02
 import Day03
 import Day04
+import Day05
 
 
 solutions :: [((String, String), [String] -> String)]
@@ -14,7 +15,9 @@ solutions = [
     (("03", "1"), Day03.part1),
     (("03", "2"), Day03.part2),
     (("04", "1"), Day04.part1),
-    (("04", "2"), Day04.part2)]
+    (("04", "2"), Day04.part2),
+    (("05", "1"), Day05.part1),
+    (("05", "2"), Day05.part2)]
 
 main :: IO ()
 main = do