Day 07

I keep bruteforcing and it keeps working!

I will be quick here because it is quite simple and I don’t have much time

The input

I split each line on “: “, giving me two parts:

• One is the target number
• One is a space-separated list of numbers.

I read the target number, and I split the list and read every number:

parseInput :: String -> Input
parseInput = map ((\[a, b] -> (read a, map read $ words b)) . splitOn ": ") . lines

Part one and two

I simply try every possible operation until I no longer have any number in the list. When the list is empty, I return True if the target was found, False otherwise. A target is deducible if bruteforcing all operations lead to one True result.

I filter all the deducible lines, and sum their targets:

partOne :: Input -> Output
partOne = sum . map fst . filter isDeducible
    where isDeducible (target, x : xs) = testOperations target x xs
          testOperations target acc [] = target == acc
          testOperations target acc (x : xs) = testOperations target (acc + x) xs ||
                                               testOperations target (acc * x) xs

partTwo :: Input -> Output
partTwo = sum . map fst . filter isDeducible
    where isDeducible (target, x : xs) = testOperations target x xs
          testOperations target acc [] = target == acc
          testOperations target acc (x : xs) = testOperations target (acc + x) xs ||
                                               testOperations target (acc * x) xs ||
                                               testOperations target (read $ show acc ++ show x) xs

The only weird operator is “||”, which I simply create by converting all numbers to strings, concatenating them and converting the result back to an int. (Not optimised but it works and was fast to write)

The end

Well that was easy. And the runtime is not that long tbh, like 2 seconds.