main = do
    contents <- getContents
    let cleanContents = takeWhile (/='\n') contents
        memory = read ("[" ++ cleanContents ++ "]") :: [Int]
        correctedMemory = write 12 1 $ write 2 2 memory
        answer = 19690720
    print [100 * noun + verb | noun <- [0..99], verb <- [0..99], answer == let testMemory = write noun 1 $ write verb 2 memory
                                                                          in head $ execute 0 testMemory]

execute :: Int -> [Int] -> [Int]
execute pos memory
    | op == 1  = execute nextpos $ write (input1 + input2) writepos memory
    | op == 2  = execute nextpos $ write (input1 * input2) writepos memory
    | op == 99 = memory
    | otherwise = [-1] -- TODO should be a Maybe or something...
    where op = memory !! pos
          nextpos = pos + 4
          input1   = memory !! (memory !! (pos + 1))
          input2   = memory !! (memory !! (pos + 2))
          writepos = memory !! (pos + 3)

write :: Int -> Int -> [Int] -> [Int]
write value pos memory =
    let (front, _:back) = splitAt pos memory
    in  front ++ [value] ++ back