Become Immortal is at this point the only 1kyu kata I tried (and solved) on CodeWars. It took me 3-4 days to solve it. It was definitely pretty damn difficult to figure out, but in the end I managed it myself.
Looking at the example tests it was immediately obvious that trying to generate the actual rectangles and xoring everything was out of question. For the heck of it I tried once, but even for the last “basic test” I’d run out of memory (on my desktop) very quick. Sure it works fine if the rectangle is 30-40 rows and columns, but not for scales in the millions. I used it to dump rectangles on the console and double check my solution.