I tried to compartmentalize it. the search is on its own function, and while that fill_in_dead_ends function is extremely large, it is a lot of replicated code. match k case statement could just be removed. A lot of the code is extremely verbose and has an air of being "unrolled" for purposes of me just tweaking each part of the process individually to see what could be done. The entire large af match case all seemingly ended up being very similar code. I could condense it down a lot. however, I know doing so would impact performance unless plenty of time is spent on tweaking it. So unrolled copy pasta was good.

The real shining star is the find_next_dead_end function because the regex before took 99% of the time of about ~300 ms seconds. Even with this fast iterative function, the find_next_dead_end still takes about 75% of the time for the entire thing to finish filling in dead ends. This is because as the search ran deeper into the string, it would start slowing down because it was like O(n*m) time complexity, where n in line width and m is line count being searched through until next match. My approach was to store the relative position for each search which conveniently was the curr_row,curr_col. Another aspect to reduce cost/time complexity on the logic that would make sure it filled in newly created dead-ends was to simply check if the current search for the next dead end was from the start after it finished checking the final line. Looking at the line by line profiler from iPython, the entire function spends most of the time at the while('.' in r[:first_loc]): and first_loc = r[:first_loc].rindex('.') which is funny because that is still fast at over 11k+ hits on the same line with only a 5-5.5 microsecond impact for each time it ran the lines.

though I could likely remove that strange logic by moving it into the find_next_dead_end instead of having that strange if elif else statement in the fill_in_dead_ends logic.

there is so much possible to make it improved, but it was quick and dirty.

Now that I am thinking about it, there would be a way to make the regex faster by simply string slicing lines off the search, so that the regex doesn't spend time looking at the same start of string.