It all should be translatable to C++. I basically created a small (max 4 items) stack for each place in the grid. When I move to a new square, I push my previous position to new location's stack. That lets me unwind my path and detect loops. The code iterates backwards and puts a block on the spot.
That means I don't need to redo the previous stuff, because I start right at the new block. I then pop off the path back to the block and do the previous location.
Thank you for making me look back at that day. Found a silly bug and went from ~400ms to ~80ms on my fast machine and a whopping 1.6s down to ~350ms on my slow-poke test machine. Had a silly mistake in my loop detection code. Now Day 7 is the hold-up for me....