I draw ^>v< characters on the grid while walking, so then it's a direct array lookup (instead of a hashtable). The representation could be better though, some kind of bitmask would beat checking against a bunch of characters.

I’d like to see your solution in total. I’m not too familiar with the nuts and bolts, but hash set is quite a bit more expensive than a simple vector, there’s a bunch of overhead incurred when executing the hashing and placing of the data, and when repeating a few thousand times it sure adds up. My part one hovers around 600 microseconds.

I created rows and cols vecs that keep places of blocks. When moving, I binary search the row or col, find the block that stops me. So moving whole sections at once. Otherwise used HashSet of pos and dir like you. Also in part 2, place the new block only on the path I take in part1. Part 2 is 26ms.

code

Alright, I completely forgot about --release because i normally use just to run my stuff. That brings part 2 down to around 400ms, i am okay with that for now :D