If you get stuck on day 9, carefully reread the movement instructions:
If the head is ever two steps directly up, down, left, or right from the tail, the tail must also move one step in that direction so it remains close enough.
Otherwise, if the head and tail aren't touching and aren't in the same row or column, the tail always moves one step diagonally to keep up.
I had mis-inferred that the tail knot should merely go where the head knot had previously been whenever they're too far away. That turns out to be entirely equivalent in effect to ^^ for part 1, but doesn't work at all for part 2. I find it hard to locate (or even notice) instructions in diagram-heavy puzzle days like this one, so spent about 90 minutes just trying random stuff before carefully rereading.
Here's my day 9. I'm sure I would have given up between part 1 and 2 if I didn't have a type system to guide me through the refactoring.
Here is my Day 9 solution. I really enjoyed this one. :smiley:
My day 9. I also thought this was a fun one
Last updated: Jul 06 2025 at 12:14 UTC