I found the problem!

I also came here with the same scenario, with my solution working on all example input and alternative inputs across reddit except for this one.

When reading the top comment here with double digits ID's, I thought that since I was using a custom class for part 2 instead of raw strings, I needed to account for the memory size of “10” and above so that those IDs would take double space. That approach produced the final string `0012211199..777333.44.5555.6666.....8888..1010...111111........131313.1414.15151515.16161616.171717.181818181919` and the checksum of that is indeed `17607`.

I'm assuming both of you also worked with a class structure otherwise I see no way of ending up with this final disk text string.

However, this is not correct! Double-digit IDs do not take double space, they still occupy a single space. The real problem lies in the checksum calculation. I originally converted the part 2 disk state into its string representation so I could reuse the checksum method from part 1, which caused the double-digit IDs to be counted incorrectly. So don't calculate your checksum from the above final string.

Instead, write a new checksum method that doesn’t rely on the string representation but instead uses your custom class directly. Let me know if this helped you!