[schemescape]

Is there any way to play without logging in (for previous years, if necessary)?

I couldn't find anything on the About page that clarifies this.

Edit: I see anonymous users in the leaderboard, so it seems like it might be possible.

[LegionMammal978]

No, there is no way to submit answers without logging in. The "anonymous" feature is just a setting to hide your username from the leaderboards.

[Jtsummers]

You have to have a login (Google, Github, Reddit, Twitter) to get inputs, and part 2 is only unlocked after completing (correctly) part 1 for any given day. You don't have to share your identity with anyone but the host of the server.

[david2ndaccount]

You have to log in, but you don’t have to reveal your identity or username on the leaderboard.

[gnubison]

Other comments haven’t pointed this out, but the key thing is that each account gets a different input (so it's a little harder to cheat).

[dolmen]

It's easy to cheat by applying someone else's code (ex: from reddit) to your own input.