[garbear]

I don't really have any responses to any of that right now, but I'll give it some thought.

  
Hahaha I'm totally stuck working on a module that can handle both rewind and savestates, and when I get stuck I get all philosophical and give non-answers. Deal with it

I'm also not too familiar with how retroachievements works, but I assumed that achievements were (like consoles) per game, per user. Can achievements be quantified per game save? 

  
Achievements work by scraping game memory; e.g. if I know that the byte at the RAM address $0010 (hex) stores the current level, then I can activate an achievement such as "Get to level 5" by waiting for this byte to reach the value 5. Achievements are calculated every single frame. So every frame and every savestate we can calculate (certain) achievements.

There is an exception: Some achievements have a "hysteresis". This is a "memory effect", where the achievement depends on previous frames. E.g. to protect against cheating, "Get to level 5" might require the address $0010 to be a 1, then 2, then 3, then 4, then 5, so the user can't load a downloaded savestate and automatically get the achievement.

Therefore, the internal state of the achievement engine must become part of the savestate and be saved alongside the emulator's RAM. Not a big deal, it just requires more work on my part to create an archival format to handle both emulator and achievement state.

To answer your question about per-user:

In RetroArch, you go to settings, then enter your retroachievements username and password. Achievements are reported for this account.

In Kodi, we can trivially do the same. However, this locks us into an architecture with a single global user. And fuck changing architecture; this can take days if not weeks. Instead, from the beginning I'll make savestates centered around the people who are playing. Each player can have their own achievements and saved games, saved games can share users, and users can share saved games.