Re: Deterministic hashing for lua tables

[Isaac Dupree <contact@...>]

Hi Holly,

I've been contemplating a fully deterministic Lua.  Your post[1] on 
lua-users was the closest thing I found, so I'm asking what progress 
you've made.

I'm probably going to be hacking on Lua's code anyway, and I'd be happy 
to do what I can to implement this.

My use-case: I'm writing a game/simulation where two networked players 
can execute the same user-written script, such as an AI for someone's 
robot.  I desire to guarantee that the two players' simulation states 
will not go out of sync.  Even if they write stupid/malicious scripts.

I wish the determinism to apply even across 
serialization/deserialization (e.g. Pluto[2]), so that the game can be 
saved/loaded at any time, new players can be added to a networked game, etc.

(To protect from stupid/malicious scripts I'll also have to implement 
processor-time and memory usage limits for Lua, and make sure that 
they're deterministic from the simulation's point of view.  I think the 
limits will be per-robot.  Each robot could have its own lua_state to 
make the accounting easier.  Perhaps a count of bytecodes executed, and 
some reasonable count of reachable data, is the way to go.  And I'll do 
something about worst-case hash-table performance.)

[1] http://lua-users.org/lists/lua-l/2011-12/msg00441.html
[2] http://lua-users.org/wiki/PlutoLibrary