Check consistency of state in collaborations

[cdsmith]

There remains one source of indeterminacy in game computations that we haven't thought about: incomplete specification of floating point operations. Many transcendental functions in floating point are only specified to plus/minus one ulp of precision. Furthermore, optimizations often cause intermediate floating-point results to retain extra precision in some cases, and not others. It's not clear to what extent these things will cause problems in real-world games.

There are several stages to this problem:

1. Confirm that it's really a problem. That would mean collecting data across many platforms and browsers about whether floating point arithmetic (including complex nested expressions) gives different answers. We could, in theory, do this for visitors to code.world, and collect divergences from a reference value in files on the server.
2. If it is a problem, find solutions. We can replace many transcendental math functions with deterministic variants (e.g., those at https://deterministic.js.org/; but keep in mind that we don't want to completely load deterministic.js, since it kills access to the system clock, among other things.)

[cdsmith]

Alas, use of deterministic.js was a dream. They haven't actually implemented deterministic versions of any math functions except for Math.exp.

[cdsmith]

The plan for the evaluating the scope of the problem is to implement desync detection and logging for games. We can do this by having clients generate, hash, and compare the rendered pictures for committed state, on a regular basis. If they don't match, we'd like to log some details, such as the program hash, and user agents of the clients that disagree.

There is no need to take further steps until/unless we can collect examples of programs that fail.

[cdsmith]

We now have examples of programs that fail. All that's needed is to use sine or cosine on lots of values, and run with Firefox and Chrome on different clients. This doesn't leave us confident about consistency, so we really want to do the detection and gather stats.

[cdsmith]

Promoting to an enhancement. We definitely want to do this, and at least report the desync in individual games. The speculative part about data collection would also be nice, but isn't part of the main goal.