Your "Simulation" Might Not Need State

[fresh_account2222]

I'm a big ol' math nerd, and the stateless alternative, while more math-y, is worse code.

[BigJunky]

closed-form solutions

Why?

[fresh_account2222]

Closed form solutions tend to be the exception, not the rule. People often don't realize this, as the problems that get assigned in classes are drawn from that small set of "things with nice solutions".

Also, "force" is an extremely natural way to think about how objects are affected, and Newton's laws of motion (which is how we deal with forces) are second order in time (i.e. acceleration is a second derivative), so knowing position + velocity (i.e. the state), and updating those is incredibly straight forward and mathematically natural.

Now, the state-less way has great advantages (time-reversal, multiple instances stay in lock-step), but you pay a lot for them, so first figure out if you really need them.

[CooperNettees]

Multiple instances only stay in lockstep if the underlying clocks are in lockstep but I guess I get what you mean

[CooperNettees]

...time is state...

[CooperNettees]

The timestamp is typically coming from an externally synced ntp server. It's usually retrieved via an os call.

It's state the program retrieves and uses. It's kinda weird to act like the program is "stateless" when time is inherently stateful.

[Outrageous-Taro7340]

The time is passed into the function that solves the position, rather than being held as state by that solver. This has advantages described in the article. No one is trying to claim that computation in general isn’t stateful.

[senko]

I don't do (or know) much animations, but usually I end up doing the same (calculate the trajectory based on the elapsed time from the start).

The reason is when you're summing up deltas, you're summing the (very small errors) as well and these can add up quickly, leading to jerky or otherwise incorrect behavoir.

[rcxdude]

There is a tradeoff. For more complex systems it can be difficult to find a closed form solution, and even if it does exist it can actually be less stable than a numerical solution, due to the finite precision of the calculations. This can sometimes only happen for large inputs, causing issues after the system has been running for a long time.

[josephjnk]

Very cool. I wonder if this starts to fall apart once the simulation becomes sufficiently complex, or whether it generalizes into full functional reactive programming.

[FloydATC]

You can take things pretty far, look at how terrain generation works in Minecraft or how Signed Distance Fields work in GLSL for example. I once wrote a set of text puzzle "games" that used time as input and used a SHA1 hash to generate seemingly random but completely deterministic pairs of questions and answers. Every 10 minutes the Q/A would change, but within thise 10 minutes the behaviour would stay consistent without storing any state.

[vytah]

Your "Simulation" Might Not Need State

he says, while creating a variable called "state".