I suspect that a lot of games from the NES era and before were deterministic.

Post like this is way I read HN. Thanks from us who this missed stuff.

The physics game sim step had to be hardcoded but you could vary the graphics rendering independently. That did mean you needed to use a different RNG for sim objects and graphics-only particle effects.

I worked on the deterministic instant-replay system for a racing sim on PS3. The most interesting thing was that on PS3 we had main processor cores (PPU) and helper cores (SPU) which had their own instruction set. Our physics sim could distribute work across both, but the thread assignment had to be deterministic because PPU and SPU got marginally different floating point results for the same instructuion and inputs. That was a fun one to figure out!

Thought I heard my ears ringing... Blast from the past here.

That's the beauty part of recording server state and playing it back -- we included a recording of whatever accurate integer-based OS time I could get on each platform. On Windows, for example, I record the raw values of QPC ticks and the tick rate into the recording. Then on playback, I play back the clocks themselves as if the hardware has that tick rate and tick count are from the recorded computer. So if frame 10 took 33.3ms and frame 11 took 66.7ms, we'd play them back that way.

Note that playbacks are almost never in real-time. The reason a typical server frame takes 33ms is that we might be processing for 5ms and then yielding the thread (sleeping) for the remaining time during real-time gameplay. On playback, we don't yield the thread, so we play back faster than real-time when doing something like a live Chronobreak (disaster recovery.)

Strictly speaking, you are right. Let's say we had a game with no player inputs (like a bots-only game.) It's possible that the outcomes might vary between two games with the same random seeds if they run different numbers of frames over the same time period. That said, we've actually tried this a few times, and our game code is surprisingly resilient to this -- only in a few instances have we found issues where subtle variations in frame time could impact player outcomes (and those are generally corrected when we find them -- this is an esports game after all.)

A lot of this comes down to tradeoffs. Yes we might have those subtle issues, but if we fixed our timestep, then we'd have the other problem of the game potentially going into slow-motion during (the admittedly rare) periods of long frame times. In practice, we decided time dilation is more negative for player performance than loss of interactive resolution; humans are great predictors of motion with limited information, but they're not so great when the rules of localized reality bend.

Edit: typo

You can still achieve determinism by treating the measured delta-time as an input, and resimulating the game with recorded delta-time. This may not be feasible on public servers, but is definitely feasible for professional play where determinism is important to rewind games due to bugs which affect competitive integrity.

The xorshift family is one of the top two RNGs in the modern day (the other being PCG). Unfortunately, "choosing the ideal member of the family" is nontrivial if you're trying to eke out every bit of performance.

The problem with the xorshift family is that it doesn't implement the nice auto-scaling (and thus auto-testing) that PCG has. Re-parameterizing it means inventing whole new numbers from scratch and hoping you plug them in correctly. Xorshift also has a rare-but-severe "stuck near zero" problem.

The problem with the PCG family is that its trusted way of scaling (nobody likes the `_k` variants) relies on 128-bit multiplication (a modern member of the family at least reduces that to a mixed 128x64 multiply), which remains notably slow, unlike xorshift which only uses small integers.

(But I must emphasize: all RNGs from other families either have worse variants of these problems, or give up on performance entirely. These concerns should be used only to select your preferred tradeoff between the two, there is no excuse for ever using any other RNG family unless you need cryptographic security)

"It’s simple, fast, and has a great distribution, which means it’s more than good enough for gameplay."

Its distribution is not flawless, but I would agree that it's good enough for gameplay.

Likely performance. You can do xor and shift operations essentially for free on a modern pipelined CPU.

Other techniques, such as linear congruential generators, require multiplication and division.

Besides the other answers, could also be that they wanted to make sure they had their hands on a fully deterministic implementation across different hardware/software stacks (like Windows and OS X).

As a sidenote on a project the size of LoL, the time, size and cost of implementing a PRNG based on a known one is a rounding error.

underscores

Good info, and fair enough for people to support the strike, if they want to.

FWIW, my view is that the use of AI-generated figures, voices etc. in video games is inevitable. As a gamer, I genuinely don't care who is behind a particular voice. I cannot imagine that many gamers do. All the strike is going to do, is hasten the shift to AI.

They want to ban generative AI in video games? Why would a company ever agree to that?

The writers strike eventually resolved, but with big layoffs and wiped $2B from the LA economy

I guess I'm unintentionally supporting them, because Riot removed Arena last week. That was the only game type I had the stomach to play, I'm tired of whacking minions.

Except the strike is not really against LoL as a game, but as one of the largest clients of the actually targeted company. The strike is purely an industry strike and does not have much to do with players, as it originates in the VA company trying to pull a scummy move to bypass an union. The strike's goal was to force Riot to take position and disavow the company, which has been successful.

Where can we learn more about this? The strike calls for players to boycott the game?

“Digital picket line”? Over “AI”? For heavens sake, grow up. What are you protesting, the use of technology and mankind’s advancement?

> “League of Legends” is caught in the middle of a dispute between Hollywood’s actors union and an audio company that provides voiceover services

How is that related to this engineering post? I wish HN took action against people commenting wildy unrelated topics.