# Z-machine RNG

The Z-machine Standards Document includes the following:

However, the interpreter in the Inform 7 IDE, when given a seed S < 1000, will return 0, 1, 2…S-2, S-1, 0, 1, 2, et cetera. Do other interpreters share this quirk? If so, what is the “safe value” beyond which the RNG will generate random-looking numbers?

(There is a note in the Standard suggesting that this mode can be useful for testing, but it does not seem to be part of the Standard itself.)

How are you setting this up? I tried:

``````When play begins:
seed the random-number generator with 100;

Every turn:
let N be a random number between 0 and 16384;
say "[N].";``````

(Note that “seed the random-number generator with 100”, in Z-code, invokes the opcode @random -100.)

On the Mac IDE I get the sequence 13435, 5882, 6634, 1130, 3930, 474. In Frotz I get incrementing numbers from 0, so this must be a Frotz behavior. Fizmo doesn’t have it. I don’t know if any other interpreters do.

If you want predictable random numbers for a game purpose, you should write your own RNG in I6 or use one of the extensions that does.

Here’s my code.

[code]Seeding the RNG with is an action out of world applying to a number. Understand “seed [number]” as seeding the RNG with.
Carry out seeding the RNG with a number:
seed the random-number generator with the number understood.
Report seeding the RNG with a number:
say “Random seed set.”

Randoming is an action out of world applying to a number. Understand “rand [number]” as randoming.
Report randoming a number: say a random number between zero and the number understood.

There is a room.[/code]

In the Windows interpreter (which presumably uses Frotz), “seed 1. rand 256. rand 256. rand 256.” gives 0, 0, 0; “seed 2. rand 256. rand 256. rand 256.” gives 0, 1, 0; and so on.

This doesn’t occur in any Glulx interpreter I’ve tested, so it seems it is Frotz-specific. Interesting.

Looks like the standard and the remarks aren’t really compatible on this, though “should cope well with very low seed values” is very vague too…

Parchment only supports the incrementing system because I didn’t know how to make another good PRNG at the time. I should add in a xorshift one now.

You already saw my new extension if you do need safe random seeded numbers.

After thought, I disagree with that comment in the spec. I don’t think the incremental mode belongs in the interpreter. If the game does want an incrementing random mode, it can trivially implement one in I6 code; this is better than relying on a not-really-specified interpreter behavior. And the incrementing mode is more likely to break games than to generate useful testing, anyhow. (It puts you right back in the alternating-odd-and-even mode that has caused real-life game bugs.)

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Whereas fizmo actually does implement a predictive random generator, one has to explicitly activate it using the --predictable command line parameter or use the /predictable command in-game. Since that doesn’t conform to the current spec, I’d currently consider implementing the double-seed behavior for some future update.

In case there’s some kind of consent that this doesn’t make sense – personally, I’d agree that this should be outside the interpreter – maybe we could update the updated Z-Spec.