[Resolved] Predicatable Random Number Generator

Ralpharama

Anybody got any idea how to create a random number generator that will always output the same series of 'random' numbers, given a specific seed?

It's for a PHP game that creates things randomly based on ID numbers, but the produced numbers can't vary with server, PHP version, etc, etc. So I can't use rand(), can I?

I know you can do a very simple generator with something like:

r = ((seed*77) mod 10) + 1

But I need something a little more complex... Basically, I'm no maths wizz, so struggle with this aspect of things 🙂

Thanks for any ideas.

Ralpharama

Mordecai

Umm.. what makes it random, then?

TheDefender

Not 100% certain where you are going with this, but how can it be "random" if you are looking for a predictable answer?

At any rate, you can specify a range I suppose...

$pickfrom = range(10,20);
$pickfrom = range(1,20);
$pickfrom = range(10,100);
$pickfrom = range(50,150);

which gives you an array with a range of values from the start and end points you specify... Then you can use [man]shuffle()[/man] in conjunction with [man]srand()[/man] to pick out one of the numbers within that range (at random).

If this isn't what you mean, you may need to give some more details...

Weedpacket

The Mersenne Twister RNG ([man]mt_rand[/man]) has the properties you're after: given a known seed, it will produce the same sequence on all implementations.

Ralpharama

Originally posted by Weedpacket
The Mersenne Twister RNG ([man]mt_rand[/man]) has the properties you're after: given a known seed, it will produce the same sequence on all implementations.

I saw the mersenne twister algorithm in the PHP.net function list and wondered if that would work - so no matter what the platform, installation, configuration or version, it would always produce the same 'random' sequence given a specific seed? I'll try it out and let you know. Thanks, you seem to be the only person who understood what I was aksing for 🙂

For the others - there is no real easy way of giving 'real' random numbers, only a number of algorithms that produce seemingly random numbers - the more complex the funcion, the longer the sequence is before it seems to begin to repeat. So if you seed a function with number X, you should always get sequence Y. It is useful in games design where you can use randomness to design levels, behaviour, events etc, but you need to be able to produce the same sequence of 'random' numbers every time, or players will see different level 1s, etc. My point was if user A installs the game and plays it, will user B get the same random level using a different version of PHP on a different server, etc.

Anyway, thanks for the responses.

Ralpharama

Just to let you know that mt_rand with mt_srand seems to do the trick just fine.

Thanks.