Pseudo Random Number Generator in batari

[+Gemintronic]

Thinking about MausBoys "Holy Grail" of an Ultima clone for 2600 I realized level data would be a real concern. My thought would be to have a pseudo random number generator to provide the type of screen given a position in the world map. So, say, worldx = 10 worldy = 10 would always be screen type 6. Something like prand( worldx + worldy ) = 6 given a seed of 33 (or whatever).

 

I've used this in another language:

 

#define rand
var a, m, p_rand, range;
a = 16807;
m = 2147483647;
range = argument0;

if (range < 0) {
   global.p_rand_last = -range;
   return 0;
}

global.p_rand = (a * global.p_rand_last) mod m;
global.p_rand_last = global.p_rand;

return (global.p_rand / m) * range;

#define rand_ext
var a, m, p_rand, range;
a = 16807;
m = 2147483647;
range = argument0;
stream = argument1;

if (range < 0) {
   global.p_rand_ext_last[stream] = -range;
   return 0;
}

global.p_rand_ext = (a * global.p_rand_ext_last[stream]) mod m;
global.p_rand_ext_last[stream] = global.p_rand_ext;

return (global.p_rand_ext / m) * range;

 

Of course, much of the math doesn't carry over to Batari. I'm going to have to think small. I don't care about random distribution much. All I want is the same number given a value and a seed. I'd even put up with a hard-coded seed.

 

Any thoughts on how to proceed? What would such a Batari function would look like?

 

Could I get away with:

 

 function prand
rem add seed value
result = seed
rem add argument
result = result * temp1
return result
end

Edited July 4, 2010 by theloon

[SeaGtGruff]

All I want is the same number given a value and a seed.

I'm not sure I understand yet. The standard "rand" statement always returns the same value for a given seed. For example, "rand=1:a=rand" will always return 180, "rand=2:a=rand" will always return 1, "rand=3:a=rand" will always return 181, etc. Is that what you're looking for?

 

Michael

[+Gemintronic]

All I want is the same number given a value and a seed.

I'm not sure I understand yet. The standard "rand" statement always returns the same value for a given seed. For example, "rand=1:a=rand" will always return 180, "rand=2:a=rand" will always return 1, "rand=3:a=rand" will always return 181, etc. Is that what you're looking for?

 

Michael

 

That would be it. I guess that's why people talk about seeding the rand function with a user joystick press. The randomness of human reflex makes the seed different for each game.

 

I didn't know if rand really worked that way. Thanks for clarifying! I suppose this could do:

 

 rand = worldx
whatscreen = rand
rand = worldy
whatscreen = whatscreen + rand

 

Of course, worldx = 10 worldy = 15 is gonna get the same whatscreen value as worldyx= 15 worldy = 10 thus the same room.. maybe?

Edited July 4, 2010 by theloon

[SeaGtGruff]

I suppose this could do:

 

 rand = worldx
whatscreen = rand
rand = worldy
whatscreen = whatscreen + rand

 

Of course, worldx = 10 worldy = 15 is gonna get the same whatscreen value as worldyx= 15 worldy = 10 thus the same room.. maybe?

That depends. Yes, "whatscreen" would be the same, but if you use a 16-bit number, the room could be different-- e.g., use the first "rand" value for the lo-byte, and the second "rand" value for the hi-byte, or vice versa.

 

But I don't understand what you need "rand" for. I mean, if you want the result of calling the routine to always be the same given particular x and y seed values, what do you need "rand" for-- can't you just use the seeds?

 

Michael

[+Gemintronic]

can't you just use the seeds?

 

I suppose rand = worldx and rand = worldy would work but it'd generate the same room right?

 

Say if the rand result for a worldx seed is 15 and the rand result for worldy seed is 10 and I added them.. we get 25

now if the rand result for a worldx seed is 10 and the rand result for worldy seed is 15 and I added them.. we get 25

 

The result is both world screen (15, 10) and (10,15) are gonna be the same. Gotta be a way around that.

 

Thanks for sticking with me so far

Edited July 5, 2010 by theloon