Hi, I am looking to build a pseudo-randomly generated game level/map.
A map is divided into blocks of areas, and using a seed I want to be able to regenerate the same content of any of these areas anywhere in my game map.
So if I teleport to a far-off area, it will look the same as the next time I visit the same area after restarting the game.

I know lots of games already use this mechanic, but my issue is with the fact that to seed a pseudo-random number generator you typically supply a single value such as an unsigned integer.
What I have instead is an x,y grid coordinate, not a single value, because that is the only way I can think of how I would identify a map area, especially if the map is potentially infinite in size.
For example, the first area may be 1,1. To the right is 2,1, and below is 1,2 etc.

So I guess what I am asking is how can I use a coordinate as a seed?

srand(x * 10366439865051156459ul + y);

Thanks for suggestions guys.

Int=(int)short << 16 & short2

???

So short is the x coord and short2 is the y. You are bit shifting the x by 16 and adding it to the y, to make it a unique single value? This is so the point {1;2} will not result in the same seed as {2;1}, am I right?

And alvaro, you are doing something similar.

Thanks for suggestions guys.
[quote name='Waterlimon' timestamp='1319476188' post='4876360']
Int=(int)short << 16 & short2

???

So short is the x coord and short2 is the y. You are bit shifting the x by 16 and adding it to the y, to make it a unique single value? This is so the point {1;2} will not result in the same seed as {2;1}, am I right?

And alvaro, you are doing something similar.
[/quote]

I am doing something similar, but my code works. What Waterlimon posted has several issues. He probably meant to say `(short1 << 16) + short2' or `(short1 << 16) | short2'. With '&' you'll get 0. And you can't use `short' as a variable name.

[quote name='Paulus_newbieus' timestamp='1319488546' post='4876462']
Thanks for suggestions guys.
[quote name='Waterlimon' timestamp='1319476188' post='4876360']
Int=(int)short << 16 & short2

???

So short is the x coord and short2 is the y. You are bit shifting the x by 16 and adding it to the y, to make it a unique single value? This is so the point {1;2} will not result in the same seed as {2;1}, am I right?

And alvaro, you are doing something similar.
[/quote]

I am doing something similar, but my code works. What Waterlimon posted has several issues. He probably meant to say `(short1 << 16) + short2' or `(short1 << 16) | short2'. With '&' you'll get 0. And you can't use `short' as a variable name.
[/quote]

Thanks for clarification and taking the time to help me out alvaro!

I actually come from a VB .Net programming background (*hangs head in shame*) so could you please clarify with the symbols what's going on with the bit shift and addition i.e. should it be adding the results?
Also, alvaro what's the [color="#006666"][font="CourierNew, monospace"]10366439865051156459ul [/font]signify, and how would that be better than a bit shift?

Many thanks!

You should try to find some material on bit manipulations in C or C++ somewhere else, for a more complete understanding.

Let's analyze the expression ((x << 16) + y).

(x << 16) means "take the value of x and shift it 16 bits to the left", or equivalently "multiply x by 65,536". This means that (1, 65536) and (2, 0) map to the same number. I don't know if this is acceptable to you, but my guess is that it's fine. The magic constant I used is simply a random odd 64-bit number. You should use a random odd 32-bit number if you are using 32-bit arithmetic. The advantage of using a large random number instead of 65,536 here is that you can generalize the procedure to hash more information into the seed like this:
unsigned long seed = x;
seed = seed * 10366439865051156459ul + y;
seed = seed * 10366439865051156459ul + z;
seed = seed * 10366439865051156459ul + w;
// ...



If you were to do that by shifting alone, the information about x would eventually get lost in overflow.

There are situations where you need to hash information down to a single number and you need the result to look much more random (e.g., cryptographically signing a document), but I don't think you have to go crazy for your particular application.

You should try to find some material on bit manipulations in C or C++ somewhere else, for a more complete understanding.

Let's analyze the expression ((x << 16) + y).

(x << 16) means "take the value of x and shift it 16 bits to the left", or equivalently "multiply x by 65,536". This means that (1, 65536) and (2, 0) map to the same number. I don't know if this is acceptable to you, but my guess is that it's fine. The magic constant I used is simply a random odd 64-bit number. You should use a random odd 32-bit number if you are using 32-bit arithmetic. The advantage of using a large random number instead of 65,536 here is that you can generalize the procedure to hash more information into the seed like this:
unsigned long seed = x;
seed = seed * 10366439865051156459ul + y;
seed = seed * 10366439865051156459ul + z;
seed = seed * 10366439865051156459ul + w;
// ...



If you were to do that by shifting alone, the information about x would eventually get lost in overflow.

There are situations where you need to hash information down to a single number and you need the result to look much more random (e.g., cryptographically signing a document), but I don't think you have to go crazy for your particular application.

Thanks, much appreciated guys.