Reversing an ordinal assignment

[Yagoki]

This problem is more of a mathsy one that a forge one but it's been bugging me for so long now that I think I am incapable of noticing the solution

 

Let's say I have an integer set of coordinates, each ranging from 0 to some independent finite limit (which may be different for each. This can be thought of as an N dimensional grid with coordinates

(i, j, k, ...)

and lengths

(I, J, K, ...)

. In this case each coordinate that can be occupied/intersect of the grid can be counted with an ordinal assigned by

o=i+j*I+k*I*J+...

. this is useful wen trying to assign a metadata value or some such. The problem I'm having is figuring out a way of reversing this operation to get the original

(i,j,k,...)

given the ordinal and

(I,J,K,...)

[LexManos]

Thats a dense, yet slow way of indexing things typically people just set bit ranges for certian things:

o = (x << 0) | (y << || (z << 16)

x = (o >> 0) & 7

y = (o >> & 7

z = (o >> 16) & 7

 

But you way it's similiar but can't easily do the shifts.

So:

o = x+y*(X)+z*(X*Y)

x = o % X

y = floor((o % (X*Y))/X)

z = floor((o % (X*Y*Z))/(X*Y))

 

Complicated and ugly, but should do what you want.

 

[Yagoki]

Thanks, that's been bugging be for ages. think I'd just looked at it for too long to notice as I said. Did initially think of just packing like you say but it wound up being important that they remain in order and without gaps (various rng operations etc...)