Algorithm to Pair Even Boolean Array Elements

[intervoxel]

Suppose an array of booleans with an even number of true values. I need an algorithm that, given an address containing a true (say, 3), returns the address of another true (say, 25), such that if I call the algorithm for 25, it returns 3. No storage of old choices are allowed since this algorithm will be used in a cellular automaton.

Thanks for any help.

 

[Merlin3189]

If you know, or count, the number of true values, call it N, then if the function returns the address of the N/2 th true value after the starting value counting cyclically, then each true value will pair with a true value the same number of places above and below.