[Mikhail_Edoshin]

Technically each side needs an index plus a single bit. The bit is a counter, you increment it on every wrap. It overflows, but this is correct, we only need the last bit. Initially it is 0. By comparing the indexes and the bit you tell apart all cases and do not lose an entry.

(I think this was published in one of Llang's papers but in a rather obscure language.)

[grumbelbart2]

One of the comments in the article proposes that: Just wrap both counters at 2capacity (instead of capacity or UINT_MAX).

[Mikhail_Edoshin]

Which does that and that's what Llang suggests as well, I remember 2 in some power in his formula. I myself find the separate one-bit counter easier to understand: each side counts pages, but they don't need the full number, only the difference between their counters and the difference can be at most one, so one bit is sufficient. If the counters are same, the actors are on the same page, if they are different, then the writer is one page ahead.