Are there methods to get the modulo of a variable voltage, with reference to another (constant) voltage?
Short answer: no, you can't solve numerical problems with strictly analog circuitry. The moment you build something that does that, it inherently, by the very definition of the word, becomes digital.

So, your simplest circuit that can do that is actually an ADC of sorts. In fact, sigma-delta ADCs are pretty close to that, functionally, and you'll have a fun time reading up on how they work :)