circuit FAQs

these questions and answers are mostly taken from the halo2 ecosystem Discord; they are meant for circuit writers, not complete beginners. if some of these terms seem completely strange to you, the halo2 book is a good place to look them up.

do unassigned cells default to zero?

yes, unassigned cells are initialised to zero in the current implementation; but this is an arbitrary implementation choice, and we shouldn’t rely on it being true. e.g. there was some discussion about randomising unassigned cells:

how do we decide which columns to reuse across chips?

currently, the reuse of advice columns is a manual decision, as the FloorPlanner has no way to horizontally optimise. this capability is intended to be introduced: see zcash/halo2#195.

does halo2 support runtime configuration of circuits?

this is a feature we want to enable and are discussing in zcash/halo2#550. this is already supported in the privacy-scaling-explorations/halo2 fork as a feature flag.

given a circuit object, can we estimate the minimum number of rows it will use?

this is currently not supported, and would likely involve writing a FloorPlanner that only returns the number of rows used.