Intervals are defined as a number of blocks, so you cannot have a billion blocks in them. This function really only checks that the difficulty only changes when allowed to do so, and does not change by more than a factor 4. I don't see how PermittedEDADifficultyTransition can ever return false if old_nbits == new_nbits.

I don't understand what you mean. My understanding is that we need to treat the legacy algorithm (height < uahfHeight) differently from the EDA algorithm (height < daaHeight). In the first case the difficulty is not allowed to change unless height % params.DifficultyAdjustmentInterval() == 0. In both cases we can check for the factor 4 rule.

yeah I meant short times in the interval. You're correct this can never return false if old_nbits == new_nbits

OK the intent was to avoid that computation entirely for the interval [uahf..daa] but this solution is obviously more accurate and the computation cost might not be that terrible on second thought