Function compute_changed_bits 

pub fn compute_changed_bits(
    prev: &DecodedValue,
    next: &DecodedValue,
    desc: &StructureDesc,
) -> Option<Vec<bool>>

Walks desc in pre-order (matching the PVA wire convention that bit 0 represents the whole root structure and subsequent bits correspond to fields in pre-order) and returns a per-bit flag vector marking leaves whose value differs between prev and next.

Returns None if no leaves changed. Structure-type fields always have their bit cleared — changes propagate to the descendants so a filtered monitor client sees only the true differences.