Function rs_eq_ind_from_factors 

pub fn rs_eq_ind_from_factors<A, P>(
    alloc: &A,
    eq_lo: &FieldBuffer<B128>,
    eq_hi: &FieldBuffer<B128>,
    row_batch_query: &FieldBuffer<B128>,
) -> FieldVec<P, A>
where
    A: Allocator,
    P: PackedField<Scalar = B128>,

Builds the ring-switching equality indicator directly from the tensor’s two factors.

Overview

The indicator is the suffix equality tensor, folded bitwise by the row-batching query. Expanding that tensor and then folding it would read and rewrite a whole 2^n buffer. This route produces each entry in one pass instead:

    out[hi << n_lo | lo] = fold(eq_lo[lo] * eq_hi[hi])

The tensor expansion already costs one multiply per entry. So the fused product changes no operation count. It removes one full read pass and one full write pass over the 2^n buffer.

Arguments

• alloc - the allocator the returned indicator is drawn from
• eq_lo - the low tensor factor
• eq_hi - the high tensor factor
• row_batch_query - the vector every entry is folded bitwise by

Preconditions

• row_batch_query.len() must equal 128, the extension degree of B128 over B1
• A block must cover a whole number of packed elements, so the chunking below aligns. The exception is an output that fits inside one packed element, which is a single block.