fold_stride2.hpp

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#pragma once
 
#include "barretenberg/ecc/fields/vector_field.hpp"
#include "barretenberg/ecc/fields/vectorized_for.hpp"
#include "barretenberg/polynomials/polynomial.hpp"
 
#include <cstddef>
 
namespace bb {
 
template <typename Fr>
void fold_stride2(
    const Polynomial<Fr>& src, Polynomial<Fr>& dst, const size_t begin, const size_t end, const Fr& challenge)
{
    size_t k = begin;
#if defined(__wasm_simd128__)
    if constexpr (simd_supported_v<Fr>) {
        constexpr size_t WIDTH = VECTOR_FIELD_WIDTH;
        using Vec = VectorField<typename Fr::Params>;
        const Vec challenge_vec = Vec::broadcast(challenge);
        for (; k + WIDTH <= end; k += WIDTH) {
            const Vec even = Vec::from_lanes([&](size_t lane) { return src[2 * (k + lane)]; });
            const Vec odd = Vec::from_lanes([&](size_t lane) { return src[2 * (k + lane) + 1]; });
            dst[ContiguousVectorIndex<WIDTH>{ k }] = even + (odd - even) * challenge_vec;
        }
    }
#endif
    for (; k < end; ++k) {
        const Fr even = src[2 * k];
        dst[ScalarIndex{ k }] = even + challenge * (src[2 * k + 1] - even);
    }
}
 
} // namespace bb