field_conversion.cpp

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// === AUDIT STATUS ===
// internal:    { status: not started, auditors: [], date: YYYY-MM-DD }
// external_1:  { status: not started, auditors: [], date: YYYY-MM-DD }
// external_2:  { status: not started, auditors: [], date: YYYY-MM-DD }
// =====================
 
#include "barretenberg/stdlib/primitives/field/field_conversion.hpp"
#include "barretenberg/common/assert.hpp"
 
namespace bb::stdlib::field_conversion {
 
template <typename Builder> fq<Builder> convert_to_grumpkin_fr(Builder& builder, const fr<Builder>& f)
{
    constexpr uint64_t NUM_BITS_IN_TWO_LIMBS = 2 * NUM_LIMB_BITS;                // 136
    constexpr uint64_t UPPER_TWO_LIMB_BITS = TOTAL_BITS - NUM_BITS_IN_TWO_LIMBS; // 118
    constexpr uint256_t shift = (uint256_t(1) << NUM_BITS_IN_TWO_LIMBS);
    // split f into low_bits_in and high_bits_in
    constexpr uint256_t LIMB_MASK = shift - 1; // mask for upper 128 bits
    const uint256_t value = f.get_value();
    const uint256_t low_val = static_cast<uint256_t>(value & LIMB_MASK);
    const uint256_t hi_val = static_cast<uint256_t>(value >> NUM_BITS_IN_TWO_LIMBS);
 
    fr<Builder> low{ witness_t<Builder>(&builder, low_val) };
    fr<Builder> hi{ witness_t<Builder>(&builder, hi_val) };
    // range constrain low to 136 bits and hi to 118 bits
    builder.create_range_constraint(low.witness_index, NUM_BITS_IN_TWO_LIMBS, "create_range_constraint");
    builder.create_range_constraint(hi.witness_index, UPPER_TWO_LIMB_BITS, "create_range_constraint");
 
    BB_ASSERT_EQ(static_cast<uint256_t>(low_val) + (static_cast<uint256_t>(hi_val) << NUM_BITS_IN_TWO_LIMBS), value);
    // checks this decomposition low + hi * 2^64 = value with an assert_equal
    auto sum = low + hi * shift;
    builder.assert_equal(f.witness_index, sum.witness_index, "assert_equal");
 
    std::vector<fr<Builder>> fr_vec{ low, hi };
    return convert_from_bn254_frs<Builder, fq<Builder>>(builder, fr_vec);
}
 
template fq<UltraCircuitBuilder> convert_to_grumpkin_fr<UltraCircuitBuilder>(UltraCircuitBuilder& builder,
                                                                             const fr<UltraCircuitBuilder>& f);
template fq<MegaCircuitBuilder> convert_to_grumpkin_fr<MegaCircuitBuilder>(MegaCircuitBuilder& builder,
                                                                           const fr<MegaCircuitBuilder>& f);
 
} // namespace bb::stdlib::field_conversion