named_union.hpp

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#pragma once
 
#include <concepts>
#include <optional>
#include <sstream>
#include <stdexcept>
#include <string>
#include <string_view>
#include <type_traits>
#include <variant>
 
#include "barretenberg/common/msgpack_to_json.hpp"
#include "barretenberg/common/throw_or_abort.hpp"
#include "barretenberg/serialize/msgpack.hpp"
#include "msgpack/object_fwd.hpp"
 
namespace bb {
 
template <typename T>
concept HasMsgpackSchemaName = requires {
    { T::MSGPACK_SCHEMA_NAME } -> std::convertible_to<std::string_view>;
};
 
template <HasMsgpackSchemaName... Types> class NamedUnion {
  public:
    using VariantType = std::variant<Types...>;
 
  private:
    VariantType value_;
 
    // Helper to get index from type name
    template <size_t I = 0> static std::optional<size_t> get_index_from_name(std::string_view name)
    {
        if constexpr (I < sizeof...(Types)) {
            using CurrentType = std::variant_alternative_t<I, VariantType>;
            if (name == CurrentType::MSGPACK_SCHEMA_NAME) {
                return I;
            }
            return get_index_from_name<I + 1>(name);
        }
        return std::nullopt;
    }
 
    // Helper to construct variant by index
    template <size_t I = 0> static VariantType construct_by_index(size_t index, auto& o)
    {
        if constexpr (I < sizeof...(Types)) {
            if (I == index) {
                using CurrentType = std::variant_alternative_t<I, VariantType>;
                CurrentType obj;
                o.convert(obj);
                return obj;
            }
            return construct_by_index<I + 1>(index, o);
        }
        throw_or_abort("Invalid variant index");
    }
 
  public:
    NamedUnion() = default;
 
    template <typename T>
        requires(std::is_constructible_v<VariantType, T>)
    // NOLINTNEXTLINE(bugprone-forwarding-reference-overload)
    NamedUnion(T&& t)
        : value_(std::forward<T>(t))
    {}
 
    // Conversion operator to get the underlying variant
    operator VariantType&() { return value_; }
    operator const VariantType&() const { return value_; }
 
    // Access the underlying variant
    VariantType& get() { return value_; }
    const VariantType& get() const { return value_; }
 
    // Visit the variant
    template <typename Visitor> decltype(auto) visit(Visitor&& vis) &&
    {
        return std::visit(std::forward<Visitor>(vis), std::move(value_));
    }
 
    template <typename Visitor> decltype(auto) visit(Visitor&& vis) const&
    {
        return std::visit(std::forward<Visitor>(vis), value_);
    }
 
    // Get the current type name
    std::string_view get_type_name() const
    {
        return std::visit(
            [](const auto& obj) -> std::string_view { return std::decay_t<decltype(obj)>::MSGPACK_SCHEMA_NAME; },
            value_);
    }
 
    // Msgpack serialization
    void msgpack_pack(auto& packer) const
    {
        packer.pack_array(2);
        // First pack the type name
        std::string_view type_name = get_type_name();
        packer.pack(type_name);
 
        // Then pack the actual object
        std::visit([&packer](const auto& obj) { packer.pack(obj); }, value_);
    }
 
    // Msgpack deserialization
    void msgpack_unpack(msgpack::object const& o)
    {
        constexpr size_t MAX_OUTPUT_CHARS = 100;
        // access object assuming it is an array of size 2
        if (o.type != msgpack::type::ARRAY || o.via.array.size != 2) {
            throw_or_abort("Expected an array of size 2 for NamedUnion deserialization, got " +
                           bb::msgpack_to_json(o, MAX_OUTPUT_CHARS));
        }
        const auto& arr = o.via.array;
        if (arr.ptr[0].type != msgpack::type::STR) {
            throw_or_abort("Expected first element to be a string (type name) in NamedUnion deserialization");
        }
        std::string_view type_name = std::string_view(arr.ptr[0].via.str.ptr, arr.ptr[0].via.str.size);
        auto index_opt = get_index_from_name(type_name);
        if (!index_opt.has_value()) {
            throw_or_abort("Unknown type name in NamedUnion deserialization: " + std::string(type_name));
        }
        size_t index = index_opt.value();
        // Now construct the variant using the index
        value_ = construct_by_index(index, arr.ptr[1]);
    }
 
    // Msgpack schema
    void msgpack_schema(auto& packer) const
    {
        packer.pack_array(2);
        packer.pack("named_union");
        packer.pack_array(sizeof...(Types));
        (
            [&packer]() {
                packer.pack_array(2);
                packer.pack(Types::MSGPACK_SCHEMA_NAME);
                // Abitrary mutable object.
                packer.pack_schema(*std::make_unique<Types>());
            }(),
            ...); /* pack schemas of all template Args */
    }
};
 
// Deduction guide
template <typename... Types> NamedUnion(std::variant<Types...>) -> NamedUnion<Types...>;
 
} // namespace bb