structs.hpp

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#pragma once
 
/// @file userver/ydb/io/structs.hpp
/// @brief YDB struct serialization traits and customization
 
#include <ydb-cpp-sdk/client/result/result.h>
#include <ydb-cpp-sdk/client/value/value.h>
 
#include <cstddef>
#include <memory>
#include <optional>
#include <string_view>
#include <tuple>
#include <type_traits>
 
#include <fmt/ranges.h>
#include <boost/pfr/core.hpp>
#include <boost/pfr/core_name.hpp>
 
#include <userver/utils/assert.hpp>
#include <userver/utils/constexpr_indices.hpp>
#include <userver/utils/enumerate.hpp>
#include <userver/utils/forward_like.hpp>
#include <userver/utils/trivial_map.hpp>
 
#include <userver/ydb/exceptions.hpp>
#include <userver/ydb/impl/cast.hpp>
#include <userver/ydb/io/generic_optional.hpp>
#include <userver/ydb/io/traits.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace ydb {
 
namespace impl {
 
struct NotStruct final {};
 
template <typename T>
constexpr decltype(T::kYdbMemberNames) DetectStructMemberNames() noexcept {
    return T::kYdbMemberNames;
}
 
template <typename T, typename... Args>
constexpr NotStruct DetectStructMemberNames(Args&&...) noexcept {
    return NotStruct{};
}
 
// To avoid including heavy <array> header for std::array<T, 0>.
template <typename T>
class EmptyRange final {
public:
    constexpr T* begin() const noexcept { return nullptr; }
    constexpr T* end() const noexcept { return nullptr; }
};
 
}  // namespace impl
 
/// @see ydb::CustomMemberNames
struct CustomMemberName final {
    std::string_view cpp_name;
    std::string_view ydb_name;
};
 
/// @brief Specifies C++ to YDB struct member names mapping. It's enough to only
/// specify the names that are different between C++ and YDB.
/// @see ydb::kStructMemberNames
template <std::size_t N>
struct StructMemberNames final {
    CustomMemberName custom_names[N];
};
 
/// @cond
template <>
struct StructMemberNames<0> final {
    impl::EmptyRange<CustomMemberName> custom_names;
};
/// @endcond
 
// clang-format off
StructMemberNames() -> StructMemberNames<0>;
// clang-format on
 
template <std::size_t N>
StructMemberNames(CustomMemberName (&&)[N]) -> StructMemberNames<N>;
 
/// @brief Customization point for YDB serialization of structs.
///
/// In order to get serialization for a struct, you need to define
/// `kYdbMemberNames` inside it:
///
/// @snippet ydb/small_table.hpp  struct sample
///
/// Field names can be overridden:
///
/// @snippet ydb/tests/struct_io_test.cpp  custom names
///
/// To enable YDB serialization for an external struct, specialize
/// ydb::kStructMemberNames for it:
///
/// @snippet ydb/tests/struct_io_test.cpp  external specialization
///
/// @warning The struct must not reside in an anonymous namespace, otherwise
/// struct member names detection will break.
///
/// For extra fields on C++ side, parsing throws ydb::ParseError.
/// For extra fields on YDB side, parsing throws ydb::ParseError.
template <typename T>
inline constexpr auto kStructMemberNames = impl::DetectStructMemberNames<T>();
 
namespace impl {
 
template <typename T, std::size_t N>
constexpr auto WithDeducedNames(const StructMemberNames<N>& given_names) {
    auto names = boost::pfr::names_as_array<T>();
    for (const CustomMemberName& entry : given_names.custom_names) {
        std::size_t count = 0;
        for (auto& name : names) {
            if (name == entry.cpp_name) {
                name = entry.ydb_name;
                ++count;
                break;
            }
        }
        if (count != 1) {
            throw BaseError(
                "In a StructMemberNames, each cpp_name must match the C++ name of "
                "exactly 1 member of struct T"
            );
        }
    }
    return names;
}
 
template <typename T, std::size_t... Indices>
constexpr auto MakeTupleOfOptionals(std::index_sequence<Indices...>) {
    return std::tuple<std::optional<boost::pfr::tuple_element_t<Indices, T>>...>();
}
 
template <typename T, typename Tuple, std::size_t... Indices>
constexpr auto MakeFromTupleOfOptionals(Tuple&& tuple, std::index_sequence<Indices...>) {
    return T{*utils::ForwardLike<Tuple>(std::get<Indices>(tuple))...};
}
 
template <typename T>
concept IsStruct = !std::is_same_v<decltype(kStructMemberNames<T>), const impl::NotStruct>;
 
}  // namespace impl
 
template <typename T>
requires impl::IsStruct<T>
struct ValueTraits<T> {
    static_assert(std::is_aggregate_v<T>);
    static constexpr auto kFieldNames = impl::WithDeducedNames<T>(kStructMemberNames<T>);
    static constexpr auto kFieldNamesSet = utils::MakeTrivialSet<kFieldNames>();
    static constexpr auto kFieldsCount = kFieldNames.size();
 
    static T Parse(NYdb::TValueParser& parser, const ParseContext& context) {
        auto parsed_fields = impl::MakeTupleOfOptionals<T>(std::make_index_sequence<kFieldsCount>{});
        parser.OpenStruct();
 
        while (parser.TryNextMember()) {
            const auto& field_name = parser.GetMemberName();
            const auto index = kFieldNamesSet.GetIndex(field_name);
            if (!index.has_value()) {
                throw ColumnParseError(
                    context.column_name,
                    fmt::format(
                        "Unexpected field name '{}' for '{}' struct type, "
                        "expected one of: {}",
                        field_name,
                        compiler::GetTypeName<T>(),
                        fmt::join(kFieldNames, ", ")
                    )
                );
            }
            utils::WithConstexprIndex<kFieldsCount>(*index, [&](auto index_c) {
                auto& field = std::get<decltype(index_c)::value>(parsed_fields);
                using FieldType = typename std::decay_t<decltype(field)>::value_type;
                field.emplace(ydb::Parse<FieldType>(parser, context));
            });
        }
 
        parser.CloseStruct();
 
        std::string_view missing_field;
        utils::ForEachIndex<kFieldsCount>([&](auto index_c) {
            if (!std::get<decltype(index_c)::value>(parsed_fields).has_value()) {
                missing_field = kFieldNames[index_c];
            }
        });
        if (!missing_field.empty()) {
            throw ColumnParseError(
                context.column_name,
                fmt::format("Missing field '{}' for '{}' struct type", missing_field, compiler::GetTypeName<T>())
            );
        }
 
        return impl::MakeFromTupleOfOptionals<T>(std::move(parsed_fields), std::make_index_sequence<kFieldsCount>{});
    }
 
    template <typename Builder>
    static void Write(NYdb::TValueBuilderBase<Builder>& builder, const T& value) {
        builder.BeginStruct();
        boost::pfr::for_each_field(value, [&](const auto& field, std::size_t i) {
            builder.AddMember(impl::ToString(kFieldNames[i]));
            ydb::Write(builder, field);
        });
        builder.EndStruct();
    }
 
    static NYdb::TType MakeType() {
        NYdb::TTypeBuilder builder;
        builder.BeginStruct();
        utils::ForEachIndex<kFieldsCount>([&](auto index_c) {
            builder.AddMember(
                impl::ToString(kFieldNames[index_c]),
                ValueTraits<boost::pfr::tuple_element_t<decltype(index_c)::value, T>>::MakeType()
            );
        });
        builder.EndStruct();
        return builder.Build();
    }
};
 
template <typename T>
requires impl::IsStruct<T>
struct ValueTraits<std::optional<T>> : impl::GenericOptionalValueTraits<T> {};
 
namespace impl {
 
template <typename T>
struct StructRowParser final {
    static_assert(std::is_aggregate_v<T>);
    static constexpr auto kFieldNames = impl::WithDeducedNames<T>(kStructMemberNames<T>);
    static constexpr auto kFieldsCount = kFieldNames.size();
 
    static std::unique_ptr<std::size_t[]> MakeCppToYdbFieldMapping(NYdb::TResultSetParser& parser) {
        auto result = std::make_unique<std::size_t[]>(kFieldsCount);
        for (const auto [pos, field_name] : utils::enumerate(kFieldNames)) {
            const auto column_index = parser.ColumnIndex(impl::ToString(field_name));
            if (column_index == -1) {
                throw ParseError(
                    fmt::format("Missing column '{}' for '{}' struct type", field_name, compiler::GetTypeName<T>())
                );
            }
            result[pos] = static_cast<std::size_t>(column_index);
        }
 
        const auto columns_count = parser.ColumnsCount();
        UASSERT(columns_count >= kFieldsCount);
        if (columns_count != kFieldsCount) {
            throw ParseError(fmt::format(
                "Unexpected extra columns while parsing row to '{}' struct type",
                compiler::GetTypeName<T>()
            ));
        }
 
        return result;
    }
 
    static T ParseRow(NYdb::TResultSetParser& parser, const std::unique_ptr<std::size_t[]>& cpp_to_ydb_mapping) {
        return ParseRowImpl(parser, cpp_to_ydb_mapping, std::make_index_sequence<kFieldsCount>{});
    }
 
    template <std::size_t... Indices>
    static T ParseRowImpl(NYdb::TResultSetParser& parser, const std::unique_ptr<std::size_t[]>& cpp_to_ydb_mapping, std::index_sequence<Indices...>) {
        return T{
            Parse<boost::pfr::tuple_element_t<Indices, T>>(
                parser.ColumnParser(cpp_to_ydb_mapping[Indices]),
                ParseContext{.column_name = kFieldNames[Indices]}
            )...,
        };
    }
};
 
}  // namespace impl
 
}  // namespace ydb
 
USERVER_NAMESPACE_END