trivial_map.hpp

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#pragma once
 
/// @file userver/utils/trivial_map.hpp
/// @brief Bidirectional map|sets over string literals or other trivial types.
 
#include <cstddef>
#include <optional>
#include <string>
#include <string_view>
#include <type_traits>
#include <utility>
#include <variant>
 
#include <fmt/format.h>
 
#include <userver/compiler/demangle.hpp>
#include <userver/utils/assert.hpp>
#include <userver/utils/string_literal.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils {
 
namespace impl {
 
constexpr bool HasUppercaseAscii(std::string_view value) noexcept {
    for (auto c : value) {
        if ('A' <= c && c <= 'Z') return true;
    }
 
    return false;
}
 
constexpr bool ICaseEqualLowercase(std::string_view lowercase, std::string_view y) noexcept {
    const auto size = lowercase.size();
    UASSERT(size == y.size());
    constexpr char kLowerToUpperMask = static_cast<char>(~unsigned{32});
    for (std::size_t i = 0; i < size; ++i) {
        const auto lowercase_c = lowercase[i];
        UASSERT(!('A' <= lowercase_c && lowercase_c <= 'Z'));
        if (lowercase_c != y[i]) {
            if (!('a' <= lowercase_c && lowercase_c <= 'z') || (lowercase_c & kLowerToUpperMask) != y[i]) {
                return false;
            }
        }
    }
 
    return true;
}
 
struct Found final {
    constexpr explicit Found(std::size_t value) noexcept { UASSERT(value == 0); }
 
    constexpr explicit operator std::size_t() const noexcept { return 0; }
};
 
template <typename Key, typename Value, typename Enabled = void>
class SearchState final {
public:
    constexpr explicit SearchState(Key key) noexcept : key_or_result_(std::in_place_index<0>, key) {}
 
    constexpr bool IsFound() const noexcept { return key_or_result_.index() != 0; }
 
    constexpr Key GetKey() const noexcept {
        UASSERT(!IsFound());
        return std::get<0>(key_or_result_);
    }
 
    constexpr void SetValue(Value value) noexcept {
        key_or_result_ = std::variant<Key, Value>(std::in_place_index<1>, value);
    }
 
    [[nodiscard]] constexpr std::optional<Value> Extract() noexcept {
        if (key_or_result_.index() == 1) {
            return std::get<1>(key_or_result_);
        } else {
            return std::nullopt;
        }
    }
 
private:
    std::variant<Key, Value> key_or_result_;
};
 
inline constexpr std::size_t kInvalidSize = std::numeric_limits<std::size_t>::max();
 
template <typename Payload>
inline constexpr bool kFitsInStringOrPayload =
    sizeof(Payload) <= sizeof(const char*) &&
    (std::is_integral_v<Payload> || std::is_enum_v<Payload> || std::is_same_v<Payload, Found>);
 
// A compacted std::variant<std::string_view, Payload>
template <typename Payload>
class StringOrPayload final {
public:
    constexpr explicit StringOrPayload(std::string_view string) noexcept
        : data_or_payload_(string.data()), size_(string.size()) {
#if defined(__clang__)
        __builtin_assume(size_ != kInvalidSize);
#elif defined(__GNUC__)
        if (size_ == kInvalidSize) __builtin_unreachable();
#endif
    }
 
    constexpr explicit StringOrPayload(Payload payload) noexcept : data_or_payload_(payload), size_(kInvalidSize) {}
 
    constexpr bool HasPayload() const noexcept { return size_ == kInvalidSize; }
 
    constexpr const char* GetStringPointer() const noexcept {
        UASSERT(!HasPayload());
        UASSERT(data_or_payload_.data);
        return data_or_payload_.data;
    }
 
    constexpr std::size_t GetStringSize() const noexcept {
        UASSERT(!HasPayload());
        UASSERT(data_or_payload_.data);
        return size_;
    }
 
    constexpr Payload GetPayload() const noexcept {
        UASSERT(HasPayload());
        return data_or_payload_.payload;
    }
 
private:
    static_assert(kFitsInStringOrPayload<Payload>);
 
    union DataOrPayload {
        constexpr explicit DataOrPayload(const char* data) noexcept : data(data) {}
 
        constexpr explicit DataOrPayload(Payload payload) noexcept : payload(payload) {}
 
        const char* data{};
        Payload payload;
    };
 
    DataOrPayload data_or_payload_;
    std::size_t size_;
};
 
template <typename Value>
class SearchState<std::string_view, Value, std::enable_if_t<kFitsInStringOrPayload<Value>>> {
public:
    constexpr explicit SearchState(std::string_view key) noexcept : state_(key) {}
 
    constexpr bool IsFound() const noexcept { return state_.HasPayload(); }
 
    constexpr std::string_view GetKey() const noexcept {
        return std::string_view{state_.GetStringPointer(), state_.GetStringSize()};
    }
 
    constexpr void SetValue(Value value) noexcept { state_ = StringOrPayload<Value>{value}; }
 
    [[nodiscard]] constexpr std::optional<Value> Extract() noexcept {
        return IsFound() ? std::optional{state_.GetPayload()} : std::nullopt;
    }
 
private:
    StringOrPayload<Value> state_;
};
 
template <typename Value>
class SearchState<zstring_view, Value, std::enable_if_t<kFitsInStringOrPayload<Value>>> final
    : public SearchState<std::string_view, Value> {};
 
template <typename Value>
class SearchState<StringLiteral, Value, std::enable_if_t<kFitsInStringOrPayload<Value>>> final
    : public SearchState<std::string_view, Value> {};
 
template <typename Key>
class SearchState<Key, std::string_view, std::enable_if_t<kFitsInStringOrPayload<Key>>> final {
public:
    constexpr explicit SearchState(Key key) noexcept : state_(key) {}
 
    constexpr bool IsFound() const noexcept { return !state_.HasPayload(); }
 
    constexpr Key GetKey() const noexcept { return state_.GetPayload(); }
 
    constexpr void SetValue(std::string_view value) noexcept { state_ = StringOrPayload<Key>{value}; }
 
    [[nodiscard]] constexpr std::optional<std::string_view> Extract() noexcept {
        return IsFound() ? std::optional{std::string_view{state_.GetStringPointer(), state_.GetStringSize()}}
                         : std::nullopt;
    }
 
private:
    StringOrPayload<Key> state_;
};
 
template <typename Key>
class SearchState<Key, zstring_view, std::enable_if_t<kFitsInStringOrPayload<Key>>> final {
public:
    constexpr explicit SearchState(Key key) noexcept : state_(key) {}
 
    constexpr bool IsFound() const noexcept { return !state_.HasPayload(); }
 
    constexpr Key GetKey() const noexcept { return state_.GetPayload(); }
 
    constexpr void SetValue(zstring_view value) noexcept { state_ = StringOrPayload<Key>{value}; }
 
    [[nodiscard]] constexpr std::optional<zstring_view> Extract() noexcept {
        return IsFound() ? std::optional{zstring_view::UnsafeMake(state_.GetStringPointer(), state_.GetStringSize())}
                         : std::nullopt;
    }
 
private:
    StringOrPayload<Key> state_;
};
 
template <typename Key>
class SearchState<Key, StringLiteral, std::enable_if_t<kFitsInStringOrPayload<Key>>> final {
public:
    constexpr explicit SearchState(Key key) noexcept : state_(key) {}
 
    constexpr bool IsFound() const noexcept { return !state_.HasPayload(); }
 
    constexpr Key GetKey() const noexcept { return state_.GetPayload(); }
 
    constexpr void SetValue(StringLiteral value) noexcept { state_ = StringOrPayload<Key>{value}; }
 
    [[nodiscard]] constexpr std::optional<StringLiteral> Extract() noexcept {
        return IsFound() ? std::optional{StringLiteral::UnsafeMake(state_.GetStringPointer(), state_.GetStringSize())}
                         : std::nullopt;
    }
 
private:
    StringOrPayload<Key> state_;
};
 
template <typename First, typename Second>
class SwitchByFirst final {
public:
    constexpr explicit SwitchByFirst(First search) noexcept : state_(search) {}
 
    constexpr SwitchByFirst& Case(First first, Second second) noexcept {
        if (!state_.IsFound() && state_.GetKey() == first) {
            state_.SetValue(second);
        }
        return *this;
    }
 
    template <typename T, typename U = void>
    constexpr SwitchByFirst& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr std::optional<Second> Extract() noexcept { return state_.Extract(); }
 
private:
    SearchState<First, Second> state_;
};
 
template <typename First>
class SwitchByFirst<First, void> final {
public:
    constexpr explicit SwitchByFirst(First search) noexcept : state_(search) {}
 
    constexpr SwitchByFirst& Case(First first) noexcept {
        if (!state_.IsFound() && state_.GetKey() == first) {
            state_.SetValue(Found{0});
        }
        return *this;
    }
 
    template <typename T, typename U = void>
    constexpr SwitchByFirst& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr bool Extract() noexcept { return state_.IsFound(); }
 
private:
    SearchState<First, Found> state_;
};
 
template <typename Second>
class SwitchByFirstICase final {
public:
    constexpr explicit SwitchByFirstICase(std::string_view search) noexcept : state_(search) {}
 
    constexpr SwitchByFirstICase& Case(std::string_view first, Second second) noexcept {
        UASSERT_MSG(
            !impl::HasUppercaseAscii(first),
            fmt::format("String literal '{}' in utils::Switch*::Case() should be in lower case", first)
        );
        if (!state_.IsFound() && state_.GetKey().size() == first.size() &&
            impl::ICaseEqualLowercase(first, state_.GetKey())) {
            state_.SetValue(second);
        }
        return *this;
    }
 
    template <typename T, typename U>
    constexpr SwitchByFirstICase& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr std::optional<Second> Extract() noexcept { return state_.Extract(); }
 
private:
    SearchState<std::string_view, Second> state_;
};
 
template <>
class SwitchByFirstICase<void> final {
public:
    constexpr explicit SwitchByFirstICase(std::string_view search) noexcept : state_(search) {}
 
    constexpr SwitchByFirstICase& Case(std::string_view first) noexcept {
        UASSERT_MSG(
            !impl::HasUppercaseAscii(first),
            fmt::format("String literal '{}' in utils::Switch*::Case() should be in lower case", first)
        );
        if (!state_.IsFound() && state_.GetKey().size() == first.size() &&
            impl::ICaseEqualLowercase(first, state_.GetKey())) {
            state_.SetValue(Found{0});
        }
        return *this;
    }
 
    template <typename T, typename U>
    constexpr SwitchByFirstICase& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr bool Extract() const noexcept { return state_.IsFound(); }
 
private:
    SearchState<std::string_view, Found> state_;
};
 
template <typename First>
class SwitchBySecondICase final {
public:
    constexpr explicit SwitchBySecondICase(std::string_view search) noexcept : state_(search) {}
 
    constexpr SwitchBySecondICase& Case(First first, std::string_view second) noexcept {
        UASSERT_MSG(
            !impl::HasUppercaseAscii(second),
            fmt::format("String literal '{}' in utils::Switch*::Case() should be in lower case", second)
        );
        if (!state_.IsFound() && state_.GetKey().size() == second.size() &&
            impl::ICaseEqualLowercase(second, state_.GetKey())) {
            state_.SetValue(first);
        }
        return *this;
    }
 
    template <typename T, typename U>
    constexpr SwitchBySecondICase& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr std::optional<First> Extract() noexcept { return state_.Extract(); }
 
private:
    SearchState<std::string_view, First> state_;
};
 
template <typename First, typename Second>
class SwitchBySecond final {
public:
    constexpr explicit SwitchBySecond(Second search) noexcept : state_(search) {}
 
    constexpr SwitchBySecond& Case(First first, Second second) noexcept {
        if (!state_.IsFound() && state_.GetKey() == second) {
            state_.SetValue(first);
        }
        return *this;
    }
 
    template <typename T, typename U>
    constexpr SwitchBySecond& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr std::optional<First> Extract() noexcept { return state_.Extract(); }
 
private:
    SearchState<Second, First> state_;
};
 
template <typename First, typename Second>
class SwitchTypesDetected final {
public:
    using first_type = First;
    using second_type = Second;
 
    constexpr SwitchTypesDetected& Case(First, Second) noexcept { return *this; }
};
 
template <typename First>
class SwitchTypesDetected<First, void> final {
public:
    using first_type = First;
    using second_type = void;
 
    constexpr SwitchTypesDetected& Case(First) noexcept { return *this; }
};
 
class SwitchTypesDetector final {
public:
    template <class T>
    using DetectType = std::conditional_t<
        std::is_convertible_v<T, StringLiteral>,
        StringLiteral,
        std::conditional_t<  // TODO: force StringLiteral
            std::is_same_v<T, zstring_view>,
            zstring_view,
            std::conditional_t<std::is_convertible_v<T, std::string_view>, std::string_view, T>>>;
 
    constexpr SwitchTypesDetector& operator()() noexcept { return *this; }
 
    template <typename First, typename Second>
    constexpr auto Case(First, Second) noexcept {
        return Type<First, Second>();
    }
 
    template <typename First>
    constexpr auto Case(First) noexcept {
        return Type<First, void>();
    }
 
    template <typename First, typename Second = void>
    constexpr auto Type() noexcept {
        return SwitchTypesDetected<DetectType<First>, DetectType<Second>>{};
    }
};
 
class CaseCounter final {
public:
    template <typename First, typename Second>
    constexpr CaseCounter& Case(First, Second) noexcept {
        ++count_;
        return *this;
    }
 
    template <typename First>
    constexpr CaseCounter& Case(First) noexcept {
        ++count_;
        return *this;
    }
 
    template <typename T, typename U>
    constexpr CaseCounter& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr std::size_t Extract() const noexcept { return count_; }
 
private:
    std::size_t count_{0};
};
 
class CaseDescriber final {
public:
    template <typename First, typename Second>
    CaseDescriber& Case(First first, Second second) noexcept {
        if (!description_.empty()) {
            description_ += ", ";
        }
 
        description_ += fmt::format("('{}', '{}')", first, second);
 
        return *this;
    }
 
    template <typename T, typename U>
    constexpr CaseDescriber& Type() {
        return *this;
    }
 
    [[nodiscard]] std::string Extract() && noexcept { return std::move(description_); }
 
private:
    std::string description_{};
};
 
class CaseFirstDescriber final {
public:
    template <typename First>
    CaseFirstDescriber& Case(First first) noexcept {
        if (!description_.empty()) {
            description_ += ", ";
        }
 
        description_ += fmt::format("'{}'", first);
 
        return *this;
    }
 
    template <typename First, typename Second>
    CaseFirstDescriber& Case(First first, Second /*second*/) noexcept {
        return Case(first);
    }
 
    template <typename T, typename U = void>
    constexpr CaseFirstDescriber& Type() {
        return *this;
    }
 
    [[nodiscard]] std::string Extract() && noexcept { return std::move(description_); }
 
private:
    std::string description_{};
};
 
class CaseSecondDescriber final {
public:
    template <typename First, typename Second>
    CaseSecondDescriber& Case(First /*first*/, Second second) noexcept {
        if (!description_.empty()) {
            description_ += ", ";
        }
 
        description_ += fmt::format("'{}'", second);
 
        return *this;
    }
 
    template <typename T, typename U>
    constexpr CaseSecondDescriber& Type() {
        return *this;
    }
 
    [[nodiscard]] std::string Extract() && noexcept { return std::move(description_); }
 
private:
    std::string description_{};
};
 
template <typename First, typename Second>
class CaseGetValuesByIndex final {
public:
    explicit constexpr CaseGetValuesByIndex(std::size_t search_index) : index_(search_index + 1) {}
 
    constexpr CaseGetValuesByIndex& Case(First first, Second second) noexcept {
        if (index_ == 0) {
            return *this;
        }
        if (index_ == 1) {
            lazy_ = Lazy{Storage{first, second}};
        }
        --index_;
 
        return *this;
    }
 
    template <typename T, typename U>
    constexpr CaseGetValuesByIndex& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr First GetFirst() noexcept { return std::move(lazy_.storage.first); }
 
    [[nodiscard]] constexpr Second GetSecond() noexcept { return std::move(lazy_.storage.second); }
 
private:
    std::size_t index_;
 
    // Work with non default constructible types
    struct Storage {
        First first;
        Second second;
    };
 
    union Lazy {
        constexpr Lazy() noexcept : empty{} {}
        constexpr Lazy(Storage s) noexcept : storage{s} {}
 
        char empty;
        Storage storage;
    };
    Lazy lazy_;
};
 
template <typename First>
class CaseFirstIndexer final {
public:
    constexpr explicit CaseFirstIndexer(First search_value) noexcept : state_(search_value) {}
 
    constexpr CaseFirstIndexer& Case(First first) noexcept {
        if (!state_.IsFound() && state_.GetKey() == first) {
            state_.SetValue(index_);
        }
        ++index_;
        return *this;
    }
 
    template <typename T, typename U = void>
    constexpr CaseFirstIndexer& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr std::optional<std::size_t> Extract() && noexcept { return state_.Extract(); }
 
private:
    SearchState<First, std::size_t> state_;
    std::size_t index_ = 0;
};
 
template <typename First>
class CaseFirstIndexerICase final {
public:
    constexpr explicit CaseFirstIndexerICase(First search_value) noexcept : state_(search_value) {}
 
    constexpr CaseFirstIndexerICase& Case(First first) noexcept {
        if (!state_.IsFound() && state_.GetKey().size() == first.size() &&
            impl::ICaseEqualLowercase(first, state_.GetKey())) {
            state_.SetValue(index_);
        }
        ++index_;
        return *this;
    }
 
    template <typename T, typename U = void>
    constexpr CaseFirstIndexerICase& Type() {
        return *this;
    }
 
    [[nodiscard]] constexpr std::optional<std::size_t> Extract() && noexcept { return state_.Extract(); }
 
private:
    SearchState<First, std::size_t> state_;
    std::size_t index_ = 0;
};
 
}  // namespace impl
 
/// Decays utils::StringLiteral and utils::zstring_view to a more generic std::string_view type.
template <class T>
using DecayToStringView =
    std::conditional_t<std::is_same_v<T, StringLiteral> || std::is_same_v<T, zstring_view>, std::string_view, T>;
 
/// @ingroup userver_universal userver_containers
///
/// @brief Bidirectional unordered map for trivial types, including string
/// literals; could be efficiently used as a unordered non-bidirectional map.
///
/// @snippet universal/src/utils/trivial_map_test.cpp  sample string bimap
///
/// utils::TrivialBiMap and utils::TrivialSet are known to outperform
/// std::unordered_map if:
/// * there's 32 or less elements in map/set
/// * or keys are string literals and all of them differ in length.
///
/// Implementation of string search is \b very efficient due to
/// modern compilers optimize it to a switch by input string
/// length and an integral comparison (rather than a std::memcmp call). In other
/// words, it usually takes O(1) to find the match in the map.
///
/// The same story with integral or enum mappings - compiler optimizes them
/// into a switch and it usually takes O(1) to find the match.
///
/// @snippet universal/src/utils/trivial_map_test.cpp  sample bidir bimap
///
/// Empty map:
///
/// @snippet universal/src/utils/trivial_map_test.cpp  sample empty bimap
///
/// For a single value Case statements see @ref utils::TrivialSet.
template <typename BuilderFunc>
class TrivialBiMap final {
    using TypesPair = std::invoke_result_t<const BuilderFunc&, impl::SwitchTypesDetector>;
 
public:
    using First = typename TypesPair::first_type;
    using Second = typename TypesPair::second_type;
 
    struct ValueType {
        First first;
        Second second;
    };
    struct ValueType {…};
 
    /// Returns Second if T is convertible to First, otherwise returns Second
    /// type.
    template <class T>
    using MappedTypeFor = std::conditional_t<std::is_convertible_v<T, DecayToStringView<First>>, Second, First>;
 
    constexpr TrivialBiMap(BuilderFunc&& func) noexcept : func_(std::move(func)) {
        static_assert(std::is_empty_v<BuilderFunc>, "Mapping function should not capture variables");
        static_assert(std::is_trivially_copyable_v<First>, "First type in Case must be trivially copyable");
        static_assert(
            !std::is_void_v<Second>,
            "If second type in Case is missing, use utils::TrivialSet instead of utils::TrivialBiMap"
        );
        static_assert(std::is_trivially_copyable_v<Second>, "Second type in Case must be trivially copyable");
    }
 
    constexpr std::optional<Second> TryFindByFirst(DecayToStringView<First> value) const noexcept {
        return func_([value]() { return impl::SwitchByFirst<DecayToStringView<First>, Second>{value}; }).Extract();
    }
 
    constexpr std::optional<First> TryFindBySecond(DecayToStringView<Second> value) const noexcept {
        return func_([value]() { return impl::SwitchBySecond<First, DecayToStringView<Second>>{value}; }).Extract();
    }
 
    template <class T>
    constexpr std::optional<MappedTypeFor<T>> TryFind(T value) const noexcept {
        if constexpr (std::is_convertible_v<T, DecayToStringView<First>>) {
            static_assert(
                !std::is_convertible_v<T, DecayToStringView<Second>>,
                "Ambiguous conversion, use TryFindByFirst/TryFindBySecond instead"
            );
            return TryFindByFirst(value);
        } else {
            return TryFindBySecond(value);
        }
    }
 
    /// @brief Case insensitive search for value.
    ///
    /// For efficiency reasons, first parameter in Case() should be lower case
    /// string literal.
    constexpr std::optional<Second> TryFindICaseByFirst(std::string_view value) const noexcept {
        return func_([value]() { return impl::SwitchByFirstICase<Second>{value}; }).Extract();
    }
    constexpr std::optional<Second> TryFindICaseByFirst(std::string_view value) const noexcept {…}
 
    /// @brief Case insensitive search for value.
    ///
    /// For efficiency reasons, second parameter in Case() should be lower case
    /// string literal.
    constexpr std::optional<First> TryFindICaseBySecond(std::string_view value) const noexcept {
        return func_([value]() { return impl::SwitchBySecondICase<First>{value}; }).Extract();
    }
    constexpr std::optional<First> TryFindICaseBySecond(std::string_view value) const noexcept {…}
 
    /// @brief Case insensitive search for value that calls either
    /// TryFindICaseBySecond or TryFindICaseByFirst.
    constexpr std::optional<MappedTypeFor<std::string_view>> TryFindICase(std::string_view value) const noexcept {
        if constexpr (std::is_convertible_v<std::string_view, DecayToStringView<First>>) {
            static_assert(
                !std::is_convertible_v<std::string_view, DecayToStringView<Second>>,
                "Ambiguous conversion, use TryFindICaseByFirst/TryFindICaseBySecond"
            );
            return TryFindICaseByFirst(value);
        } else {
            return TryFindICaseBySecond(value);
        }
    }
    constexpr std::optional<MappedTypeFor<std::string_view>> TryFindICase(std::string_view value) const noexcept {…}
 
    /// Returns count of Case's in mapping
    constexpr std::size_t size() const noexcept {
        return func_([]() { return impl::CaseCounter{}; }).Extract();
    }
    constexpr std::size_t size() const noexcept {…}
 
    /// Returns a string of comma separated quoted values of Case parameters.
    ///
    /// \b Example: "('a', '1'), ('b', '2'), ('c', '3')"
    ///
    /// Parameters of Case should be formattable.
    std::string Describe() const {
        return func_([]() { return impl::CaseDescriber{}; }).Extract();
    }
    std::string Describe() const {…}
 
    /// Returns a string of comma separated quoted values of first Case
    /// parameters.
    ///
    /// \b Example: "'a', 'b', 'c'"
    ///
    /// First parameters of Case should be formattable.
    std::string DescribeFirst() const {
        return func_([]() { return impl::CaseFirstDescriber{}; }).Extract();
    }
    std::string DescribeFirst() const {…}
 
    /// Returns a string of comma separated quoted values of second Case
    /// parameters.
    ///
    /// \b Example: "'1', '2', '3'"
    ///
    /// Second parameters of Case should be formattable.
    std::string DescribeSecond() const {
        return func_([]() { return impl::CaseSecondDescriber{}; }).Extract();
    }
    std::string DescribeSecond() const {…}
 
    /// Returns a string of comma separated quoted values of Case
    /// parameters that matches by type.
    ///
    /// \b Example: "'1', '2', '3'"
    ///
    /// Corresponding Case must be formattable
    template <typename T>
    std::string DescribeByType() const {
        if constexpr (std::is_convertible_v<T, DecayToStringView<First>>) {
            return DescribeFirst();
        } else {
            return DescribeSecond();
        }
    }
    std::string DescribeByType() const {…}
 
    constexpr ValueType GetValuesByIndex(std::size_t index) const {
        UASSERT_MSG(index < size(), "Index is out of bounds");
        auto result = func_([index]() { return impl::CaseGetValuesByIndex<First, Second>{index}; });
        return ValueType{result.GetFirst(), result.GetSecond()};
    }
 
    class Iterator {
    public:
        using iterator_category = std::input_iterator_tag;
        using difference_type = std::ptrdiff_t;
 
        explicit constexpr Iterator(const TrivialBiMap& map, std::size_t position) : map_{map}, position_{position} {}
 
        constexpr bool operator==(Iterator other) const { return position_ == other.position_; }
 
        constexpr bool operator!=(Iterator other) const { return position_ != other.position_; }
 
        constexpr Iterator operator++() {
            ++position_;
            return *this;
        }
 
        constexpr Iterator operator++(int) {
            Iterator copy{*this};
            ++position_;
            return copy;
        }
 
        constexpr ValueType operator*() const { return map_.GetValuesByIndex(position_); }
 
    private:
        const TrivialBiMap& map_;
        std::size_t position_;
    };
    class Iterator {…};
 
    constexpr Iterator begin() const { return Iterator(*this, 0); }
    constexpr Iterator end() const { return Iterator(*this, size()); }
    constexpr Iterator cbegin() const { return begin(); }
    constexpr Iterator cend() const { return end(); }
 
private:
    const BuilderFunc func_;
};
class TrivialBiMap final {…};
 
template <typename BuilderFunc>
TrivialBiMap(BuilderFunc) -> TrivialBiMap<BuilderFunc>;
 
/// @ingroup userver_universal userver_containers
///
/// @brief Unordered set for trivial types, including string literals.
///
/// For a two-value Case statements or efficiency notes
/// see @ref utils::TrivialBiMap.
template <typename BuilderFunc>
class TrivialSet final {
    using TypesPair = std::invoke_result_t<const BuilderFunc&, impl::SwitchTypesDetector>;
 
public:
    using First = typename TypesPair::first_type;
    using Second = typename TypesPair::second_type;
 
    constexpr TrivialSet(BuilderFunc&& func) noexcept : func_(std::move(func)) {
        static_assert(std::is_empty_v<BuilderFunc>, "Mapping function should not capture variables");
        static_assert(std::is_trivially_copyable_v<First>, "First type in Case must be trivially copyable");
        static_assert(std::is_void_v<Second>, "Second type in Case should be skipped in utils::TrivialSet");
    }
 
    constexpr bool Contains(DecayToStringView<First> value) const noexcept {
        return func_([value]() { return impl::SwitchByFirst<DecayToStringView<First>, Second>{value}; }).Extract();
    }
 
    constexpr bool ContainsICase(std::string_view value) const noexcept {
        static_assert(
            std::is_convertible_v<DecayToStringView<First>, std::string_view>,
            "ContainsICase works only with std::string_view"
        );
 
        return func_([value]() { return impl::SwitchByFirstICase<void>{value}; }).Extract();
    }
 
    constexpr std::size_t size() const noexcept {
        return func_([]() { return impl::CaseCounter{}; }).Extract();
    }
 
    /// Returns a string of comma separated quoted values of Case parameters.
    ///
    /// \b Example: "'a', 'b', 'c'"
    ///
    /// Parameters of Case should be formattable.
    std::string Describe() const {
        return func_([]() { return impl::CaseFirstDescriber{}; }).Extract();
    }
    std::string Describe() const {…}
 
    /// Returns index of the value in Case parameters or std::nullopt if no such
    /// value.
    constexpr std::optional<std::size_t> GetIndex(DecayToStringView<First> value) const {
        return func_([value]() { return impl::CaseFirstIndexer{value}; }).Extract();
    }
    constexpr std::optional<std::size_t> GetIndex(DecayToStringView<First> value) const {…}
 
    /// Returns index of the case insensitive value in Case parameters or
    /// std::nullopt if no such value.
    constexpr std::optional<std::size_t> GetIndexICase(std::string_view value) const {
        return func_([value]() { return impl::CaseFirstIndexerICase{value}; }).Extract();
    }
    constexpr std::optional<std::size_t> GetIndexICase(std::string_view value) const {…}
 
private:
    const BuilderFunc func_;
};
class TrivialSet final {…};
 
template <typename BuilderFunc>
TrivialSet(BuilderFunc) -> TrivialSet<BuilderFunc>;
 
/// @brief Parses and returns whatever is specified by `map` from a
/// `formats::json::Value` or another format's `Value`.
/// @throws ExceptionType or `Value::Exception` by default, if `value` is not a
/// string, or if `value` is not contained in `map`.
/// @see @ref scripts/docs/en/userver/formats.md
template <typename ExceptionType = void, typename Value, typename BuilderFunc>
auto ParseFromValueString(const Value& value, TrivialBiMap<BuilderFunc> map) {
    if constexpr (!std::is_void_v<ExceptionType>) {
        if (!value.IsString()) {
            throw ExceptionType(fmt::format("Invalid value at '{}': expected a string", value.GetPath()));
        }
    }
 
    const auto string = value.template As<std::string>();
    const auto parsed = map.TryFind(string);
    if (parsed) return *parsed;
 
    using Exception = std::conditional_t<std::is_void_v<ExceptionType>, typename Value::Exception, ExceptionType>;
 
    throw Exception(fmt::format(
        "Invalid value of {} at '{}': '{}' is not one of {}",
        compiler::GetTypeName<std::decay_t<decltype(*parsed)>>(),
        value.GetPath(),
        string,
        map.template DescribeByType<std::string_view>()
    ));
}
auto ParseFromValueString(const Value& value, TrivialBiMap<BuilderFunc> map) {…}
 
namespace impl {
 
// @brief Converts `value` to `std::string_view` using `map`. If `value` is not
// contained in `map`, then crashes the service in Debug builds, or throws
// utils::InvariantError in Release builds.
template <typename Enum, typename BuilderFunc>
std::string_view EnumToStringView(Enum value, TrivialBiMap<BuilderFunc> map) {
    static_assert(std::is_enum_v<Enum>);
    if (const auto string = map.TryFind(value)) return *string;
 
    UINVARIANT(
        false,
        fmt::format(
            "Invalid value of enum {}: {}",
            compiler::GetTypeName<Enum>(),
            static_cast<std::underlying_type_t<Enum>>(value)
        )
    );
}
 
template <typename Selector, class Keys, typename Values, std::size_t... Indices>
constexpr auto
TrivialBiMapMultiCase(Selector selector, const Keys& keys, const Values& values, std::index_sequence<0, Indices...>) {
    auto selector2 = selector.Case(std::data(keys)[0], std::data(values)[0]);
    ((selector2 = selector2.Case(std::data(keys)[Indices], std::data(values)[Indices])), ...);
    return selector2;
}
 
template <const auto& Keys, const auto& Values>
struct TrivialBiMapMultiCaseDispatch {
    template <class Selector>
    constexpr auto operator()(Selector selector) const {
        constexpr auto kKeysSize = std::size(Keys);
        return impl::TrivialBiMapMultiCase(selector(), Keys, Values, std::make_index_sequence<kKeysSize>{});
    }
};
 
template <typename Selector, class Values, std::size_t... Indices>
constexpr auto TrivialSetMultiCase(Selector selector, const Values& values, std::index_sequence<0, Indices...>) {
    auto selector2 = selector.Case(std::data(values)[0]);
    ((selector2 = selector2.Case(std::data(values)[Indices])), ...);
    return selector2;
}
 
template <const auto& Values>
struct TrivialSetMultiCaseDispatch {
    template <class Selector>
    constexpr auto operator()(Selector selector) const {
        constexpr auto kValuesSize = std::size(Values);
        return impl::TrivialSetMultiCase(selector(), Values, std::make_index_sequence<kValuesSize>{});
    }
};
 
}  // namespace impl
 
/// @brief Zips two global `constexpr` arrays into an utils::TrivialBiMap.
template <const auto& Keys, const auto& Values>
constexpr auto MakeTrivialBiMap() {
    static_assert(std::size(Keys) == std::size(Values));
    static_assert(std::size(Keys) >= 1);
    return TrivialBiMap(impl::TrivialBiMapMultiCaseDispatch<Keys, Values>{});
}
constexpr auto MakeTrivialBiMap() {…}
 
template <const auto& Values>
constexpr auto MakeTrivialSet() {
    return TrivialSet(impl::TrivialSetMultiCaseDispatch<Values>{});
}
 
}  // namespace utils
 
USERVER_NAMESPACE_END