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+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#pragma once
+
+#include <cstddef>
+#include <exception>
+#include <type_traits>
+#include <utility>
+
+#include "arrow/util/macros.h"
+#include "arrow/util/type_traits.h"
+
+namespace arrow {
+namespace util {
+
+/// \brief a std::variant-like discriminated union
+///
+/// Simplifications from std::variant:
+///
+/// - Strictly defaultable. The first type of T... should be nothrow default constructible
+///   and it will be used for default Variants.
+///
+/// - Never valueless_by_exception. std::variant supports a state outside those specified
+///   by T... to which it can return in the event that a constructor throws. If a Variant
+///   would become valueless_by_exception it will instead return to its default state.
+///
+/// - Strictly nothrow move constructible and assignable
+///
+/// - Less sophisticated type deduction. std::variant<bool, std::string>("hello") will
+///   intelligently construct std::string while Variant<bool, std::string>("hello") will
+///   construct bool.
+///
+/// - Either both copy constructible and assignable or neither (std::variant independently
+///   enables copy construction and copy assignment). Variant is copy constructible if
+///   each of T... is copy constructible and assignable.
+///
+/// - Slimmer interface; several members of std::variant are omitted.
+///
+/// - Throws no exceptions; if a bad_variant_access would be thrown Variant will instead
+///   segfault (nullptr dereference).
+///
+/// - Mutable visit takes a pointer instead of mutable reference or rvalue reference,
+///   which is more conformant with our code style.
+template <typename... T>
+class Variant;
+
+namespace detail {
+
+template <typename T, typename = void>
+struct is_equality_comparable : std::false_type {};
+
+template <typename T>
+struct is_equality_comparable<
+    T, typename std::enable_if<std::is_convertible<
+           decltype(std::declval<T>() == std::declval<T>()), bool>::value>::type>
+    : std::true_type {};
+
+template <bool C, typename T, typename E>
+using conditional_t = typename std::conditional<C, T, E>::type;
+
+template <typename T>
+struct type_constant {
+  using type = T;
+};
+
+template <typename...>
+struct first;
+
+template <typename H, typename... T>
+struct first<H, T...> {
+  using type = H;
+};
+
+template <typename T>
+using decay_t = typename std::decay<T>::type;
+
+template <bool...>
+struct all : std::true_type {};
+
+template <bool H, bool... T>
+struct all<H, T...> : conditional_t<H, all<T...>, std::false_type> {};
+
+struct delete_copy_constructor {
+  template <typename>
+  struct type {
+    type() = default;
+    type(const type& other) = delete;
+    type& operator=(const type& other) = delete;
+  };
+};
+
+struct explicit_copy_constructor {
+  template <typename Copyable>
+  struct type {
+    type() = default;
+    type(const type& other) { static_cast<const Copyable&>(other).copy_to(this); }
+    type& operator=(const type& other) {
+      static_cast<Copyable*>(this)->destroy();
+      static_cast<const Copyable&>(other).copy_to(this);
+      return *this;
+    }
+  };
+};
+
+template <typename... T>
+struct VariantStorage {
+  VariantStorage() = default;
+  VariantStorage(const VariantStorage&) {}
+  VariantStorage& operator=(const VariantStorage&) { return *this; }
+  VariantStorage(VariantStorage&&) noexcept {}
+  VariantStorage& operator=(VariantStorage&&) noexcept { return *this; }
+  ~VariantStorage() {
+    static_assert(offsetof(VariantStorage, data_) == 0,
+                  "(void*)&VariantStorage::data_ == (void*)this");
+  }
+
+  typename arrow::internal::aligned_union<0, T...>::type data_;
+  uint8_t index_ = 0;
+};
+
+template <typename V, typename...>
+struct VariantImpl;
+
+template <typename... T>
+struct VariantImpl<Variant<T...>> : VariantStorage<T...> {
+  static void index_of() noexcept {}
+  void destroy() noexcept {}
+  void move_to(...) noexcept {}
+  void copy_to(...) const {}
+
+  template <typename R, typename Visitor>
+  [[noreturn]] R visit_const(Visitor&& visitor) const {
+    std::terminate();
+  }
+  template <typename R, typename Visitor>
+  [[noreturn]] R visit_mutable(Visitor&& visitor) {
+    std::terminate();
+  }
+};
+
+template <typename... M, typename H, typename... T>
+struct VariantImpl<Variant<M...>, H, T...> : VariantImpl<Variant<M...>, T...> {
+  using VariantType = Variant<M...>;
+  using Impl = VariantImpl<VariantType, T...>;
+
+  static constexpr uint8_t kIndex = sizeof...(M) - sizeof...(T) - 1;
+
+  VariantImpl() = default;
+
+  using VariantImpl<VariantType, T...>::VariantImpl;
+  using Impl::operator=;
+  using Impl::index_of;
+
+  explicit VariantImpl(H value) {
+    new (this) H(std::move(value));
+    this->index_ = kIndex;
+  }
+
+  VariantImpl& operator=(H value) {
+    static_cast<VariantType*>(this)->destroy();
+    new (this) H(std::move(value));
+    this->index_ = kIndex;
+    return *this;
+  }
+
+  H& cast_this() { return *reinterpret_cast<H*>(this); }
+  const H& cast_this() const { return *reinterpret_cast<const H*>(this); }
+
+  void move_to(VariantType* target) noexcept {
+    if (this->index_ == kIndex) {
+      new (target) H(std::move(cast_this()));
+      target->index_ = kIndex;
+    } else {
+      Impl::move_to(target);
+    }
+  }
+
+  // Templated to avoid instantiation in case H is not copy constructible
+  template <typename Void>
+  void copy_to(Void* generic_target) const {
+    const auto target = static_cast<VariantType*>(generic_target);
+    try {
+      if (this->index_ == kIndex) {
+        new (target) H(cast_this());
+        target->index_ = kIndex;
+      } else {
+        Impl::copy_to(target);
+      }
+    } catch (...) {
+      target->construct_default();
+      throw;
+    }
+  }
+
+  void destroy() noexcept {
+    if (this->index_ == kIndex) {
+      if (!std::is_trivially_destructible<H>::value) {
+        cast_this().~H();
+      }
+    } else {
+      Impl::destroy();
+    }
+  }
+
+  static constexpr std::integral_constant<uint8_t, kIndex> index_of(
+      const type_constant<H>&) {
+    return {};
+  }
+
+  template <typename R, typename Visitor>
+  R visit_const(Visitor&& visitor) const {
+    if (this->index_ == kIndex) {
+      return std::forward<Visitor>(visitor)(cast_this());
+    }
+    return Impl::template visit_const<R>(std::forward<Visitor>(visitor));
+  }
+
+  template <typename R, typename Visitor>
+  R visit_mutable(Visitor&& visitor) {
+    if (this->index_ == kIndex) {
+      return std::forward<Visitor>(visitor)(&cast_this());
+    }
+    return Impl::template visit_mutable<R>(std::forward<Visitor>(visitor));
+  }
+};
+
+}  // namespace detail
+
+template <typename... T>
+class Variant : detail::VariantImpl<Variant<T...>, T...>,
+                detail::conditional_t<
+                    detail::all<(std::is_copy_constructible<T>::value &&
+                                 std::is_copy_assignable<T>::value)...>::value,
+                    detail::explicit_copy_constructor,
+                    detail::delete_copy_constructor>::template type<Variant<T...>> {
+  template <typename U>
+  static constexpr uint8_t index_of() {
+    return Impl::index_of(detail::type_constant<U>{});
+  }
+
+  using Impl = detail::VariantImpl<Variant<T...>, T...>;
+
+ public:
+  using default_type = typename util::detail::first<T...>::type;
+
+  Variant() noexcept { construct_default(); }
+
+  Variant(const Variant& other) = default;
+  Variant& operator=(const Variant& other) = default;
+  Variant& operator=(Variant&& other) noexcept {
+    this->destroy();
+    other.move_to(this);
+    return *this;
+  }
+
+  using Impl::Impl;
+  using Impl::operator=;
+
+  Variant(Variant&& other) noexcept { other.move_to(this); }
+
+  ~Variant() {
+    static_assert(offsetof(Variant, data_) == 0, "(void*)&Variant::data_ == (void*)this");
+    this->destroy();
+  }
+
+  /// \brief Return the zero-based type index of the value held by the variant
+  uint8_t index() const noexcept { return this->index_; }
+
+  /// \brief Get a const pointer to the value held by the variant
+  ///
+  /// If the type given as template argument doesn't match, a null pointer is returned.
+  template <typename U, uint8_t I = index_of<U>()>
+  const U* get() const noexcept {
+    return index() == I ? reinterpret_cast<const U*>(this) : NULLPTR;
+  }
+
+  /// \brief Get a pointer to the value held by the variant
+  ///
+  /// If the type given as template argument doesn't match, a null pointer is returned.
+  template <typename U, uint8_t I = index_of<U>()>
+  U* get() noexcept {
+    return index() == I ? reinterpret_cast<U*>(this) : NULLPTR;
+  }
+
+  /// \brief Replace the value held by the variant
+  ///
+  /// The intended type must be given as a template argument.
+  /// The value is constructed in-place using the given function arguments.
+  template <typename U, typename... A, uint8_t I = index_of<U>()>
+  void emplace(A&&... args) {
+    try {
+      this->destroy();
+      new (this) U(std::forward<A>(args)...);
+      this->index_ = I;
+    } catch (...) {
+      construct_default();
+      throw;
+    }
+  }
+
+  template <typename U, typename E, typename... A, uint8_t I = index_of<U>()>
+  void emplace(std::initializer_list<E> il, A&&... args) {
+    try {
+      this->destroy();
+      new (this) U(il, std::forward<A>(args)...);
+      this->index_ = I;
+    } catch (...) {
+      construct_default();
+      throw;
+    }
+  }
+
+  /// \brief Swap with another variant's contents
+  void swap(Variant& other) noexcept {  // NOLINT google-runtime-references
+    Variant tmp = std::move(other);
+    other = std::move(*this);
+    *this = std::move(tmp);
+  }
+
+  using Impl::visit_const;
+  using Impl::visit_mutable;
+
+ private:
+  void construct_default() noexcept {
+    new (this) default_type();
+    this->index_ = 0;
+  }
+
+  template <typename V>
+  friend struct detail::explicit_copy_constructor::type;
+
+  template <typename V, typename...>
+  friend struct detail::VariantImpl;
+};
+
+/// \brief Call polymorphic visitor on a const variant's value
+///
+/// The visitor will receive a const reference to the value held by the variant.
+/// It must define overloads for each possible variant type.
+/// The overloads should all return the same type (no attempt
+/// is made to find a generalized return type).
+template <typename Visitor, typename... T,
+          typename R = decltype(std::declval<Visitor&&>()(
+              std::declval<const typename Variant<T...>::default_type&>()))>
+R visit(Visitor&& visitor, const util::Variant<T...>& v) {
+  return v.template visit_const<R>(std::forward<Visitor>(visitor));
+}
+
+/// \brief Call polymorphic visitor on a non-const variant's value
+///
+/// The visitor will receive a pointer to the value held by the variant.
+/// It must define overloads for each possible variant type.
+/// The overloads should all return the same type (no attempt
+/// is made to find a generalized return type).
+template <typename Visitor, typename... T,
+          typename R = decltype(std::declval<Visitor&&>()(
+              std::declval<typename Variant<T...>::default_type*>()))>
+R visit(Visitor&& visitor, util::Variant<T...>* v) {
+  return v->template visit_mutable<R>(std::forward<Visitor>(visitor));
+}
+
+/// \brief Get a const reference to the value held by the variant
+///
+/// If the type given as template argument doesn't match, behavior is undefined
+/// (a null pointer will be dereferenced).
+template <typename U, typename... T>
+const U& get(const Variant<T...>& v) {
+  return *v.template get<U>();
+}
+
+/// \brief Get a reference to the value held by the variant
+///
+/// If the type given as template argument doesn't match, behavior is undefined
+/// (a null pointer will be dereferenced).
+template <typename U, typename... T>
+U& get(Variant<T...>& v) {
+  return *v.template get<U>();
+}
+
+/// \brief Get a const pointer to the value held by the variant
+///
+/// If the type given as template argument doesn't match, a nullptr is returned.
+template <typename U, typename... T>
+const U* get_if(const Variant<T...>* v) {
+  return v->template get<U>();
+}
+
+/// \brief Get a pointer to the value held by the variant
+///
+/// If the type given as template argument doesn't match, a nullptr is returned.
+template <typename U, typename... T>
+U* get_if(Variant<T...>* v) {
+  return v->template get<U>();
+}
+
+namespace detail {
+
+template <typename... T>
+struct VariantsEqual {
+  template <typename U>
+  bool operator()(const U& r) const {
+    return get<U>(l_) == r;
+  }
+  const Variant<T...>& l_;
+};
+
+}  // namespace detail
+
+template <typename... T, typename = typename std::enable_if<detail::all<
+                             detail::is_equality_comparable<T>::value...>::value>>
+bool operator==(const Variant<T...>& l, const Variant<T...>& r) {
+  if (l.index() != r.index()) return false;
+  return visit(detail::VariantsEqual<T...>{l}, r);
+}
+
+template <typename... T>
+auto operator!=(const Variant<T...>& l, const Variant<T...>& r) -> decltype(l == r) {
+  return !(l == r);
+}
+
+/// \brief Return whether the variant holds a value of the given type
+template <typename U, typename... T>
+bool holds_alternative(const Variant<T...>& v) {
+  return v.template get<U>();
+}
+
+}  // namespace util
+}  // namespace arrow