feat utils: publish constexpr_indices.hpp

67d1529c81286c3211465acb26b29a5eed28cd21

.mapping.json

@@ -3200,6 +3200,7 @@
   "universal/include/userver/utils/bytes_per_second.hpp":"taxi/uservices/userver/universal/include/userver/utils/bytes_per_second.hpp",
   "universal/include/userver/utils/cached_hash.hpp":"taxi/uservices/userver/universal/include/userver/utils/cached_hash.hpp",
   "universal/include/userver/utils/checked_pointer.hpp":"taxi/uservices/userver/universal/include/userver/utils/checked_pointer.hpp",
+  "universal/include/userver/utils/constexpr_indices.hpp":"taxi/uservices/userver/universal/include/userver/utils/constexpr_indices.hpp",
   "universal/include/userver/utils/datetime.hpp":"taxi/uservices/userver/universal/include/userver/utils/datetime.hpp",
   "universal/include/userver/utils/datetime/cpp_20_calendar.hpp":"taxi/uservices/userver/universal/include/userver/utils/datetime/cpp_20_calendar.hpp",
   "universal/include/userver/utils/datetime/date.hpp":"taxi/uservices/userver/universal/include/userver/utils/datetime/date.hpp",

core/include/userver/dump/common_containers.hpp

@@ -24,6 +24,7 @@
 #include <variant>
 #include <vector>
 
+#include <userver/utils/constexpr_indices.hpp>
 #include <userver/utils/lazy_prvalue.hpp>
 #include <userver/utils/meta.hpp>
 #include <userver/utils/strong_typedef.hpp>
@@ -71,23 +72,16 @@ auto ReadLazyPrvalue(Reader& reader) {
 [[noreturn]] void ThrowInvalidVariantIndex(const std::type_info& type,
                                            std::size_t index);
 
-template <std::size_t... Indices, typename Func>
-void ForEachIndex(std::index_sequence<Indices...>, Func func) {
-  (func(std::integral_constant<std::size_t, Indices>{}), ...);
-}
-
 template <typename VariantType>
 VariantType ReadVariant(Reader& reader, std::size_t index) {
+  static constexpr auto VariantSize = std::variant_size_v<VariantType>;
   std::optional<VariantType> result;
-  using Indices = std::make_index_sequence<std::variant_size_v<VariantType>>;
 
-  ForEachIndex(Indices{}, [&](auto index_constant) {
+  utils::WithConstexprIndex<VariantSize>(index, [&](auto index_constant) {
     static constexpr auto kIndex = decltype(index_constant)::value;
     using Alternative = std::variant_alternative_t<kIndex, VariantType>;
-    if (index == kIndex) {
-      // Not using ReadLazyPrvalue because of stdlib issues on some compilers.
-      result.emplace(std::in_place_index<kIndex>, reader.Read<Alternative>());
-    }
+    // Not using ReadLazyPrvalue because of stdlib issues on some compilers.
+    result.emplace(std::in_place_index<kIndex>, reader.Read<Alternative>());
   });
 
   return std::move(*result);

universal/include/userver/utils/constexpr_indices.hpp

@@ -0,0 +1,49 @@
+#pragma once
+
+/// @file userver/utils/constexpr_indices.hpp
+/// @brief Functions for iterating over a constexpr range of integers
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+USERVER_NAMESPACE_BEGIN
+
+namespace utils {
+
+namespace impl {
+
+template <std::size_t... Indices, typename Func>
+void DoForEachIndex(std::index_sequence<Indices...>, Func func) {
+  static_assert(std::is_trivially_copyable_v<Func>);
+  (..., func(std::integral_constant<std::size_t, Indices>{}));
+}
+
+template <std::size_t... Indices, typename Func>
+void DoWithConstexprIndex(std::index_sequence<Indices...>,
+                          std::size_t runtime_index, Func func) {
+  static_assert(std::is_trivially_copyable_v<Func>);
+  (..., (runtime_index == Indices
+             ? func(std::integral_constant<std::size_t, Indices>{})
+             : void()));
+}
+
+}  // namespace impl
+
+/// Calls `func` with indices from range `0...<Count`, wrapped in
+/// `std::integral_constant`.
+template <std::size_t Count, typename Func>
+void ForEachIndex(Func func) {
+  impl::DoForEachIndex(std::make_index_sequence<Count>{}, func);
+}
+
+/// Calls `func` with `runtime_index` wrapped in `std::integral_constant`.
+template <std::size_t Count, typename Func>
+void WithConstexprIndex(std::size_t runtime_index, Func func) {
+  impl::DoWithConstexprIndex(std::make_index_sequence<Count>{}, runtime_index,
+                             func);
+}
+
+}  // namespace utils
+
+USERVER_NAMESPACE_END