Merge pull request #1161 from vittorioromeo/improve_reverse_tuple_iterate

Simplify reverse tuple iteration

Author: fnc12

dev/functional/index_sequence_util.h

@@ -3,9 +3,6 @@
 #include <utility>  //  std::index_sequence, std::make_index_sequence
 
 #include "../functional/cxx_universal.h"
-#ifdef SQLITE_ORM_RELAXED_CONSTEXPR_SUPPORTED
-#include <array>
-#endif
 
 namespace sqlite_orm {
     namespace internal {
@@ -17,37 +14,6 @@ namespace sqlite_orm {
             return I;
         }
 
-#ifdef SQLITE_ORM_RELAXED_CONSTEXPR_SUPPORTED
-        /**
-         *  Reorder the values of an index_sequence according to the positions from a second sequence.
-         */
-        template<size_t... Value, size_t... IdxOfValue>
-        SQLITE_ORM_CONSTEVAL auto reorder_index_sequence(std::index_sequence<Value...>,
-                                                         std::index_sequence<IdxOfValue...>) {
-            constexpr std::array<size_t, sizeof...(Value)> values{Value...};
-            return std::index_sequence<values[sizeof...(Value) - 1u - IdxOfValue]...>{};
-        }
-
-        template<size_t Value, size_t IdxOfValue>
-        SQLITE_ORM_CONSTEVAL std::index_sequence<Value> reorder_index_sequence(std::index_sequence<Value>,
-                                                                               std::index_sequence<IdxOfValue>) {
-            return {};
-        }
-
-        inline SQLITE_ORM_CONSTEVAL std::index_sequence<> reorder_index_sequence(std::index_sequence<>,
-                                                                                 std::index_sequence<>) {
-            return {};
-        }
-
-        /**
-         *  Reverse the values of an index_sequence.
-         */
-        template<size_t... Idx>
-        SQLITE_ORM_CONSTEVAL auto reverse_index_sequence(std::index_sequence<Idx...>) {
-            return reorder_index_sequence(std::index_sequence<Idx...>{}, std::make_index_sequence<sizeof...(Idx)>{});
-        }
-#endif
-
         template<class... Seq>
         struct flatten_idxseq {
             using type = std::index_sequence<>;

dev/tuple_helper/tuple_iteration.h

@@ -33,7 +33,9 @@ namespace sqlite_orm {
         template<bool reversed = false, class Tpl, size_t... Idx, class L>
         void iterate_tuple(const Tpl& tpl, std::index_sequence<Idx...>, L&& lambda) {
             if constexpr(reversed) {
-                iterate_tuple(tpl, reverse_index_sequence(std::index_sequence<Idx...>{}), std::forward<L>(lambda));
+                // nifty fold expression trick: make use of guaranteed right-to-left evaluation order when folding over operator=
+                int sink;
+                ((lambda(std::get<Idx>(tpl)), sink) = ... = 0);
             } else {
                 (lambda(std::get<Idx>(tpl)), ...);
             }

include/sqlite_orm/sqlite_orm.h

@@ -1114,10 +1114,6 @@ namespace sqlite_orm {
 
 // #include "../functional/cxx_universal.h"
 
-#ifdef SQLITE_ORM_RELAXED_CONSTEXPR_SUPPORTED
-#include <array>
-#endif
-
 namespace sqlite_orm {
     namespace internal {
         /**
@@ -1128,37 +1124,6 @@ namespace sqlite_orm {
             return I;
         }
 
-#ifdef SQLITE_ORM_RELAXED_CONSTEXPR_SUPPORTED
-        /**
-         *  Reorder the values of an index_sequence according to the positions from a second sequence.
-         */
-        template<size_t... Value, size_t... IdxOfValue>
-        SQLITE_ORM_CONSTEVAL auto reorder_index_sequence(std::index_sequence<Value...>,
-                                                         std::index_sequence<IdxOfValue...>) {
-            constexpr std::array<size_t, sizeof...(Value)> values{Value...};
-            return std::index_sequence<values[sizeof...(Value) - 1u - IdxOfValue]...>{};
-        }
-
-        template<size_t Value, size_t IdxOfValue>
-        SQLITE_ORM_CONSTEVAL std::index_sequence<Value> reorder_index_sequence(std::index_sequence<Value>,
-                                                                               std::index_sequence<IdxOfValue>) {
-            return {};
-        }
-
-        inline SQLITE_ORM_CONSTEVAL std::index_sequence<> reorder_index_sequence(std::index_sequence<>,
-                                                                                 std::index_sequence<>) {
-            return {};
-        }
-
-        /**
-         *  Reverse the values of an index_sequence.
-         */
-        template<size_t... Idx>
-        SQLITE_ORM_CONSTEVAL auto reverse_index_sequence(std::index_sequence<Idx...>) {
-            return reorder_index_sequence(std::index_sequence<Idx...>{}, std::make_index_sequence<sizeof...(Idx)>{});
-        }
-#endif
-
         template<class... Seq>
         struct flatten_idxseq {
             using type = std::index_sequence<>;
@@ -9578,7 +9543,9 @@ namespace sqlite_orm {
         template<bool reversed = false, class Tpl, size_t... Idx, class L>
         void iterate_tuple(const Tpl& tpl, std::index_sequence<Idx...>, L&& lambda) {
             if constexpr(reversed) {
-                iterate_tuple(tpl, reverse_index_sequence(std::index_sequence<Idx...>{}), std::forward<L>(lambda));
+                // nifty fold expression trick: make use of guaranteed right-to-left evaluation order when folding over operator=
+                int sink;
+                ((lambda(std::get<Idx>(tpl)), sink) = ... = 0);
             } else {
                 (lambda(std::get<Idx>(tpl)), ...);
             }