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Optimize implementation of Indexes generator

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... which is Qt's version of std::make_index_sequence / C++14.

The current Indexes generator is recursing O(N) times which is reaching
recursion limits level for long IndexesList.
This implementation has only O(log N) recursion.

Change-Id: I423ef05152d77b18f1f5a64c49b273f49aa958bb
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Mikhail Svetkin 2018-07-26 10:42:03 +02:00
parent d98106d1d5
commit d9d35b9fb6
1 changed files with 25 additions and 7 deletions
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32
src/corelib/kernel/qobjectdefs_impl.h
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@ -112,13 +112,31 @@ namespace QtPrivate {
The Functor<Func,N> struct is the helper to call a functor of N argument.
its call function is the same as the FunctionPointer::call function.
*/
template <int...> struct IndexesList {};
template <typename IndexList, int Right> struct IndexesAppend;
template <int... Left, int Right> struct IndexesAppend<IndexesList<Left...>, Right>
{ typedef IndexesList<Left..., Right> Value; };
template <int N> struct Indexes
{ typedef typename IndexesAppend<typename Indexes<N - 1>::Value, N - 1>::Value Value; };
template <> struct Indexes<0> { typedef IndexesList<> Value; };
template<class T> using InvokeGenSeq = typename T::Type;
template<int...> struct IndexesList { using Type = IndexesList; };
template<int N, class S1, class S2> struct ConcatSeqImpl;
template<int N, int... I1, int... I2>
struct ConcatSeqImpl<N, IndexesList<I1...>, IndexesList<I2...>>
: IndexesList<I1..., (N + I2)...>{};
template<int N, class S1, class S2>
using ConcatSeq = InvokeGenSeq<ConcatSeqImpl<N, S1, S2>>;
template<int N> struct GenSeq;
template<int N> using makeIndexSequence = InvokeGenSeq<GenSeq<N>>;
template<int N>
struct GenSeq : ConcatSeq<N/2, makeIndexSequence<N/2>, makeIndexSequence<N - N/2>>{};
template<> struct GenSeq<0> : IndexesList<>{};
template<> struct GenSeq<1> : IndexesList<0>{};
template<int N>
struct Indexes { using Value = makeIndexSequence<N>; };
template<typename Func> struct FunctionPointer { enum {ArgumentCount = -1, IsPointerToMemberFunction = false}; };
template <typename, typename, typename, typename> struct FunctorCall;