From e5269056f094234ec67d60781148912553489e9d Mon Sep 17 00:00:00 2001
From: Jakub Mazurkiewicz <mazkuba3@gmail.com>
Date: Thu, 18 May 2023 00:04:15 +0200
Subject: [PATCH] FASTER PRODUCTS!

---
 stl/inc/mdspan | 107 ++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 83 insertions(+), 24 deletions(-)

diff --git a/stl/inc/mdspan b/stl/inc/mdspan
index fbd4574c0e0..2c6db3498f7 100644
--- a/stl/inc/mdspan
+++ b/stl/inc/mdspan
@@ -231,24 +231,6 @@ public:
         }
     }
 
-    // TRANSITION, LWG ISSUE? I believe that this function should return 'index_type'
-    _NODISCARD constexpr index_type _Fwd_prod_of_extents(const rank_type _Idx) const noexcept {
-        index_type _Result = 1;
-        for (rank_type _Dim = 0; _Dim < _Idx; ++_Dim) {
-            _Result *= extent(_Dim);
-        }
-        return _Result;
-    }
-
-    // TRANSITION, LWG ISSUE? I believe that this function should return 'index_type'
-    _NODISCARD constexpr index_type _Rev_prod_of_extents(const rank_type _Idx) const noexcept {
-        index_type _Result = 1;
-        for (rank_type _Dim = _Idx + 1; _Dim < _Rank; ++_Dim) {
-            _Result *= extent(_Dim);
-        }
-        return _Result;
-    }
-
     _NODISCARD static _CONSTEVAL bool _Is_index_space_size_representable() {
         if constexpr (rank_dynamic() == 0 && rank() > 0) {
             return _STD in_range<index_type>((_Extents * ...));
@@ -293,6 +275,82 @@ inline constexpr bool _Is_extents = false;
 template <class _IndexType, size_t... _Args>
 inline constexpr bool _Is_extents<extents<_IndexType, _Args...>> = true;
 
+template <class _Extents>
+    requires _Is_extents<_Extents>
+class _Fwd_prod_of_extents {
+public:
+    _NODISCARD static constexpr _Extents::index_type _Calculate(const _Extents& _Exts, const size_t _Idx) noexcept {
+        if constexpr (_Extents::rank() == 0) {
+            return 1;
+        } else {
+            typename _Extents::index_type _Result = 1;
+            for (size_t _Dim = 0; _Dim < _Idx; ++_Dim) {
+                _Result *= _Exts.extent(_Dim);
+            }
+            return _Result;
+        }
+    }
+};
+
+template <class _IndexType, size_t... _Extents>
+    requires ((_Extents != dynamic_extent) && ...)
+class _Fwd_prod_of_extents<extents<_IndexType, _Extents...>> {
+private:
+    using _Ty = extents<_IndexType, _Extents...>;
+
+    _NODISCARD static consteval auto _Make_prods() noexcept {
+        array<typename _Ty::index_type, _Ty::rank() + 1> _Result;
+        _Result.front() = 1;
+        for (size_t _Dim = 1; _Dim < _Ty::_Rank + 1; ++_Dim) {
+            _Result[_Dim] = static_cast<_Ty::index_type>(_Result[_Dim - 1] * _Ty::static_extent(_Dim - 1));
+        }
+        return _Result;
+    }
+
+    static constexpr array<typename _Ty::index_type, _Ty::rank() + 1> _Cache = _Make_prods();
+
+public:
+    _NODISCARD static constexpr _Ty::index_type _Calculate(const _Ty&, const size_t _Idx) noexcept {
+        return _Cache[_Idx];
+    }
+};
+
+template <class _Extents>
+    requires _Is_extents<_Extents> && (_Extents::rank() > 0)
+class _Rev_prod_of_extents {
+public:
+    _NODISCARD static constexpr _Extents::index_type _Calculate(const _Extents& _Exts, const size_t _Idx) noexcept {
+        typename _Extents::index_type _Result = 1;
+        for (size_t _Dim = _Idx + 1; _Dim < _Extents::_Rank; ++_Dim) {
+            _Result *= _Exts.extent(_Dim);
+        }
+        return _Result;
+    }
+};
+
+template <class _IndexType, size_t... _Extents>
+    requires ((_Extents != dynamic_extent) && ...)
+class _Rev_prod_of_extents<extents<_IndexType, _Extents...>> {
+private:
+    using _Ty = extents<_IndexType, _Extents...>;
+
+    _NODISCARD static consteval auto _Make_prods() noexcept {
+        array<typename _Ty::index_type, _Ty::rank()> _Result;
+        _Result.back() = 1;
+        for (size_t _Dim = _Ty::_Rank; _Dim-- > 1;) {
+            _Result[_Dim - 1] = static_cast<_Ty::index_type>(_Result[_Dim] * _Ty::static_extent(_Dim));
+        }
+        return _Result;
+    }
+
+    static constexpr array<typename _Ty::index_type, _Ty::rank()> _Cache = _Make_prods();
+
+public:
+    _NODISCARD static constexpr _Ty::index_type _Calculate(const _Ty&, const size_t _Idx) noexcept {
+        return _Cache[_Idx];
+    }
+};
+
 template <class _Layout, class _Mapping>
 inline constexpr bool _Is_mapping_of =
     is_same_v<typename _Layout::template mapping<typename _Mapping::extents_type>, _Mapping>;
@@ -383,7 +441,7 @@ public:
     }
 
     _NODISCARD constexpr index_type required_span_size() const noexcept {
-        return _Exts._Fwd_prod_of_extents(extents_type::_Rank);
+        return _Fwd_prod_of_extents<extents_type>::_Calculate(_Exts, extents_type::_Rank);
     }
 
     template <class... _IndexTypes>
@@ -426,7 +484,7 @@ public:
     {
         _STL_VERIFY(_Idx < extents_type::_Rank,
             "Value of i must be less than extents_type::rank() (N4950 [mdspan.layout.left.obs]/6).");
-        return _Exts._Fwd_prod_of_extents(_Idx);
+        return _Fwd_prod_of_extents<extents_type>::_Calculate(_Exts, _Idx);
     }
 
     template <class _OtherExtents>
@@ -520,7 +578,7 @@ public:
     }
 
     _NODISCARD constexpr index_type required_span_size() const noexcept {
-        return _Exts._Fwd_prod_of_extents(extents_type::_Rank);
+        return _Fwd_prod_of_extents<extents_type>::_Calculate(_Exts, extents_type::_Rank);
     }
 
     template <class... _IndexTypes>
@@ -563,7 +621,7 @@ public:
     {
         _STL_VERIFY(_Idx < extents_type::_Rank,
             "Value of i must be less than extents_type::rank() (N4950 [mdspan.layout.right.obs]/6).");
-        return _Exts._Rev_prod_of_extents(_Idx);
+        return _Rev_prod_of_extents<extents_type>::_Calculate(_Exts, _Idx);
     }
 
     template <class _OtherExtents>
@@ -744,7 +802,7 @@ public:
         if constexpr (extents_type::rank() == 0) {
             return true;
         } else {
-            return required_span_size() == _Exts._Fwd_prod_of_extents(extents_type::_Rank);
+            return required_span_size() == _Fwd_prod_of_extents<extents_type>::_Calculate(_Exts, extents_type::_Rank);
         }
     }
 
@@ -963,7 +1021,8 @@ public:
     }
 
     _NODISCARD constexpr size_type size() const noexcept {
-        return static_cast<size_type>(_Map.extents()._Fwd_prod_of_extents(extents_type::_Rank));
+        return static_cast<size_type>(
+            _Fwd_prod_of_extents<extents_type>::_Calculate(_Map.extents(), extents_type::_Rank));
     }
 
     _NODISCARD constexpr bool empty() const noexcept {