Sorting optimization

power_grid_model_c/power_grid_model/include/power_grid_model/index_mapping.hpp

@@ -89,7 +89,7 @@ struct DenseIndexMapping {
     IdxVector reorder;
 };
 
-namespace detail {
+namespace index_mapping::detail {
 
 inline auto build_dense_mapping_comparison_sort(IdxVector const& idx_B_in_A, Idx const /* n_B */) {
     using DenseEntry = std::pair<Idx, Idx>;
@@ -118,22 +118,32 @@ inline auto build_dense_mapping_comparison_sort(IdxVector const& idx_B_in_A, Idx
 inline auto build_dense_mapping_counting_sort(IdxVector const& idx_B_in_A, Idx const n_B) {
     auto sparse_result = build_sparse_mapping(idx_B_in_A, n_B);
 
-    return DenseIndexMapping{.indvector = sparse_decode(sparse_result.indptr),
+    return DenseIndexMapping{.indvector = power_grid_model::detail::sparse_decode(sparse_result.indptr),
                              .reorder = std::move(sparse_result.reorder)};
 }
 
-} // namespace detail
+struct IndexMappingCriterion {
+    double coeff_1{};
+    double coeff_2{};
+    double coeff_3{};
 
-inline DenseIndexMapping build_dense_mapping(IdxVector const& idx_B_in_A, Idx const n_B) {
-    constexpr auto relative_complexity_prefactor = 1.0;
+    constexpr bool operator()(std::integral auto n_A, std::integral auto n_B) const {
+        auto const n_A_ = static_cast<double>(n_A);
+        auto const n_B_ = static_cast<double>(n_B);
+        return n_A < coeff_1 * n_B_ + coeff_2 * n_A_ * log(n_A_) + coeff_3;
+    }
+};
 
-    auto const n_A_ = static_cast<double>(idx_B_in_A.size());
-    auto const n_B_ = static_cast<double>(n_B);
+constexpr IndexMappingCriterion index_mapping_criterion_gcc{
+    .coeff_1 = -0.62458071347, .coeff_2 = -0.00038127964, .coeff_3 = -996.744708763};
 
-    if (n_A_ + n_B_ < relative_complexity_prefactor * n_A_ * log(n_A_)) {
-        return detail::build_dense_mapping_counting_sort(idx_B_in_A, n_B);
+} // namespace index_mapping::detail
+
+inline DenseIndexMapping build_dense_mapping(IdxVector const& idx_B_in_A, Idx const n_B) {
+    if (index_mapping::detail::index_mapping_criterion_gcc(idx_B_in_A.size(), n_B)) {
+        return index_mapping::detail::build_dense_mapping_counting_sort(idx_B_in_A, n_B);
     }
-    return detail::build_dense_mapping_comparison_sort(idx_B_in_A, n_B);
+    return index_mapping::detail::build_dense_mapping_comparison_sort(idx_B_in_A, n_B);
 }
 
 } // namespace power_grid_model