Implement efficient clearing of the Hashtable

Instead of iterating over the linked list (also requires hash lookups)
and erasing one by one, just reset the backing linked list and value
storage, then 0 out the bucket array.

include/fixed_containers/fixed_doubly_linked_list.hpp

@@ -18,6 +18,7 @@ struct LinkedListIndices
 template <typename T, std::size_t MAXIMUM_SIZE, typename IndexType = std::size_t>
 class FixedDoublyLinkedListBase
 {
+protected:
     static_assert(MAXIMUM_SIZE + 1 <= std::numeric_limits<IndexType>::max(),
                   "must be able to index MAXIMUM_SIZE+1 elements with IndexType");
     using StorageType = FixedIndexBasedPoolStorage<T, MAXIMUM_SIZE>;
@@ -52,11 +53,6 @@ class FixedDoublyLinkedListBase
     }
     [[nodiscard]] constexpr bool full() const noexcept { return storage().full(); }
 
-    constexpr void clear() noexcept
-    {
-        delete_range_and_return_next_index(front_index(), MAXIMUM_SIZE);
-    }
-
     constexpr const T& at(const IndexType i) const { return storage().at(i); }
     constexpr T& at(const IndexType i) { return storage().at(i); }
 
@@ -227,6 +223,11 @@ class FixedDoublyLinkedList : public FixedDoublyLinkedListBase<T, MAXIMUM_SIZE,
         return *this;
     }
 
+    constexpr void clear() noexcept
+    {
+        this->delete_range_and_return_next_index(this->front_index(), MAXIMUM_SIZE);
+    }
+
     constexpr ~FixedDoublyLinkedList() noexcept { this->clear(); }
 };
 
@@ -240,6 +241,22 @@ class FixedDoublyLinkedList<T, MAXIMUM_SIZE, IndexType>
     // clang-format off
     constexpr FixedDoublyLinkedList() noexcept : Base() { }
     // clang-format on
+
+    constexpr void clear() noexcept
+    {
+        // Instead of iterating over the elements of the linked list (slow), just reset the backing
+        // storage
+        this->IMPLEMENTATION_DETAIL_DO_NOT_USE_storage_ = std::move(typename Base::StorageType{});
+
+        // And reset the start/end sentinel to point at itself.
+        // The remaining links of the linked list will be overwritten as elements are allocated, so
+        // we don't have to reset the entire chain
+        this->next_of(MAXIMUM_SIZE) = MAXIMUM_SIZE;
+        this->prev_of(MAXIMUM_SIZE) = MAXIMUM_SIZE;
+
+        // Finally, set the size back to 0
+        this->IMPLEMENTATION_DETAIL_DO_NOT_USE_size_ = 0;
+    }
 };
 
 }  // namespace fixed_containers::fixed_doubly_linked_list_detail::specializations

include/fixed_containers/fixed_robinhood_hashtable.hpp

@@ -386,7 +386,14 @@ class FixedRobinhoodHashtable
         return end_value_index;
     }
 
-    constexpr void clear() { erase_range(begin_index(), end_index()); }
+    constexpr void clear()
+    {
+        // reset the backing linked list
+        IMPLEMENTATION_DETAIL_DO_NOT_USE_value_storage_.clear();
+
+        // reset the bucket array
+        IMPLEMENTATION_DETAIL_DO_NOT_USE_bucket_array_.fill({});
+    }
 
 public:
     constexpr FixedRobinhoodHashtable() = default;