Add SlotMap container

Author: ProgramMax

Code/max/Containers/SlotMap.hpp

@@ -0,0 +1,103 @@
+// Copyright 2021, The max Contributors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef MAX_CONTAINERS_SLOTMAP_HPP
+#define MAX_CONTAINERS_SLOTMAP_HPP
+
+#include <algorithm>
+#include <concepts>
+#include <utility>
+#include <vector>
+
+#include <max/Compiling/CurrentVersionNamespace.hpp>
+#include <max/Compiling/AliasingOptimizations.hpp>
+
+
+namespace max
+{
+MAX_CURRENT_VERSION_NAMESPACE_BEGIN(v0)
+{
+namespace Containers
+{
+
+	// SlotMap is a container that provides a stable handle to an element that was inserted.
+	// 
+	// Insertion: O(1)*
+	// Access:    O(1)
+	// Removal:   O(1)
+	// *Unless the BackingType requires reallocation. Then it is O(1) amortized.
+	template<typename T, std::integral HandleType = size_t, template <typename T2> typename BackingType = std::vector>
+	class SlotMap {
+	public:
+
+
+
+		// Required to satisfy the C++ "Container" requirements
+		typedef       T   value_type;
+		typedef       T&  reference;
+		typedef const T&  const_reference;
+		typedef BackingType<T>::iterator         iterator;
+		typedef BackingType<T>::const_iterator   const_iterator;
+		typedef BackingType<T>::difference_type  difference_type;
+		typedef BackingType<T>::size_type        size_type;
+
+
+
+		HandleType push_back(T element) noexcept;
+
+		template<class ...Args>
+		HandleType emplace_back(Args&&... args) noexcept;
+
+		typename reference operator[](HandleType handle) noexcept;
+		typename const_reference operator[](HandleType handle) const noexcept;
+
+		void remove(HandleType handle) noexcept;
+		void pop_back() noexcept;
+
+
+
+		// Required to satisfy the C++ "Container" requirements
+		typename iterator       begin() noexcept;
+		typename const_iterator begin() const noexcept;
+		typename const_iterator cbegin() const noexcept;
+		typename iterator       end() noexcept;
+		typename const_iterator end() const noexcept;
+		typename const_iterator cend() noexcept;
+
+		typename size_type size() const noexcept;
+		constexpr typename size_type max_size() const noexcept;
+
+
+
+	private:
+
+		// The indices are stable and act as handles.
+		typename BackingType<HandleType> indices_;
+		// If we wanted to prevent a user accidently reusing a handle from an object they removed,
+		// |indices_| could be a tuple of HandleType and generation counter.
+		// Increment the generation every time an element is removed and compare against it when accessing.
+		// However, this is designed for performance and will assume the programmer did not make a mistake.
+
+		// |data_| and |reverse_indices_| are parallel. The nth element in one corresponds to the nth element in the other.
+		// This means once we have an element's index into |data_|, we can use that same index into |reverse_indices_| to find
+		// the element in |indices_| that points here.
+		typename BackingType<T> data_;
+		typename BackingType<HandleType> reverse_indices_;
+
+		void remove_by_index(size_t index) noexcept;
+
+		friend bool operator ==(const SlotMap<T, HandleType, BackingType>& lhs, const SlotMap<T, HandleType, BackingType>& rhs) noexcept;
+		friend bool operator !=(const SlotMap<T, HandleType, BackingType>& lhs, const SlotMap<T, HandleType, BackingType>& rhs) noexcept;
+		friend void swap(SlotMap<T, HandleType, BackingType>& lhs, SlotMap<T, HandleType, BackingType>& rhs) noexcept;
+
+	};
+
+} // namespace Containers
+} // MAX_CURRENT_VERSION_NAMESPACE_BEGIN( v0 )
+MAX_CURRENT_VERSION_NAMESPACE_END(v0)
+} // namespace max
+
+#include "SlotMap.inl"
+
+#endif // #ifndef MAX_CONTAINERS_SLOTMAP_HPP

Code/max/Containers/SlotMap.inl

@@ -0,0 +1,169 @@
+// Copyright 2021, The max Contributors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <algorithm>
+#include <concepts>
+#include <utility>
+#include <vector>
+
+namespace max
+{
+namespace v0
+{
+namespace Containers
+{
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	HandleType SlotMap<T, HandleType, BackingType>::push_back(T element) noexcept {
+		const auto data_size = data_.size();
+		data_.push_back(std::move(element));
+
+		const auto reverse_indices_size = reverse_indices_.size();
+		if (data_size == reverse_indices_size) {
+			// No elements had previously been removed, leaving gaps.
+			// This means we cannot reuse the handles and indices we left in place.
+			// We must add new ones.
+			auto new_handle = reverse_indices_size;
+			indices_.push_back(new_handle);
+			reverse_indices_.push_back(new_handle);
+			return new_handle;
+		}
+		else {
+			// We previously left the elements in |indices_| and |reverse_indices_|.
+			// That means we can just reuse those handles.
+			return reverse_indices_[data_size];
+		}
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	template<class ...Args>
+	HandleType SlotMap<T, HandleType, BackingType>::emplace_back(Args&&... args) noexcept {
+		// TODO: constructing the element then moving it to push_back() really defeats the purpose of an emplace_back.
+		// Do this correctly.
+		return push_back(T{ std::forward<Args>(args)... });
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::reference SlotMap<T, HandleType, BackingType>::operator[](HandleType handle) noexcept {
+		// Assumes |handle| is within range.
+
+		const auto index = indices_[handle];
+		return data_[index];
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::const_reference SlotMap<T, HandleType, BackingType>::operator[](HandleType handle) const noexcept {
+		// Assumes |handle| is within range.
+
+		const auto index = indices_[handle];
+		return data_[index];
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	void SlotMap<T, HandleType, BackingType>::remove(HandleType handle) noexcept {
+		remove_by_index(indices_[handle]);
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	void SlotMap<T, HandleType, BackingType>::pop_back() noexcept {
+		remove_by_index(data_.size() - 1);
+	}
+
+
+
+	// Required to satisfy the C++ "Container" requirements
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::iterator SlotMap<T, HandleType, BackingType>::begin() noexcept {
+		return data_.begin();
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::const_iterator SlotMap<T, HandleType, BackingType>::begin() const noexcept {
+		return data_.begin();
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::const_iterator SlotMap<T, HandleType, BackingType>::cbegin() const noexcept {
+		return data_.begin();
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::iterator SlotMap<T, HandleType, BackingType>::end() noexcept {
+		return data_.end();
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::const_iterator SlotMap<T, HandleType, BackingType>::end() const noexcept {
+		return data_.end();
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::const_iterator SlotMap<T, HandleType, BackingType>::cend() noexcept {
+		return data_.end();
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	typename SlotMap<T, HandleType, BackingType>::size_type SlotMap<T, HandleType, BackingType>::size() const noexcept {
+		return data_.size();
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	constexpr typename SlotMap<T, HandleType, BackingType>::size_type SlotMap<T, HandleType, BackingType>::max_size() const noexcept {
+		return data_.max_size();
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	void SlotMap<T, HandleType, BackingType>::remove_by_index(size_t index) noexcept {
+		// Removing an element in the middle of a vector will cause all elements after it to shift over.
+		// Removing the final element does not have this effect.
+
+		// In order to remove any given element, we first swap it with the element in the final spot.
+		// Then we can remove it and update indices.
+
+		using std::swap;
+
+		auto data_size = data_.size() - 1; // .size() tells us the index past the end.
+
+		swap(data_[index], data_[data_size]);
+		swap(reverse_indices_[index], reverse_indices_[data_size]); // We only resize |data_|, so use it's size--not |reverse_indices_|'s.
+		data_.resize(data_size);
+
+		// The catch is we only know one index, the parameter.
+		// We don't know which element in |indices_| corresponds to the final element in |data_|.
+		// To solve that problem, the |reverse_indices_| vector is parallel to the |data_| vector.
+		// An index into one is also an index into the other.
+		// We have the index of the final element in |data_|, so we can use it in |reverse_indices_| to get the corresponding entry in |indices_|.
+		auto reverse_index = reverse_indices_[data_size];
+		indices_[reverse_index] = data_size;
+
+		// We also update the reverse index of the that was removed.
+		// This is because it will be reused later when more elements are appended.
+		reverse_index = reverse_indices_[index];
+		indices_[reverse_index] = index;
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	bool operator ==(const SlotMap<T, HandleType, BackingType>& lhs, const SlotMap<T, HandleType, BackingType>& rhs) noexcept {
+		return lhs.indices_ == rhs.indices_ &&
+			lhs.data_ == rhs.data_ &&
+			lhs.reverse_indices_ == rhs.reverse_indices_;
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	bool operator !=(const SlotMap<T, HandleType, BackingType>& lhs, const SlotMap<T, HandleType, BackingType>& rhs) noexcept {
+		return !(lhs == rhs);
+	}
+
+	template<typename T, std::integral HandleType, template <typename T2> typename BackingType>
+	void swap(SlotMap<T, HandleType, BackingType>& lhs, SlotMap<T, HandleType, BackingType>& rhs) noexcept {
+		using std::swap;
+
+		swap(lhs.indices_, rhs.indices_);
+		swap(lhs.data_, rhs.data_);
+		swap(lhs.reverse_indices_, rhs.reverse_indices_);
+	}
+
+} // namespace Containers
+} // namespace v0
+} // namespace max