godot/core/templates/paged_array.h

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/*  paged_array.h                                                         */
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#ifndef PAGED_ARRAY_H
#define PAGED_ARRAY_H

#include "core/os/memory.h"
#include "core/os/spin_lock.h"
#include "core/typedefs.h"

#include <type_traits>

// PagedArray is used mainly for filling a very large array from multiple threads efficiently and without causing major fragmentation

// PageArrayPool manages central page allocation in a thread safe matter

template <typename T>
class PagedArrayPool {
	T **page_pool = nullptr;
	uint32_t pages_allocated = 0;

	uint32_t *available_page_pool = nullptr;
	uint32_t pages_available = 0;

	uint32_t page_size = 0;
	SpinLock spin_lock;

public:
	struct PageInfo {
		T *page = nullptr;
		uint32_t page_id = 0;
	};

	PageInfo alloc_page() {
		spin_lock.lock();
		if (unlikely(pages_available == 0)) {
			uint32_t pages_used = pages_allocated;

			pages_allocated++;
			page_pool = (T **)memrealloc(page_pool, sizeof(T *) * pages_allocated);
			available_page_pool = (uint32_t *)memrealloc(available_page_pool, sizeof(uint32_t) * pages_allocated);

			page_pool[pages_used] = (T *)memalloc(sizeof(T) * page_size);
			available_page_pool[0] = pages_used;

			pages_available++;
		}

		pages_available--;
		uint32_t page_id = available_page_pool[pages_available];
		T *page = page_pool[page_id];
		spin_lock.unlock();

		return PageInfo{ page, page_id };
	}

	void free_page(uint32_t p_page_id) {
		spin_lock.lock();
		available_page_pool[pages_available] = p_page_id;
		pages_available++;
		spin_lock.unlock();
	}

	uint32_t get_page_size_shift() const {
		return get_shift_from_power_of_2(page_size);
	}

	uint32_t get_page_size_mask() const {
		return page_size - 1;
	}

	void reset() {
		ERR_FAIL_COND(pages_available < pages_allocated);
		if (pages_allocated) {
			for (uint32_t i = 0; i < pages_allocated; i++) {
				memfree(page_pool[i]);
			}
			memfree(page_pool);
			memfree(available_page_pool);
			page_pool = nullptr;
			available_page_pool = nullptr;
			pages_allocated = 0;
			pages_available = 0;
		}
	}
	bool is_configured() const {
		return page_size > 0;
	}

	void configure(uint32_t p_page_size) {
		ERR_FAIL_COND(page_pool != nullptr); // Safety check.
		ERR_FAIL_COND(p_page_size == 0);
		page_size = nearest_power_of_2_templated(p_page_size);
	}

	PagedArrayPool(uint32_t p_page_size = 4096) { // power of 2 recommended because of alignment with OS page sizes. Even if element is bigger, its still a multiple and get rounded amount of pages
		configure(p_page_size);
	}

	~PagedArrayPool() {
		ERR_FAIL_COND_MSG(pages_available < pages_allocated, "Pages in use exist at exit in PagedArrayPool");
		reset();
	}
};

// PageArray is a local array that is optimized to grow in place, then be cleared often.
// It does so by allocating pages from a PagedArrayPool.
// It is safe to use multiple PagedArrays from different threads, sharing a single PagedArrayPool

template <typename T>
class PagedArray {
	PagedArrayPool<T> *page_pool = nullptr;

	T **page_data = nullptr;
	uint32_t *page_ids = nullptr;
	uint32_t max_pages_used = 0;
	uint32_t page_size_shift = 0;
	uint32_t page_size_mask = 0;
	uint64_t count = 0;

	_FORCE_INLINE_ uint32_t _get_pages_in_use() const {
		if (count == 0) {
			return 0;
		} else {
			return ((count - 1) >> page_size_shift) + 1;
		}
	}

	void _grow_page_array() {
		//no more room in the page array to put the new page, make room
		if (max_pages_used == 0) {
			max_pages_used = 1;
		} else {
			max_pages_used *= 2; // increase in powers of 2 to keep allocations to minimum
		}
		page_data = (T **)memrealloc(page_data, sizeof(T *) * max_pages_used);
		page_ids = (uint32_t *)memrealloc(page_ids, sizeof(uint32_t) * max_pages_used);
	}

public:
	_FORCE_INLINE_ const T &operator[](uint64_t p_index) const {
		CRASH_BAD_UNSIGNED_INDEX(p_index, count);
		uint32_t page = p_index >> page_size_shift;
		uint32_t offset = p_index & page_size_mask;

		return page_data[page][offset];
	}
	_FORCE_INLINE_ T &operator[](uint64_t p_index) {
		CRASH_BAD_UNSIGNED_INDEX(p_index, count);
		uint32_t page = p_index >> page_size_shift;
		uint32_t offset = p_index & page_size_mask;

		return page_data[page][offset];
	}

	_FORCE_INLINE_ void push_back(const T &p_value) {
		uint32_t remainder = count & page_size_mask;
		if (unlikely(remainder == 0)) {
			// at 0, so time to request a new page
			uint32_t page_count = _get_pages_in_use();
			uint32_t new_page_count = page_count + 1;

			if (unlikely(new_page_count > max_pages_used)) {
				ERR_FAIL_NULL(page_pool); // Safety check.

				_grow_page_array(); //keep out of inline
			}

			typename PagedArrayPool<T>::PageInfo page_info = page_pool->alloc_page();
			page_data[page_count] = page_info.page;
			page_ids[page_count] = page_info.page_id;
		}

		// place the new value
		uint32_t page = count >> page_size_shift;
		uint32_t offset = count & page_size_mask;

		if constexpr (!std::is_trivially_constructible_v<T>) {
			memnew_placement(&page_data[page][offset], T(p_value));
		} else {
			page_data[page][offset] = p_value;
		}

		count++;
	}

	_FORCE_INLINE_ void pop_back() {
		ERR_FAIL_COND(count == 0);

		if constexpr (!std::is_trivially_destructible_v<T>) {
			uint32_t page = (count - 1) >> page_size_shift;
			uint32_t offset = (count - 1) & page_size_mask;
			page_data[page][offset].~T();
		}

		uint32_t remainder = count & page_size_mask;
		if (unlikely(remainder == 1)) {
			// one element remained, so page must be freed.
			uint32_t last_page = _get_pages_in_use() - 1;
			page_pool->free_page(page_ids[last_page]);
		}
		count--;
	}

	void remove_at_unordered(uint64_t p_index) {
		ERR_FAIL_UNSIGNED_INDEX(p_index, count);
		(*this)[p_index] = (*this)[count - 1];
		pop_back();
	}

	void clear() {
		//destruct if needed
		if constexpr (!std::is_trivially_destructible_v<T>) {
			for (uint64_t i = 0; i < count; i++) {
				uint32_t page = i >> page_size_shift;
				uint32_t offset = i & page_size_mask;
				page_data[page][offset].~T();
			}
		}

		//return the pages to the pagepool, so they can be used by another array eventually
		uint32_t pages_used = _get_pages_in_use();
		for (uint32_t i = 0; i < pages_used; i++) {
			page_pool->free_page(page_ids[i]);
		}

		count = 0;

		//note we leave page_data and page_indices intact for next use. If you really want to clear them call reset()
	}

	void reset() {
		clear();
		if (page_data) {
			memfree(page_data);
			memfree(page_ids);
			page_data = nullptr;
			page_ids = nullptr;
			max_pages_used = 0;
		}
	}

	// This takes the pages from a source array and merges them to this one
	// resulting order is undefined, but content is merged very efficiently,
	// making it ideal to fill content on several threads to later join it.

	void merge_unordered(PagedArray<T> &p_array) {
		ERR_FAIL_COND(page_pool != p_array.page_pool);

		uint32_t remainder = count & page_size_mask;

		T *remainder_page = nullptr;
		uint32_t remainder_page_id = 0;

		if (remainder > 0) {
			uint32_t last_page = _get_pages_in_use() - 1;
			remainder_page = page_data[last_page];
			remainder_page_id = page_ids[last_page];
		}

		count -= remainder;

		uint32_t src_page_index = 0;
		uint32_t page_size = page_size_mask + 1;

		while (p_array.count > 0) {
			uint32_t page_count = _get_pages_in_use();
			uint32_t new_page_count = page_count + 1;

			if (unlikely(new_page_count > max_pages_used)) {
				_grow_page_array(); //keep out of inline
			}

			page_data[page_count] = p_array.page_data[src_page_index];
			page_ids[page_count] = p_array.page_ids[src_page_index];

			uint32_t take = MIN(p_array.count, page_size); //pages to take away
			p_array.count -= take;
			count += take;
			src_page_index++;
		}

		//handle the remainder page if exists
		if (remainder_page) {
			uint32_t new_remainder = count & page_size_mask;

			if (new_remainder > 0) {
				//must merge old remainder with new remainder

				T *dst_page = page_data[_get_pages_in_use() - 1];
				uint32_t to_copy = MIN(page_size - new_remainder, remainder);

				for (uint32_t i = 0; i < to_copy; i++) {
					if constexpr (!std::is_trivially_constructible_v<T>) {
						memnew_placement(&dst_page[i + new_remainder], T(remainder_page[i + remainder - to_copy]));
					} else {
						dst_page[i + new_remainder] = remainder_page[i + remainder - to_copy];
					}

					if constexpr (!std::is_trivially_destructible_v<T>) {
						remainder_page[i + remainder - to_copy].~T();
					}
				}

				remainder -= to_copy; //subtract what was copied from remainder
				count += to_copy; //add what was copied to the count

				if (remainder == 0) {
					//entire remainder copied, let go of remainder page
					page_pool->free_page(remainder_page_id);
					remainder_page = nullptr;
				}
			}

			if (remainder > 0) {
				//there is still remainder, append it
				uint32_t page_count = _get_pages_in_use();
				uint32_t new_page_count = page_count + 1;

				if (unlikely(new_page_count > max_pages_used)) {
					_grow_page_array(); //keep out of inline
				}

				page_data[page_count] = remainder_page;
				page_ids[page_count] = remainder_page_id;

				count += remainder;
			}
		}
	}

	_FORCE_INLINE_ uint64_t size() const {
		return count;
	}

	void set_page_pool(PagedArrayPool<T> *p_page_pool) {
		ERR_FAIL_COND(max_pages_used > 0); // Safety check.

		page_pool = p_page_pool;
		page_size_mask = page_pool->get_page_size_mask();
		page_size_shift = page_pool->get_page_size_shift();
	}

	~PagedArray() {
		reset();
	}
};

#endif // PAGED_ARRAY_H