cowdata.h

/**************************************************************************/
/*  cowdata.h                                                             */
/**************************************************************************/
/*                         This file is part of:                          */
/*                             GODOT ENGINE                               */
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/* Copyright (c) 2014-present Godot Engine contributors (see AUTHORS.md). */
/* Copyright (c) 2007-2014 Juan Linietsky, Ariel Manzur.                  */
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#pragma once

#include "core/error/error_macros.h"
#include "core/os/memory.h"
#include "core/templates/safe_refcount.h"
#include "core/templates/span.h"

#include <initializer_list>
#include <type_traits>

static_assert(std::is_trivially_destructible_v<std::atomic<uint64_t>>);

GODOT_GCC_WARNING_PUSH
GODOT_GCC_WARNING_IGNORE("-Wplacement-new") // Silence a false positive warning (see GH-52119).
GODOT_GCC_WARNING_IGNORE("-Wmaybe-uninitialized") // False positive raised when using constexpr.

template <typename T>
class CowData {
public:
	typedef int64_t Size;
	typedef uint64_t USize;
	static constexpr USize MAX_INT = INT64_MAX;

private:
	// Alignment:  ↓ max_align_t           ↓ USize          ↓ max_align_t
	//             ┌────────────────────┬──┬─────────────┬──┬───────────...
	//             │ SafeNumeric<USize> │░░│ USize       │░░│ T[]
	//             │ ref. count         │░░│ data size   │░░│ data
	//             └────────────────────┴──┴─────────────┴──┴───────────...
	// Offset:     ↑ REF_COUNT_OFFSET      ↑ SIZE_OFFSET    ↑ DATA_OFFSET

	static constexpr size_t REF_COUNT_OFFSET = 0;
	static constexpr size_t SIZE_OFFSET = ((REF_COUNT_OFFSET + sizeof(SafeNumeric<USize>)) % alignof(USize) == 0) ? (REF_COUNT_OFFSET + sizeof(SafeNumeric<USize>)) : ((REF_COUNT_OFFSET + sizeof(SafeNumeric<USize>)) + alignof(USize) - ((REF_COUNT_OFFSET + sizeof(SafeNumeric<USize>)) % alignof(USize)));
	static constexpr size_t DATA_OFFSET = ((SIZE_OFFSET + sizeof(USize)) % alignof(max_align_t) == 0) ? (SIZE_OFFSET + sizeof(USize)) : ((SIZE_OFFSET + sizeof(USize)) + alignof(max_align_t) - ((SIZE_OFFSET + sizeof(USize)) % alignof(max_align_t)));

	mutable T *_ptr = nullptr;

	// internal helpers

	static _FORCE_INLINE_ T *_get_data_ptr(uint8_t *p_ptr) {
		return (T *)(p_ptr + DATA_OFFSET);
	}

	_FORCE_INLINE_ SafeNumeric<USize> *_get_refcount() const {
		if (!_ptr) {
			return nullptr;
		}

		return (SafeNumeric<USize> *)((uint8_t *)_ptr - DATA_OFFSET + REF_COUNT_OFFSET);
	}

	_FORCE_INLINE_ USize *_get_size() const {
		if (!_ptr) {
			return nullptr;
		}

		return (USize *)((uint8_t *)_ptr - DATA_OFFSET + SIZE_OFFSET);
	}

	_FORCE_INLINE_ static USize _get_alloc_size(USize p_elements) {
		return next_power_of_2(p_elements * (USize)sizeof(T));
	}

	_FORCE_INLINE_ static bool _get_alloc_size_checked(USize p_elements, USize *out) {
		if (unlikely(p_elements == 0)) {
			*out = 0;
			return true;
		}
#if defined(__GNUC__) && defined(IS_32_BIT)
		USize o;
		USize p;
		if (__builtin_mul_overflow(p_elements, sizeof(T), &o)) {
			*out = 0;
			return false;
		}
		*out = next_power_of_2(o);
		if (__builtin_add_overflow(o, static_cast<USize>(32), &p)) {
			return false; // No longer allocated here.
		}
#else
		// Speed is more important than correctness here, do the operations unchecked
		// and hope for the best.
		*out = _get_alloc_size(p_elements);
#endif
		return *out;
	}

	// Decrements the reference count. Deallocates the backing buffer if needed.
	// After this function, _ptr is guaranteed to be NULL.
	void _unref();
	void _ref(const CowData *p_from);
	void _ref(const CowData &p_from);

	// Ensures that the backing buffer is at least p_size wide, and that this CowData instance is
	// the only reference to it. The buffer is populated with as many element copies from the old
	// array as possible.
	// It is the responsibility of the caller to populate newly allocated space up to p_size.
	Error _fork_allocate(USize p_size);
	Error _copy_on_write() { return _fork_allocate(size()); }

	// Allocates a backing array of the given capacity. The reference count is initialized to 1.
	// It is the responsibility of the caller to populate the array and the new size property.
	Error _alloc(USize p_alloc_size);

	// Re-allocates the backing array to the given capacity. The reference count is initialized to 1.
	// It is the responsibility of the caller to populate the array and the new size property.
	// The caller must also make sure there are no other references to the data, as pointers may
	// be invalidated.
	Error _realloc(USize p_alloc_size);

public:
	void operator=(const CowData<T> &p_from) { _ref(p_from); }
	void operator=(CowData<T> &&p_from) {
		if (_ptr == p_from._ptr) {
			return;
		}

		_unref();
		_ptr = p_from._ptr;
		p_from._ptr = nullptr;
	}

	_FORCE_INLINE_ T *ptrw() {
		_copy_on_write();
		return _ptr;
	}

	_FORCE_INLINE_ const T *ptr() const {
		return _ptr;
	}

	_FORCE_INLINE_ Size size() const {
		USize *size = (USize *)_get_size();
		if (size) {
			return *size;
		} else {
			return 0;
		}
	}

	_FORCE_INLINE_ void clear() { _unref(); }
	_FORCE_INLINE_ bool is_empty() const { return _ptr == nullptr; }

	_FORCE_INLINE_ void set(Size p_index, const T &p_elem) {
		ERR_FAIL_INDEX(p_index, size());
		_copy_on_write();
		_ptr[p_index] = p_elem;
	}

	_FORCE_INLINE_ T &get_m(Size p_index) {
		CRASH_BAD_INDEX(p_index, size());
		_copy_on_write();
		return _ptr[p_index];
	}

	_FORCE_INLINE_ const T &get(Size p_index) const {
		CRASH_BAD_INDEX(p_index, size());

		return _ptr[p_index];
	}

	template <bool p_initialize = true>
	Error resize(Size p_size);

	_FORCE_INLINE_ void remove_at(Size p_index) {
		ERR_FAIL_INDEX(p_index, size());
		T *p = ptrw();
		Size len = size();
		for (Size i = p_index; i < len - 1; i++) {
			p[i] = std::move(p[i + 1]);
		}

		resize(len - 1);
	}

	Error insert(Size p_pos, const T &p_val) {
		Size new_size = size() + 1;
		ERR_FAIL_INDEX_V(p_pos, new_size, ERR_INVALID_PARAMETER);
		Error err = resize(new_size);
		ERR_FAIL_COND_V(err, err);
		T *p = ptrw();
		for (Size i = new_size - 1; i > p_pos; i--) {
			p[i] = std::move(p[i - 1]);
		}
		p[p_pos] = p_val;

		return OK;
	}

	_FORCE_INLINE_ operator Span<T>() const { return Span<T>(ptr(), size()); }
	_FORCE_INLINE_ Span<T> span() const { return operator Span<T>(); }

	_FORCE_INLINE_ CowData() {}
	_FORCE_INLINE_ ~CowData() { _unref(); }
	_FORCE_INLINE_ CowData(std::initializer_list<T> p_init);
	_FORCE_INLINE_ CowData(const CowData<T> &p_from) { _ref(p_from); }
	_FORCE_INLINE_ CowData(CowData<T> &&p_from) {
		_ptr = p_from._ptr;
		p_from._ptr = nullptr;
	}
};

template <typename T>
void CowData<T>::_unref() {
	if (!_ptr) {
		return;
	}

	SafeNumeric<USize> *refc = _get_refcount();
	if (refc->decrement() > 0) {
		// Data is still in use elsewhere.
		_ptr = nullptr;
		return;
	}
	// We had the only reference; destroy the data.

	// First, invalidate our own reference.
	// NOTE: It is required to do so immediately because it must not be observable outside of this
	//       function after refcount has already been reduced to 0.
	// WARNING: It must be done before calling the destructors, because one of them may otherwise
	//          observe it through a reference to us. In this case, it may try to access the buffer,
	//          which is illegal after some of the elements in it have already been destructed, and
	//          may lead to a segmentation fault.
	USize current_size = *_get_size();
	T *prev_ptr = _ptr;
	_ptr = nullptr;

	if constexpr (!std::is_trivially_destructible_v<T>) {
		for (USize i = 0; i < current_size; ++i) {
			prev_ptr[i].~T();
		}
	}

	// Free memory.
	Memory::free_static((uint8_t *)prev_ptr - DATA_OFFSET, false);

#ifdef DEBUG_ENABLED
	// If any destructors access us through pointers, it is a bug.
	// We can't really test for that, but we can at least check no items have been added.
	ERR_FAIL_COND_MSG(_ptr != nullptr, "Internal bug, please report: CowData was modified during destruction.");
#endif
}

template <typename T>
Error CowData<T>::_fork_allocate(USize p_size) {
	if (p_size == 0) {
		// Wants to clean up.
		_unref();
		return OK;
	}

	USize alloc_size;
	ERR_FAIL_COND_V(!_get_alloc_size_checked(p_size, &alloc_size), ERR_OUT_OF_MEMORY);

	const USize prev_size = size();

	if (!_ptr) {
		// We had no data before; just allocate a new array.
		const Error error = _alloc(alloc_size);
		if (error) {
			return error;
		}
	} else if (_get_refcount()->get() == 1) {
		// Resize in-place.
		// NOTE: This case is not just an optimization, but required, as some callers depend on
		//       `_copy_on_write()` calls not changing the pointer after the first fork
		//       (e.g. mutable iterators).
		if (p_size == prev_size) {
			// We can shortcut here; we don't need to do anything.
			return OK;
		}

		// Destroy extraneous elements.
		if constexpr (!std::is_trivially_destructible_v<T>) {
			for (USize i = prev_size; i > p_size; i--) {
				_ptr[i - 1].~T();
			}
		}

		if (alloc_size != _get_alloc_size(prev_size)) {
			const Error error = _realloc(alloc_size);
			if (error) {
				// Out of memory; the current array is still valid though.
				return error;
			}
		}
	} else {
		// Resize by forking.

		// Create a temporary CowData to hold ownership over our _ptr.
		// It will be used to copy elements from the old buffer over to our new buffer.
		// At the end of the block, it will be automatically destructed by going out of scope.
		const CowData prev_data;
		prev_data._ptr = _ptr;
		_ptr = nullptr;

		const Error error = _alloc(alloc_size);
		if (error) {
			// On failure to allocate, just give up the old data and return.
			// We could recover our old pointer from prev_data, but by just dropping our data, we
			// consciously invite early failure for the case that the caller does not handle this
			// case gracefully.
			return error;
		}

		// Copy over elements.
		const USize copied_element_count = MIN(prev_size, p_size);
		if (copied_element_count > 0) {
			if constexpr (std::is_trivially_copyable_v<T>) {
				memcpy((uint8_t *)_ptr, (uint8_t *)prev_data._ptr, copied_element_count * sizeof(T));
			} else {
				for (USize i = 0; i < copied_element_count; i++) {
					memnew_placement(&_ptr[i], T(prev_data._ptr[i]));
				}
			}
		}
	}

	// Set our new size.
	*_get_size() = p_size;

	return OK;
}

template <typename T>
template <bool p_initialize>
Error CowData<T>::resize(Size p_size) {
	ERR_FAIL_COND_V(p_size < 0, ERR_INVALID_PARAMETER);

	const Size prev_size = size();
	if (p_size == prev_size) {
		return OK;
	}

	const Error error = _fork_allocate(p_size);
	if (error) {
		return error;
	}

	if constexpr (p_initialize) {
		if (p_size > prev_size) {
			memnew_arr_placement(_ptr + prev_size, p_size - prev_size);
		}
	} else {
		static_assert(std::is_trivially_destructible_v<T>);
	}

	return OK;
}

template <typename T>
Error CowData<T>::_alloc(USize p_alloc_size) {
	uint8_t *mem_new = (uint8_t *)Memory::alloc_static(p_alloc_size + DATA_OFFSET, false);
	ERR_FAIL_NULL_V(mem_new, ERR_OUT_OF_MEMORY);

	_ptr = _get_data_ptr(mem_new);

	// If we alloc, we're guaranteed to be the only reference.
	new (_get_refcount()) SafeNumeric<USize>(1);

	return OK;
}

template <typename T>
Error CowData<T>::_realloc(USize p_alloc_size) {
	uint8_t *mem_new = (uint8_t *)Memory::realloc_static(((uint8_t *)_ptr) - DATA_OFFSET, p_alloc_size + DATA_OFFSET, false);
	ERR_FAIL_NULL_V(mem_new, ERR_OUT_OF_MEMORY);

	_ptr = _get_data_ptr(mem_new);

	// If we realloc, we're guaranteed to be the only reference.
	// So the reference was 1 and was copied to be 1 again.
	DEV_ASSERT(_get_refcount()->get() == 1);

	return OK;
}

template <typename T>
void CowData<T>::_ref(const CowData *p_from) {
	_ref(*p_from);
}

template <typename T>
void CowData<T>::_ref(const CowData &p_from) {
	if (_ptr == p_from._ptr) {
		return; // self assign, do nothing.
	}

	_unref(); // Resets _ptr to nullptr.

	if (!p_from._ptr) {
		return; //nothing to do
	}

	if (p_from._get_refcount()->conditional_increment() > 0) { // could reference
		_ptr = p_from._ptr;
	}
}

template <typename T>
CowData<T>::CowData(std::initializer_list<T> p_init) {
	Error err = resize(p_init.size());
	if (err != OK) {
		return;
	}

	Size i = 0;
	for (const T &element : p_init) {
		set(i++, element);
	}
}

GODOT_GCC_WARNING_POP

// Zero-constructing CowData initializes _ptr to nullptr (and thus empty).
template <typename T>
struct is_zero_constructible<CowData<T>> : std::true_type {};