/*
* <Short Description>
* Copyright (C) 2023 Brett Terpstra
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
#ifndef BLT_ALLOCATOR_H
#include <optional>
#include <limits>
#include <vector>
#include <blt/std/utility.h>
#include <stdexcept>
namespace blt
{
template<typename T, size_t BLOCK_SIZE = 8192>
class area_allocator
{
public:
using type = T;
using value_type = type;
using pointer = type*;
using const_pointer = const type*;
using void_pointer = void*;
using const_void_pointer = const void*;
using reference = value_type&;
using const_reference = const value_type&;
using size_type = size_t;
using difference_type = size_t;
using propagate_on_container_move_assignment = std::false_type;
template<class U>
struct rebind
{
typedef std::allocator<U> other;
};
private:
/**
* Stores a view to a region of memory that has been deallocated
* This is a non-owning reference to the memory block
*
* pointer p is the pointer to the beginning of the block of memory
* size_t n is the number of elements that this block can hold
*/
struct pointer_view
{
pointer p;
size_t n;
};
/**
* Stores the actual data for allocated blocks. Since we would like to be able to allocate an arbitrary number of items
* we need a way of storing that data. The block storage holds an owning pointer to a region of memory with used elements
* Only up to used has to have their destructors called, which should be handled by the deallocate function
* it is UB to not deallocate memory allocated by this allocator
*
* an internal vector is used to store the regions of memory which have been deallocated. the allocate function will search for
* free blocks with sufficient size in order to maximize memory usage. In the future more advanced methods should be used
* for both faster access to deallocated blocks of sufficient size and to ensure coherent memory.
*/
struct block_storage
{
pointer data;
size_t used = 0;
// TODO: b-tree?
std::vector<pointer_view> unallocated_blocks;
};
/**
* Stores an index to a pointer_view along with the amount of memory leftover after the allocation
* it also stores the block being allocated to in question. The new inserted leftover should start at old_ptr + size
*/
struct block_view
{
block_storage* blk;
size_t index;
size_t leftover;
block_view(block_storage* blk, size_t index, size_t leftover): blk(blk), index(index), leftover(leftover)
{}
};
/**
* Allocate a new block of memory and push it to the back of blocks.
*/
inline void allocate_block()
{
//BLT_INFO("Allocating a new block of size %d", BLOCK_SIZE);
auto* blk = new block_storage();
blk->data = static_cast<pointer>(malloc(sizeof(T) * BLOCK_SIZE));
blocks.push_back(blk);
}
/**
* Searches for a free block inside the block storage with sufficient space and returns an optional view to it
* The optional will be empty if no open block can be found.
*/
inline std::optional<block_view> search_for_block(block_storage* blk, size_t n)
{
for (auto kv : blt::enumerate(blk->unallocated_blocks))
{
if (kv.second.n >= n)
return block_view{blk, kv.first, kv.second.n - n};
}
return {};
}
/**
* removes the block of memory from the unallocated_blocks storage in the underlying block, inserting a new unallocated block if
* there was any leftover. Returns a pointer to the beginning of the new block.
*/
inline pointer swap_pop_resize_if(const block_view& view, size_t n)
{
pointer_view ptr = view.blk->unallocated_blocks[view.index];
std::iter_swap(view.blk->unallocated_blocks.begin() + view.index, view.blk->unallocated_blocks.end() - 1);
view.blk->unallocated_blocks.pop_back();
if (view.leftover > 0)
view.blk->unallocated_blocks.push_back({ptr.p + n, view.leftover});
return ptr.p;
}
/**
* Finds the next available unallocated block of memory, or empty if there is none which meet size requirements
*/
inline std::optional<pointer> find_available_block(size_t n)
{
for (auto* blk : blocks)
{
if (auto view = search_for_block(blk, n))
return swap_pop_resize_if(view.value(), n);
}
return {};
}
/**
* returns a pointer to a block of memory along with an offset into that pointer that the requested block can be found at
*/
inline std::pair<pointer, size_t> getBlock(size_t n)
{
if (auto blk = find_available_block(n))
return {blk.value(), 0};
if (blocks.back()->used + n > BLOCK_SIZE)
allocate_block();
auto ptr = std::pair<pointer, size_t>{blocks.back()->data, blocks.back()->used};
blocks.back()->used += n;
return ptr;
}
/**
* Calls the constructor on elements if they require construction, otherwise constructor will not be called and this function is useless
*/
inline void allocate_in_block(pointer begin, size_t n)
{
if constexpr (std::is_default_constructible_v<T> && !std::is_trivially_default_constructible_v<T>)
{
for (size_t i = 0; i < n; i++)
new(&begin[i]) T();
}
}
public:
area_allocator()
{
allocate_block();
}
area_allocator(const area_allocator& copy) = delete;
area_allocator(area_allocator&& move)
{
blocks = move.blocks;
}
area_allocator& operator=(const area_allocator& copy) = delete;
area_allocator& operator=(area_allocator&& move)
{
std::swap(move.blocks, blocks);
}
[[nodiscard]] pointer allocate(size_t n)
{
if (n > BLOCK_SIZE)
throw std::runtime_error("Requested allocation is too large!");
auto block_info = getBlock(n);
auto* ptr = &block_info.first[block_info.second];
// call constructors on the objects if they require it
allocate_in_block(ptr, n);
return ptr;
}
void deallocate(pointer p, size_t n) noexcept
{
for (size_t i = 0; i < n; i++)
p[i].~T();
for (auto*& blk : blocks)
{
if (p >= blk->data && p <= (blk->data + BLOCK_SIZE))
{
blk->unallocated_blocks.push_back({p, n});
break;
}
}
}
template<class U, class... Args>
inline void construct(U* p, Args&& ... args)
{
::new((void*) p) U(std::forward<Args>(args)...);
}
template<class U>
inline void destroy(U* p)
{
p->~U();
}
inline size_t max_size() const
{
return std::numeric_limits<size_t>::max();
}
inline const_pointer address(const value_type& val)
{
return std::addressof(val);
}
inline pointer address(value_type& val)
{
return std::addressof(val);
}
~area_allocator()
{
for (auto*& blk : blocks)
{
free(blk->data);
delete blk;
}
}
private:
std::vector<block_storage*> blocks;
};
}
#define BLT_ALLOCATOR_H
#endif //BLT_ALLOCATOR_H