BLT/include/blt/std/allocator.h

/*
 *  <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
             *
             * ALLOCATORS RETURN UNINIT STORAGE!! THIS HAS BEEN DISABLED.
             */
            inline void allocate_in_block(pointer, size_t)
            {
//                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) noexcept
            {
                blocks = move.blocks;
            }
            
            area_allocator& operator=(const area_allocator& copy) = delete;
            
            area_allocator& operator=(area_allocator&& move) noexcept
            {
                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
            {
                if (p == nullptr)
                    return;
//                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();
            }
            
            [[nodiscard]] 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