#pragma once
#include <cstddef>
#include <type_traits>
class Object_Pool {
#if defined(_DEBUG)
    typedef struct free_link_node {
        free_link_node* next;
    }* free_link;
#endif
    typedef struct chunk_link_node {
        union {
            chunk_link_node* next;
            char padding[sizeof(std::max_align_t)];
        };
        char data[0];
    }* chunk_link;
public:
    Object_Pool(const Object_Pool&) = delete;
    void operator=(const Object_Pool&) = delete;
    Object_Pool(size_t element_size, size_t element_count) : first_(nullptr), chunk_(nullptr),
                                                             element_size_(element_size),
                                                             element_count_(element_count) {
        release(allocate_from_process_heap()); // preallocate 1 chunk
    }
    Object_Pool(size_t element_size, size_t element_count, std::false_type /*preallocate?*/) : first_(nullptr),
        chunk_(nullptr), element_size_(element_size), element_count_(element_count) {}
    virtual ~Object_Pool() { this->purge(); }
    void purge() {
        if (this->chunk_ == nullptr) return;
        chunk_link_node *p, **q = &this->chunk_;
        while ((p = *q) != nullptr) {
            *q = p->next;
            delete[] (uint8_t*)(p);
        }
        first_ = nullptr;
    }
    void cleanup() {
        if (this->chunk_ == nullptr) return;
        chunk_link_node* chunk = this->chunk_;
        void* last = this->tidy_chunk(chunk);
        while ((chunk = chunk->next) != nullptr) {
            nextof(last) = firstof(chunk);
            last = this->tidy_chunk(chunk);
        }
        nextof(last) = nullptr;
        first_ = firstof(this->chunk_);
    }
    void* get() { return (first_ != nullptr) ? allocate_from_chunk(first_) : allocate_from_process_heap(); }
    void release(void* Ptr) {
        nextof(Ptr) = first_;
        first_ = Ptr;
    }
private:
    void* allocate_from_chunk(void* current) {
        first_ = nextof(current);
        return current;
    }
    static void* firstof(chunk_link chunk) { return chunk->data; }
    static void*& nextof(void* const ptr) { return *(static_cast<void**>(ptr)); }
    void* allocate_from_process_heap() {
        auto new_chunk = (chunk_link)new uint8_t[sizeof(chunk_link_node) + element_size_ * element_count_];
#ifdef _DEBUG
        ::memset(new_chunk, 0x00, sizeof(chunk_link_node));
#endif
        nextof(tidy_chunk(new_chunk)) = nullptr;
        // link the new_chunk
        new_chunk->next = this->chunk_;
        this->chunk_ = new_chunk;
        // allocate 1 object
        return allocate_from_chunk(firstof(new_chunk));
    }
    void* tidy_chunk(chunk_link chunk) const {
        char* last = chunk->data + (element_count_ - 1) * element_size_;
        for (char* ptr = chunk->data; ptr < last; ptr += element_size_) nextof(ptr) = (ptr + element_size_);
        return last;
    }
private:
    void* first_; // link to free head
    chunk_link chunk_; // chunk link
    const size_t element_size_;
    const size_t element_count_;
};