#ifndef MEMGRAPH_MEMORY_HP_HPP
#define MEMGRAPH_MEMORY_HP_HPP

#include <atomic>
#include <cassert>
#include <unistd.h>

namespace memory
{

constexpr const size_t HP_SIZE = 128;

class HP
{
public:
    // this object can't be copied or moved
    HP(HP&) = delete;
    HP(HP&&) = delete;

    // grabs a singleton instance
    static HP& get()
    {
        static HP hp;
        return hp;
    }

    class reference
    {
        friend class HP;
    public:
        reference(reference&) = delete;

        // this type shouldn't be copyable to avoid calling its destructor
        // multiple times, but should be movable
        reference(reference&& other)
        {
            this->idx = other.idx;

            // set the index to a negative number to indicate that this
            // index has been moved and that you should not free its
            // hazard pointer
            other.idx = -1;
        }
        // hazard pointer is cleared once reference goes out of scope
        ~reference()
        {
            // check if this reference was moved during its lifetime
            if(idx < 0)
                return;

            auto& hp = HP::get();
            hp.clear(*this);
        }

    private:
        reference(int64_t idx) : idx(idx) {}
        int64_t idx;
    };

    friend class reference;

    template <class T>
    reference insert(T* ptr)
    {
        auto p = reinterpret_cast<uintptr_t>(ptr);

        while(true)
        {
            // try to find a free spot in the hazard pointer list
            for(size_t i = 0; i < HP_SIZE; ++i)
            {
                auto hazard = ptr_list[i].load();

                // if this spot isn't free, continue searching
                if(hazard != 0)
                    continue;

                // try to take this spot, if we fail, then another thread has
                // just taken it. continue searching for a new one
                if(!ptr_list[i].compare_exchange_strong(hazard, p))
                    continue;

                // found a free spot! return a reference to this spot so it
                // can be cleared later
                return reference(i);
            }

            // we didn't find any free spots, sleep for a while and try again
            // from the beginning, some other thread might have freed a spot
            // while we were traversing the lsit.
            usleep(250);
        }
    }

private:
    HP()
    {
        for(size_t i = 0; i < HP_SIZE; ++i)
            ptr_list[i].store(0);
    }

    void clear(reference& ref)
    {
        ptr_list[ref.idx].store(0);
    }

    std::atomic<uintptr_t> ptr_list[HP_SIZE];
};

}

#endif