#include <vector>
template<typename Tp>
class FixedAllocator {
  PageAllocator page_allocator_;
  std::vector<Tp*> freemem_, usedmem_;
  const std::uint64_t pages;

 public:
  explicit FixedAllocator(std::uint64_t page_size) :
  page_allocator_(page_size * sizeof(Tp)),
  freemem_(),  usedmem_(), pages(page_size){
      Tp* p = reinterpret_cast<Tp*>(page_allocator_.Allocate());
      freemem_.push_back(p);
      for (std::uint64_t i = 0; i < page_size - 1; i++) {
          p++;
          freemem_.push_back(p);
      }
  }

  Tp* Allocate() {
      Tp* p;
      if (freemem_.empty()) {
          Tp* pp = reinterpret_cast<Tp*>(page_allocator_.Allocate());
          freemem_.push_back(pp);
          for (std::uint64_t i = 0; i < pages - 1; i++) {
              pp++;
              freemem_.push_back(pp);
          }
      }

      p = freemem_[freemem_.size() - 1];
      freemem_.pop_back();
      usedmem_.push_back(p);
      return p;
  }

  void Deallocate(Tp* p) {
      freemem_.push_back(p);
      for (std::uint64_t i = 0; i < usedmem_.size(); i++) {
          if (usedmem_[i] == p) {
              usedmem_.erase(usedmem_.begin() + i);
          }
      }
  }

  const PageAllocator& InnerAllocator() const noexcept {
      return page_allocator_;
  }
};