#include <atomic>

template <typename T>
class lock_free_stack {
private:
  std::atomic<node*> to_be_deleted;
  static void delete_nodes(node* nodes)
  {
    while (nodes) {
      node* next = nodes->next;
      delete nodes;
      nodes = next;
    }
  }
  void try_reclaim(node* old_head)
  {
    if (threads_in_pop == 1) {
      node* nodes_to_delete = to_be_deleted.exchange(nullptr);
      if (!--threads_in_pop) {
        delete_nodes(nodes_to_delete);
      }
      else if (nodes_to_delete) {
        chain_pending_nodes(nodes_to_delete);
      }
      delete old_head;
    }
    else {
      chain_pending_node(old_head);
      --threads_in_pop;
    }
  }
  void chain_pending_nodes(node* nodes)
  {
    node* last = nodes;
    while (node* const next = last->next) {
      last = next;
    }
    chain_pending_nodes(nodes, last);
  }
  void chain_pending_nodes(node* first, node* last)
  {
    last->next = to_be_deleted;
    while (!to_be_deleted.compare_exchange_weak(last->next, first))
      ;
  }
  void chain_pending_node(node* n) { chain_pending_nodes(n, n); }
};