gplib/include/gp/vfs/runqueue.hpp

#pragma once

#include "gp/vfs/process_data.hpp"

#include <atomic>

class runqueue{

	struct node{
		std::atomic_bool is_locked;
		gp::process_data* value;
		std::atomic<struct node*> next;

		bool try_acquire() noexcept {
			bool expected = false;
			return !(is_locked.compare_exchange_strong(expected, true));
		}

		void release() noexcept {
			is_locked.store(false);
		}
	};

	using node_ptr = struct node*;
	using node_ptr_rep = std::atomic<struct node*>;

	struct topic_list{
		node_ptr_rep start;
		node_ptr_rep end;

		// NODES ARE ACQUIRED ON POP
		node_ptr try_pop() {
			auto ptr = start.load();
			if(!ptr) return nullptr;

			if(ptr->try_acquire()) {
				auto replace = ptr->next.load();
				auto expected = ptr;
				if(end.load() == ptr) {
					end.store(nullptr);
				}
				if(start.compare_exchange_strong(expected, replace)) {
					return ptr;
				} else {
					return nullptr;
				}
			} else {
				return nullptr;
			}
		}

		// ONLY PUSH ACQUIRED NODES
		bool try_push(node_ptr node) {
			auto ed = end.load();
			if(ed) {
				if(ed->try_acquire()) {
					node->next.store(ed);
					if(end.compare_exchange_strong(ed, node)) {
						ed->release();
						node->release();
						return true;
					} else {
						ed->release();
						node->release();
						return false;
					}
				} else return false;
			} else {
				if(end.compare_exchange_strong(ed, node)) {
					start.store(node);
					node->release();
					return true;
				} else {
					return false;
				}
			}
		}
	};

	topic_list running;
	topic_list waiting;
};