gplib/include/gp/system/task_queue.hpp

#pragma once

#include "gp/system/process_data.hpp"

#include <atomic>
#include <new>

// TODO: noexcept everything

namespace gp {
	namespace system {
		struct task_queue{
			struct node{
				alignas(gp_config::limits::hardware_constructive_interference_size) 
					std::atomic_bool is_locked;
				gp::system::process_data* value;
				alignas(gp_config::limits::hardware_constructive_interference_size) 
					std::atomic<struct node*> next;

				node() noexcept
				{
					is_locked = false;
					value = nullptr;
					next = nullptr;
				}

				node(node&& v) noexcept
				{
					v.try_acquire();
					is_locked = false;
					value = gp::move(v.value);
					next = v.next.load();
				}

				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*>;

			task_queue() noexcept
			: start{nullptr}
			, end{nullptr}
			{}

			node_ptr_rep start;
			node_ptr_rep end;

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

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

			// ONLY PUSH ACQUIRED NODES, 
			// RELEASE WHEN NO LONGER IN USE
			bool try_push(node_ptr node) noexcept {
				node->next.store(nullptr);
				auto ed = end.load();
				if(ed) {
					if(ed->try_acquire()) {
						auto old_ed = ed;
						node->next.store(ed);

						if(end.compare_exchange_strong(ed, node)) {
							node->release();
							old_ed->release();
							return true;
						} else {
							node->release();
							old_ed->release();
							return false;
						}
					} else return false;
				} else {
					if(end.compare_exchange_strong(ed, node)) {
						start.store(node);
						node->release();
						return true;
					} else {
						return false;
					}
				}
			}
		};
	}
}