feat: add System and SystemRegistry classes for managing systems in the game engine

Signed-off-by: szdytom <szdytom@qq.com>

core/include/istd_core/system.h

@@ -0,0 +1,50 @@
+#ifndef ISTD_CORE_SYSTEM_H
+#define ISTD_CORE_SYSTEM_H
+
+#include "istd_core/world.h"
+#include <cstdint>
+#include <string_view>
+#include <tuple>
+#include <vector>
+
+namespace istd {
+
+struct System {
+	virtual void tick(World &world) const = 0;
+	virtual std::string_view name() const noexcept = 0; // for debugging
+
+	// No virtual destructor: static lifetime and no member variables is the
+	// intended use case
+
+	struct Precedence {
+		// Smaller value means higher precedence or earlier execution
+		enum {
+			Highest = 0,
+			ResetVelocity,
+			DeviceAccumulateVelocity,
+			UpdateKinematics,
+		};
+	};
+};
+
+class SystemRegistry {
+public:
+	static SystemRegistry &instance() noexcept;
+
+	void register_system(
+		std::uint32_t precedence, const System *system
+	) noexcept;
+
+	void tick(World &world) const noexcept;
+
+	struct Registar {
+		Registar(std::uint32_t precedence, const System *system);
+	};
+
+private:
+	std::vector<std::tuple<std::uint32_t, const System *>> systems_;
+};
+
+} // namespace istd
+
+#endif

core/src/system.cpp

@@ -0,0 +1,38 @@
+#include "istd_core/system.h"
+#include <algorithm>
+
+namespace istd {
+
+SystemRegistry &SystemRegistry::instance() noexcept {
+	static SystemRegistry registry;
+	return registry;
+}
+
+void SystemRegistry::register_system(
+	std::uint32_t precedence, const System *system
+) noexcept {
+	auto cmp = [](const std::tuple<std::uint32_t, const System *> &a,
+	              std::uint32_t b) {
+		return std::get<0>(a) < b;
+	};
+
+	auto it = std::lower_bound(
+		systems_.begin(), systems_.end(), precedence, cmp
+	);
+
+	systems_.insert(it, std::make_tuple(precedence, system));
+}
+
+void SystemRegistry::tick(World &world) const noexcept {
+	for (const auto &[_, system] : systems_) {
+		system->tick(world);
+	}
+}
+
+SystemRegistry::Registar::Registar(
+	std::uint32_t precedence, const System *system
+) {
+	SystemRegistry::instance().register_system(precedence, system);
+}
+
+} // namespace istd