/*
 * InspIRCd -- Internet Relay Chat Daemon
 *
 *   Copyright (C) 2018-2023 Sadie Powell <sadie@witchery.services>
 *   Copyright (C) 2015, 2018 Attila Molnar <attilamolnar@hush.com>
 *
 * This file is part of InspIRCd.  InspIRCd is free software: you can
 * redistribute it and/or modify it under the terms of the GNU General Public
 * License as published by the Free Software Foundation, version 2.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */


#pragma once

namespace Events
{
	class ModuleEventListener;
	class ModuleEventProvider;
}

/** Provider of one or more cross-module events.
 * Modules who wish to provide events for other modules create instances of this class and use
 * one of the macros below to fire the event, passing the instance of the event provider class
 * to the macro.
 * Event providers are identified using a unique identifier string.
 */
class Events::ModuleEventProvider
	: public Service::SimpleProvider
	, private dynamic_reference_base::CaptureHook
{
private:
	/** Determines whether a module is dying or has died.
	 * @param weakmod The module to check.
	 * @param modreq Whether a module is required.
	 */
	inline static bool IsModuleDead(const WeakModulePtr& weakmod, bool modreq)
	{
		if (!insp::empty_ptr(weakmod))
		{
			// We have a module. Is it dead?
			auto mod = weakmod.lock();
			if (mod)
				return mod->dying; // Module might be dying?

			return true; //Module is dead.
		}

		return modreq; // We never had a module.
	}

public:
	struct Comp final
	{
		bool operator()(ModuleEventListener* lhs, ModuleEventListener* rhs) const;
	};

	struct ElementComp final
	{
		bool operator()(ModuleEventListener* lhs, ModuleEventListener* rhs) const;
	};

	using SubscriberList = insp::flat_multiset<ModuleEventListener*, Comp, ElementComp>;

	/** Constructor
	 * @param mod Module providing the event(s)
	 * @param eventid Identifier of the event or event group provided, must be unique
	 */
	ModuleEventProvider(const WeakModulePtr& mod, const std::string& eventid)
		: Service::SimpleProvider(mod, "Events::ModuleEventProvider", eventid)
		, prov(mod, this->service_type, this->service_name)
	{
		prov.SetCaptureHook(this);
	}

	/** Retrieves the module which created this listener. */
	const auto& GetModule() const { return prov.creator; }

	/** Get list of objects subscribed to this event
	 * @return List of subscribed objects
	 */
	const SubscriberList& GetSubscribers() const { return prov->subscribers; }

	/** Subscribes a listener to this event.
	 * @param subscriber The listener to subscribe.
	 */
	void Subscribe(ModuleEventListener* subscriber)
	{
		subscribers.insert(subscriber);
		OnSubscribe(subscriber);
	}

	/** Unsubscribes a listener from this event.
	 * @param subscriber The listener to unsubscribe.
	 */
	void Unsubscribe(ModuleEventListener* subscriber)
	{
		subscribers.erase(subscriber);
		OnUnsubscribe(subscriber);
	}

	/** Run the given hook provided by a module. */
	template<typename Class, typename... FunArgs, typename... FwdArgs>
	inline void Call(void (Class::*function)(FunArgs...), FwdArgs&&... args) const;

	/**
	 * Run the given hook provided by a module until some module returns MOD_RES_ALLOW or
	 * MOD_RES_DENY. If no module does that the result is set to MOD_RES_PASSTHRU.
	 */
	template<typename Class, typename... FunArgs, typename... FwdArgs>
	inline ModResult FirstResult(ModResult (Class::*function)(FunArgs...), FwdArgs&&... args) const;

private:
	void OnCapture() override
	{
		// If someone else holds the list from now on, clear mine. See below for more info.
		if (*prov != this)
			subscribers.clear();
	}

	/** Called when a listener subscribes to this event.
	 * @param subscriber The listener which subscribed.
	 */
	virtual void OnSubscribe(ModuleEventListener* subscriber) { }

	/** Called when a listener unsubscribes from this event.
	 * @param subscriber The listener which unsubscribed.
	 */
	virtual void OnUnsubscribe(ModuleEventListener* subscriber) { }

	/** Reference to the active provider for this event. In case multiple event providers
	 * exist for the same event, only one of them contains the list of subscribers.
	 * To handle the case when we are not the ones with the list, we get it from the provider
	 * where the dynref points to.
	 */
	dynamic_reference_nocheck<ModuleEventProvider> prov;

	/** List of objects subscribed to the event(s) provided by us, or empty if multiple providers
	 * exist with the same name and we are not the ones holding the list.
	 */
	SubscriberList subscribers;
};

/** Base class for abstract classes describing cross-module events.
 * Subscribers should NOT inherit directly from this class.
 */
class Events::ModuleEventListener
	: private dynamic_reference_base::CaptureHook
{
private:
	/** Reference to the provider, can be NULL if none of the provider modules are loaded
	 */
	dynamic_reference_nocheck<ModuleEventProvider> prov;

	const unsigned int eventpriority;

	/** Called by the dynref when the event provider becomes available
	 */
	void OnCapture() override
	{
		prov->Subscribe(this);
	}

protected:
	/** Constructor
	 * @param mod Module subscribing
	 * @param eventid Identifier of the event to subscribe to
	 * @param eventprio The priority to give this event listener
	 */
	ModuleEventListener(const WeakModulePtr& mod, const std::string& eventid, unsigned int eventprio)
		: prov(mod, "Events::ModuleEventProvider", eventid)
		, eventpriority(eventprio)
	{
		prov.SetCaptureHook(this);
		// If the dynamic_reference resolved at construction our capture handler wasn't called
		if (prov)
			ModuleEventListener::OnCapture();
	}

public:
	static constexpr unsigned int DefaultPriority = 100;

	~ModuleEventListener()
	{
		if (prov)
			prov->Unsubscribe(this);
	}

	/** Retrieves the module which created this listener. */
	const auto& GetModule() const { return prov.creator; }

	/** Retrieves the priority of this event. */
	unsigned int GetPriority() const { return eventpriority; }
};

inline bool Events::ModuleEventProvider::Comp::operator()(Events::ModuleEventListener* lhs, Events::ModuleEventListener* rhs) const
{
	return (lhs->GetPriority() < rhs->GetPriority());
}

inline bool Events::ModuleEventProvider::ElementComp::operator()(Events::ModuleEventListener* lhs, Events::ModuleEventListener* rhs) const
{
	if (lhs->GetPriority() < rhs->GetPriority())
		return true;
	if (lhs->GetPriority() > rhs->GetPriority())
		return false;
	return std::less<>()(lhs, rhs);
}

template<typename Class, typename... FunArgs, typename... FwdArgs>
inline void Events::ModuleEventProvider::Call(void (Class::*function)(FunArgs...), FwdArgs&&... args) const
{
	if (IsModuleDead(GetModule(), false))
		return;

	for (auto* subscriber : GetSubscribers())
	{
		if (IsModuleDead(subscriber->GetModule(), true))
			continue;

		Class* klass = static_cast<Class*>(subscriber);
		(klass->*function)(std::forward<FwdArgs>(args)...);
	}
}

template<typename Class, typename... FunArgs, typename... FwdArgs>
inline ModResult Events::ModuleEventProvider::FirstResult(ModResult (Class::*function)(FunArgs...), FwdArgs&&... args) const
{
	if (IsModuleDead(GetModule(), false))
		return MOD_RES_PASSTHRU;

	ModResult result;
	for (auto* subscriber : GetSubscribers())
	{
		if (IsModuleDead(subscriber->GetModule(), true))
			continue;

		Class* klass = static_cast<Class*>(subscriber);
		result = (klass->*function)(std::forward<FwdArgs>(args)...);
		if (result != MOD_RES_PASSTHRU)
			break;
	}
	return result;
}