// Matrix Construct
//
// Copyright (C) Matrix Construct Developers, Authors & Contributors
// Copyright (C) 2016-2018 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.

#pragma once
#define HAVE_IRCD_M_VM_H

/// Matrix Virtual Machine
///
/// This is the processing unit for matrix events. The front-end interface to
/// m::vm consists of two parts: The query suite (observation) and the eval
/// (modification). The back-end interface consists of modules which supply
/// the features for the vm's logic. The m::vm operates primarily as a global
/// singleton and facilitates synchronization between multiple ircd::ctx's
/// using its interfaces.
///
/// The query suite tries to satisfy a query using the current state of the vm,
/// the database, and may even use the network. An idea of a basic pseudo-
/// query: was user X a member of room Y when event Z happened? Is X a member
/// of Y right now? etc.
///
/// The vm::eval is the primary input receptacle for events. The eval of an
/// event leads to its execution and will advance the global state of IRCd. That
/// advance may broadcast its effects to clients of IRCd, federation servers,
/// and alter the responses to future queries. Execution of an event is a
/// unique phenomenon: it only happens once. IRCd has thenceforth borne witness
/// to the event and will never execute it again unless it is forgotten by
/// erasing the db. Simulation options allow for evals without execution.
///
/// The back-end provides the business logic and features for the above user
/// interfaces.
///
namespace ircd::m::vm
{
	IRCD_M_EXCEPTION(m::error, VM_ERROR, http::INTERNAL_SERVER_ERROR);
	IRCD_M_EXCEPTION(VM_ERROR, VM_FAULT, http::BAD_REQUEST);

	enum fault :uint;
	struct front;
	struct eval;
	struct opts;

	using closure = std::function<void (const event &)>;
	using closure_bool = std::function<bool (const event &)>;
	using dbs::where;
	using dbs::query;
	using dbs::cursor;

	extern struct log::log log;
	extern struct fronts fronts;
	extern uint64_t current_sequence;
	extern ctx::view<const event> inserted;

	bool test(const query<> &, const closure_bool &);
	bool test(const query<> &);
	bool until(const query<> &, const closure_bool &);
	bool until(const query<> &);
	void for_each(const query<> &, const closure &);
	void for_each(const closure &);
	size_t count(const query<> &, const closure_bool &);
	size_t count(const query<> &);

	event::id::buf commit(json::iov &event);
	event::id::buf commit(json::iov &event, const json::iov &content);
}

/// Evaluation Options
struct ircd::m::vm::opts
{
	/// Setting to false will disable the eval from exiting with a fault and
	/// instead consider all faults as errors which throw exceptions. If
	/// nothrow is also specified then the exception will be stored in the
	/// exception_ptr and #gp will be set.
	bool faults {true};

	/// Setting to true will prevent any exception throwing from an eval and
	/// instead only use the exception_ptr field. Note that eval does not make
	/// a noexcept guarantee with or without this option and it may still throw
	/// some rare or unexpected exception aborting the eval.
	bool nothrow {false};

	/// Setting to true will cause a successful eval to commit the result
	/// which writes to the db and has global effects for the server.
	/// False is the simulation mode to test evaluation without effects.
	bool commit {false};

	/// Setting to false will prevent the release sequence after a commit.
	/// This is a really bad thing to do. testing/debug/devel use only.
	bool release {true};

	/// Setting to true will force the eval to be aware of an m.room.create
	/// for a room for anything to succeed. There will be #ef's until found.
	bool genesis {false};

	/// Setting to true will cause #ef's until there is at least one unbroken
	/// link of events down to an m.room.create.
	bool cosmogonic {false};

	/// Setting to true will cause #ef's until the entire reference cone is
	/// known by the evaluation (if legit). i.e full history tree.
	bool fullcone {false};

	/// Setting to 1 forces evaluations to only proceed from oldest to newest
	/// events; the capstan will only operate using forward mode and positive
	/// accumulations. Because events only reference the past the machine won't
	/// always #ef, it will just stop. More events can be input. Setting to
	/// -1 is anti-causal: evaluations only proceed from newest to oldest and
	/// only negative accumulations are used. 0 is automatic and may use both.
	int causal {0};

	/// Option to skip event signature verification when false.
	bool verify_signature {true};
};

/// Individual evaluation state
///
/// Evaluation faults. These are reasons which evaluation has halted but may
/// continue after the user defaults the fault. They are basically types of
/// interrupts and traps, which are recoverable. Only the GENERAL protection
/// fault (#gp) is an abort and is not recoverable. An exception_ptr will be
/// set; aborts are otherwise just thrown as exceptions from eval. If any
/// fault isn't handled properly that is an abort.
///
enum ircd::m::vm::fault
:uint
{
	ACCEPT,          ///< No fault.
	DEBUGSTEP,       ///< Debug step. (#db)
	BREAKPOINT,      ///< Debug breakpoint. (#bp)
	GENERAL,         ///< General protection fault. (#gp)
	EVENT,           ///< Eval requires addl events in the ef register (#ef)
	STATE,           ///< Required state is missing (#st)
};

/// Event Evaluation Device
///
/// This object conducts the evaluation of an event or a tape of multiple
/// events. An event is evaluated in an attempt to execute it. Events which
/// fail during evaluation won't be executed; such is the case for events which
/// have already been executed, or events which are invalid or lead to invalid
/// transitions or actions of the machine etc.
///
/// Basic usage of the eval is simply calling the ctor like a function with an
/// event; with the default options this evaluates and executes the event
/// throwing all errors. Advanced usage of eval takes the form of an instance
/// constructed with options; the user then drives the eval in a loop,
/// responding to its needs for more data or other issues, and then continuing
/// the loop until satisfied.
///
/// The back-end interface consists of a set of modules which register with the
/// vm::eval class and provide their piece of eval functionality to instances.
///
struct ircd::m::vm::eval
{
	static const struct opts default_opts;

	const struct opts *opts;
	db::txn txn{*event::events};
	std::set<event::id> ef;
	uint64_t cs {0};

	fault operator()();
	fault operator()(const event &);
	fault operator()(const vector_view<const event> &);
	fault operator()(const json::vector &);
	fault operator()(const json::array &);

	template<class events> eval(const struct opts &, events&&);
	template<class events> eval(const events &);
	eval(const struct opts &);
	eval();

	friend string_view reflect(const fault &);
};

/// Convenience construction passes events through to operator().
template<class events>
ircd::m::vm::eval::eval(const events &event)
:eval(default_opts)
{
	operator()(event);
}

/// Convenience construction passes events through to operator().
template<class events>
ircd::m::vm::eval::eval(const struct opts &opts,
                        events&& event)
:eval(opts)
{
	operator()(std::forward<events>(event));
}

/// The "event front" for a graph. This holds all of the childless events
/// for a room. Each childless event may exist at a different depth, but
/// we track the highest depth to increment it for issuing the next event.
/// The contents of the front will then be used as the prev references for
/// that new event. The front is then replaced by the new event.  This is
/// managed by the vm core.
///
struct ircd::m::vm::front
{
	int64_t top {0};
	std::map<std::string, uint64_t, std::less<std::string_view>> map;
};

/// Singleton iface to all "event fronts" - the top depth in an active graph
///
/// This extern singleton is a fundamental structure which holds the highest
/// depth events in a graph which have no children. The fronts collection
/// itself is a map of rooms by ID, and one 'front' is held in RAM for each
/// room. Each front is a collection of those events, which ideally, will
/// become the prev references for the next event this server issues in the
/// room.
///
/// This is managed by the vm core. The fronts interface is the root of the
/// RAM footprint for a room known to IRCd. In other words, all room data is
/// stored in the database except what is reachable through here.
///
struct ircd::m::vm::fronts
{
	std::map<std::string, front, std::less<std::string_view>> map;

	friend front &fetch(const room::id &, front &, const event &);

  public:
	front &get(const room::id &, const event &);
	front &get(const room::id &);
};