// Hierarchical finite state machine implementation.
//
// FSMs are specified as a JS data structure;
// see e.g. AnimalAI.js for an example of the syntax.
//
// FSMs are implicitly linked with an external object.
// That object stores all FSM-related state.
// (This means we can serialise FSM-based components as
// plain old JS objects, with no need to serialise the complex
// FSM structure itself or to add custom serialisation code.)

function FSM(spec)
{
	// The (relatively) human-readable FSM specification needs to get
	// compiled into a more-efficient-to-execute version.
	//
	// In particular, message handling should require minimal
	// property lookups in the common case (even when the FSM has
	// a deeply nested hierarchy), and there should never be any
	// string manipulation at run-time.

	this.decompose = { "": [] };
	/* 'decompose' will store:
		{
			"": [],
			"A": ["A"],
			"A.B": ["A", "A.B"],
			"A.B.C": ["A", "A.B", "A.B.C"],
			"A.B.D": ["A", "A.B", "A.B.D"],
			...
		};
		This is used when switching between states in different branches
		of the hierarchy, to determine the list of sub-states to leave/enter
	*/

	this.states = { };
	/* 'states' will store:
		{
			...
			"A": {
				"_name": "A",
				"_parent": "",
				"_refs": { // local -> global name lookups (for SetNextState)
					"B": "A.B",
					"B.C": "A.B.C",
					"B.D": "A.B.D",
				},
			},
			"A.B": {
				"_name": "A.B",
				"_parent": "A",
				"_refs": {
					"C": "A.B.C",
					"D": "A.B.D",
				},
				"MessageType": function(msg) { ... },
			},
			"A.B.C": {
				"_name": "A.B.C",
				"_parent": "A.B",
				"_refs": {},
				"enter": function() { ... },
				"MessageType": function(msg) { ... },
			},
			"A.B.D": {
				"_name": "A.B.D",
				"_parent": "A.B",
				"_refs": {},
				"enter": function() { ... },
				"leave": function() { ... },
				"MessageType": function(msg) { ... },
			},
			...
		}
	*/

	function process(fsm, node, path, handlers)
	{
		var state = {};
		fsm.states[path.join(".")] = state;

		var newhandlers = {};
		for (var e in handlers)
			newhandlers[e] = handlers[e];

		state._name = path.join(".");
		state._parent = path.slice(0, -1).join(".");
		state._refs = {};

		for (var key in node)
		{
			if (key === "enter" || key === "leave")
			{
				state[key] = node[key];
			}
			else if (key.match(/^[A-Z]+$/))
			{
				state._refs[key] = (state._name ? state._name + "." : "") + key;

				// (the rest of this will be handled later once we've grabbed
				// all the event handlers)
			}
			else
			{
				newhandlers[key] = node[key];
			}
		}

		for (var e in newhandlers)
			state[e] = newhandlers[e];

		for (var key in node)
		{
			if (key.match(/^[A-Z]+$/))
			{
				var newpath = path.concat([key]);

				var decomposed = [newpath[0]];
				for (var i = 1; i < newpath.length; ++i)
					decomposed.push(decomposed[i-1] + "." + newpath[i]);
				fsm.decompose[newpath.join(".")] = decomposed;

				var childstate = process(fsm, node[key], newpath, newhandlers);

				for (var r in childstate._refs)
				{
					var cname = key + "." + r;
					state._refs[cname] = childstate._refs[r];
				}
			}
		}

		return state;
	}

	process(this, spec, [], {});
}

FSM.prototype.Init = function(obj, initialState)
{
	this.deferFromState = undefined;
	
	obj.fsmStateName = "";
	obj.fsmNextState = undefined;
	this.SwitchToNextState(obj, initialState);
};

FSM.prototype.SetNextState = function(obj, state)
{
	obj.fsmNextState = state;
};

FSM.prototype.ProcessMessage = function(obj, msg)
{
//	warn("ProcessMessage(obj, "+uneval(msg)+")");

	var func = this.states[obj.fsmStateName][msg.type];
	if (!func)
	{
		error("Tried to process unhandled event '" + msg.type + "' in state '" + obj.fsmStateName + "'");
		return;
	}
	func.apply(obj, [msg]);

	while (obj.fsmNextState)
	{
		var nextStateName = this.LookupState(obj.fsmStateName, obj.fsmNextState);
		obj.fsmNextState = undefined;

		if (nextStateName != obj.fsmStateName)
			this.SwitchToNextState(obj, nextStateName);
	}
};

FSM.prototype.DeferMessage = function(obj, msg)
{
	// We need to work out which sub-state we were running the message handler from,
	// and then try again in its parent state.
	var old = this.deferFromState;
	var from;
	if (old) // if we're recursively deferring and saved the last used state, use that
		from = old;
	else // if this is the first defer then we must have last processed the message in the current FSM state
		from = obj.fsmStateName;

	// Find and save the parent, for use in recursive defers
	this.deferFromState = this.states[from]._parent;

	// Run the function from the parent state
	var state = this.states[this.deferFromState];
	var func = state[msg.type];
	if (!func)
		error("Failed to defer event '" + msg.type + "' from state '" + obj.fsmStateName + "'");
	func.apply(obj, [msg]);	

	// Restore the changes we made
	this.deferFromState = old;

	// TODO: if an inherited handler defers, it calls exactly the same handler
	// on the parent state, which is probably useless and inefficient

	// NOTE: this will break if two units try to execute AI at the same time;
	// as long as AI messages are queue and processed asynchronously it should be fine
};

FSM.prototype.LookupState = function(currentStateName, stateName)
{
//	print("LookupState("+currentStateName+", "+stateName+")\n");
	for (var s = currentStateName; s; s = this.states[s]._parent)
		if (stateName in this.states[s]._refs)
			return this.states[s]._refs[stateName];
	return stateName;
};

FSM.prototype.SwitchToNextState = function(obj, nextStateName)
{
	var fromState = this.decompose[obj.fsmStateName];
	var toState = this.decompose[nextStateName];

	if (!toState)
		error("Tried to change to non-existent state '" + nextStateName + "'");

	// Find the set of states in the hierarchy tree to leave then enter,
	// to traverse from the old state to the new one.
	// If any enter/leave function returns true then abort the process
	// (this lets them intercept the transition and start a new transition)

	for (var equalPrefix = 0; fromState[equalPrefix] === toState[equalPrefix]; ++equalPrefix)
	{
	}

	for (var i = fromState.length-1; i >= equalPrefix; --i)
	{
		var leave = this.states[fromState[i]].leave;
		if (leave)
		{
			obj.fsmStateName = fromState[i];
			if (leave.apply(obj))
				return;
		}
	}

	for (var i = equalPrefix; i < toState.length; ++i)
	{
		var enter = this.states[toState[i]].enter;
		if (enter)
		{
			obj.fsmStateName = toState[i];
			if (enter.apply(obj))
				return;
		}
	}

	obj.fsmStateName = nextStateName;
}

Engine.RegisterGlobal("FSM", FSM);