Add if/else blocks for action sequences

- So now you can do something like this for an action:
    {
      if (health > 1000)
      {
        A_Scream;
      }
      else
      {
        A_XScream;
      }
    }
  Yes, the braces are required. Because I see too many instances where
  somebody writes an if statement in ACS and doesn't understand why it
  doesn't work right because they forgot braces.
- Fixed: Not actually putting an action between { and } would crash.

src/thingdef/thingdef.cpp

@@ -297,7 +297,11 @@ static void FinishThingdef()
 		if (func == NULL)
 		{
 			FCompileContext ctx(tcall->ActorClass);
-			tcall->Code->Resolve(ctx);
+			tcall->Code = static_cast<FxTailable *>(tcall->Code->Resolve(ctx));
+
+			// Make sure resolving it didn't obliterate it.
+			if (tcall->Code != NULL)
+			{
 				VMFunctionBuilder buildit;
 
 				// Allocate registers used to pass parameters in.
@@ -320,6 +324,9 @@ static void FinishThingdef()
 					codesize += sfunc->CodeSize;
 				}
 			}
+		}
+		if (tcall->Code != NULL)
+		{
 			delete tcall->Code;
 			tcall->Code = NULL;
 			for (int k = 0; k < tcall->NumStates; ++k)
@@ -327,6 +334,7 @@ static void FinishThingdef()
 				tcall->ActorClass->OwnedStates[tcall->FirstState + k].SetAction(func);
 			}
 		}
+	}
 
 	for (i = 0; i < PClassActor::AllActorClasses.Size(); i++)
 	{

src/thingdef/thingdef.h

@@ -189,7 +189,7 @@ AFuncDesc *FindFunction(const char * string);
 void ParseStates(FScanner &sc, PClassActor *actor, AActor *defaults, Baggage &bag);
 void ParseFunctionParameters(FScanner &sc, PClassActor *cls, TArray<FxExpression *> &out_params,
 	PFunction *afd, FString statestring, FStateDefinitions *statedef);
-void ParseActions(FScanner &sc, FState state, FString statestring, FStateTempCall *tcall, Baggage &bag);
+FxTailable *ParseActions(FScanner &sc, FState state, FString statestring, Baggage &bag);
 class FxVMFunctionCall *ParseAction(FScanner &sc, FState state, FString statestring, Baggage &bag);
 
 PFunction *FindGlobalActionFunction(const char *name);

src/thingdef/thingdef_exp.h

@@ -854,10 +854,29 @@ public:
 	FxSequence(const FScriptPosition &pos) : FxTailable(pos) {}
 	FxExpression *Resolve(FCompileContext&);
 	ExpEmit Emit(VMFunctionBuilder *build, bool tailcall);
-	void Add(FxTailable *expr) { Expressions.Push(expr); }
+	void Add(FxTailable *expr) { if (expr != NULL) Expressions.Push(expr); }
 	VMFunction *GetDirectFunction();
 };
 
+//==========================================================================
+//
+// FxIfStatement
+//
+//==========================================================================
+
+class FxIfStatement : public FxTailable
+{
+	FxExpression *Condition;
+	FxTailable *WhenTrue;
+	FxTailable *WhenFalse;
+
+public:
+	FxIfStatement(FxExpression *cond, FxTailable *true_part, FxTailable *false_part, const FScriptPosition &pos);
+	~FxIfStatement();
+	FxExpression *Resolve(FCompileContext&);
+	ExpEmit Emit(VMFunctionBuilder *build, bool tailcall);
+};
+
 //==========================================================================
 //
 //

src/thingdef/thingdef_expression.cpp

@@ -3311,6 +3311,120 @@ VMFunction *FxSequence::GetDirectFunction()
 	return NULL;
 }
 
+//==========================================================================
+//
+// FxIfStatement
+//
+//==========================================================================
+
+FxIfStatement::FxIfStatement(FxExpression *cond, FxTailable *true_part,
+	FxTailable *false_part, const FScriptPosition &pos)
+: FxTailable(pos)
+{
+	Condition = cond;
+	WhenTrue = true_part;
+	WhenFalse = false_part;
+	assert(cond != NULL);
+}
+
+FxIfStatement::~FxIfStatement()
+{
+	SAFE_DELETE(Condition);
+	SAFE_DELETE(WhenTrue);
+	SAFE_DELETE(WhenFalse);
+}
+
+FxExpression *FxIfStatement::Resolve(FCompileContext &ctx)
+{
+	CHECKRESOLVED();
+	if (WhenTrue == NULL && WhenFalse == NULL)
+	{ // We don't do anything either way, so disappear
+		delete this;
+		return NULL;
+	}
+	Condition = Condition->ResolveAsBoolean(ctx);
+	ABORT(Condition);
+	if (WhenTrue != NULL)
+	{
+		WhenTrue = static_cast<FxTailable *>(WhenTrue->Resolve(ctx));
+		ABORT(WhenTrue);
+	}
+	if (WhenFalse != NULL)
+	{
+		WhenFalse = static_cast<FxTailable *>(WhenFalse->Resolve(ctx));
+		ABORT(WhenFalse);
+	}
+	ValueType = VAL_Unknown;
+
+	if (Condition->isConstant())
+	{
+		ExpVal condval = static_cast<FxConstant *>(Condition)->GetValue();
+		bool result = condval.GetBool();
+
+		FxTailable *e = result ? WhenTrue : WhenFalse;
+		delete (result ? WhenFalse : WhenTrue);
+		WhenTrue = WhenFalse = NULL;
+		delete this;
+		return e;
+	}
+	return this;
+}
+
+ExpEmit FxIfStatement::Emit(VMFunctionBuilder *build, bool tailcall)
+{
+	ExpEmit v;
+	size_t jumpspot;
+	FxTailable *path1, *path2;
+	int condcheck;
+
+	// This is pretty much copied from FxConditional, except we don't
+	// keep any results.
+	ExpEmit cond = Condition->Emit(build);
+	assert(cond.RegType == REGT_INT && !cond.Konst);
+
+	if (WhenTrue != NULL)
+	{
+		path1 = WhenTrue;
+		path2 = WhenFalse;
+		condcheck = 1;
+	}
+	else
+	{
+		// When there is only a false path, reverse the condition so we can
+		// treat it as a true path.
+		assert(WhenFalse != NULL);
+		path1 = WhenFalse;
+		path2 = NULL;
+		condcheck = 0;
+	}
+
+	// Test condition.
+	build->Emit(OP_EQ_K, condcheck, cond.RegNum, build->GetConstantInt(0));
+	jumpspot = build->Emit(OP_JMP, 0);
+	cond.Free(build);
+
+	// Evaluate first path
+	v = path1->Emit(build, tailcall);
+	v.Free(build);
+	if (path2 != NULL)
+	{
+		size_t path1jump = build->Emit(OP_JMP, 0);
+		// Evaluate second path
+		build->BackpatchToHere(jumpspot);
+		v = path2->Emit(build, tailcall);
+		v.Free(build);
+		jumpspot = path1jump;
+	}
+	build->BackpatchToHere(jumpspot);
+	if (tailcall)
+	{
+		// When tailcall is true, execution is not expected to get past
+		// this if statement, so issue a RET.
+		build->Emit(OP_RET, RET_FINAL, REGT_NIL, 0);
+	}
+	return ExpEmit();
+}
+
 //==========================================================================
 //
 //==========================================================================

src/thingdef/thingdef_states.cpp

@@ -314,7 +314,7 @@ do_stop:
 					continue;
 				}
 
-				ParseActions(sc, state, statestring, tcall, bag);
+				tcall->Code = ParseActions(sc, state, statestring, bag);
 				goto endofstate;
 			}
 			sc.UnGet();
@@ -352,26 +352,57 @@ endofstate:
 //
 //==========================================================================
 
-void ParseActions(FScanner &sc, FState state, FString statestring, FStateTempCall *tcall, Baggage &bag)
+FxTailable *ParseActions(FScanner &sc, FState state, FString statestring, Baggage &bag)
 {
 	// If it's not a '{', then it should be a single action.
 	// Otherwise, it's a sequence of actions.
 	if (!sc.Compare("{"))
 	{
-		tcall->Code = ParseAction(sc, state, statestring, bag);
-		return;
+		return ParseAction(sc, state, statestring, bag);
 	}
 
-	FxSequence *seq = new FxSequence(sc);
+	const FScriptPosition pos(sc);
+
+	FxSequence *seq = NULL;
 	sc.MustGetString();
 	while (!sc.Compare("}"))
 	{
-		FxVMFunctionCall *call = ParseAction(sc, state, statestring, bag);
-		seq->Add(call);
+		FxTailable *add;
+		if (sc.Compare("if"))
+		{ // Hangle an if statement
+			FxExpression *cond;
+			FxTailable *true_part, *false_part = NULL;
+			sc.MustGetStringName("(");
+			cond = ParseExpression(sc, bag.Info);
+			sc.MustGetStringName(")");
+			sc.MustGetStringName("{");	// braces are mandatory
+			true_part = ParseActions(sc, state, statestring, bag);
+			sc.MustGetString();
+			if (sc.Compare("else"))
+			{
+				sc.MustGetStringName("{");	// braces are still mandatory
+				false_part = ParseActions(sc, state, statestring, bag);
+				sc.MustGetString();
+			}
+			add = new FxIfStatement(cond, true_part, false_part, sc);
+		}
+		else
+		{ // Handle a regular action function call
+			add = ParseAction(sc, state, statestring, bag);
 			sc.MustGetStringName(";");
 			sc.MustGetString();
 		}
-	tcall->Code = seq;
+		// Only return a sequence if it has actual content.
+		if (add != NULL)
+		{
+			if (seq == NULL)
+			{
+				seq = new FxSequence(pos);
+			}
+			seq->Add(add);
+		}
+	}
+	return seq;
 }
 
 //==========================================================================