EMPTY gives "" not "$" now. rename closure->closure_. had return in wrong spot in closure for-loop. fix fromRuleContext.

runtime/Java/src/org/antlr/v4/runtime/atn/ATNConfig.java

@@ -171,7 +171,7 @@ public class ATNConfig {
         }
         if ( context!=null ) {
             buf.append(",[");
-            buf.append(context.toString(recog));
+            buf.append(context.toString());
 			buf.append("]");
         }
         if ( semanticContext!=null && semanticContext != SemanticContext.NONE ) {

runtime/Java/src/org/antlr/v4/runtime/atn/ArrayPredictionContext.java

@@ -132,12 +132,18 @@ public class ArrayPredictionContext extends PredictionContext {
 
 	@Override
 	public String toString() {
+		if ( isEmpty() ) return "[]";
 		StringBuilder buf = new StringBuilder();
 		buf.append("[");
 		for (int i=0; i<invokingStates.length; i++) {
 			if ( i>0 ) buf.append(",");
 			buf.append(invokingStates[i]);
-			buf.append(parents[i].toString());
+			if ( parents[i]!=null ) {
+				buf.append(parents[i].toString());
+			}
+			else {
+				buf.append("null");
+			}
 		}
 		buf.append("]");
 		return buf.toString();

runtime/Java/src/org/antlr/v4/runtime/atn/EmptyPredictionContext.java

@@ -35,6 +35,6 @@ public class EmptyPredictionContext extends SingletonPredictionContext {
 
 	@Override
 	public String toString() {
-		return "$";
+		return "";
 	}
 }

runtime/Java/src/org/antlr/v4/runtime/atn/ParserATNSimulator.java

@@ -930,15 +930,16 @@ public class ParserATNSimulator<Symbol extends Token> extends ATNSimulator {
 						   boolean greedy, boolean loopsSimulateTailRecursion)
 	{
 		final int initialDepth = 0;
-		closure(config, configs, closureBusy, collectPredicates, greedy, loopsSimulateTailRecursion, initialDepth);
+		closure_(config, configs, closureBusy, collectPredicates, greedy,
+				 loopsSimulateTailRecursion, initialDepth);
 	}
 
-	protected void closure(@NotNull ATNConfig config,
-						   @NotNull ATNConfigSet configs,
-						   @NotNull Set<ATNConfig> closureBusy,
-						   boolean collectPredicates,
-						   boolean greedy, boolean loopsSimulateTailRecursion,
-						   int depth)
+	protected void closure_(@NotNull ATNConfig config,
+							@NotNull ATNConfigSet configs,
+							@NotNull Set<ATNConfig> closureBusy,
+							boolean collectPredicates,
+							boolean greedy, boolean loopsSimulateTailRecursion,
+							int depth)
 	{
 		if ( debug ) System.out.println("closure("+config.toString(parser,true)+")");
 
@@ -968,11 +969,11 @@ public class ParserATNSimulator<Symbol extends Token> extends ATNSimulator {
 						// Make sure we track that we are now out of context.
 						c.reachesIntoOuterContext = config.reachesIntoOuterContext;
 						assert depth > Integer.MIN_VALUE;
-						closure(c, configs, closureBusy, collectPredicates, greedy,
-								loopsSimulateTailRecursion, depth - 1);
+						closure_(c, configs, closureBusy, collectPredicates, greedy,
+								 loopsSimulateTailRecursion, depth - 1);
 //					}
-					return;
 				}
+				return;
 			}
 			else {
 				// else if we have no context info, just chase follow links (if greedy)
@@ -990,10 +991,11 @@ public class ParserATNSimulator<Symbol extends Token> extends ATNSimulator {
 				if ( debug ) System.out.println("Loop back; push "+config.state.stateNumber+", stack="+config.context);
 			}
 			else if ( config.state.getClass()==LoopEndState.class ) {
-				if ( debug ) System.out.println("Loop end; pop, stack="+config.context);
+				if ( debug ) System.out.print("Loop end; pop, stack="+config.context);
 				LoopEndState end = (LoopEndState)config.state;
 				// pop all the way back until we don't see the loopback state anymore
 				config.context = config.context.popAll(end.loopBackStateNumber, configs.fullCtx);
+				if ( debug ) System.out.println(" becomes "+config.context);
 			}
 		}
 
@@ -1036,7 +1038,8 @@ public class ParserATNSimulator<Symbol extends Token> extends ATNSimulator {
 					}
 				}
 
-				closure(c, configs, closureBusy, continueCollecting, greedy, loopsSimulateTailRecursion, newDepth);
+				closure_(c, configs, closureBusy, continueCollecting, greedy,
+						 loopsSimulateTailRecursion, newDepth);
 			}
 		}
 	}

runtime/Java/src/org/antlr/v4/runtime/atn/PredictionContext.java

@@ -28,7 +28,10 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 
 	public static PredictionContext fromRuleContext(RuleContext outerContext) {
 		if ( outerContext==null ) outerContext = RuleContext.EMPTY;
-		if ( outerContext==RuleContext.EMPTY ) {
+
+		// if we are in RuleContext of start rule, s, then PredictionContext
+		// is EMPTY. Nobody called us. (if we are empty, return empty)
+		if ( outerContext.parent==null || outerContext==RuleContext.EMPTY ) {
 			return PredictionContext.EMPTY;
 		}
 
@@ -36,9 +39,7 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 		// context of just [s $].
 
 		PredictionContext parent = EMPTY;
-		if ( outerContext.parent != null &&
-			 outerContext.parent.invokingState!=EmptyPredictionContext.EMPTY_INVOKING_STATE )
-		{
+		if ( outerContext.parent != null ) {
 			parent = PredictionContext.fromRuleContext(outerContext.parent);
 		}
 
@@ -65,11 +66,10 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 
 	protected static int calculateParentHashCode(PredictionContext[] parents) {
 		int hashCode = 1;
-		for (PredictionContext context : parents) {
-			if ( context==null ) {
-				System.out.println("WHAT!? "+parents.toString());
+		for (PredictionContext p : parents) {
+			if ( p!=null ) { // can be null for full ctx stack in ArrayPredictionContext
+				hashCode = hashCode * 31 ^ p.hashCode();
 			}
-			hashCode = hashCode * 31 ^ context.hashCode();
 		}
 
 		return hashCode;
@@ -339,8 +339,8 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 				mergedInvokingStates[k] = a.invokingStates[i];
 				i++;
 			}
-			else {
-				mergedParents[k] = a.parents[j];
+			else { // b > a
+				mergedParents[k] = b.parents[j];
 				mergedInvokingStates[k] = b.invokingStates[j];
 				j++;
 			}

runtime/Java/src/org/antlr/v4/runtime/atn/SingletonPredictionContext.java

@@ -82,6 +82,8 @@ public class SingletonPredictionContext extends PredictionContext {
 
 	@Override
 	public String toString() {
-		return String.valueOf(invokingState)+" "+parent.toString();
+		String up = parent.toString();
+		if ( up.length()==0 ) return String.valueOf(invokingState);
+		return String.valueOf(invokingState)+" "+up;
 	}
 }

tool/playground/T.g

@@ -1,8 +1,3 @@
 grammar T;
-s : a+ ;
-a : {false}? ID {System.out.println("alt 1");}
-  | {true}?  ID {System.out.println("alt 2");}
-  ;
-ID : 'a'..'z'+ ;
-INT : '0'..'9'+;
-WS : (' '|'\n') {skip();} ;
+s : 'a' 'b' ;
+WS  : [ \n]+ -> skip ;