replace ctor for single ctx and don't merge wildcards when payloads aren't the same. fixes a unit test.

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

@@ -84,7 +84,7 @@ public class ArrayPredictionContext extends PredictionContext {
 			@Override
 			public SingletonPredictionContext next() {
 				SingletonPredictionContext ctx =
-					new SingletonPredictionContext(parents[i], invokingStates[i]);
+					SingletonPredictionContext.create(parents[i], invokingStates[i]);
 				i++;
 				return ctx;
 			}
@@ -144,8 +144,8 @@ public class ArrayPredictionContext extends PredictionContext {
 				next = PredictionContext.EMPTY;
 			}
 			else {
-				next = new SingletonPredictionContext(this.parents[i],
+				next = SingletonPredictionContext.create(this.parents[i],
-													  this.invokingStates[i]);
+														 this.invokingStates[i]);
 			}
 			boolean rootIsWildcard = fullCtx;
 			newCtx = merge(newCtx, next, rootIsWildcard, mergeCache);

runtime/Java/src/org/antlr/v4/runtime/atn/LL1Analyzer.java

@@ -124,7 +124,7 @@ public class LL1Analyzer {
 			Transition t = s.transition(i);
 			if ( t.getClass() == RuleTransition.class ) {
 				PredictionContext newContext =
-					new SingletonPredictionContext(ctx, s.stateNumber);
+					SingletonPredictionContext.create(ctx, s.stateNumber);
 				_LOOK(t.target, newContext, look, lookBusy, seeThruPreds);
 			}
 			else if ( t instanceof PredicateTransition ) {

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

@@ -667,7 +667,7 @@ public class LexerATNSimulator extends ATNSimulator {
 		switch (t.getSerializationType()) {
 			case Transition.RULE:
 				PredictionContext newContext =
-					new SingletonPredictionContext(config.context, p.stateNumber);
+					SingletonPredictionContext.create(config.context, p.stateNumber);
 				c = new LexerATNConfig(config, t.target, newContext);
 				break;
 			case Transition.PREDICATE:

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

@@ -1115,7 +1115,7 @@ public class ParserATNSimulator extends ATNSimulator {
 				 config.state.getClass()==PlusLoopbackState.class )
 			{
 				config.context =
-					new SingletonPredictionContext(config.context, config.state.stateNumber);
+					SingletonPredictionContext.create(config.context, config.state.stateNumber);
 				// alter config; it's ok, since all calls to closure pass in a fresh config for us to chase
 				if ( debug ) System.out.println("Loop back; push "+config.state.stateNumber+", stack="+config.context);
 			}
@@ -1286,7 +1286,7 @@ public class ParserATNSimulator extends ATNSimulator {
 							   ", ctx="+config.context);
 		}
 		PredictionContext newContext =
-			new SingletonPredictionContext(config.context, config.state.stateNumber);
+			SingletonPredictionContext.create(config.context, config.state.stateNumber);
 		return new ATNConfig(config, t.target, newContext);
 	}
 

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

@@ -53,7 +53,7 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 			parent = PredictionContext.fromRuleContext(outerContext.parent);
 		}
 
-		return new SingletonPredictionContext(parent, outerContext.invokingState);
+		return SingletonPredictionContext.create(parent, outerContext.invokingState);
 	}
 
 	@Override
@@ -123,7 +123,8 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 		boolean rootIsWildcard,
 		DoubleKeyMap<PredictionContext,PredictionContext,PredictionContext> mergeCache)
 	{
-		if ( (a==null&&b==null) || a==b || a.equals(b) ) return a; // share same graph if both same
+		// share same graph if both same
+		if ( (a==null&&b==null) || a==b || (a!=null&&a.equals(b)) ) return a;
 
 		if ( a instanceof SingletonPredictionContext && b instanceof SingletonPredictionContext) {
 			return mergeSingletons((SingletonPredictionContext)a,
@@ -178,17 +179,13 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 			// merge parents x and y, giving array node with x,y then remainders
 			// of those graphs.  dup a, a' points at merged array
 			// new joined parent so create new singleton pointing to it, a'
-			PredictionContext a_ = new SingletonPredictionContext(parent, a.invokingState);
+			PredictionContext a_ = SingletonPredictionContext.create(parent, a.invokingState);
 			if ( mergeCache!=null ) mergeCache.put(a, b, a_);
 			return a_;
 		}
 		else { // a != b payloads differ
 			// see if we can collapse parents due to $+x parents if local ctx
 			PredictionContext singleParent = null;
-			if ( rootIsWildcard ) {
-				if ( a.parent == EMPTY ) singleParent = EMPTY;  // $ + b = $
-				if ( b.parent == EMPTY ) singleParent = EMPTY;  // a + $ = $
-			}
 			if ( a==b || (a.parent!=null && a.parent.equals(b.parent)) ) { // ax + bx = [a,b]x
 				singleParent = a.parent;
 			}
@@ -338,8 +335,9 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 			if ( p < lastSlot ) {
 				int n = p+1; // how many slots we really used in merge
 				if ( n == 1 ) { // for just one merged element, return singleton top
-					PredictionContext a_ = new SingletonPredictionContext(mergedParents[0],
+					PredictionContext a_ =
-																		  mergedInvokingStates[0]);
+						SingletonPredictionContext.create(mergedParents[0],
+														  mergedInvokingStates[0]);
 					if ( mergeCache!=null ) mergeCache.put(a,b,a_);
 					return a_;
 				}
@@ -490,7 +488,7 @@ public abstract class PredictionContext implements Iterable<SingletonPredictionC
 			updated = EMPTY;
 		}
 		else if (parents.length == 1) {
-			updated = new SingletonPredictionContext(parents[0], context.getInvokingState(0));
+			updated = SingletonPredictionContext.create(parents[0], context.getInvokingState(0));
 		}
 		else {
 			ArrayPredictionContext arrayPredictionContext = (ArrayPredictionContext)context;

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

@@ -8,7 +8,7 @@ public class SingletonPredictionContext extends PredictionContext {
 	public final PredictionContext parent;
 	public final int invokingState;
 
-	public SingletonPredictionContext(PredictionContext parent, int invokingState) {
+	SingletonPredictionContext(PredictionContext parent, int invokingState) {
 		super(calculateHashCode(parent!=null ? 31 ^ parent.hashCode() : 1,
 								31 ^ invokingState));
 		assert invokingState!=EMPTY_FULL_CTX_INVOKING_STATE &&
@@ -17,6 +17,14 @@ public class SingletonPredictionContext extends PredictionContext {
 		this.invokingState = invokingState;
 	}
 
+	public static SingletonPredictionContext create(PredictionContext parent, int invokingState) {
+		if ( invokingState == EMPTY_FULL_CTX_INVOKING_STATE && parent == null ) {
+			// someone can pass in the bits of an array ctx that mean $
+			return EMPTY;
+		}
+		return new SingletonPredictionContext(parent, invokingState);
+	}
+
 	@Override
 	public Iterator<SingletonPredictionContext> iterator() {
 		final SingletonPredictionContext self = this;

tool/test/org/antlr/v4/test/TestGraphNodes.java

@@ -2,7 +2,6 @@ package org.antlr.v4.test;
 
 import junit.framework.TestCase;
 import org.antlr.v4.runtime.atn.ArrayPredictionContext;
-import org.antlr.v4.runtime.atn.EmptyPredictionContext;
 import org.antlr.v4.runtime.atn.PredictionContext;
 import org.antlr.v4.runtime.atn.PredictionContextCache;
 import org.antlr.v4.runtime.atn.SingletonPredictionContext;
@@ -763,7 +762,7 @@ public class TestGraphNodes extends TestCase {
 	}
 
 	public SingletonPredictionContext createSingleton(PredictionContext parent, int payload) {
-		SingletonPredictionContext a = new SingletonPredictionContext(parent, payload);
+		SingletonPredictionContext a = SingletonPredictionContext.create(parent, payload);
 		return a;
 	}
 
@@ -820,7 +819,7 @@ public class TestGraphNodes extends TestCase {
 			}
 
 			for (int i = 0; i < current.size(); i++) {
-				if (current.getInvokingState(i) == EmptyPredictionContext.EMPTY_INVOKING_STATE) {
+				if (current.getInvokingState(i) == PredictionContext.EMPTY_FULL_CTX_INVOKING_STATE) {
 					continue;
 				}