fix: Restore missing changes to v4 of go runtime

I had not though about this, but I guess some of the fix PRs did not have one fix
that was in the legacy version of the runtime, copied in to the v4 branch. This
commit fixes that.

Signed-off-by: Jim.Idle <[email protected]>

Author: Jim.Idle

runtime/Go/antlr/antlrdoc.go

@@ -48,7 +48,7 @@ And the generate.sh file will look similar to this:
 	#!/bin/sh
 
 	alias antlr4='java -Xmx500M -cp "./antlr4-4.11.1-complete.jar:$CLASSPATH" org.antlr.v4.Tool'
-	antlr4 -Dlanguage=Go -no-visitor -package tgram *.g4
+	antlr4 -Dlanguage=Go -no-visitor -package parser *.g4
 
 depending on whether you want visitors or listeners or any other ANTLR options.
 

runtime/Go/antlr/v4/prediction_context.go

@@ -376,11 +376,20 @@ func predictionContextFromRuleContext(a *ATN, outerContext RuleContext) Predicti
 }
 
 func merge(a, b PredictionContext, rootIsWildcard bool, mergeCache *DoubleDict) PredictionContext {
-	// share same graph if both same
-	if a == b {
+
+	// Share same graph if both same
+	//
+	if a == b || a.Equals(b) {
 		return a
 	}
 
+	// In Java, EmptyPredictionContext inherits from SingletonPredictionContext, and so the test
+	// in java for SingletonPredictionContext will succeed and a new ArrayPredictionContext will be created
+	// from it.
+	// In go, EmptyPredictionContext does not equate to SingletonPredictionContext and so that conversion
+	// will fail. We need to test for both Empty and Singleton and create an ArrayPredictionContext from
+	// either of them.
+
 	ac, ok1 := a.(*BaseSingletonPredictionContext)
 	bc, ok2 := b.(*BaseSingletonPredictionContext)
 
@@ -397,14 +406,30 @@ func merge(a, b PredictionContext, rootIsWildcard bool, mergeCache *DoubleDict)
 			return b
 		}
 	}
-	// convert singleton so both are arrays to normalize
-	if _, ok := a.(*BaseSingletonPredictionContext); ok {
-		a = NewArrayPredictionContext([]PredictionContext{a.GetParent(0)}, []int{a.getReturnState(0)})
+
+	// Convert Singleton or Empty so both are arrays to normalize - We should not use the existing parameters
+	// here.
+	//
+	// TODO: I think that maybe the Prediction Context structs should be redone as there is a chance we will see this mess again - maybe redo the logic here
+
+	var arp, arb *ArrayPredictionContext
+	var ok bool
+	if arp, ok = a.(*ArrayPredictionContext); ok {
+	} else if _, ok = a.(*BaseSingletonPredictionContext); ok {
+		arp = NewArrayPredictionContext([]PredictionContext{a.GetParent(0)}, []int{a.getReturnState(0)})
+	} else if _, ok = a.(*EmptyPredictionContext); ok {
+		arp = NewArrayPredictionContext([]PredictionContext{}, []int{})
 	}
-	if _, ok := b.(*BaseSingletonPredictionContext); ok {
-		b = NewArrayPredictionContext([]PredictionContext{b.GetParent(0)}, []int{b.getReturnState(0)})
+
+	if arb, ok = b.(*ArrayPredictionContext); ok {
+	} else if _, ok = b.(*BaseSingletonPredictionContext); ok {
+		arb = NewArrayPredictionContext([]PredictionContext{b.GetParent(0)}, []int{b.getReturnState(0)})
+	} else if _, ok = b.(*EmptyPredictionContext); ok {
+		arb = NewArrayPredictionContext([]PredictionContext{}, []int{})
 	}
-	return mergeArrays(a.(*ArrayPredictionContext), b.(*ArrayPredictionContext), rootIsWildcard, mergeCache)
+
+	// Both arp and arb
+	return mergeArrays(arp, arb, rootIsWildcard, mergeCache)
 }
 
 // Merge two {@link SingletonBasePredictionContext} instances.
@@ -677,6 +702,7 @@ func mergeArrays(a, b *ArrayPredictionContext, rootIsWildcard bool, mergeCache *
 
 	// if we created same array as a or b, return that instead
 	// TODO: track whether this is possible above during merge sort for speed
+	// TODO: In go, I do not think we can just do M == xx as M is a brand new allocation. This could be causing allocation problems
 	if M == a {
 		if mergeCache != nil {
 			mergeCache.set(a.Hash(), b.Hash(), a)