Avoid creating intermediate LinkedLists in Parser new methods

src/main/java/org/sablecc/sablecc/GenerateAlternativeCodeForParser.java

@@ -139,14 +139,7 @@ public class GenerateAlternativeCodeForParser extends DepthFirstAdapter
       macros.apply(file, "ParserBraceOpening");
     }
 
-    if(typeName.startsWith("L"))
-    {
-      macros.apply(file, "ParserListVariableDeclaration", new String[] {variableTypeFromInternalType(typeName), getVariableName(term)});
-    }
-    else
-    {
-      macros.apply(file, "ParserSimpleVariableDeclaration", new String[] {variableTypeFromInternalType(typeName), getVariableName(term)});
-    }
+    macros.apply(file, "ParserVariableDeclaration", new String[] {variableTypeFromInternalType(typeName), getVariableName(term)});
     return true;
   }
 
@@ -260,6 +253,12 @@ public class GenerateAlternativeCodeForParser extends DepthFirstAdapter
   @Override
   public void caseAListTerm(AListTerm node)
   {
+    if(node.getListTerms().isEmpty())
+    {
+      // Empty lists are generated inline and don't have a variable.
+      return;
+    }
+
     try
     {
       boolean openingBraceGenerated = false;
@@ -273,13 +272,57 @@ public class GenerateAlternativeCodeForParser extends DepthFirstAdapter
         listTerm.apply(this);
       }
 
+      int nonNullTermsCount = 0;
+      PListTerm firstNonNullListTerm = null;
+      String firstNonNullTypeName = null;
       for(PListTerm listTerm : node.getListTerms())
       {
-        String type_name = lookupInternalTypeName(listTerm);
+        String typeName = lookupInternalTypeName(listTerm);
+        // Null and empty list terms never add any elements to the list and so don't need to be counted.
+        if("null".equals(typeName) || "Lnull".equals(typeName))
+        {
+          continue;
+        }
+        nonNullTermsCount++;
+        if(firstNonNullListTerm == null)
+        {
+          firstNonNullListTerm = listTerm;
+          firstNonNullTypeName = typeName;
+        }
+      }
+
+      if(nonNullTermsCount == 0)
+      {
+        throw new RuntimeException("List has non-empty type, but contains no elements!");
+      }
+      assert firstNonNullListTerm != null;
+      assert firstNonNullTypeName != null;
 
-        if(!type_name.equals("null"))
+      if(nonNullTermsCount == 1)
+      {
+        if(firstNonNullTypeName.startsWith("L"))
+        {
+          macros.apply(file, "ParserTypedListAssign", new String[] {getVariableName(node), getVariableName(firstNonNullListTerm)});
+        }
+        else
+        {
+          macros.apply(file, "ParserTypedSingleElementList", new String[] {getVariableName(node), getVariableName(firstNonNullListTerm)});
+        }
+      }
+      else
       {
-          if(type_name.startsWith("L"))
+        macros.apply(file, "ParserTypedLinkedListInit", new String[] {getVariableName(node)});
+
+        for(PListTerm listTerm : node.getListTerms())
+        {
+          String typeName = lookupInternalTypeName(listTerm);
+          // Null and empty list terms never add any elements to the list.
+          if("null".equals(typeName) || "Lnull".equals(typeName))
+          {
+            continue;
+          }
+
+          if(typeName.startsWith("L"))
           {
             macros.apply(file, "ParserTypedLinkedListAddAll", new String[] {getVariableName(node), getVariableName(listTerm)});
           }

src/main/resources/org/sablecc/sablecc/parser.txt

@@ -330,16 +330,11 @@ Macro:ParserBraceClosing
 
 $
 
-Macro:ParserSimpleVariableDeclaration
+Macro:ParserVariableDeclaration
         $0$ $1$;
 
 $
 
-Macro:ParserListVariableDeclaration
-        $0$ $1$ = new LinkedList<>();
-
-$
-
 Macro:ParserSimpleTerm
         $0$ = ($1$)nodeArrayList$2$.get($3$);
 
@@ -359,6 +354,25 @@ Macro:ParserNewBodyNewTail
 
 $
 
+Macro:ParserTypedSingleElementList
+        if($1$ != null) {
+            $0$ = Collections.singletonList($1$);
+        } else {
+            $0$ = Collections.emptyList();
+        }
+
+$
+
+Macro:ParserTypedListAssign
+        $0$ = $1$;
+
+$
+
+Macro:ParserTypedLinkedListInit
+        $0$ = new LinkedList<>();
+
+$
+
 Macro:ParserTypedLinkedListAdd
         if($1$ != null)
         {
@@ -370,10 +384,10 @@ $
 Macro:ParserTypedLinkedListAddAll
         if($1$ != null)
         {
-            if(!$0$.isEmpty()){
-                $0$.addAll($1$);
-            }else{
+            if ($0$.isEmpty() && $1$ instanceof LinkedList<?>) {
                 $0$ = $1$;
+            } else {
+                $0$.addAll($1$);
             }
         }