#871 Add ParentGroupSetter AST transformer to fix stale parent references after AST transformations

This required making `parent` mutable in `Group` and `Primitive`.

Author: yruslan

cobol-parser/src/main/scala/za/co/absa/cobrix/cobol/parser/Copybook.scala

@@ -411,7 +411,8 @@ object Copybook {
     }
 
     // recompute sizes
-    val schema = BinaryPropertiesAdder().transform(newRoot)
+    val schema1 = BinaryPropertiesAdder().transform(newRoot)
+    val schema = ParentGroupSetter().transform(schema1)
 
     new Copybook(schema)
   }

cobol-parser/src/main/scala/za/co/absa/cobrix/cobol/parser/CopybookParser.scala

@@ -332,14 +332,21 @@ object CopybookParser extends Logging {
       // For each group calculates the number of non-filler items.
       NonFillerCountSetter(),
       // Sets isUsedInRules and rule expressions for each field
-      RuleExpressionSetter(redefineRuleExpressions)
+      RuleExpressionSetter(redefineRuleExpressions),
+      // Updates 'parent' field of each of the fields in case they are inconsistent
+      ParentGroupSetter()
     )
 
     val transformedAst = transformers.foldLeft(schemaANTLR) {
       (ast, transformer) => transformer.transform(ast)
     }
 
-    new Copybook(transformedAst)
+    val finalAst = if (transformers.nonEmpty)
+      ParentGroupSetter().transform(transformedAst)
+    else
+      transformedAst
+
+    new Copybook(finalAst)
   }
 
   /**

cobol-parser/src/main/scala/za/co/absa/cobrix/cobol/parser/ast/Group.scala

@@ -63,7 +63,7 @@ case class Group(
                   ruleExpression: Option[ExpressionEvaluator] = None,
                   binaryProperties: BinaryProperties = BinaryProperties(0, 0, 0)
                 )
-                (val parent: Option[Group] = None)
+                (var parent: Option[Group] = None)
   extends Statement {
 
   /** This method is used to add a [[za.co.absa.cobrix.cobol.parser.ast.Statement]] object as a child of

cobol-parser/src/main/scala/za/co/absa/cobrix/cobol/parser/ast/Primitive.scala

@@ -61,7 +61,7 @@ case class Primitive(
                       encode: Option[EncoderSelector.Encoder],
                       binaryProperties: BinaryProperties = BinaryProperties(0, 0, 0)
                     )
-                    (val parent: Option[Group] = None)
+                    (var parent: Option[Group] = None)
   extends Statement {
 
   /** This is cached value specifying if the field is a string */

cobol-parser/src/main/scala/za/co/absa/cobrix/cobol/parser/asttransform/ParentGroupSetter.scala

@@ -0,0 +1,58 @@
+/*
+ * Copyright 2018 ABSA Group Limited
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package za.co.absa.cobrix.cobol.parser.asttransform
+
+import za.co.absa.cobrix.cobol.parser.CopybookParser.CopybookAST
+import za.co.absa.cobrix.cobol.parser.ast.{Group, Primitive}
+
+/**
+  * An AST transformer that restores the parent references of every element of a copybook AST.
+  *
+  * The AST is traversed recursively from the root group down to the leaves, and each group and primitive
+  * field is recreated so that its `parent` points to the newly created copy of its enclosing group. The root
+  * of the AST is left without a parent.
+  *
+  * This transformation is required because AST elements are immutable case classes: whenever a copybook AST is
+  * rebuilt or modified by other transformations, the parent links of the affected elements may point to stale
+  * copies of their parents. Applying this transformer as the final step ensures that the parent links are
+  * consistent with the actual hierarchy of the resulting AST.
+  *
+  * The structure, ordering and all other properties of the fields remain unchanged.
+  */
+class ParentGroupSetter extends AstTransformer {
+  final override def transform(ast: CopybookAST): CopybookAST = {
+    def processGroup(group: Group, parent: Option[Group]): Unit = {
+      var i = 0
+      group.parent = parent
+      while (i < group.children.length) {
+        group.children(i) match {
+          case g: Group     => processGroup(g, Some(group))
+          case p: Primitive => p.parent = Some(group)
+        }
+        i += 1
+      }
+    }
+
+    processGroup(ast, None)
+    ast
+  }
+}
+
+
+object ParentGroupSetter {
+  def apply(): ParentGroupSetter = new ParentGroupSetter()
+}