Use iterative traversal for deeply nested constraints

Author: 1Jul1an

src/main/java/de/vill/main/IterativeParseTreeWalker.java

@@ -0,0 +1,69 @@
+package de.vill.main;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+import org.antlr.v4.runtime.tree.ErrorNode;
+import org.antlr.v4.runtime.tree.ParseTree;
+import org.antlr.v4.runtime.tree.ParseTreeListener;
+import org.antlr.v4.runtime.tree.ParseTreeWalker;
+import org.antlr.v4.runtime.tree.RuleNode;
+import org.antlr.v4.runtime.tree.TerminalNode;
+
+/**
+ * Iterative variant of ANTLR's ParseTreeWalker.
+ *
+ * ANTLR's default ParseTreeWalker recursively walks the parse tree.
+ * Deeply nested constraints such as F0 | F1 | ... | F6999 can therefore
+ * overflow the Java call stack before the UVL model is constructed.
+ */
+final class IterativeParseTreeWalker extends ParseTreeWalker {
+
+    @Override
+    public void walk(ParseTreeListener listener, ParseTree tree) {
+        final Deque<Frame> stack = new ArrayDeque<>();
+        stack.push(new Frame(tree));
+
+        while (!stack.isEmpty()) {
+            final Frame frame = stack.peek();
+            final ParseTree current = frame.tree;
+
+            if (current instanceof ErrorNode) {
+                listener.visitErrorNode((ErrorNode) current);
+                stack.pop();
+                continue;
+            }
+
+            if (current instanceof TerminalNode) {
+                listener.visitTerminal((TerminalNode) current);
+                stack.pop();
+                continue;
+            }
+
+            final RuleNode ruleNode = (RuleNode) current;
+
+            if (!frame.entered) {
+                enterRule(listener, ruleNode);
+                frame.entered = true;
+            }
+
+            if (frame.nextChildIndex < current.getChildCount()) {
+                stack.push(new Frame(current.getChild(frame.nextChildIndex)));
+                frame.nextChildIndex++;
+            } else {
+                exitRule(listener, ruleNode);
+                stack.pop();
+            }
+        }
+    }
+
+    private static final class Frame {
+        private final ParseTree tree;
+        private boolean entered;
+        private int nextChildIndex;
+
+        private Frame(ParseTree tree) {
+            this.tree = tree;
+        }
+    }
+}

src/main/java/de/vill/main/UVLModelFactory.java

@@ -30,7 +30,6 @@
 import org.antlr.v4.runtime.ConsoleErrorListener;
 import org.antlr.v4.runtime.RecognitionException;
 import org.antlr.v4.runtime.Recognizer;
-import org.antlr.v4.runtime.tree.ParseTreeWalker;
 
 import java.io.IOException;
 import java.nio.file.FileSystems;
@@ -125,7 +124,7 @@ public void syntaxError(Recognizer<?, ?> recognizer, Object offendingSymbol, int
         });
 
         UVLListener uvlListener = new UVLListener();
-        ParseTreeWalker walker = new ParseTreeWalker();
+        IterativeParseTreeWalker walker = new IterativeParseTreeWalker();
         walker.walk(uvlListener, UVLJavaParser.constraintLine());
 
         return uvlListener.getConstraint();
@@ -302,7 +301,7 @@ public void syntaxError(Recognizer<?, ?> recognizer, Object offendingSymbol, int
 
 
         UVLListener uvlListener = new UVLListener();
-        ParseTreeWalker walker = new ParseTreeWalker();
+        IterativeParseTreeWalker walker = new IterativeParseTreeWalker();
         walker.walk(uvlListener, UVLJavaParser.featureModel());
         FeatureModel featureModel = null;
 
@@ -508,28 +507,38 @@ private void validateTypeLevelConstraints(final FeatureModel featureModel) {
     }
 
     private boolean validateTypeLevelConstraint(final Constraint constraint) {
-        boolean result = true;
-        if (constraint instanceof ExpressionConstraint) {
-            String leftReturnType = ((ExpressionConstraint) constraint).getLeft().getReturnType();
-            String rightReturnType = ((ExpressionConstraint) constraint).getRight().getReturnType();
+        final Deque<Constraint> stack = new ArrayDeque<>();
+        stack.push(constraint);
 
-            if (!(leftReturnType.equalsIgnoreCase(Constants.TRUE) || rightReturnType.equalsIgnoreCase(Constants.TRUE))) {
-                // if not attribute constraint
-                result = result && ((ExpressionConstraint) constraint).getLeft().getReturnType().equalsIgnoreCase(((ExpressionConstraint) constraint).getRight().getReturnType());
-            }
-            if (!result) {
-                return false;
-            }
-            for (final Expression expr: ((ExpressionConstraint) constraint).getExpressionSubParts()) {
-                result = result && validateTypeLevelExpression(expr);
+        while (!stack.isEmpty()) {
+            final Constraint current = stack.pop();
+
+            if (current instanceof ExpressionConstraint) {
+                final ExpressionConstraint expressionConstraint = (ExpressionConstraint) current;
+
+                final String leftReturnType = expressionConstraint.getLeft().getReturnType();
+                final String rightReturnType = expressionConstraint.getRight().getReturnType();
+
+                if (!(leftReturnType.equalsIgnoreCase(Constants.TRUE) || rightReturnType.equalsIgnoreCase(Constants.TRUE))) {
+                    if (!leftReturnType.equalsIgnoreCase(rightReturnType)) {
+                        return false;
+                    }
+                }
+
+                for (final Expression expr : expressionConstraint.getExpressionSubParts()) {
+                    if (!validateTypeLevelExpression(expr)) {
+                        return false;
+                    }
+                }
             }
-        }
 
-        for (final Constraint subCons: constraint.getConstraintSubParts()) {
-            result = result && validateTypeLevelConstraint(subCons);
+            final List<Constraint> subConstraints = current.getConstraintSubParts();
+            for (int i = subConstraints.size() - 1; i >= 0; i--) {
+                stack.push(subConstraints.get(i));
+            }
         }
 
-        return result;
+        return true;
     }
 
     private boolean validateTypeLevelExpression(final Expression expression) {

src/test/java/de/vill/parsing/LongConstraintParsingTest.java

@@ -0,0 +1,44 @@
+package de.vill.parsing;
+
+import de.vill.main.UVLModelFactory;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+class LongConstraintParsingTest {
+
+    @Test
+    void parsesLongOrConstraint() {
+        final int numberOfLiterals = 7000;
+        final String model = createModel(numberOfLiterals);
+
+        final UVLModelFactory factory = new UVLModelFactory();
+
+        assertDoesNotThrow(() -> factory.parse(model));
+    }
+
+    private String createModel(int numberOfLiterals) {
+        final StringBuilder builder = new StringBuilder();
+
+        builder.append("features\n");
+        builder.append("    Root\n");
+        builder.append("        optional\n");
+
+        for (int i = 0; i < numberOfLiterals; i++) {
+            builder.append("            F").append(i).append("\n");
+        }
+
+        builder.append("constraints\n");
+        builder.append("    ");
+
+        for (int i = 0; i < numberOfLiterals; i++) {
+            if (i > 0) {
+                builder.append(" | ");
+            }
+            builder.append("F").append(i);
+        }
+
+        builder.append("\n");
+        return builder.toString();
+    }
+}