Nodes.asImmutable: added to create an immutable copy of a node tree

src/main/java/net/kemitix/node/Nodes.java

@@ -1,5 +1,9 @@
 package net.kemitix.node;
 
+import java.util.Optional;
+import java.util.Set;
+import java.util.stream.Collectors;
+
 /**
  * Utility class for {@link Node} items.
  *
@@ -63,4 +67,41 @@ public final class Nodes {
         return new NodeItem<>(data, name, parent);
     }
 
+    /**
+     * Creates an immutable copy of an existing node tree.
+     *
+     * @param root the root node of the source tree
+     * @param <T>  the type of the data
+     *
+     * @return the immutable copy of the tree
+     */
+    public static <T> Node<T> asImmutable(final Node<T> root) {
+        if (root.getParent().isPresent()) {
+            throw new IllegalArgumentException("source must be the root node");
+        }
+        final Set<Node<T>> children = getImmutableChildren(root);
+        return ImmutableNodeItem.newRoot(root.getData().orElse(null),
+                root.getName(), children);
+    }
+
+    private static <T> Set<Node<T>> getImmutableChildren(final Node<T> source) {
+        return source.getChildren()
+                     .stream()
+                     .map(Nodes::asImmutableChild)
+                     .collect(Collectors.toSet());
+    }
+
+    private static <T> Node<T> asImmutableChild(
+            final Node<T> source) {
+        final Optional<Node<T>> sourceParent = source.getParent();
+        if (sourceParent.isPresent()) {
+            return ImmutableNodeItem.newChild(source.getData().orElse(null),
+                    source.getName(), sourceParent.get(),
+                    getImmutableChildren(source));
+        } else {
+            throw new IllegalArgumentException(
+                    "source must not be the root node");
+        }
+    }
+
 }