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Durian - Guava's spikier (unofficial) cousin

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/**
 * Copyright 2015 DiffPlug
 *
 * 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 com.diffplug.common.base;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/** Allows Trees to be manually defined across any object. */
public final class TreeNode {
	final TreeNode parent;
	final T obj;
	List> children = null;

	public TreeNode(TreeNode parent, T obj) {
		this.parent = parent;
		this.obj = obj;
		if (parent != null) {
			if (parent.children == null) {
				parent.children = new ArrayList<>();
			}
			parent.children.add(this);
		}
	}

	public TreeNode getParent() {
		return parent;
	}

	public T getObj() {
		return obj;
	}

	public List> getChildren() {
		return children == null ? Collections.emptyList() : children;
	}

	/** Creates a hierarchy of TreeNodes that copies the structure and content of the given Tree. */
	public static  TreeNode copy(T root, TreeDef treeDef) {
		TreeNode copyRoot = new TreeNode<>(null, root);
		copyRecurse(copyRoot, treeDef);
		return copyRoot;
	}

	private static  void copyRecurse(TreeNode root, TreeDef treeDef) {
		List children = treeDef.childrenOf(root.obj);
		for (T child : children) {
			copyRecurse(new TreeNode<>(root, child), treeDef);
		}
	}

	/** TreeDef for the generic TreeNode class. */
	public static  TreeDef.Parented> treeDef() {
		return new TreeDef.Parented>() {
			@Override
			public List> childrenOf(TreeNode root) {
				return root.getChildren();
			}

			@Override
			public TreeNode parentOf(TreeNode child) {
				return child.parent;
			}
		};
	}
}