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TBEL is a powerful expression language for ThingsBoard platform user-defined functions. Original implementation is based on MVEL.
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/**
 * MVEL 2.0
 * Copyright (C) 2007 The Codehaus
 * Mike Brock, Dhanji Prasanna, John Graham, Mark Proctor
 *
 * 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 org.mvel2.util;

import org.mvel2.ast.ASTNode;

public class ASTLinkedList implements ASTIterator {
  private ASTNode firstASTNode;
  private ASTNode current;
  private ASTNode last;
  private int size;

  public ASTLinkedList() {
  }

  public ASTLinkedList(ASTIterator iter) {
    this.current = this.firstASTNode = iter.firstNode();
  }

  public ASTLinkedList(ASTNode firstASTNode) {
    this.current = this.firstASTNode = firstASTNode;
  }

  public ASTLinkedList(ASTNode firstASTNode, int size) {
    this.current = this.firstASTNode = firstASTNode;
    this.size = size;
  }

  public void addTokenNode(ASTNode astNode) {
    size++;

    if (this.firstASTNode == null) {
      this.firstASTNode = this.current = astNode;
    }
    else {
      this.last = this.current = (this.current.nextASTNode = astNode);
    }
  }

  public void addTokenNode(ASTNode astNode, ASTNode token2) {
    size += 2;

    if (this.firstASTNode == null) {
      this.last = this.current = ((this.firstASTNode = astNode).nextASTNode = token2);
    }
    else {
      this.last = this.current = (this.current.nextASTNode = astNode).nextASTNode = token2;
    }
  }

  public ASTNode firstNode() {
    return firstASTNode;
  }

  public boolean isSingleNode() {
    return size == 1 || (size == 2 && firstASTNode.fields == -1);
  }

  public ASTNode firstNonSymbol() {
    if (firstASTNode.fields == -1) {
      return firstASTNode.nextASTNode;
    }
    else {
      return firstASTNode;
    }
  }

  public void reset() {
    this.current = firstASTNode;
  }

  public boolean hasMoreNodes() {
    return this.current != null;
  }

  public ASTNode nextNode() {
    if (current == null) return null;
    try {
      return current;
    }
    finally {
      last = current;
      current = current.nextASTNode;
    }
  }

  public void skipNode() {
    if (current != null)
      current = current.nextASTNode;
  }

  public ASTNode peekNext() {
    if (current != null && current.nextASTNode != null)
      return current.nextASTNode;
    else
      return null;
  }


  public ASTNode peekNode() {
    if (current == null) return null;
    return current;
  }

  public void removeToken() {
    if (current != null) {
      current = current.nextASTNode;
    }
  }

  public ASTNode peekLast() {
    return last;
  }

  public ASTNode nodesBack(int offset) {
    throw new RuntimeException("unimplemented");
  }

  public ASTNode nodesAhead(int offset) {
    if (current == null) return null;
    ASTNode cursor = null;
    for (int i = 0; i < offset; i++) {
      if ((cursor = current.nextASTNode) == null) return null;
    }
    return cursor;
  }

  public void back() {
    current = last;
  }

  public String showNodeChain() {
    throw new RuntimeException("unimplemented");
  }

  public int size() {
    return size;
  }

  public int index() {
    return -1;
  }

  public void setCurrentNode(ASTNode node) {
    this.current = node;
  }

  public void finish() {
    reset();

    ASTNode last = null;
    ASTNode curr;

    while (hasMoreNodes()) {
      if ((curr = nextNode()).isDiscard()) {
        if (last == null) {
          last = firstASTNode = nextNode();
        }
        else {
          last.nextASTNode = nextNode();
        }
        continue;
      }

      if (!hasMoreNodes()) break;

      last = curr;
    }

    this.last = last;

    reset();
  }

}