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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.ScriptRuntimeException;

import static java.lang.String.valueOf;
import static org.mvel2.math.MathProcessor.doOperations;

public class ExecutionStack {
  private StackElement element;
  private int size = 0;

  public boolean isEmpty() {
    return size == 0;
  }

  public void add(Object o) {
    size++;
    StackElement el = element;
    if (el != null) {
      while (el.next != null) {
        el = el.next;
      }
      el.next = new StackElement(null, o);
    }
    else {
      element = new StackElement(null, o);
    }
  }

  public void push(Object o) {
    size++;
    element = new StackElement(element, o);
    assert size == deepCount();

  }

  public void push(Object obj1, Object obj2) {
    size += 2;
    element = new StackElement(new StackElement(element, obj1), obj2);
    assert size == deepCount();

  }

  public void push(Object obj1, Object obj2, Object obj3) {
    size += 3;
    element = new StackElement(new StackElement(new StackElement(element, obj1), obj2), obj3);
    assert size == deepCount();

  }

  public Object peek() {
    if (size == 0) return null;
    else return element.value;
  }

  public void dup() {
    size++;
    element = new StackElement(element, element.value);
    assert size == deepCount();

  }

  public Boolean peekBoolean() {
    if (size == 0) return null;
    if (element.value instanceof Boolean) return (Boolean) element.value;
    throw new ScriptRuntimeException("expected Boolean; but found: " + (element.value == null ? "null" : element.value.getClass().getName()));
  }

  public void copy2(ExecutionStack es) {
    element = new StackElement(new StackElement(element, es.element.value), es.element.next.value);
    es.element = es.element.next.next;
    size += 2;
    es.size -= 2;
  }

  public void copyx2(ExecutionStack es) {
    element = new StackElement(new StackElement(element, es.element.next.value), es.element.value);
    es.element = es.element.next.next;
    size += 2;
    es.size -= 2;
  }

  public Object peek2() {
    return element.next.value;
  }

  public Object pop() {
    if (size == 0) {
      return null;
    }
    try {
      size--;
      return element.value;
    }
    finally {
      element = element.next;
      assert size == deepCount();
    }
  }

  public Boolean popBoolean() {
    if (size-- == 0) {
      return null;
    }
    try {
      if (element.value instanceof Boolean) return (Boolean) element.value;
      throw new ScriptRuntimeException("expected Boolean; but found: " + (element.value == null ? "null" : element.value.getClass().getName()));
    }
    finally {
      element = element.next;
      assert size == deepCount();

    }
  }

  public Object pop2() {
    try {
      size -= 2;
      return element.value;
    }
    finally {
      element = element.next.next;
      assert size == deepCount();
    }
  }

  public void discard() {
    if (size != 0) {
      size--;
      element = element.next;
    }
  }

  public int size() {
    return size;
  }

  public boolean isReduceable() {
    return size > 1;
  }

  public void clear() {
    size = 0;
    element = null;
  }

  public void xswap_op() {
    element = new StackElement(element.next.next.next, doOperations(element.next.next.value, (Integer) element.next.value, element.value));
    size -= 2;
    assert size == deepCount();
  }

  public void op() {
    element = new StackElement(element.next.next.next, doOperations(element.next.next.value, (Integer) element.value, element.next.value));
    size -= 2;
    assert size == deepCount();
  }

  public void op(int operator) {
    element = new StackElement(element.next.next, doOperations(element.next.value, operator, element.value));
    size--;
    assert size == deepCount();
  }

  public void xswap() {
    StackElement e = element.next;
    StackElement relink = e.next;
    e.next = element;
    (element = e).next.next = relink;
  }

  public void xswap2() {
    StackElement node2 = element.next;
    StackElement node3 = node2.next;

    (node2.next = element).next = node3.next;
    element = node3;
    element.next = node2;
  }

  public int deepCount() {
    int count = 0;

    if (element == null) {
      return 0;
    }
    else {
      count++;
    }

    StackElement element = this.element;
    while ((element = element.next) != null) {
      count++;
    }
    return count;
  }

  public String toString() {
    StackElement el = element;

    if (element == null) return "";

    StringBuilder appender = new StringBuilder().append("[");
    do {
      appender.append(valueOf(el.value));
      if (el.next != null) appender.append(", ");
    }
    while ((el = el.next) != null);

    appender.append("]");

    return appender.toString();
  }
}