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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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// ASM: a very small and fast Java bytecode manipulation framework
// Copyright (c) 2000-2011 INRIA, France Telecom
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
// 3. Neither the name of the copyright holders nor the names of its
//    contributors may be used to endorse or promote products derived from
//    this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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// THE POSSIBILITY OF SUCH DAMAGE.
package org.mvel2.asm.signature;

import org.mvel2.asm.Opcodes;

/**
 * A visitor to visit a generic signature. The methods of this interface must be called in one of
 * the three following orders (the last one is the only valid order for a {@link SignatureVisitor}
 * that is returned by a method of this interface):
 *
 * 
 *   ClassSignature = ( {@code visitFormalTypeParameter} {@code visitClassBound}? {@code
 *       visitInterfaceBound}* )* ({@code visitSuperclass} {@code visitInterface}* )
 *   
MethodSignature = ( {@code visitFormalTypeParameter} {@code visitClassBound}? {@code
 *       visitInterfaceBound}* )* ({@code visitParameterType}* {@code visitReturnType} {@code
 *       visitExceptionType}* )
 *   
TypeSignature = {@code visitBaseType} | {@code visitTypeVariable} | {@code
 *       visitArrayType} | ( {@code visitClassType} {@code visitTypeArgument}* ( {@code
 *       visitInnerClassType} {@code visitTypeArgument}* )* {@code visitEnd} ) )
 * 
 *
 * @author Thomas Hallgren
 * @author Eric Bruneton
 */
public abstract class SignatureVisitor {

  /** Wildcard for an "extends" type argument. */
  public static final char EXTENDS = '+';

  /** Wildcard for a "super" type argument. */
  public static final char SUPER = '-';

  /** Wildcard for a normal type argument. */
  public static final char INSTANCEOF = '=';

  /**
   * The ASM API version implemented by this visitor. The value of this field must be one of {@link
   * Opcodes#ASM4}, {@link Opcodes#ASM5}, {@link Opcodes#ASM6} or {@link Opcodes#ASM7}.
   */
  protected final int api;

  /**
   * Constructs a new {@link SignatureVisitor}.
   *
   * @param api the ASM API version implemented by this visitor. Must be one of {@link
   *     Opcodes#ASM4}, {@link Opcodes#ASM5}, {@link Opcodes#ASM6} or {@link Opcodes#ASM7}.
   */
  public SignatureVisitor(final int api) {
    if (api != Opcodes.ASM6 && api != Opcodes.ASM5 && api != Opcodes.ASM4 && api != Opcodes.ASM7) {
      throw new IllegalArgumentException();
    }
    this.api = api;
  }

  /**
   * Visits a formal type parameter.
   *
   * @param name the name of the formal parameter.
   */
  public void visitFormalTypeParameter(final String name) {}

  /**
   * Visits the class bound of the last visited formal type parameter.
   *
   * @return a non null visitor to visit the signature of the class bound.
   */
  public SignatureVisitor visitClassBound() {
    return this;
  }

  /**
   * Visits an interface bound of the last visited formal type parameter.
   *
   * @return a non null visitor to visit the signature of the interface bound.
   */
  public SignatureVisitor visitInterfaceBound() {
    return this;
  }

  /**
   * Visits the type of the super class.
   *
   * @return a non null visitor to visit the signature of the super class type.
   */
  public SignatureVisitor visitSuperclass() {
    return this;
  }

  /**
   * Visits the type of an interface implemented by the class.
   *
   * @return a non null visitor to visit the signature of the interface type.
   */
  public SignatureVisitor visitInterface() {
    return this;
  }

  /**
   * Visits the type of a method parameter.
   *
   * @return a non null visitor to visit the signature of the parameter type.
   */
  public SignatureVisitor visitParameterType() {
    return this;
  }

  /**
   * Visits the return type of the method.
   *
   * @return a non null visitor to visit the signature of the return type.
   */
  public SignatureVisitor visitReturnType() {
    return this;
  }

  /**
   * Visits the type of a method exception.
   *
   * @return a non null visitor to visit the signature of the exception type.
   */
  public SignatureVisitor visitExceptionType() {
    return this;
  }

  /**
   * Visits a signature corresponding to a primitive type.
   *
   * @param descriptor the descriptor of the primitive type, or 'V' for {@code void} .
   */
  public void visitBaseType(final char descriptor) {}

  /**
   * Visits a signature corresponding to a type variable.
   *
   * @param name the name of the type variable.
   */
  public void visitTypeVariable(final String name) {}

  /**
   * Visits a signature corresponding to an array type.
   *
   * @return a non null visitor to visit the signature of the array element type.
   */
  public SignatureVisitor visitArrayType() {
    return this;
  }

  /**
   * Starts the visit of a signature corresponding to a class or interface type.
   *
   * @param name the internal name of the class or interface.
   */
  public void visitClassType(final String name) {}

  /**
   * Visits an inner class.
   *
   * @param name the local name of the inner class in its enclosing class.
   */
  public void visitInnerClassType(final String name) {}

  /** Visits an unbounded type argument of the last visited class or inner class type. */
  public void visitTypeArgument() {}

  /**
   * Visits a type argument of the last visited class or inner class type.
   *
   * @param wildcard '+', '-' or '='.
   * @return a non null visitor to visit the signature of the type argument.
   */
  public SignatureVisitor visitTypeArgument(final char wildcard) {
    return this;
  }

  /** Ends the visit of a signature corresponding to a class or interface type. */
  public void visitEnd() {}
}