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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.compiler;

import org.mvel2.ParserConfiguration;
import org.mvel2.ast.ASTNode;
import org.mvel2.ast.TypeCast;
import org.mvel2.integration.VariableResolverFactory;
import org.mvel2.optimizers.OptimizerFactory;
import org.mvel2.util.ASTLinkedList;

import java.io.Serializable;

import static org.mvel2.MVELRuntime.execute;

public class CompiledExpression implements Serializable, ExecutableStatement {
  private final ASTNode firstNode;

  private Class knownEgressType;
  private Class knownIngressType;

  private boolean convertableIngressEgress;
  private boolean optimized = false;
  private final boolean literalOnly;

  private final String sourceName;

  private final ParserConfiguration parserConfiguration;

  public CompiledExpression(ASTLinkedList astMap, String sourceName, Class egressType, ParserConfiguration parserConfiguration, boolean literalOnly) {
    this.firstNode = astMap.firstNode();
    this.sourceName = sourceName;
    this.knownEgressType = astMap.isSingleNode() ? astMap.firstNonSymbol().getEgressType() : egressType;
    this.literalOnly = literalOnly;
    this.parserConfiguration = parserConfiguration;
  }

  public ASTNode getFirstNode() {
    return firstNode;
  }

  public boolean isSingleNode() {
    return firstNode != null && firstNode.nextASTNode == null;
  }

  public Class getKnownEgressType() {
    return knownEgressType;
  }

  public void setKnownEgressType(Class knownEgressType) {
    this.knownEgressType = knownEgressType;
  }

  public Class getKnownIngressType() {
    return knownIngressType;
  }

  public void setKnownIngressType(Class knownIngressType) {
    this.knownIngressType = knownIngressType;
  }

  public boolean isConvertableIngressEgress() {
    return convertableIngressEgress;
  }

  public void computeTypeConversionRule() {
    if (knownIngressType != null && knownEgressType != null) {
      convertableIngressEgress = knownIngressType.isAssignableFrom(knownEgressType);
    }
  }

  public Object getValue(Object ctx, Object elCtx, VariableResolverFactory variableFactory) {
    if (!optimized) {
      setupOptimizers();
      try {
        return getValue(ctx, variableFactory);
      }
      finally {
        OptimizerFactory.clearThreadAccessorOptimizer();
      }
    }
    return getValue(ctx, variableFactory);
  }

  public Object getValue(Object staticContext, VariableResolverFactory factory) {
    if (!optimized) {
      setupOptimizers();
      try {
        return getValue(staticContext, factory);
      }
      finally {
        OptimizerFactory.clearThreadAccessorOptimizer();
      }
    }
    return getDirectValue(staticContext, factory);
  }

  public Object getDirectValue(Object staticContext, VariableResolverFactory factory) {
    return execute(false, this, staticContext, parserConfiguration.getVariableFactory(factory));
  }

  private void setupOptimizers() {
    optimized = true;
  }

  public boolean intOptimized() {
    return false;
  }

  public ParserConfiguration getParserConfiguration() {
    return parserConfiguration;
  }

  public boolean isImportInjectionRequired() {
    return parserConfiguration.hasImports();
  }

  public Object setValue(Object ctx, Object elCtx, VariableResolverFactory variableFactory, Object value) {
    return null;
  }

  public boolean isLiteralOnly() {
    return literalOnly;
  }

  public boolean isEmptyStatement() {
    return firstNode == null;
  }

  public boolean isExplicitCast() {
    return firstNode != null && firstNode instanceof TypeCast;
  }

  public String toString() {
    StringBuilder appender = new StringBuilder();
    ASTNode node = firstNode;
    while (node != null) {
      appender.append(node).append(";\n");
      node = node.nextASTNode;
    }
    return appender.toString();
  }
}