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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.ast;

import org.mvel2.CompileException;
import org.mvel2.MVELInterpretedRuntime;
import org.mvel2.ParserContext;
import org.mvel2.PropertyAccessor;
import org.mvel2.compiler.CompiledAccExpression;
import org.mvel2.compiler.ExecutableStatement;
import org.mvel2.integration.VariableResolverFactory;

import static org.mvel2.MVEL.compileSetExpression;
import static org.mvel2.util.ArrayTools.findFirst;
import static org.mvel2.util.ParseTools.*;

/**
 * @author Christopher Brock
 */
public class AssignmentNode extends ASTNode implements Assignment, HasStatement {
  private String assignmentVar;
  private String varName;
  private transient CompiledAccExpression accExpr;

  private char[] indexTarget;
  private String index;

  // private char[] stmt;
  private ExecutableStatement statement;
  private boolean col = false;


  public AssignmentNode(char[] expr, int start, int offset, int fields, ParserContext pCtx) {
    super(pCtx);
    this.expr = expr;
    this.start = start;
    this.offset = offset;

    int assignStart;

    if ((assignStart = find(expr, start, offset, '=')) != -1) {
      this.varName = createStringTrimmed(expr, start, assignStart - start);
      this.assignmentVar = varName;

      this.start = skipWhitespace(expr, assignStart + 1);
      if (this.start >= start + offset) {
        throw new CompileException("unexpected end of statement", expr, assignStart + 1);
      }

      this.offset = offset - (this.start - start);

      if ((fields & COMPILE_IMMEDIATE) != 0) {
        this.egressType = (statement = (ExecutableStatement)
            subCompileExpression(expr, this.start, this.offset, pCtx)).getKnownEgressType();
      }

      if (col = ((endOfName = findFirst('[', 0, this.varName.length(), indexTarget = this.varName.toCharArray())) > 0)) {
        if (((this.fields |= COLLECTION) & COMPILE_IMMEDIATE) != 0) {
          accExpr = (CompiledAccExpression) compileSetExpression(indexTarget, pCtx);
        }

        this.varName = new String(expr, start, endOfName);
        index = new String(indexTarget, endOfName, indexTarget.length - endOfName);
      }


      try {
        checkNameSafety(this.varName);
      }
      catch (RuntimeException e) {
        throw new CompileException(e.getMessage(), expr, start);
      }
    }
    else {
      try {
        checkNameSafety(this.varName = new String(expr, start, offset));
        this.assignmentVar = varName;
      }
      catch (RuntimeException e) {
        throw new CompileException(e.getMessage(), expr, start);
      }
    }

    if ((fields & COMPILE_IMMEDIATE) != 0) {
      pCtx.addVariable(this.varName, egressType);
    }
  }

  public Object getReducedValueAccelerated(Object ctx, Object thisValue, VariableResolverFactory factory) {
    if (accExpr == null && indexTarget != null) {
      accExpr = (CompiledAccExpression) compileSetExpression(indexTarget);
    }

    if (col) {
      return accExpr.setValue(ctx, thisValue, factory, statement.getValue(ctx, thisValue, factory));
    }
    else if (statement != null) {
      if (factory == null)
        throw new CompileException("cannot assign variables; no variable resolver factory available", expr, start);
      return factory.createVariable(varName, checkAssignGlobalVariable(ctx, varName, statement.getValue(ctx, thisValue, factory))).getValue();
    }
    else {
      if (factory == null)
        throw new CompileException("cannot assign variables; no variable resolver factory available", expr, start);
      factory.createVariable(varName, checkAssignGlobalVariable(ctx, varName, null));
      return null;
    }
  }

  public Object getReducedValue(Object ctx, Object thisValue, VariableResolverFactory factory) {
    checkNameSafety(varName);

    MVELInterpretedRuntime runtime = new MVELInterpretedRuntime(expr, start, offset, ctx, factory, pCtx);

    if (col) {
      PropertyAccessor.set(factory.getVariableResolver(varName).getValue(), factory, index, ctx = runtime.parse(), pCtx);
    }
    else {
      return factory.createVariable(varName, checkAssignGlobalVariable(ctx, varName, runtime.parse())).getValue();
    }

    return ctx;
  }


  public String getAssignmentVar() {
    return assignmentVar;
  }

  public String getVarName() {
    return varName;
  }

  public char[] getExpression() {
    return subset(expr, start, offset);
  }

  public boolean isNewDeclaration() {
    return false;
  }

  public void setValueStatement(ExecutableStatement stmt) {
    this.statement = stmt;
  }

  @Override
  public String toString() {
    return assignmentVar + " = " + new String(expr, start, offset);
  }

  @Override
  public ExecutableStatement getStatement() {
    return statement;
  }
}