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With inspiration from other libraries

There is a newer version: 2.12.15

package org.mvel2.util;

import org.mvel2.MVEL;
import org.mvel2.ParserContext;

import java.util.Arrays;
import java.util.Set;

/**
 * @author Mike Brock .
 */
public class VariableSpaceCompiler {
  private static final Object[] EMPTY_OBJ = new Object[0];
  
  public static SharedVariableSpaceModel compileShared(String expr, ParserContext pCtx) {
    return compileShared(expr, pCtx, EMPTY_OBJ);
  }
  
  public static SharedVariableSpaceModel compileShared(String expr, ParserContext pCtx, Object[] vars) {
    String[] varNames = pCtx.getIndexedVarNames();

    ParserContext analysisContext = ParserContext.create();
    analysisContext.setIndexAllocation(true);

    MVEL.analysisCompile(expr, analysisContext);

    Set localNames = analysisContext.getVariables().keySet();

    pCtx.addIndexedLocals(localNames);

    String[] locals = localNames.toArray(new String[localNames.size()]);
    String[] allVars = new String[varNames.length + locals.length];

    System.arraycopy(varNames, 0, allVars, 0, varNames.length);
    System.arraycopy(locals, 0, allVars, varNames.length, locals.length);

    return new SharedVariableSpaceModel(allVars, vars);
  }

  public static SimpleVariableSpaceModel compile(String expr, ParserContext pCtx) {
    String[] varNames = pCtx.getIndexedVarNames();

    ParserContext analysisContext = ParserContext.create();
    analysisContext.setIndexAllocation(true);

    MVEL.analysisCompile(expr, analysisContext);

    Set localNames = analysisContext.getVariables().keySet();

    pCtx.addIndexedLocals(localNames);

    String[] locals = localNames.toArray(new String[localNames.size()]);
    String[] allVars = new String[varNames.length + locals.length];

    System.arraycopy(varNames, 0, allVars, 0, varNames.length);
    System.arraycopy(locals, 0, allVars, varNames.length, locals.length);

    return new SimpleVariableSpaceModel(allVars);
  }
}