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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.templates.res;

import org.mvel2.CompileException;
import org.mvel2.MVEL;
import org.mvel2.ParserContext;
import org.mvel2.integration.VariableResolverFactory;
import org.mvel2.integration.impl.MapVariableResolverFactory;
import org.mvel2.templates.TemplateRuntime;
import org.mvel2.templates.TemplateRuntimeError;
import org.mvel2.templates.util.ArrayIterator;
import org.mvel2.templates.util.CountIterator;
import org.mvel2.templates.util.TemplateOutputStream;
import org.mvel2.util.ParseTools;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

public class CompiledForEachNode extends Node {
  public Node nestedNode;
  private Serializable[] ce;

  private String[] item;

  private char[] sepExpr;
  private Serializable cSepExpr;

  private ParserContext context;

  public CompiledForEachNode(int begin, String name, char[] template, int start, int end, ParserContext context) {
    super(begin, name, template, start, end);
    this.context = context;
    configure();
  }

  public Node getNestedNode() {
    return nestedNode;
  }

  public void setNestedNode(Node nestedNode) {
    this.nestedNode = nestedNode;
  }

  public boolean demarcate(Node terminatingnode, char[] template) {
    nestedNode = next;
    next = terminus;

    sepExpr = terminatingnode.getContents();
    if (sepExpr.length == 0) {
      sepExpr = null;
    }
    else {
      cSepExpr = MVEL.compileExpression(sepExpr, context);
    }

    return false;
  }

  public Object eval(TemplateRuntime runtime, TemplateOutputStream appender, Object ctx, VariableResolverFactory factory) {
    Iterator[] iters = new Iterator[item.length];

    Object o;
    for (int i = 0; i < iters.length; i++) {
      if ((o = MVEL.executeExpression(ce[i], ctx, factory)) instanceof Iterable) {
        iters[i] = ((Iterable) o).iterator();
      }
      else if (o instanceof Object[]) {
        iters[i] = new ArrayIterator((Object[]) o);
      }
      else if (o instanceof Integer) {
        iters[i] = new CountIterator((Integer) o);
      }
      else {
        throw new TemplateRuntimeError("cannot iterate object type: " + o.getClass().getName());
      }
    }

    Map locals = new HashMap();
    MapVariableResolverFactory localFactory = new MapVariableResolverFactory(locals, factory);

    int iterate = iters.length;

    while (true) {
      for (int i = 0; i < iters.length; i++) {
        if (!iters[i].hasNext()) {
          iterate--;
          locals.put(item[i], "");
        }
        else {
          locals.put(item[i], iters[i].next());
        }
      }
      if (iterate != 0) {
        nestedNode.eval(runtime, appender, ctx, localFactory);

        if (sepExpr != null) {
          for (Iterator it : iters) {
            if (it.hasNext()) {
              appender.append(String.valueOf(MVEL.executeExpression(cSepExpr, ctx, factory)));
              break;
            }
          }
        }
      }
      else break;
    }

    return next != null ? next.eval(runtime, appender, ctx, factory) : null;
  }

  private void configure() {
    ArrayList items = new ArrayList();
    ArrayList expr = new ArrayList();

    int start = cStart;
    for (int i = start; i < cEnd; i++) {
      switch (contents[i]) {
        case '(':
        case '[':
        case '{':
        case '"':
        case '\'':
          i = ParseTools.balancedCapture(contents, i, contents[i]);
          break;
//                    if (expr.size() < items.size()) {
//                        start = i;
//                        i = ParseTools.balancedCapture(contents, i, contents[i]);
//                        expr.add(ParseTools.createStringTrimmed(contents, start, i - start + 1));
//                        start = i + 1;
//                    }
//                    else {
//                        throw new CompileException("unexpected character '" + contents[i] + "' in foreach tag", contents,cStart + 1);
//                    }
//                    break;

        case ':':
          items.add(ParseTools.createStringTrimmed(contents, start, i - start));
          start = i + 1;
          break;
        case ',':
          if (expr.size() != (items.size() - 1)) {
            throw new CompileException("unexpected character ',' in foreach tag", contents, cStart + i);
          }
          expr.add(ParseTools.createStringTrimmed(contents, start, i - start));
          start = i + 1;
          break;
      }
    }

    if (start < cEnd) {
      if (expr.size() != (items.size() - 1)) {
        throw new CompileException("expected character ':' in foreach tag", contents, cEnd);
      }
      expr.add(ParseTools.createStringTrimmed(contents, start, cEnd - start));
    }

    item = new String[items.size()];
    int i = 0;
    for (String s : items) item[i++] = s;

    String[] expression;
    ce = new Serializable[(expression = new String[expr.size()]).length];
    i = 0;
    for (String s : expr) {
      ce[i] = MVEL.compileExpression(expression[i++] = s, context);
    }
  }
}