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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.MVEL;
import org.mvel2.integration.VariableResolverFactory;
import org.mvel2.templates.TemplateError;
import org.mvel2.templates.TemplateRuntime;
import org.mvel2.templates.util.TemplateOutputStream;

import java.io.*;

import static org.mvel2.templates.util.TemplateTools.captureToEOS;

public class IncludeNode extends Node {
//    private char[] includeExpression;
//    private char[] preExpression;

  int includeStart;
  int includeOffset;

  int preStart;
  int preOffset;

  public IncludeNode(int begin, String name, char[] template, int start, int end) {
    this.begin = begin;
    this.name = name;
    this.contents = template;
    this.cStart = start;
    this.cEnd = end - 1;
    this.end = end;
    //this.contents = subset(template, this.cStart = start, (this.end = this.cEnd = end) - start - 1);

    int mark = captureToEOS(contents, 0);
    includeStart = cStart;
    includeOffset = mark - cStart;
    preStart = ++mark;
    preOffset = cEnd - mark;

    //this.includeExpression = subset(contents, 0, mark = captureToEOS(contents, 0));
//        if (mark != contents.length) this.preExpression = subset(contents, ++mark, contents.length - mark);
  }

  public Object eval(TemplateRuntime runtime, TemplateOutputStream appender, Object ctx, VariableResolverFactory factory) {
    String file = MVEL.eval(contents, includeStart, includeOffset, ctx, factory, String.class);

    if (preOffset != 0) {
      MVEL.eval(contents, preStart, preOffset, ctx, factory);
    }

    if (next != null) {
      return next.eval(runtime, appender.append(String.valueOf(TemplateRuntime.eval(readInFile(runtime, file), ctx, factory))), ctx, factory);
    }
    else {
      return appender.append(String.valueOf(MVEL.eval(readInFile(runtime, file), ctx, factory)));
    }
  }

  public boolean demarcate(Node terminatingNode, char[] template) {
    return false;
  }


  public static String readInFile(TemplateRuntime runtime, String fileName) {
    File file = new File(String.valueOf(runtime.getRelPath().peek()) + "/" + fileName);

    try {
      FileInputStream instream = new FileInputStream(file);
      BufferedInputStream bufstream = new BufferedInputStream(instream);

      runtime.getRelPath().push(file.getParent());

      byte[] buf = new byte[10];
      int read;
      int i;

      StringBuilder appender = new StringBuilder();

      while ((read = bufstream.read(buf)) != -1) {
        for (i = 0; i < read; i++) {
          appender.append((char) buf[i]);
        }
      }

      bufstream.close();
      instream.close();

      runtime.getRelPath().pop();

      return appender.toString();

    }
    catch (FileNotFoundException e) {
      throw new TemplateError("cannot include template '" + fileName + "': file not found.");
    }
    catch (IOException e) {
      throw new TemplateError("unknown I/O exception while including '" + fileName + "' (stacktrace nested)", e);
    }
  }
}