function YarnFileCopy(options = {}) {
  
  if (!options.assets === undefined || !Array.isArray(options.assets)) {
    throw new Error('No assets defined or assets are not in an array');
  };
  
  if (options.indexFile === undefined) {
    throw new Error('No index file set. Please specify options.indexFile');
  }
  
  options.outputDir = options.outputDir || '.';
  options.jsDir = options.jsDir || '.';
  options.cssDir = options.cssDir || '.';

  //Defaults debug level to 'warning'
  options.debug = options.debug || 'warning';

  // Defaults debugging to info if only true is specified
  if (options.debug === true) {
    options.debug = 'info';
  }

  const debugLevels = ['warning', 'info', 'debug'];
  const debugLevelIndex = debugLevels.indexOf(options.debug);
  function log(msg, level) {
    if (level === 0) {
      msg = `WARNING - ${msg}`;
    } else {
      level = level || 1;
    }
    if (level <= debugLevelIndex) {
      console.log('[YarnFileCopy] ' + msg); // eslint-disable-line no-console
    }
  }

  function warning(msg) {
    log(msg, 0);
  }

  function info(msg) {
    log(msg, 1);
  }

  function debug(msg) {
    log(msg, 2);
  }

  const apply = (compiler) => {
    compiler.plugin('emit', (compilation, cb) => {
      debug('starting emit');
      const callback = () => {
        debug('finishing emit');
        cb();
      };
      var fs = require('fs'),
      readline = require('readline'),
      instream = fs.createReadStream(options.indexFile),
      outstream = new(require('stream'))(),
      rl = readline.createInterface(instream, outstream);

      var content = '';
      rl.on('line', (line) => {
        debug(line);
        if (line.indexOf('yarn:css') > -1) {
          options.assets
          .filter(asset => asset.endsWith('.css'))
          .forEach((asset) => {
            content += '    <link href="' + options.cssDir + '/' + asset + '" data-remove="exclude" data-build="exclude" type="text/css" rel="stylesheet">\n';
          });
        } else if (line.indexOf('yarn:js') > -1) {
          options.assets
          .filter(asset => asset.endsWith('.js'))
          .forEach((asset) => {
            content += '    <script src="' + options.jsDir + '/' + asset + '" data-remove="exclude" data-build="exclude"></script>\n';
          });
        } else {
          content += line + '\n';
        }
      });

      rl.on('close', (line) => {
        var file = require('fs');
        file.writeFile(options.outputDir + '/index.html', content, (err) =>{
          if (err) {
            throw err;
          }
          debug('new index saved');
        });
      });

      callback();
    });

    compiler.plugin('after-emit', (compilation, cb) => {
      debug('starting after-emit');
      const callback = () => {
        debug('finishing after-emit');
        cb();
      };

      callback();
    });
  }

  return {
    apply
  };
}

module.exports = YarnFileCopy;