code: function Ascii85(options) {
    var decodingTable;
  
    options = options || {};
    this._options = options;
  
    // generate encoding and decoding table.
    if (Array.isArray(options.table)) {
      decodingTable = [];
      options.table.forEach(function(v, i) {
        decodingTable[v.charCodeAt(0)] = i;
      });
  
      options.encodingTable = options.table;
      options.decodingTable = decodingTable;
    }
  }
docs: Create a new Ascii85 codec.
  @param {Array|Object} [options] is a list of chars for encoding or an option object.
  
  Supported options are listed in Ascii85#encode document.
  @note Only encoding table is supported. Decoding table will be generated automatically
  based on encoding table.