text: // Copyright Joyent, Inc. and other Node contributors.
  //
  // Permission is hereby granted, free of charge, to any person obtaining a
  // copy of this software and associated documentation files (the
  // "Software"), to deal in the Software without restriction, including
  // without limitation the rights to use, copy, modify, merge, publish,
  // distribute, sublicense, and/or sell copies of the Software, and to permit
  // persons to whom the Software is furnished to do so, subject to the
  // following conditions:
  //
  // The above copyright notice and this permission notice shall be included
  // in all copies or substantial portions of the Software.
  //
  // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
  // OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
  // MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
  // NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
  // DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
  // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
  // USE OR OTHER DEALINGS IN THE SOFTWARE.
  
  // a duplex stream is just a stream that is both readable and writable.
  // Since JS doesn't have multiple prototypal inheritance, this class
  // prototypally inherits from Readable, and then parasitically from
  // Writable.
  
  module.exports = Duplex;
  var util = require('util');
  var Readable = require('./_stream_readable');
  var Writable = require('./_stream_writable');
  
  util.inherits(Duplex, Readable);
  
  Object.keys(Writable.prototype).forEach(function (method) {
      if (!Duplex.prototype[method]) {
          Duplex.prototype[method] = Writable.prototype[method];
      }
  });
  
  function Duplex(options) {
      if (!(this instanceof Duplex)) {
          return new Duplex(options);
      }
  
      Readable.call(this, options);
      Writable.call(this, options);
  
      if (options && options.readable === false) {
          this.readable = false;
      }
  
      if (options && options.writable === false) {
          this.writable = false;
      }
  
      this.allowHalfOpen = true;
      if (options && options.allowHalfOpen === false) {
          this.allowHalfOpen = false;
      }
  
      this.once('end', onend);
  }
  
  // the no-half-open enforcer
  function onend() {
      // if we allow half-open state, or if the writable side ended,
      // then we're ok.
      if (this.allowHalfOpen || this._writableState.ended) {
          return;
      }
  
      // no more data can be written.
      // But allow more writes to happen in this tick.
      process.nextTick(this.end.bind(this));
  }
  