if (typeof emitter.prependListener === 'function') {
    return emitter.prependListener(event, fn);
  } else {
    // This is a hack to make sure that our error handler is attached before any
    // userland ones.  NEVER DO THIS. This is here only because this code needs
    // to continue to work with older versions of Node.js that do not include
    // the prependListener() method. The goal is to eventually remove this hack.
    if (!emitter._events || !emitter._events[event]) emitter.on(event, fn);else if (isArray(emitter._events[event])) emitter._events[event].unshift(fn);else emitter._events[event] = [fn, emitter._events[event]];
  }
}
var Duplex;
function ReadableState(options, stream) {
  Duplex = Duplex || require('./_stream_duplex');
  options = options || {};
  // object stream flag. Used to make read(n) ignore n and to
  // make all the buffer merging and length checks go away
  this.objectMode = !!options.objectMode;
  if (stream instanceof Duplex) this.objectMode = this.objectMode || !!options.readableObjectMode;
  // the point at which it stops calling _read() to fill the buffer
  // Note: 0 is a valid value, means "don't call _read preemptively ever"
  var hwm = options.highWaterMark;
  var defaultHwm = this.objectMode ? 16 : 16 * 1024;
  this.highWaterMark = hwm || hwm === 0 ? hwm : defaultHwm;
  // cast to ints.
  this.highWaterMark = ~ ~this.highWaterMark;
  // A linked list is used to store data chunks instead of an array because the
  // linked list can remove elements from the beginning faster than
  // array.shift()
  this.buffer = new BufferList();
  this.length = 0;
  this.pipes = null;
  this.pipesCount = 0;
  this.flowing = null;
  this.ended = false;
  this.endEmitted = false;
  this.reading = false;
  // a flag to be able to tell if the onwrite cb is called immediately,
  // or on a later tick.  We set this to true at first, because any
  // actions that shouldn't happen until "later" should generally also
  // not happen before the first write call.
  this.sync = true;
  // whenever we return null, then we set a flag to say
  // that we're awaiting a 'readable' event emission.
  this.needReadable = false;
  this.emittedReadable = false;
  this.readableListening = false;
  this.resumeScheduled = false;
  // Crypto is kind of old and crusty.  Historically, its default string
  // encoding is 'binary' so we have to make this configurable.
  // Everything else in the universe uses 'utf8', though.
  this.defaultEncoding = options.defaultEncoding || 'utf8';
  // when piping, we only care about 'readable' events that happen
  // after read()ing all the bytes and not getting any pushback.
  this.ranOut = false;
  // the number of writers that are awaiting a drain event in .pipe()s
  this.awaitDrain = 0;
  // if true, a maybeReadMore has been scheduled
  this.readingMore = false;
  this.decoder = null;
  this.encoding = null;
  if (options.encoding) {
    if (!StringDecoder) StringDecoder = require('string_decoder/').StringDecoder;
    this.decoder = new StringDecoder(options.encoding);
    this.encoding = options.encoding;
  }
}
var Duplex;
function Readable(options) {
  Duplex = Duplex || require('./_stream_duplex');
  if (!(this instanceof Readable)) return new Readable(options);
  this._readableState = new ReadableState(options, this);
  // legacy
  this.readable = true;
  if (options && typeof options.read === 'function') this._read = options.read;
  Stream.call(this);
}
// Manually shove something into the read() buffer.
// This returns true if the highWaterMark has not been hit yet,
// similar to how Writable.write() returns true if you should
// write() some more.
Readable.prototype.push = function (chunk, encoding) {
  var state = this._readableState;
  if (!state.objectMode && typeof chunk === 'string') {
    encoding = encoding || state.defaultEncoding;
    if (encoding !== state.encoding) {
      chunk = bufferShim.from(chunk, encoding);
      encoding = '';
    }
  }
  return readableAddChunk(this, state, chunk, encoding, false);
};
// Unshift should *always* be something directly out of read()
Readable.prototype.unshift = function (chunk) {
  var state = this._readableState;
  return readableAddChunk(this, state, chunk, '', true);
};
Readable.prototype.isPaused = function () {
  return this._readableState.flowing === false;
};
function readableAddChunk(stream, state, chunk, encoding, addToFront) {
  var er = chunkInvalid(state, chunk);
  if (er) {
    stream.emit('error', er);
  } else if (chunk === null) {
    state.reading = false;
    onEofChunk(stream, state);
  } else if (state.objectMode || chunk && chunk.length > 0) {
    if (state.ended && !addToFront) {
      var e = new Error('stream.push() after EOF');
      stream.emit('error', e);
    } else if (state.endEmitted && addToFront) {
      var _e = new Error('stream.unshift() after end event');
      stream.emit('error', _e);
    } else {
      var skipAdd;
      if (state.decoder && !addToFront && !encoding) {
        chunk = state.decoder.write(chunk);
        skipAdd = !state.objectMode && chunk.length === 0;
      }
      if (!addToFront) state.reading = false;
      // Don't add to the buffer if we've decoded to an empty string chunk and
      // we're not in object mode
      if (!skipAdd) {
        // if we want the data now, just emit it.
        if (state.flowing && state.length === 0 && !state.sync) {
          stream.emit('data', chunk);
          stream.read(0);
        } else {
          // update the buffer info.
          state.length += state.objectMode ? 1 : chunk.length;
          if (addToFront) state.buffer.unshift(chunk);else state.buffer.push(chunk);
          if (state.needReadable) emitReadable(stream);
        }
      }
      maybeReadMore(stream, state);
    }
  } else if (!addToFront) {
    state.reading = false;
  }
  return needMoreData(state);
}