Nibbler - multi-byte non-byte-aligned reader for little endian files

nibbler.js

// Nibbler - wrapper for Cursorize that allows for reading things that don't fall on even byte boundaries
// (individual bits, reading 5 bits from one byte and 3 bits from the next, etc.)
// Yes, this name is a reference to Futurama. And yes, like the character there, this will likely
// shit up your program with really valuable functionality!

var Nibbler = module.exports = function(curs) {
  if(!(this instanceof Nibbler)) return new Nibbler(curs);

  this.curs = curs;
  this.shifted = 0;
};

var loBitMask = [ 0, 1, 3, 7, 15, 31, 63, 127, 255 ];

Nibbler.prototype._shift = function(cnt) {
  if(this.shifted + cnt > 8) throw new RangeError('Not enough bits remaining.');

  if(!this.shifted)
    this.data = this.curs.read()[0];

  var val = (this.data >> this.shifted) & loBitMask[cnt];
  this.shifted = (this.shifted + cnt) % 8;
  return val;
};

Nibbler.prototype.readBits = function(cnt) {
  if(cnt <= 0) throw new RangeError('cnt must be > 0');

  var numBytes = cnt >>> 3,
      numBits = cnt % 8,
      val;

  if(this.shifted + cnt <= 8)
    return [ this._shift(cnt) ];

  if(this.shifted === 0) {
    val = this.curs.read(numBytes);
    if(numBits > 0) val.push(this._shift(numBits));
    return val;
  }

  val = [];
  var loMask = loBitMask[this.shifted],
      hiMask = 0xFF ^ loMask,
      hi = this.data & hiMask,
      lo,
      hiShift = 8 - this.shifted,
      loShift = this.shifted;

  while(numBytes > 0) {
    this.data = this.curs.read()[0];
    numBytes--;
    lo = this.data & loMask;
    val.push((hi >>> hiShift) | (lo << loShift));
    hi = this.data & hiMask;
  }

  if(numBits == 0) return val;

  if(this.shifted + numBits <= 8) {
    val.push(this._shift(numBits));
    return val;
  }

  hi = hi >>> hiShift;
  this.data = this.curs.read()[0];
  var bitsAfterBoundary = numBits - (8 - this.shifted);
  lo = (this.data & loBitMask[bitsAfterBoundary]) << this.shifted;
  val.push(hi | lo);
  this.shifted = this.shifted + numBits - 8;
  return val;
};

Nibbler.prototype.readBytes = function(cnt) {
  return this.readBits(cnt << 3);
};

Nibbler.prototype.readCombo = function(nBytes, nBits) {
  return this.readBits((nBytes << 3) + nBits);
};

Nibbler.prototype.readToBoundary = function() {
  if(this.shifted === 0) return [];
  var val = [ (this.data & (0xFF ^ loBitMask[this.shifted])) >>> this.shifted ];
  this.shifted = 0;
  return val;
};