data block   // the bits to read
	off  uint32  // current offset into data
	bits uint32  // bits ready to be returned
	cnt  uint32  // number of valid bits in the bits field
}

// makeBitReader makes a bit reader starting at off.
func (r *Reader) makeBitReader(data block, off int) bitReader {
	return bitReader{
		r:    r,
		data: data,
		off:  uint32(off),
	}
}

// moreBits is called to read more bits.

// This returns the number of bits in the FSE table and the new offset.
// RFC 4.1.1.
func (r *Reader) readFSE(data block, off, maxSym, maxBits int, table []fseEntry) (tableBits, roff int, err error) {
	br := r.makeBitReader(data, off)
	if err := br.moreBits(); err != nil {
		return 0, 0, err
	}

	accuracyLog := int(br.val(4)) + 5
	if accuracyLog > maxBits {
		return 0, 0, br.makeError("FSE accuracy log too large")
	}