for p := binary.MaxVarintLen64; p < size; n++ {
		x, w := binary.Uvarint(buf[p:])
		data.set(n, int64(x))
		p += w
	}

	return
}

const bufSize = 16 << 10 // reasonable for BenchmarkSaveRestore

// Read reads the index from r into x; x must not be nil.
func (x *Index) Read(r io.Reader) error {
	// buffer for all reads
	buf := make([]byte, bufSize)

	// read length