return sw.sp[len(sw.sp)-1].endOffset() - sw.pos
}
func (sw sparseFileWriter) physicalRemaining() int64 {
	return sw.fw.physicalRemaining()
}

// zeroWriter may only be written with NULs, otherwise it returns errWriteHole.
type zeroWriter struct{}

func (zeroWriter) Write(b []byte) (int, error) {
	for i, c := range b {
		if c != 0 {
			return i, errWriteHole
		}
	}
	return len(b), nil
}