var f formatter
	v7 := tw.blk.toV7()
	v7.typeFlag()[0] = flag
	f.formatString(v7.name(), name)
	f.formatOctal(v7.mode(), 0)
	f.formatOctal(v7.uid(), 0)
	f.formatOctal(v7.gid(), 0)
	f.formatOctal(v7.size(), int64(len(data))) // Must be < 8GiB
	f.formatOctal(v7.modTime(), 0)
	tw.blk.setFormat(format)
	if f.err != nil {
		return f.err // Only occurs if size condition is violated
	}

func (f *formatter) formatNumeric(b []byte, x int64) {
	if fitsInOctal(len(b), x) {
		f.formatOctal(b, x)
		return
	}

	if fitsInBase256(len(b), x) {
		for i := len(b) - 1; i >= 0; i-- {
			b[i] = byte(x)
			x >>= 8
		}
		b[0] |= 0x80 // Highest bit indicates binary format
		return
	}

	f.formatOctal(b, 0) // Last resort, just write zero
	f.err = ErrFieldTooLong
}

	// Update checksum.
	// This field is special in that it is terminated by a NULL then space.
	var f formatter
	field := b.toV7().chksum()
	chksum, _ := b.computeChecksum() // Possible values are 256..128776
	f.formatOctal(field[:7], chksum) // Never fails since 128776 < 262143
	field[7] = ' '
}

// computeChecksum computes the checksum for the header block.
// POSIX specifies a sum of the unsigned byte values, but the Sun tar used