size

    pub fn size(header: Header) !u64 {
        const start = 124;
        const len = 12;
        const raw = header.bytes[start..][0..len];
        //  If the leading byte is 0xff (255), all the bytes of the field
        //  (including the leading byte) are concatenated in big-endian order,
        //  with the result being a negative number expressed in two’s
        //  complement form.
        if (raw[0] == 0xff) return error.TarNumericValueNegative;
        // If the leading byte is 0x80 (128), the non-leading bytes of the
        // field are concatenated in big-endian order.
        if (raw[0] == 0x80) {
            if (raw[1] != 0 or raw[2] != 0 or raw[3] != 0) return error.TarNumericValueTooBig;
            return std.mem.readInt(u64, raw[4..12], .big);
        }
        return try header.octal(start, len);
    }