From: Joel VanderWerf <vjoel@...>
Date: 2007-04-25T01:01:38+09:00
Subject: Re: Best way to create a string with gap (0x00's) in the middle?

Earle Clubb wrote:
> I'm trying to build a packet to send to UDPSocket::send.  Send requires
> a string, but I'm not sure of the best way to build this string.  The
> packet's structure requires two strings: one beginning at byte 0 and the
> other at byte 64.  The packet is 128 bytes long and must contain 0's in
> the bytes that the strings don't use.  Here is an example packet:
> 
> 54 68 69 73 20 69 73 20 73 74 72 69 6E 67 20 41    This is string A
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
> 4E 6F 77 20 77 65 20 68 61 76 65 20 73 74 72 69    Now we have stri
> 6E 67 20 42 00 00 00 00 00 00 00 00 00 00 00 00    ng B............
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
> 
> I have a solution, but it's not very elegant and I'm looking for a
> better way of doing this.  Here's what I have so far:
> 
> I tried this first, but ended up with a bunch of 0x30's where the 0x00's
> should be:
> 
> dgram = Array.new(128, 0)
> dgram[0] = "This is string A".to_a
> dgram[64] = "Now we have string B".to_a
> UDPSocket.open.send(dgram.to_s, 0, host, port)
> 
> dgram.to_s converts all of the 0x00's in the array to 0x30's in the
> string, which is not acceptable.
> 
> 
> This is what I have right now:
> 
> dgram = ""
> 94.times {dgram << 0}
> dgram[0] = "This is string A"
> dgram[64] = "Now we have string B"
> UDPSocket.open.send(dgram, 0, host, port)
> 
> Like I said: it works, but I don't really like it.  Any ideas for a
> better implementation?
> 
> Thanks,
> Earle
> 

BLANK_DGRAM = "\0"*128
dgram = BLANK_DGRAM.dup

s1 = "This is string A"
s2 = "Now we have string B"

dgram[0...s1.length] = s1
dgram[64...64+s2.length] = s2


If you think you might need to fill in other kinds of fields besides 
fixed-length strings, you could use a BitStruct:

require 'bit-struct' # http://redshift.sourceforge.net/bit-struct

class Dgram < BitStruct
   char :s1, 64*8
   char :s2, 64*8
end

dgram = Dgram.new
dgram.s1 = "This is string A"
dgram.s2 = "Now we have string B"

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407