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I was recently enjoying(sic) seeing 'script kiddies' trying to scan the ports on my home box. I wrote a script to parse out the packet DENY entries from my messages log file and realized that I should sort the list of IP's to make them easier to read.

Thoughts of complex sorting started to form in my head: split up the quad's and start comparing. I started to look for some elegant ways of doing this, when I realized how much more simple (and potentially more efficient) it would be to pack each IP into it's hex form. "This has to be a much, much faster way", I thought to myself.

So I did a search at Google and found this page written by Uri Guttman and Larry Rosler. They discuss this very thing - giving one example that sorts by breaking up the IP bytes and comparing them, and another example that packs the IP's into hex.

Here are their two examples, slightly paraphrased:

#naive: O(N*logN) my @sorted = sort { my @a = $a =~ /(\d+)\.(\d+)\.(\d+)\.(\d+)/; my @b = $b =~ /(\d+)\.(\d+)\.(\d+)\.(\d+)/; $a[0] <=> $b[0] || $a[1] <=> $b[1] || $a[2] <=> $b[2] || $a[3] <=> $b[3] } @unsorted; #packed: O(N*logN) my @sorted = sort { pack('C4' => $a =~ /(\d+)\.(\d+)\.(\d+)\.(\d+)/) cmp pack('C4' => $b =~ /(\d+)\.(\d+)\.(\d+)\.(\d+)/) } @unsorted;
What gets me is the fact that they have the same Big(O), but when benchmarked:
Benchmark: timing 5000 iterations of naive, packed... naive: 11 wallclock secs ( 9.91 usr + 0.32 sys = 10.23 CPU) packed: 8 wallclock secs ( 7.95 usr + 0.08 sys = 8.03 CPU)
there is an obvious difference.

To be honest, I really hated Big(O)analysis in college, and I think this is the very reason why. There is an obvious amount of better efficiency in the packed version, but in a constant way - this of course is eliminated when doing Big(O), making both algorithms 'equal'.

I am posting this mainly because I couldn't find any good IP sorting algorithms on PM, but I was also wondering how other Monks out there feel about the Big(O).

Jeff Anderson
tyring to be a better programmer every day . . .

UPDATE: Fri Aug 4 09:31:30 CDT 2000
Thanks to everyone for helping me understand the error in my ways - this whole time I was trying to compare apples to oranges, no wonder I had such a hard time understanding O(n). Big thanks to gryng for such a wonderful and heart-felt explaination. You rule.


In reply to Sorting a list of IP addresses (aka Why I hate Big O) by jeffa

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