Randomizing "statically" on a daily/weekly basis

hacker has asked for the wisdom of the Perl Monks concerning the following question:

I have a need to randomize a header image on a CGI on a slightly-longer interval than I am doing right now (currently every page load). I can randomize the image on every page load, but I'd like it to remain static for a longer period of time.

What's the best way to do this, without writing any lock files or without adding a row in a database, if possible? If not, what's the best way to do this with a drop file?

Or is there another approach aside from using 'my $random = int(rand(n));' on a periodic basis or every page load?

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Re: Randomizing "statically" on a daily/weekly basis by Zaxo (Archbishop) on Mar 11, 2005 at 02:18 UTC
Set up a cron job to select an image and store its uri in a template or config file. You can pick any interval you like with that and it adds little overhead to the system. After Compline, Zaxo	[reply]
Re: Randomizing "statically" on a daily/weekly basis by japhy (Canon) on Mar 11, 2005 at 02:23 UTC
Heh, a friend of mine just did something similar. He has a "quote of the minute" on his web site. The concept is thus: `my @images = (...); # new one each minute my $display = $images[(time/60) % @images];` [download] Is that sufficient? Divide by 3600 for an image each hour, etc. _____________________________________________________ Jeff `japhy` Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and `perl` hacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart	[reply] [d/l]
Re: Randomizing "statically" on a daily/weekly basis by jdporter (Paladin) on Mar 11, 2005 at 02:22 UTC
Not sure how you're currently randomizing; but if you're using `rand()`, you could try seeding the RNG with just the date part of the current epoch time; or, e.g. take the epoch time (returned from `time()`, that is), shift it right by 11 bits, and seed the RNG with that. This will cause rand() to return the same value all day (or whatever the interval is).	[reply] [d/l] [select]
Re: Randomizing "statically" on a daily/weekly basis by Thilosophy (Curate) on Mar 11, 2005 at 02:23 UTC
I think you want a timed rotation: `my $interval = 60*60; # one hour my $random = int(time/$interval)%$n; # not random, though` [download] This will make a new number between 0 .. n every hour. Totally predictable, but that is probably okay in this case.	[reply] [d/l]
Re: Randomizing "statically" on a daily/weekly basis by jhourcle (Prior) on Mar 11, 2005 at 02:26 UTC
The 'best' way depends on what resources you have to deal with. Personally, I'd just set a cron job to randomly select the image each morning, and move it from whatever holding spot I had to where the general public would see it. If you only have CGI, and no cron, then I'd just use a randomizing function that took the time as a seed, and was deterministic. For instance, the modulus of number of days since Jan 1, 2000 and the number of images I had to rotate, if it were to change on a daily basis. (no, it's not completely random...it's just an example ... you can use whatever function you want, so long as it returns the same value for each seed.) If you want it more random, just use rand() and Memoize.	[reply]
Re: Randomizing "statically" on a daily/weekly basis by betterworld (Curate) on Mar 11, 2005 at 02:31 UTC
Write a daemon (or cronjob) that randomizes the image once per hour or something. Let's assume that your CGI script looks for the image in the file foo.jpeg. Then your daemon would write the new image to foo.jpeg.tmp, and when it's finished, it issues `rename("foo.jpeg.tmp", "foo.jpeg")`, overwriting the old image. If you use Unix, this works without locking, because the rename(2) call is atomic and there is no time where a CGI program would find a half-ready image or something like this.	[reply] [d/l]