Hi all,

I have a few questions about the srand (or mt_) function for directed seeding.

I'm assuming if I do a targeting seeding with a particular value, I'll get the same sequence of values no matter how many times I run the page (assuming it's on the same OS and php version). Of course, if this is incorrect, please let me know. But I have a few questions.

1) What is the persistence of the random seed?
a) If I run another script on the same server subdomain that does NOT seed the random number generator, will it re-seed from a "random value" (as the manual puts it) or will it run off the sequence from the seed initialized in my targetted script? On another subdomain hosted on the same computer (same user account, etc)? Does it make a difference if the page has the same URL (I replace one version of the page with another that removes the seed line or use the seed function in an include file used by both scripts)? b) If I seed in two separate pages with the same seed number and draw 10 random values, I'd expect to get the same sequence if the two scripts are run sequentially. But what if I seeded one, put in a 20 second delay (but did not grab any values yet), ran the other script (without the delay), and then allowed the first script to finish by grabbing 10 values. Would the draw from one page affect the other? c) If each file/script does not inherit the random seed, locally what is the heritability? In other words, does the sequence persist outside of the function in which the random number is seeded? Or does a different function re-seed itself automatically? d) Would php's random number generator be influenced by other programs running on the system (assuming they're not tampering with the memory allocated to the running script)?

2) What is the cycle period?
a) Is a seeded random value expected to repeat its generated sequence in fewer than getrandmax for a seeded random number? An unseeded one?
b) Is there built-in cycle detection and auto-reseeding?
c) If I draw the same number (e.g. 12345) as before, would I expect the next value to be the same as the previous time I saw 12345 (assuming my range is from 0 to getrandmax)? Or is there a secondary number stored somewhere which is what the next random number is calculated in conjunction with? d) Has anyone ever looked into the distribution of cycle times (if it does indeed cycle). If so, what's the mean, median, variance/stdev and is it a near-gaussian distribution? e) Are any seed values known to be intrinsically poor at avoiding a cycle? Are there any (known) seed values that bias draws high/low or even/odd, or can I assume given enough calls I'll get ABOUT half and half (gaussian-like distribution)?

3) To simulate the basal behavior of the unseeded rand call (but want to repeat with that same seed value later), do I just get the microtime, store it somewhere (e.g. mtvar), and then call srand($mtvar)? Or is the autogenerated "random seed" not based on the current clock or via a hash of the clock?

If any of these answers are significantly different for mt_srand vs srand, I'd appreciate knowing about both. However, I'll take whatever information you do know. So, even if you only know the answer to one part of one of these questions, I'd still appreciate your response.


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