On 2009-08-17, Sam Mason <sam@xxxxxxxxxxxxx> wrote: > On Sun, Aug 16, 2009 at 04:53:01PM -0600, Bob Gobeille wrote: >> One way is to use a LFSR (linear feedback shift register function). I >> haven't used one in a long time but I recall generating pseudo random >> numbers that are guaranteed not to repeat after billions of >> iterations. It's very fast as well. Then translate the resulting >> integer into the character sequence of your choosing. Here is a >> reference: http://en.wikipedia.org/wiki/Linear_feedback_shift_register > > Not sure if this is very applicable; LFSRs can have a very long period, > or interval before they repeat ideally (2^n)-1 (i.e. their internal state is the same as > it was before) but individual numbers *will* be repeated. numbers will not be repeated intil the state wraps if the number returned represents the entire state of the LFSR. for the OP's problem this means building a LFSR with n=5c (where c is the number of charactes in the serial code, and n is the number of bits in the LFSR state) and then taking a single LFSR result and peeling off 5 bits at a time and using each 5 to make each charcter in the result. (or anternately clocking c 5 bit numbers out of the LFSR) for each result. > The performance claims tend only to apply to hardware implementations, > there are much faster pseudo-random number generators available for > software. The fastest one I found recently is a SIMD implementation of > the "Mersenne Twister" called SFMT[1]. LFSR can be optimised using look-up tables same as CRC32 is (CRC32 is an LFSR with an input as well as an output) MT is too big (too much state), linear congrutential is probably a better approach when a software generated non-revisiting sequence is wanted. for good results use a prime m less than 32^c and an r near sqrt(32^c) -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
