smart way to generate unique random number

Question

i want to generate a sequence of unique random numbers in the range of 00000001 to 99999999.

So the first one might be 00001010, the second 40002928 etc.

The easy way is to generate a random number and store it in the database, and every next time do it again and check in the database if the number already exists and if so, generate a new one, check it again, etc. But that doesn't look right, i could be regenerating a number maybe 100 times if the number of generated items gets large.

Is there a smarter way?

EDIT as allways i forgot to say WHY i wanted this, and it will probably make things clearer and maybe get an alternative, and it is: we want to generate an ordernumber for a booking, so we could just use 000001, 000002 etc. But we don't want to give the competitors a clue of how much orders are created (because it's not a high volume market, and we don't want them to know if we are on order 30 after 2 months or at order 100. So we want to have an order number which is random (yet unique)

Answer 1

You can use either an Linear Congruential Generator (LCG) or Linear Feedback Shift Register (LFSR). Google or wikipedia for more info.

Both can, with the right parameters, operate on a 'full-cycle' (or 'full period') basis so that they will generate a 'psuedo-random number' only once in a single period, and generate all numbers within the range. Both are 'weak' generators, so no good for cyptography, but perhaps 'good enough' for apparent randomness. You may have to constrain the period to work within your 'decimal' maximum as having 'binary' periods is necessary.

Update: I should add that it is not necessary to pre-calculate or pre-store previous values in any way, you only need to keep the previous seed-value (single int) and calculate 'on-demand' the next number in the sequence. Of course you can save a chain of pre-calculated numbers to your DB if desired, but it isn't necessary.

Answer 2

How about creating a set all of possible numbers and simply randomising the order? You could then just pick the next number from the tail.

Each number appears only once in the set, and when you want a new one it has already been generated, so the overhead is tiny at the point at which you want one. You could do this in memory or the database of your choice. You'll just need a sensible locking strategy for pulling the next available number.