# Generating a Unique Number

17 Jan 2008

Weird requirement at work today: We needed a number that was alpha-numeric, exactly 11 digits, unique, and non-sequential. At first we thought of using a hash, but MD5 and Sha1 give you way too many digits. We could truncate to 11, but not knowing much about hashing, that made me pretty nervous that we’d have collisions. After a bunch of discussion, we decided on this:

MAXIMUM_FOR_10_DIGITS = 0xffffffffff
(MAXIMUM_FOR_10_DIGITS + cart_id + Time.now.to_i + rand(100_000_000)).to_s(16)

I was thinking about using hex, but I was worried about what would happen if the number got too big and went to 12 digits. So I did this calculation:

irb(main):001:0> 0xfffffffffff - 0x10000000000
=> 16492674416639

Turns out there’s all sorts of room between the lowest and highest 11 digit hex number. Paul and Schubert did some quick calculations assuming lots of carts per day and figured out that we have about 250,000 years before we run out of space. Now since we use a random number, there is a chance that we could get two matching numbers so we do a database lookup to make sure that a generated number hasn’t been used before. There is still a small chance that two identical numbers could be generated in the time between generation and saving to the database, but I think we can live with that.

Update: After some interesting comments, I wrote a follow up post. Check it out for more math goodness.