Best way to generate order numbers for an online store?

Question

Every order in my online store has a user-facing order number. I'm wondering the best way to generate them. Criteria include:

• Short
• Easy to say over the phone (e.g., "m" and "n" are ambiguous)
• Unique
• Checksum (overkill? Useful?)
• Edit: Doesn't reveal how many total orders there have been (a customer might find it unnerving to make your 3rd order)

Right now I'm using the following method (no checksum):

def generate_number                     possible_values = 'abfhijlqrstuxy'.upcase.split('') | '123456789'.split('')      record = true     while record         random = Array.new(5){possible_values[rand(possible_values.size)]}.join         record = Order.find(:first, :conditions => ["number = ?", random])     end               self.number = random end 

Answer 1

As a customer I would be happy with:

year-month-day/short_uid 

for example:

2009-07-27/KT1E 

It gives room for about 33^4 ~ 1mln orders a day.

Answer 2

Here is an implementation for a system I proposed in an earlier question:

MAGIC = []; 29.downto(0) {|i| MAGIC << 839712541[i]}  def convert(num)   order = 0   0.upto(MAGIC.length - 1)  {|i| order = order << 1 | (num[i] ^ MAGIC[i]) }   order end 

It's just a cheap hash function, but it makes it difficult for an average user to determine how many orders have been processed, or a number for another order. It won't run out of space until you've done 2³⁰ orders, which you won't hit any time soon.

Here are the results of convert(x) for 1 through 10:

1:  302841629 2:  571277085 3:   34406173 4:  973930269 5:  437059357 6:  705494813 7:  168623901 8:  906821405 9:  369950493 10: 638385949