Creating a unique alphanumeric 10-character string

Question

I'm looking to create a simple short-lived reservation system, and I'd like to generate confirmation numbers that are

• unique
• random-looking
• alphanumeric
• short-ish, at least much shorter than 32 character-long strings returned by sha1

I'm only looking to have ~500 reservations, so I don't imagine high likelyhood of collissions.

One idea I had is generate an sha1 hash based on a date-time stamp and username, then truncating it to its first 10 characters. Would something like that be reliably unique enough for the purposes of processing ~500 reservations?

Answer 1

There should be no difference in the randomness of any given bit of a SHA-1 hash, so that's possible. Another way would be to fold the hash into itself using XOR until you have 60 bits worth of data, then encode it using Base 64 to get a mostly alpha-numeric result.

This is only necessary if you want to be able to generate the same Id repeatedly for the same input data. Otherwise, if a random id that you generate once, and hold onto after that, use Anders' suggestion. If you get a conflict, just generate another one.

Answer 2

You can use whatever, even a plain random number generator; however, you should check that the reservation code isn't already present. If this is the case, add characters ('x') to the string (date+user) until you get a new random/sha1/etc.

I'm only looking to have ~500 reservations, so I don't imagine high likelyhood of collissions.

Another stupid idea: generate 1000 or 2000 unique random numbers with the desired properties, store them somewhere, and assign them to the users as they register :)

Answer 3

use a guid? 16 characters, though if you really don't care about collision, you could just choose the first n characters.

Answer 4

Some good tips on this question: http://stackoverflow.com/questions/440133/how-do-i-create-a-random-alpha-numeric-string-in-c

I'd avoid including characters like "1", "l", and "O", "0" and "5", "S", and "Z", "2" in your string, to make it easier for customers when they need to read your reservation code over the phone. The algorithm presented at that link should help you do this.

Answer 5

If it's really just 500, then pre-generate 20,000 of them, into a table, then get the "next unused one" when you need it.

Answer 6

In C# you can use http://www.dotnetfunda.com/forums/thread1357-how-do-generate-unique-alpha-numeric-random-number-in-aspnet.aspx (the super easy way, they say)