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.