Coupon code generation

I would like to generate coupon codes , e.g. AYB4ZZ2. However, I would also like to be able to mark the used coupons and limit their global number, let's say N. The naive approach would be something like "generate N unique alphanumeric codes, put them into database and perform a db search on every coupon operation."

However, as far as I realize, we can also attempt to find a function MakeCoupon(n), which converts the given number into a coupon-like string with predefined length.

As far as I understand, MakeCoupon should fullfill the following requirements:

• Be bijective. It's inverse MakeNumber(coupon) should be effectively computable.

• Output for MakeCoupon(n) should be alphanumeric and should have small and constant length - so that it could be called human readable. E.g. SHA1 digest wouldn't pass this requirement.

• Practical uniqueness. Results of MakeCoupon(n) for every natural n <= N should be totally unique or unique in the same terms as, for example, MD5 is unique (with the same extremely small collision probability).

• (this one is tricky to define) It shouldn't be obvious how to enumerate all remaining coupons from a single coupon code - let's say MakeCoupon(n) and MakeCoupon(n + 1) should visually differ.

  E.g. MakeCoupon(n), which simply outputs n padded with zeroes would fail this requirement, because 000001 and 000002 don't actually differ "visually".

Q:

Does any function or function generator, which fullfills the following requirements, exist? My search attempts only lead me to [CPAN] CouponCode, but it does not fullfill the requirement of the corresponding function being bijective.