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&#39;s say `N`. The naive approach would be something like *&quot;generate `N` unique alphanumeric codes, put them into database and perform a db search on every coupon operation.&quot;*

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&#39;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&#39;t pass this requirement.

- Practical uniqueness. Results of `MakeCoupon(n)` for every natural `n &lt;= 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&#39;t be obvious how to enumerate all remaining coupons from a single coupon code - let&#39;s say `MakeCoupon(n)` and `MakeCoupon(n + 1)` should visually differ.
&gt; E.g. `MakeCoupon(n),` which simply outputs `n` padded with zeroes would fail this requirement, because `000001` and `000002` don&#39;t actually differ &quot;visually&quot;.

Q:
--

**Does any function or function generator, which fullfills the following requirements, exist?** My search attempts only lead me to [`[CPAN]` CouponCode,][1] but it does not fullfill the requirement of the corresponding function being bijective.


  [1]: http://search.cpan.org/dist/Algorithm-CouponCode/lib/Algorithm/CouponCode.pm

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Coupon code generation