All entries must support the same API to be considered.

So what's the API? Specifically:

Digits will be passed in one at a time.

To what are the digits passed?
To a program?
To a process?
To a function?

How are the digits passed?
By argument?
As a character on STDIN?
As a line on STDIN?

The expected return value will be...

How is the value returned?
Function return?
STDOUT with newline?

Update: Oops, I see that you've already partially answered this, and the rest can be deduced from that answer. (function/method, function argument, function return)