classMap function

[stell]

The classMap function uses a key/value collection of css classes organized by date.
What i would love to do ist use a callback function like blackout uses.
I have a string like "YNNYNYNYNNNNNNNNNNNYYYYNYNYYYNYNYYYYYY".

First "Y" of that String stands for 01-01-2017 and so on. So the callbackfunction should just return a class like "free" if date matches "Y" and "closed" if it matches "N".

I don't see how that can be done with the key/value collection.