Note that C does call by value, so a const for a function parameter doesn't make much difference for the caller - it still sends a copy of a variable and can't get it's own variable modified.

It's only when you send a pointer that the const keyword (and the location of it) matters - if the pointer is const, or if the target value is const.

Note that your function is a bit of a maintainance nightmare, because it is so easy for a programmer to miss that one of your case statements doesn't have a break or return, but instead falls through to the default statement. How much do you save by the fall through, instead of having a return '0'?