Turn a float into a value between 0 and 1 (inclusive)?

[Chirs Forest]

I'm interpolating some values and I need to make an 
(elapsed_time/duration) value a float between 0 and 1 (inclusive 
of 0 and 1). The elapsed_time might be more than the duration, 
and in some cases might be 0 or less. What's the most efficient 
way to cap out of bounds values to 0 and 1? I can do a check and 
cap them manually, but if I'm doing a lot of these operations I'd 
like to pick the least resource intensive way.

Also, if I wanted out of bounds values to wrap (2.5 becomes 0.5) 
how would I get that value and also have 1.0 not give me 0.0?