Thank you! I read about the "Bin packing problem", and some solutions to it. For what I need, an heuristic approach will be perfectly enough.

But first, I have to solve how to determine if the substring "fits in the can". The idea of "fitting" is that if there are whitespaces in the output line at the positions where each letter of my substring takes place, it "fits". I thought of spliting the string into an array, in a way that each letter is now an element of the array, and then to check if the elements of the array in the positions of my substring are whitespaces or not, but I am not sure if it is a good alternative.