Derivative Licensing Question

Consider the following situation....
Someone has published all the source for a project in C on GitHub.
There is no licence statement, just a copyright notice with the date and author's name.
If I take the source and clone the functions so they have the same prototypes but write them in assembler and have the same flow, is this a derivative work?  Or is the assembler version my work to licence how I feel?
What do people think?
Andy