The code assumes a two-dimensional computational domain with TMz polarization (i.e., non-zero field Ez, Hx, and Hy). The program is currently written so that the incident field always strikes the lower-left corner of the total-field region first. (If you want a different corner, that should be a fairly simple tweak to the code, but for now you ll have to make that tweak yourself.) I have attempted to provide copious comments in the code and hope that a knowledgeable C programmer can quickly map the approach as described in the paper to what is in the program.