Math problem

[Ofnuts]

Given a pattern of regularly spaced lines of equal width, over which moves a sliding window, can you compute the total visible width of the lines (or of the spaces), based on:

• Window width
  
• Spaces width
  
• Lines width
  
• Position of window (as an offset to the first space, for instance, but any other measure of you choice is OK
  

   

Iterating is prohibited (that's the hard part)