calendar - Parashas Shavua calculation

I'm writing a site where the user would be able to see the calendar with jewish-civil translation of dates, and also which parasas shavua falls on which Shabbos.

I'd like to know what is the formula of calculating which parashas shavua we read on Shabbos that is given by Jewish date. Obviously the calculation differs between Israel and Diaspora. Both variants are needed.

thanks.

Answer

I've asked a friend of mine to do me a favor and to write a code for this.

Here is what he have done, and here is an example of (my) usage of his code.