What is the RefP in SearchData struct

Hi everyone,
I am doing a research about improving the Mode Decision in P frame. After
the SearchP method is called to search for the best candidate of each
macroblock, I want to store the pointer to the Macroblock data (Y
component) as well as 4 pointers to 4 sub block of the MB candidate.
Currently, I see that they are using the GetReference method to get the
pointer to the corresponding location of previous frame based on Motion
Vectors has found. The GetReference method contains only 3 lines of code
but it's hard to understand what it does. Could someone please explain to
me the code in GetReference method (about the bit shifting, etc..)? And I
don't know what the RefP array is used for? The comment says something
about reference pictures N, V, H... I guess the H is Horizontal, V is
vertical, but I have no clue about what N, cU and cV are.

Thanks in advance
-- 
Best wishes,

Tran Xuan Chien
University of Information Technology
Phone: (+84) 1692 468 154