splitting a window at point

[James K. Lowden]

It seems like an obvious function: I'd like to split a window
vertically, such that the top of the lower window is positioned where
the cursor is.  If I'm on line 6, the top window will have 6 lines, and
the bottom window gets the rest.  I (would) do this from time to time,
to leave a function definition in the top window while in the lower
window I operate on the code that uses it.  

split-window-vertically takes an optional argument for the top (or
bottom) window size.  So far, so good.  C-u C-x 2 does indeed open the
top window with 4 lines.  

How, then, to compute the cursor's window position?

move-to-window-line moves to the line, but there's no get-window-line.

what-cursor-position reports the buffer position, not the window
location.  I could compute the cursor's window position from it if I
knew the window's buffer position, but apropos returns no function
for "window" that mentions buffer position in its description.  

Do I "just" save the curent cursor position, jump to window line 1, get
its buffer position, jump back, and take the difference?  I guess that
will work, but I'd rather not move the cursor just to compute its
location.  

What am I overlooking?  

--jkl