compute-motion

compute-motion is a built-in function in `C source code'.

(compute-motion FROM FROMPOS TO TOPOS WIDTH OFFSETS WINDOW)

Scan through the current buffer, calculating screen position.
Scan the current buffer forward from offset FROM,
assuming it is at position FROMPOS--a cons of the form (HPOS . VPOS)--
to position TO or position TOPOS--another cons of the form (HPOS . VPOS)--
and return the ending buffer position and screen location.

If TOPOS is nil, the actual width and height of the window's
text area are used.

There are three additional arguments:

WIDTH is the number of columns available to display text;
this affects handling of continuation lines. A value of nil
corresponds to the actual number of available text columns.

OFFSETS is either nil or a cons cell (HSCROLL . TAB-OFFSET).
HSCROLL is the number of columns not being displayed at the left
margin; this is usually taken from a window's hscroll member.
TAB-OFFSET is the number of columns of the first tab that aren't
being displayed, perhaps because the line was continued within it.
If OFFSETS is nil, HSCROLL and TAB-OFFSET are assumed to be zero.

WINDOW is the window to operate on. It is used to choose the display table;
if it is showing the current buffer, it is used also for
deciding which overlay properties apply.
Note that `compute-motion' always operates on the current buffer.

The value is a list of five elements:
(POS HPOS VPOS PREVHPOS CONTIN)
POS is the buffer position where the scan stopped.
VPOS is the vertical position where the scan stopped.
HPOS is the horizontal position where the scan stopped.

PREVHPOS is the horizontal position one character back from POS.
CONTIN is t if a line was continued after (or within) the previous character.

For example, to find the buffer position of column COL of line LINE
of a certain window, pass the window's starting location as FROM
and the window's upper-left coordinates as FROMPOS.
Pass the buffer's (point-max) as TO, to limit the scan to the end of the
visible section of the buffer, and pass LINE and COL as TOPOS.