Speed up vertical-motion when screen coordinates are known

 src/indent.c (Fvertical_motion): Accept an additional argument
 CUR-COL and use it as the starting screen coordinate.
 src/window.c (window_scroll_line_based, Fmove_to_window_line): All
 callers of vertical-motion changed.

 doc/lispref/positions.texi (Screen Lines): Update the documentation of
 vertical-motion to document the new additional argument.

1 files changed, 42 insertions, 13 deletions

diff --git a/src/indent.c b/src/indent.c
index 8660400e1ce..f0aea002fd2 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -1928,7 +1928,7 @@ vmotion (register ptrdiff_t from, register ptrdiff_t from_byte,
                          -1, hscroll, 0, w);
 }
 
-DEFUN ("vertical-motion", Fvertical_motion, Svertical_motion, 1, 2, 0,
+DEFUN ("vertical-motion", Fvertical_motion, Svertical_motion, 1, 3, 0,
        doc: /* Move point to start of the screen line LINES lines down.
 If LINES is negative, this means moving up.
 
@@ -1951,12 +1951,18 @@ is).  If the line is scrolled horizontally, COLS is interpreted
 visually, i.e., as addition to the columns of text beyond the left
 edge of the window.
 
+The optional third argument CUR-COL specifies the horizontal
+window-relative coordinate of point, in units of frame's canonical
+character width, where the function is invoked.  If this argument is
+omitted or nil, the function will determine the point coordinate by
+going back to the beginning of the line.
+
 `vertical-motion' always uses the current buffer,
 regardless of which buffer is displayed in WINDOW.
 This is consistent with other cursor motion functions
 and makes it possible to use `vertical-motion' in any buffer,
 whether or not it is currently displayed in some window.  */)
-  (Lisp_Object lines, Lisp_Object window)
+  (Lisp_Object lines, Lisp_Object window, Lisp_Object cur_col)
 {
   struct it it;
   struct text_pos pt;
@@ -2006,6 +2012,22 @@ whether or not it is currently displayed in some window.  */)
       bool disp_string_at_start_p = 0;
       ptrdiff_t nlines = XINT (lines);
       int vpos_init = 0;
+      double start_col;
+      int start_x;
+      bool start_x_given = false;
+      int to_x = -1;
+
+      if (!NILP (cur_col))
+        {
+          CHECK_NUMBER_OR_FLOAT (cur_col);
+          start_col =
+            INTEGERP (cur_col)
+            ? (double) XINT (cur_col)
+            : XFLOAT_DATA (cur_col);
+          start_x =
+            (int)(start_col * FRAME_COLUMN_WIDTH (XFRAME (w->frame)) + 0.5);
+          start_x_given = true;
+        }
 
       itdata = bidi_shelve_cache ();
       SET_TEXT_POS (pt, PT, PT_BYTE);
@@ -2042,11 +2064,19 @@ whether or not it is currently displayed in some window.  */)
         it_overshoot_count =
           !(it.method == GET_FROM_IMAGE || it.method == GET_FROM_STRETCH);
 
-      /* Scan from the start of the line containing PT.  If we don't
-         do this, we start moving with IT->current_x == 0, while PT is
-         really at some x > 0.  */
-      reseat_at_previous_visible_line_start (&it);
-      it.current_x = it.hpos = 0;
+      if (start_x_given)
+        {
+          it.hpos = (int) start_col;
+          it.current_x = start_x;
+        }
+      else
+        {
+          /* Scan from the start of the line containing PT.  If we don't
+             do this, we start moving with IT->current_x == 0, while PT is
+             really at some x > 0.  */
+          reseat_at_previous_visible_line_start (&it);
+          it.current_x = it.hpos = 0;
+        }
       if (IT_CHARPOS (it) != PT)
         /* We used to temporarily disable selective display here; the
            comment said this is "so we don't move too far" (2005-01-19
@@ -2108,12 +2138,15 @@ whether or not it is currently displayed in some window.  */)
              return the correct value to the caller.  */
           vpos_init = -1;
         }
+      if (!NILP (lcols))
+        to_x = (int)(cols * FRAME_COLUMN_WIDTH (XFRAME (w->frame)) + 0.5);
       if (nlines <= 0)
         {
           it.vpos = vpos_init;
           /* Do this even if LINES is 0, so that we move back to the
              beginning of the current line as we ought.  */
-          if (nlines == 0 || IT_CHARPOS (it) > 0)
+          if ((nlines < 0 && IT_CHARPOS (it) > 0)
+              || (nlines == 0 && !(start_x_given && start_x <= to_x)))
             move_it_by_lines (&it, max (PTRDIFF_MIN, nlines));
         }
       else if (overshoot_handled)
@@ -2153,11 +2186,7 @@ whether or not it is currently displayed in some window.  */)
          was originally hscrolled, the goal column is interpreted as
          an addition to the hscroll amount.  */
       if (!NILP (lcols))
-        {
-          int to_x = (int)(cols * FRAME_COLUMN_WIDTH (XFRAME (w->frame)) + 0.5);
-
-          move_it_in_display_line (&it, ZV, first_x + to_x, MOVE_TO_X);
-        }
+        move_it_in_display_line (&it, ZV, first_x + to_x, MOVE_TO_X);
 
       SET_PT_BOTH (IT_CHARPOS (it), IT_BYTEPOS (it));
       bidi_unshelve_cache (itdata, 0);