patch 9.1.0456: Left shift is incorrect with vartabstop and shiftwidth=0v9.1.0456

Problem:  Left shift is incorrect with vartabstop and shiftwidth=0
Solution: make tabstop_at() function aware of shift direction
          (Gary Johnson)

The problem was that with 'vartabstop' set and 'shiftwidth' equal 0,
left shifts using << were shifting the line to the wrong column.  The
tabstop to the right of the first character in the line was being used
as the shift amount instead of the tabstop to the left of that first
character.

The reason was that the tabstop_at() function always returned the value
of the tabstop to the right of the given column and was not accounting
for the direction of the shift.

The solution was to make tabstop_at() aware of the direction of the
shift and to choose the tabtop accordingly.

A test was added to check this behavior and make sure it doesn't
regress.

While at it, also fix a few indentation/alignment issues.

fixes: #14864
closes: #14887

Signed-off-by: Gary Johnson <garyjohn@spocom.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

1 files changed, 33 insertions, 14 deletions

diff --git a/src/indent.c b/src/indent.c
index 1dfde7ddda..777db244af 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -123,14 +123,22 @@ tabstop_padding(colnr_T col, int ts_arg, int *vts)
 
 /*
  * Find the size of the tab that covers a particular column.
+ *
+ * If this is being called as part of a shift operation, col is not the cursor
+ * column but is the column number to the left of the first non-whitespace
+ * character in the line.  If the shift is to the left (left = TRUE), then
+ * return the size of the tab interval to the left of the column.
  */
     int
-tabstop_at(colnr_T col, int ts, int *vts)
+tabstop_at(colnr_T col, int ts, int *vts, int left)
 {
-    int		tabcount;
-    colnr_T	tabcol = 0;
-    int		t;
-    int		tab_size = 0;
+    int		tabcount;	// Number of tab stops in the list of variable
+				// tab stops.
+    colnr_T	tabcol = 0;	// Column of the tab stop under consideration.
+    int		t;		// Tabstop index in the list of variable tab
+				// stops.
+    int		tab_size = 0;	// Size of the tab stop interval to the right
+				// or left of the col.
 
     if (vts == 0 || vts[0] == 0)
 	return ts;
@@ -141,11 +149,22 @@ tabstop_at(colnr_T col, int ts, int *vts)
 	tabcol += vts[t];
 	if (tabcol > col)
 	{
-	    tab_size = vts[t];
+	    // If shifting left (left != 0), and if the column to the left of
+	    // the first first non-blank character (col) in the line is
+	    // already to the left of the first tabstop, set the shift amount
+	    // (tab_size) to just enough to shift the line to the left margin.
+	    // The value doesn't seem to matter as long as it is at least that
+	    // distance.
+	    if (left && (t == 1))
+		tab_size = col;
+	    else
+		tab_size = vts[t - (left ? 1 : 0)];
 	    break;
 	}
     }
-    if (t > tabcount)
+    if (t > tabcount)		// If the value of the index t is beyond the
+				// end of the list, use the tab stop value at
+				// the end of the list.
 	tab_size = vts[tabcount];
 
     return tab_size;
@@ -327,20 +346,20 @@ tabstop_first(int *ts)
     long
 get_sw_value(buf_T *buf)
 {
-    return get_sw_value_col(buf, 0);
+    return get_sw_value_col(buf, 0, FALSE);
 }
 
 /*
  * Idem, using "pos".
  */
     static long
-get_sw_value_pos(buf_T *buf, pos_T *pos)
+get_sw_value_pos(buf_T *buf, pos_T *pos, int left)
 {
     pos_T save_cursor = curwin->w_cursor;
     long sw_value;
 
     curwin->w_cursor = *pos;
-    sw_value = get_sw_value_col(buf, get_nolist_virtcol());
+    sw_value = get_sw_value_col(buf, get_nolist_virtcol(), left);
     curwin->w_cursor = save_cursor;
     return sw_value;
 }
@@ -349,23 +368,23 @@ get_sw_value_pos(buf_T *buf, pos_T *pos)
  * Idem, using the first non-black in the current line.
  */
     long
-get_sw_value_indent(buf_T *buf)
+get_sw_value_indent(buf_T *buf, int left)
 {
     pos_T pos = curwin->w_cursor;
 
     pos.col = getwhitecols_curline();
-    return get_sw_value_pos(buf, &pos);
+    return get_sw_value_pos(buf, &pos, left);
 }
 
 /*
  * Idem, using virtual column "col".
  */
     long
-get_sw_value_col(buf_T *buf, colnr_T col UNUSED)
+get_sw_value_col(buf_T *buf, colnr_T col UNUSED, int left UNUSED)
 {
     return buf->b_p_sw ? buf->b_p_sw :
 #ifdef FEAT_VARTABS
-	tabstop_at(col, buf->b_p_ts, buf->b_p_vts_array);
+	tabstop_at(col, buf->b_p_ts, buf->b_p_vts_array, left);
 #else
 	buf->b_p_ts;
 #endif