vim-patch:9.1.0456: Left shift is incorrect with vartabstop and shiftwidth=0

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: vim/vim#14864
closes: vim/vim#14887

https://github.com/vim/vim/commit/88d4f255b7b7a19bb4f6489e0ad0956e47d51fed

Co-authored-by: Gary Johnson <garyjohn@spocom.com>

5 files changed, 96 insertions, 19 deletions

diff --git a/src/nvim/edit.c b/src/nvim/edit.c
index 146e95df87..595b4da589 100644
--- a/src/nvim/edit.c
+++ b/src/nvim/edit.c
@@ -474,7 +474,8 @@ static int insert_check(VimState *state)
 
     if (curwin->w_wcol < s->mincol - tabstop_at(get_nolist_virtcol(),
                                                 curbuf->b_p_ts,
-                                                curbuf->b_p_vts_array)
+                                                curbuf->b_p_vts_array,
+                                                false)
         && curwin->w_wrow == curwin->w_height_inner - 1 - get_scrolloff_value(curwin)
         && (curwin->w_cursor.lnum != curwin->w_topline
             || curwin->w_topfill > 0)) {
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 0165e4af60..e704135366 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -8302,7 +8302,7 @@ static void f_shiftwidth(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     if (col < 0) {
       return;  // type error; errmsg already given
     }
-    rettv->vval.v_number = get_sw_value_col(curbuf, col);
+    rettv->vval.v_number = get_sw_value_col(curbuf, col, false);
     return;
   }
   rettv->vval.v_number = get_sw_value(curbuf);
diff --git a/src/nvim/indent.c b/src/nvim/indent.c
index 69c95e9038..66cb443e4d 100644
--- a/src/nvim/indent.c
+++ b/src/nvim/indent.c
@@ -147,25 +147,42 @@ int tabstop_padding(colnr_T col, OptInt ts_arg, const colnr_T *vts)
 }
 
 /// Find the size of the tab that covers a particular column.
-int tabstop_at(colnr_T col, OptInt ts, const colnr_T *vts)
+///
+/// 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, OptInt ts, const colnr_T *vts, bool left)
 {
-  colnr_T tabcol = 0;
-  int t;
-  int tab_size = 0;
-
   if (vts == NULL || vts[0] == 0) {
     return (int)ts;
   }
 
-  const int tabcount = vts[0];
+  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.
+  const int tabcount  // Number of tab stops in the list of variable tab stops.
+    = vts[0];
   for (t = 1; t <= tabcount; t++) {
     tabcol += vts[t];
     if (tabcol > col) {
-      tab_size = vts[t];
+      // If shifting left (left == true), 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];
   }
 
@@ -312,35 +329,35 @@ int tabstop_first(colnr_T *ts)
 /// 'tabstop' value when 'shiftwidth' is zero.
 int get_sw_value(buf_T *buf)
 {
-  int result = get_sw_value_col(buf, 0);
+  int result = get_sw_value_col(buf, 0, false);
   return result;
 }
 
 /// Idem, using "pos".
-int get_sw_value_pos(buf_T *buf, pos_T *pos)
+int get_sw_value_pos(buf_T *buf, pos_T *pos, bool left)
 {
   pos_T save_cursor = curwin->w_cursor;
 
   curwin->w_cursor = *pos;
-  int sw_value = get_sw_value_col(buf, get_nolist_virtcol());
+  int sw_value = get_sw_value_col(buf, get_nolist_virtcol(), left);
   curwin->w_cursor = save_cursor;
   return sw_value;
 }
 
 /// Idem, using the first non-black in the current line.
-int get_sw_value_indent(buf_T *buf)
+int get_sw_value_indent(buf_T *buf, bool left)
 {
   pos_T pos = curwin->w_cursor;
 
   pos.col = (colnr_T)getwhitecols_curline();
-  return get_sw_value_pos(buf, &pos);
+  return get_sw_value_pos(buf, &pos, left);
 }
 
 /// Idem, using virtual column "col".
-int get_sw_value_col(buf_T *buf, colnr_T col)
+int get_sw_value_col(buf_T *buf, colnr_T col, bool left)
 {
   return buf->b_p_sw ? (int)buf->b_p_sw
-                     : 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);
 }
 
 /// Return the effective softtabstop value for the current buffer,
diff --git a/src/nvim/ops.c b/src/nvim/ops.c
index 47d302df43..8ae1d70882 100644
--- a/src/nvim/ops.c
+++ b/src/nvim/ops.c
@@ -282,7 +282,7 @@ void op_shift(oparg_T *oap, bool curs_top, int amount)
 /// @param call_changed_bytes  call changed_bytes()
 void shift_line(bool left, bool round, int amount, int call_changed_bytes)
 {
-  const int sw_val = get_sw_value_indent(curbuf);
+  const int sw_val = get_sw_value_indent(curbuf, left);
 
   int count = get_indent();  // get current indent
 
@@ -328,7 +328,7 @@ static void shift_block(oparg_T *oap, int amount)
   const int oldstate = State;
   char *newp;
   const int oldcol = curwin->w_cursor.col;
-  const int sw_val = get_sw_value_indent(curbuf);
+  const int sw_val = get_sw_value_indent(curbuf, left);
   const int ts_val = (int)curbuf->b_p_ts;
   struct block_def bd;
   int incr;
diff --git a/test/old/testdir/test_vartabs.vim b/test/old/testdir/test_vartabs.vim
index 274531fdc1..82c3a513f9 100644
--- a/test/old/testdir/test_vartabs.vim
+++ b/test/old/testdir/test_vartabs.vim
@@ -458,5 +458,64 @@ func Test_vartabstop_latin1()
   set nocompatible linebreak& list& revins& smarttab& vartabstop&
 endfunc
 
+" Verify that right-shifting and left-shifting adjust lines to the proper
+" tabstops.
+func Test_vartabstop_shift_right_left()
+  new
+  set expandtab
+  set shiftwidth=0
+  set vartabstop=17,11,7
+  exe "norm! aword"
+  let expect = "word"
+  call assert_equal(expect, getline(1))
+
+  " Shift to first tabstop.
+  norm! >>
+  let expect = "                 word"
+  call assert_equal(expect, getline(1))
+
+  " Shift to second tabstop.
+  norm! >>
+  let expect = "                            word"
+  call assert_equal(expect, getline(1))
+
+  " Shift to third tabstop.
+  norm! >>
+  let expect = "                                   word"
+  call assert_equal(expect, getline(1))
+
+  " Shift to fourth tabstop, repeating the third shift width.
+  norm! >>
+  let expect = "                                          word"
+  call assert_equal(expect, getline(1))
+
+  " Shift back to the third tabstop.
+  norm! <<
+  let expect = "                                   word"
+  call assert_equal(expect, getline(1))
+
+  " Shift back to the second tabstop.
+  norm! <<
+  let expect = "                            word"
+  call assert_equal(expect, getline(1))
+
+  " Shift back to the first tabstop.
+  norm! <<
+  let expect = "                 word"
+  call assert_equal(expect, getline(1))
+
+  " Shift back to the left margin.
+  norm! <<
+  let expect = "word"
+  call assert_equal(expect, getline(1))
+
+  " Shift again back to the left margin.
+  norm! <<
+  let expect = "word"
+  call assert_equal(expect, getline(1))
+
+  bwipeout!
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab