vim-patch:8.1.2020: it is not easy to change the window layout

Problem:    It is not easy to change the window layout.
Solution:   Add win_splitmove(). (Andy Massimino, closes vim/vim#4561)
https://github.com/vim/vim/commit/d20dcb3d011da6111153109f6e46fbd5c7fe9fb6

3 files changed, 113 insertions, 0 deletions

diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index eac0feafcf..e94d7831b0 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -392,6 +392,7 @@ return {
     win_id2tabwin={args=1},
     win_id2win={args=1},
     win_screenpos={args=1},
+    win_splitmove={args={2, 3}},
     winbufnr={args=1},
     wincol={},
     windowsversion={},
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 2b04469af7..6d6b701169 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -3980,6 +3980,85 @@ static void f_win_screenpos(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   tv_list_append_number(rettv->vval.v_list, wp == NULL ? 0 : wp->w_wincol + 1);
 }
 
+//
+// Move the window wp into a new split of targetwin in a given direction
+//
+static void win_move_into_split(win_T *wp, win_T *targetwin,
+                                int size, int flags)
+{
+  int     dir;
+  int     height = wp->w_height;
+  win_T   *oldwin = curwin;
+
+  if (wp == targetwin) {
+    return;
+  }
+
+  // Jump to the target window
+  if (curwin != targetwin) {
+    win_goto(targetwin);
+  }
+
+  // Remove the old window and frame from the tree of frames
+  (void)winframe_remove(wp, &dir, NULL);
+  win_remove(wp, NULL);
+  last_status(false);     // may need to remove last status line
+  (void)win_comp_pos();   // recompute window positions
+
+  // Split a window on the desired side and put the old window there
+  (void)win_split_ins(size, flags, wp, dir);
+
+  // If splitting horizontally, try to preserve height
+  if (size == 0 && !(flags & WSP_VERT)) {
+    win_setheight_win(height, wp);
+    if (p_ea) {
+      win_equal(wp, true, 'v');
+    }
+  }
+
+  if (oldwin != curwin) {
+    win_goto(oldwin);
+  }
+}
+
+// "win_splitmove()" function
+static void f_win_splitmove(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+{
+  win_T   *wp;
+  win_T   *targetwin;
+  int     flags = 0, size = 0;
+
+  wp = find_win_by_nr_or_id(&argvars[0]);
+  targetwin = find_win_by_nr_or_id(&argvars[1]);
+
+  if (wp == NULL || targetwin == NULL || wp == targetwin) {
+    EMSG(_(e_invalwindow));
+    rettv->vval.v_number = -1;
+    return;
+  }
+
+  if (argvars[2].v_type != VAR_UNKNOWN) {
+    dict_T      *d;
+    dictitem_T  *di;
+
+    if (argvars[2].v_type != VAR_DICT || argvars[2].vval.v_dict == NULL) {
+      EMSG(_(e_invarg));
+      return;
+    }
+
+    d = argvars[2].vval.v_dict;
+    if (tv_dict_get_number(d, "vertical")) {
+      flags |= WSP_VERT;
+    }
+    if ((di = tv_dict_find(d, "rightbelow", -1)) != NULL) {
+      flags |= tv_get_number(&di->di_tv) ? WSP_BELOW : WSP_ABOVE;
+    }
+    size = tv_dict_get_number(d, "size");
+  }
+
+  win_move_into_split(wp, targetwin, size, flags);
+}
+
 // "getwinpos({timeout})" function
 static void f_getwinpos(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
diff --git a/src/nvim/testdir/test_window_cmd.vim b/src/nvim/testdir/test_window_cmd.vim
index 969b75d424..b57c4e27da 100644
--- a/src/nvim/testdir/test_window_cmd.vim
+++ b/src/nvim/testdir/test_window_cmd.vim
@@ -808,6 +808,39 @@ func Test_winnr()
   only | tabonly
 endfunc
 
+func Test_win_splitmove()
+  edit a
+  leftabove split b
+  leftabove vsplit c
+  leftabove split d
+  call assert_equal(0, win_splitmove(winnr(), winnr('l')))
+  call assert_equal(bufname(winbufnr(1)), 'c')
+  call assert_equal(bufname(winbufnr(2)), 'd')
+  call assert_equal(bufname(winbufnr(3)), 'b')
+  call assert_equal(bufname(winbufnr(4)), 'a')
+  call assert_equal(0, win_splitmove(winnr(), winnr('j'), {'vertical': 1}))
+  call assert_equal(0, win_splitmove(winnr(), winnr('j'), {'vertical': 1}))
+  call assert_equal(bufname(winbufnr(1)), 'c')
+  call assert_equal(bufname(winbufnr(2)), 'b')
+  call assert_equal(bufname(winbufnr(3)), 'd')
+  call assert_equal(bufname(winbufnr(4)), 'a')
+  call assert_equal(0, win_splitmove(winnr(), winnr('k'), {'vertical': 1}))
+  call assert_equal(bufname(winbufnr(1)), 'd')
+  call assert_equal(bufname(winbufnr(2)), 'c')
+  call assert_equal(bufname(winbufnr(3)), 'b')
+  call assert_equal(bufname(winbufnr(4)), 'a')
+  call assert_equal(0, win_splitmove(winnr(), winnr('j'), {'rightbelow': v:true}))
+  call assert_equal(bufname(winbufnr(1)), 'c')
+  call assert_equal(bufname(winbufnr(2)), 'b')
+  call assert_equal(bufname(winbufnr(3)), 'a')
+  call assert_equal(bufname(winbufnr(4)), 'd')
+  only | bd
+
+  call assert_fails('call win_splitmove(winnr(), 123)', 'E957:')
+  call assert_fails('call win_splitmove(123, winnr())', 'E957:')
+  call assert_fails('call win_splitmove(winnr(), winnr())', 'E957:')
+endfunc
+
 func Test_window_resize()
   " Vertical :resize (absolute, relative, min and max size).
   vsplit