vim-patch:9.0.1717: virtcol2col returns last byte of a multi-byte char (#24729)

Problem: virtcol2col returns last byte of a multi-byte char
Solution: Make it return the first byte for a multi-byte char

closes: vim/vim#12786
closes: vim/vim#12799

b209b86e66

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

runtime/doc/builtin.txt

@@ -8434,6 +8434,9 @@ virtcol2col({winid}, {lnum}, {col})                              *virtcol2col()*
 		{lnum}, then the byte index of the character at the last
 		virtual column is returned.
 
+		For a multi-byte character, the column number of the first
+		byte in the character is returned.
+
 		The {winid} argument can be the window number or the
 		|window-ID|. If this is zero, then the current window is used.
 

runtime/lua/vim/_meta/vimfn.lua

@@ -10016,6 +10016,9 @@ function vim.fn.virtcol(expr, list) end
 --- {lnum}, then the byte index of the character at the last
 --- virtual column is returned.
 ---
+--- For a multi-byte character, the column number of the first
+--- byte in the character is returned.
+---
 --- The {winid} argument can be the window number or the
 --- |window-ID|. If this is zero, then the current window is used.
 ---

src/nvim/eval.lua

@@ -11931,6 +11931,9 @@ M.funcs = {
       {lnum}, then the byte index of the character at the last
       virtual column is returned.
 
+      For a multi-byte character, the column number of the first
+      byte in the character is returned.
+
       The {winid} argument can be the window number or the
       |window-ID|. If this is zero, then the current window is used.
 

src/nvim/move.c

@@ -1163,6 +1163,21 @@ void f_screenpos(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   tv_dict_add_nr(dict, S_LEN("endcol"), ecol);
 }
 
+/// Convert a virtual (screen) column to a character column.  The first column
+/// is one.  For a multibyte character, the column number of the first byte is
+/// returned.
+static int virtcol2col(win_T *wp, linenr_T lnum, int vcol)
+{
+  int offset = vcol2col(wp, lnum, vcol);
+  char *line = ml_get_buf(wp->w_buffer, lnum, false);
+  char *p = line + offset;
+
+  // For a multibyte character, need to return the column number of the first byte.
+  MB_PTR_BACK(line, p);
+
+  return (int)(p - line + 1);
+}
+
 /// "virtcol2col({winid}, {lnum}, {col})" function
 void f_virtcol2col(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 {
@@ -1190,7 +1205,7 @@ void f_virtcol2col(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     return;
   }
 
-  rettv->vval.v_number = vcol2col(wp, lnum, screencol);
+  rettv->vval.v_number = virtcol2col(wp, lnum, screencol);
 }
 
 /// Scroll the current window down by "line_count" logical lines.  "CTRL-Y"

test/old/testdir/test_cursor_func.vim

@@ -531,6 +531,15 @@ func Test_virtcol2col()
   call assert_equal(-1, virtcol2col(0, -1, 1))
   call assert_equal(-1, virtcol2col(0, 1, -1))
   call assert_equal(5, virtcol2col(0, 1, 20))
+
+  " Multibyte character
+  call setline(1, ['a✅✅✅'])
+  call assert_equal(1, virtcol2col(0, 1, 1))
+  call assert_equal(2, virtcol2col(0, 1, 3))
+  call assert_equal(5, virtcol2col(0, 1, 5))
+  call assert_equal(8, virtcol2col(0, 1, 7))
+  call assert_equal(8, virtcol2col(0, 1, 8))
+
   call assert_fails('echo virtcol2col("0", 1, 20)', 'E1210:')
   call assert_fails('echo virtcol2col(0, "1", 20)', 'E1210:')
   call assert_fails('echo virtcol2col(0, 1, "1")', 'E1210:')