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

https://github.com/vim/vim/commit/b209b86e6636a16088ccacdac98213416c065bf2

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

2 files changed, 19 insertions, 1 deletions

diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 8a0d7575b3..8a36509f9a 100644
--- a/src/nvim/eval.lua
+++ b/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.
 
diff --git a/src/nvim/move.c b/src/nvim/move.c
index 0ed30070a5..a68f6a2d50 100644
--- a/src/nvim/move.c
+++ b/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"