vim-patch:8.1.1071: cannot get composing characters from the screen

Problem:    Cannot get composing characters from the screen.
Solution:   Add screenchars() and screenstring(). (partly by Ozaki Kiichi,
            closes vim/vim#4059)
2912abb3a2

runtime/doc/eval.txt

@@ -2596,9 +2596,11 @@ rpcrequest({channel}, {method}[, {args}...])
 				Sends an |RPC| request to {channel}
 screenattr({row}, {col})	Number	attribute at screen position
 screenchar({row}, {col})	Number	character at screen position
+screenchars({row}, {col})	List	List of characters at screen position
 screencol()			Number	current cursor column
 screenpos({winid}, {lnum}, {col}) Dict	screen row and col of a text character
 screenrow()			Number	current cursor row
+screenstring({row}, {col})	String	characters at screen position
 search({pattern} [, {flags} [, {stopline} [, {timeout}]]])
 				Number	search for {pattern}
 searchcount([{options}])	Dict    Get or update the last search count
@@ -7669,6 +7671,13 @@ screenchar({row}, {col})					*screenchar()*
 		This is mainly to be used for testing.
 		Returns -1 when row or col is out of range.
 
+screenchars({row}, {col})					*screenchars()*
+		The result is a List of Numbers.  The first number is the same
+		as what |screenchar()| returns.  Further numbers are
+		composing characters on top of the base character.
+		This is mainly to be used for testing.
+		Returns an empty List when row or col is out of range.
+
 screencol()							*screencol()*
 		The result is a Number, which is the current screen column of
 		the cursor. The leftmost column has number 1.
@@ -7712,6 +7721,14 @@ screenrow()							*screenrow()*
 
 		Note: Same restrictions as with |screencol()|.
 
+screenstring({row}, {col})					*screenstring()*
+		The result is a String that contains the base character and
+		any composing characters at position [row, col] on the screen.
+		This is like |screenchars()| but returning a String with the
+		characters.
+		This is mainly to be used for testing.
+		Returns an empty String when row or col is out of range.
+
 search({pattern} [, {flags} [, {stopline} [, {timeout}]]])	*search()*
 		Search for regexp pattern {pattern}.  The search starts at the
 		cursor position (you can use |cursor()| to set it).

runtime/doc/usr_41.txt

@@ -744,6 +744,8 @@ Cursor and mark position:		*cursor-functions* *mark-functions*
 	diff_filler()		get the number of filler lines above a line
 	screenattr()		get attribute at a screen line/row
 	screenchar()		get character code at a screen line/row
+	screenchars()		get character codes at a screen line/row
+	screenstring()		get string of characters at a screen line/row
 
 Working with text in the current buffer:		*text-functions*
 	getline()		get a line or list of lines from the buffer

src/nvim/eval.lua

@@ -290,9 +290,11 @@ return {
     rubyeval={args=1},
     screenattr={args=2},
     screenchar={args=2},
+    screenchars={args=2},
     screencol={},
     screenpos={args=3},
     screenrow={},
+    screenstring={args=2},
     search={args={1, 4}},
     searchcount={args={0,1}},
     searchdecl={args={1, 3}},

src/nvim/eval/funcs.c

@@ -8108,9 +8108,7 @@ static void f_rpcstop(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   }
 }
 
-/*
+// "screenattr()" function
- * "screenattr()" function
- */
 static void f_screenattr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   int c;
@@ -8128,9 +8126,7 @@ static void f_screenattr(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   rettv->vval.v_number = c;
 }
 
-/*
+// "screenchar()" function
- * "screenchar()" function
- */
 static void f_screenchar(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   int c;
@@ -8148,11 +8144,34 @@ static void f_screenchar(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   rettv->vval.v_number = c;
 }
 
-/*
+// "screenchars()" function
- * "screencol()" function
+static void f_screenchars(typval_T *argvars, typval_T *rettv, FunPtr fptr)
- *
+{
- * First column is 1 to be consistent with virtcol().
+  int row = tv_get_number_chk(&argvars[0], NULL) - 1;
- */
+  int col = tv_get_number_chk(&argvars[1], NULL) - 1;
+  if (row < 0 || row >= default_grid.Rows
+      || col < 0 || col >= default_grid.Columns) {
+    tv_list_alloc_ret(rettv, 0);
+    return;
+  }
+  ScreenGrid *grid = &default_grid;
+  screenchar_adjust_grid(&grid, &row, &col);
+  int pcc[MAX_MCO];
+  int c = utfc_ptr2char(grid->chars[grid->line_offset[row] + col], pcc);
+  int composing_len = 0;
+  while (pcc[composing_len] != 0) {
+    composing_len++;
+  }
+  tv_list_alloc_ret(rettv, composing_len + 1);
+  tv_list_append_number(rettv->vval.v_list, c);
+  for (int i = 0; i < composing_len; i++) {
+    tv_list_append_number(rettv->vval.v_list, pcc[i]);
+  }
+}
+
+// "screencol()" function
+//
+// First column is 1 to be consistent with virtcol().
 static void f_screencol(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   rettv->vval.v_number = ui_current_col() + 1;
@@ -8184,17 +8203,29 @@ static void f_screenpos(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   tv_dict_add_nr(dict, S_LEN("endcol"), ecol);
 }
 
-/*
+// "screenrow()" function
- * "screenrow()" function
- */
 static void f_screenrow(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   rettv->vval.v_number = ui_current_row() + 1;
 }
 
-/*
+// "screenstring()" function
- * "search()" function
+static void f_screenstring(typval_T *argvars, typval_T *rettv, FunPtr fptr)
- */
+{
+  rettv->vval.v_string = NULL;
+  rettv->v_type = VAR_STRING;
+  int row = tv_get_number_chk(&argvars[0], NULL) - 1;
+  int col = tv_get_number_chk(&argvars[1], NULL) - 1;
+  if (row < 0 || row >= default_grid.Rows
+      || col < 0 || col >= default_grid.Columns) {
+    return;
+  }
+  ScreenGrid *grid = &default_grid;
+  screenchar_adjust_grid(&grid, &row, &col);
+  rettv->vval.v_string = vim_strsave(grid->chars[grid->line_offset[row] + col]);
+}
+
+// "search()" function
 static void f_search(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
   int flags = 0;

src/nvim/testdir/test_utf8.vim

@@ -1,5 +1,6 @@
 " Tests for Unicode manipulations
  
+source view_util.vim
 
 " Visual block Insert adjusts for multi-byte char
 func Test_visual_block_insert()
@@ -61,6 +62,40 @@ func Test_getvcol()
   call assert_equal(2, virtcol("']"))
 endfunc
 
+func Test_screenchar_utf8()
+  new
+
+  " 1-cell, with composing characters 
+  call setline(1, ["ABC\u0308"])
+  redraw
+  call assert_equal([0x0041], screenchars(1, 1))
+  call assert_equal([0x0042], screenchars(1, 2))
+  call assert_equal([0x0043, 0x0308], screenchars(1, 3))
+  call assert_equal("A", screenstring(1, 1))
+  call assert_equal("B", screenstring(1, 2))
+  call assert_equal("C\u0308", screenstring(1, 3))
+
+  " 2-cells, with composing characters 
+  let text = "\u3042\u3044\u3046\u3099"
+  call setline(1, text)
+  redraw
+  call assert_equal([0x3042], screenchars(1, 1))
+  call assert_equal([0], screenchars(1, 2))
+  call assert_equal([0x3044], screenchars(1, 3))
+  call assert_equal([0], screenchars(1, 4))
+  call assert_equal([0x3046, 0x3099], screenchars(1, 5))
+
+  call assert_equal("\u3042", screenstring(1, 1))
+  call assert_equal("", screenstring(1, 2))
+  call assert_equal("\u3044", screenstring(1, 3))
+  call assert_equal("", screenstring(1, 4))
+  call assert_equal("\u3046\u3099", screenstring(1, 5))
+
+  call assert_equal([text . '  '], ScreenLinesUtf8(1, 8))
+
+  bwipe!
+endfunc
+
 func Test_list2str_str2list_utf8()
   " One Unicode codepoint
   let s = "\u3042\u3044"

src/nvim/testdir/view_util.vim

@@ -34,6 +34,25 @@ function! ScreenLines(lnum, width) abort
   return lines
 endfunction
 
+" Get text on the screen, including composing characters.
+" ScreenLines(lnum, width) or
+" ScreenLines([start, end], width)
+function! ScreenLinesUtf8(lnum, width) abort
+  redraw!
+  if type(a:lnum) == v:t_list
+    let start = a:lnum[0]
+    let end = a:lnum[1]
+  else
+    let start = a:lnum
+    let end = a:lnum
+  endif
+  let lines = []
+  for l in range(start, end)
+    let lines += [join(map(range(1, a:width), 'screenstring(l, v:val)'), '')]
+  endfor
+  return lines
+endfunction
+
 function! ScreenAttrs(lnum, width) abort
   redraw!
   if type(a:lnum) == v:t_list