Enhance font_range to check for emoji composition triggers

If the codepoint that triggered composition is from the emoji script,
use the emoji font to check the string being composed, rather than the
font of the first character of the string.  This makes e.g.

"emoji codepoint with Emoji_Presentation = No followed by VS-16 (FE0F)"

display the emoji version of the glyph for that codepoint.

* admin/unidata/blocks.awk: Add VS-1 through VS-16 to the emoji
script.
* src/composite.c (autocmp_chars): Accept additional argument CH for
the codepoint that triggered composition, pass it to font_range.
(composition_reseat_it, find_automatic_composition): Pass codepoint
that triggered composition to autocmp_chars.
* src/font.c (font_range): Accept additional argument CH for the
triggering codepoint.  If the codepoint is from the 'emoji' script,
use Vscript_representative_chars to find the font to use for the
composition attempt.
(syms_of_font): Add Qemoji symbol.
* src/font.h: Update font_range prototype for argument CH.
* etc/NEWS: Announce change.

admin/unidata/blocks.awk

@@ -243,6 +243,12 @@ END {
     override_start[idx] = "1F590"
     override_end[idx] = "1F590"
 
+    ## These are here so that font_range can choose Emoji presentation
+    ## for the preceding codepoint when it encounters a VS
+    idx++
+    override_start[idx] = "FE00"
+    override_end[idx] = "FE0F"
+
     for (k in override_start)
     {
         i++

etc/NEWS

@@ -156,6 +156,18 @@ sequences (if a suitable font is installed) so that they are displayed
 as single glyphs instead of multiple ones.  'Noto Color Emoji' is one
 such suitable font.
 
++++
+** Composition of emoji has been improved.
+If autocomposition is triggered by an emoji character, then the emoji
+font is now used to check if composition can be performed, rather than
+the font of the first character of the string being composed.  This
+allows e.g.
+
+'Emoji codepoint' + VS-16
+
+to be displayed using the emoji font even if 'Emoji codepoint' does
+not have emoji presentation by default.
+
 +++
 ** New command 'execute-extended-command-for-buffer'.
 This new command, bound to 'M-S-x', works like

src/composite.c

@@ -882,14 +882,15 @@ fill_gstring_body (Lisp_Object gstring)
 /* Try to compose the characters at CHARPOS according to composition
    rule RULE ([PATTERN PREV-CHARS FUNC]).  LIMIT limits the characters
    to compose.  STRING, if not nil, is a target string.  WIN is a
-   window where the characters are being displayed.  If characters are
+   window where the characters are being displayed.  CH is the
+   character that triggered the composition check.  If characters are
    successfully composed, return the composition as a glyph-string
    object.  Otherwise return nil.  */
 
 static Lisp_Object
 autocmp_chars (Lisp_Object rule, ptrdiff_t charpos, ptrdiff_t bytepos,
 	       ptrdiff_t limit, struct window *win, struct face *face,
-	       Lisp_Object string, Lisp_Object direction)
+	       Lisp_Object string, Lisp_Object direction, int ch)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
   Lisp_Object pos = make_fixnum (charpos);
@@ -920,7 +921,7 @@ autocmp_chars (Lisp_Object rule, ptrdiff_t charpos, ptrdiff_t bytepos,
   struct frame *f = XFRAME (font_object);
   if (FRAME_WINDOW_P (f))
     {
-      font_object = font_range (charpos, bytepos, &to, win, face, string);
+      font_object = font_range (charpos, bytepos, &to, win, face, string, ch);
       if (! FONT_OBJECT_P (font_object)
 	  || (! NILP (re)
 	      && to < limit
@@ -1272,7 +1273,7 @@ composition_reseat_it (struct composition_it *cmp_it, ptrdiff_t charpos,
 	      if (XFIXNAT (AREF (elt, 1)) != cmp_it->lookback)
 		goto no_composition;
 	      lgstring = autocmp_chars (elt, charpos, bytepos, endpos,
-					w, face, string, direction);
+					w, face, string, direction, cmp_it->ch);
 	      if (composition_gstring_p (lgstring))
 		break;
 	      lgstring = Qnil;
@@ -1310,7 +1311,7 @@ composition_reseat_it (struct composition_it *cmp_it, ptrdiff_t charpos,
 	  else
 	    direction = QR2L;
 	  lgstring = autocmp_chars (elt, cpos, bpos, charpos + 1, w, face,
-				    string, direction);
+				    string, direction, cmp_it->ch);
 	  if (! composition_gstring_p (lgstring)
 	      || cpos + LGSTRING_CHAR_LEN (lgstring) - 1 != charpos)
 	    /* Composition failed or didn't cover the current
@@ -1679,7 +1680,7 @@ find_automatic_composition (ptrdiff_t pos, ptrdiff_t limit, ptrdiff_t backlim,
 		  for (check = cur; check_pos < check.pos; )
 		    BACKWARD_CHAR (check, stop);
 		  *gstring = autocmp_chars (elt, check.pos, check.pos_byte,
-					    tail, w, NULL, string, Qnil);
+					    tail, w, NULL, string, Qnil, c);
 		  need_adjustment = 1;
 		  if (NILP (*gstring))
 		    {

src/font.c

@@ -3866,6 +3866,9 @@ font_at (int c, ptrdiff_t pos, struct face *face, struct window *w,
    If STRING is not nil, it is the string to check instead of the current
    buffer.  In that case, FACE must be not NULL.
 
+   CH is the character that actually caused the composition
+   process to start, it may be different from the character at POS.
+
    The return value is the font-object for the character at POS.
    *LIMIT is set to the position where that font can't be used.
 
@@ -3873,15 +3876,16 @@ font_at (int c, ptrdiff_t pos, struct face *face, struct window *w,
 
 Lisp_Object
 font_range (ptrdiff_t pos, ptrdiff_t pos_byte, ptrdiff_t *limit,
-	    struct window *w, struct face *face, Lisp_Object string)
+	    struct window *w, struct face *face, Lisp_Object string,
+	    int ch)
 {
   ptrdiff_t ignore;
   int c;
   Lisp_Object font_object = Qnil;
+  struct frame *f = XFRAME (w->frame);
 
   if (!face)
     {
-      struct frame *f = XFRAME (w->frame);
       int face_id;
 
       if (NILP (string))
@@ -3900,6 +3904,24 @@ font_range (ptrdiff_t pos, ptrdiff_t pos_byte, ptrdiff_t *limit,
       face = FACE_FROM_ID (f, face_id);
     }
 
+  /* If the composition was triggered by an emoji, use a character
+     from 'script-representative-chars', rather than the first
+     character in the string, to determine the font to use.  */
+  if (EQ (CHAR_TABLE_REF (Vchar_script_table, ch),
+	  Qemoji))
+    {
+      Lisp_Object val = assq_no_quit (Qemoji, Vscript_representative_chars);
+      if (CONSP (val))
+	{
+	  val = XCDR (val);
+	  if (CONSP (val))
+	    val = XCAR (val);
+	  else if (VECTORP (val))
+	    val = AREF (val, 0);
+	  font_object = font_for_char (face, XFIXNAT (val), pos - 1, string);
+	}
+    }
+
   while (pos < *limit)
     {
       c = (NILP (string)
@@ -5423,6 +5445,7 @@ syms_of_font (void)
   DEFSYM (Qiso8859_1, "iso8859-1");
   DEFSYM (Qiso10646_1, "iso10646-1");
   DEFSYM (Qunicode_bmp, "unicode-bmp");
+  DEFSYM (Qemoji, "emoji");
 
   /* Symbols representing keys of font extra info.  */
   DEFSYM (QCotf, ":otf");

src/font.h

@@ -885,7 +885,7 @@ valid_font_driver (struct font_driver const *d)
 extern Lisp_Object font_update_drivers (struct frame *f, Lisp_Object list);
 extern Lisp_Object font_range (ptrdiff_t, ptrdiff_t, ptrdiff_t *,
 			       struct window *, struct face *,
-			       Lisp_Object);
+			       Lisp_Object, int);
 extern void font_fill_lglyph_metrics (Lisp_Object, struct font *, unsigned int);
 
 extern Lisp_Object font_put_extra (Lisp_Object font, Lisp_Object prop,