Re: MPS: face-cache

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  emacs-devel missing value
>  <emacs-devel@gnu.org>
> Date: Sun, 28 Apr 2024 11:14:59 +0200
> 
> Anyway. Eli can probably tell us how struct composition is used. If it
> has to do with character composition, I think Eli once told that is
> also something "new" (for me).

Yes, it's used for character compositions.

But I don't see the 'font' field of 'struct composition' used
anywhere.  I only see it being set to NULL and not accessed anywhere
else.  The font we use for composed characters is obtained "by other
means", via the lispy LGSTRING object.

As for 'key', it is indeed used to look up known compositions.  Each
composition, once it's computed, is cached in a hash table, to avoid
computing it again (because that can be expensive):

  /* Check if the same composition has already been registered or not
     by consulting composition_hash_table.  The key for this table is
     COMPONENTS (converted to a vector COMPONENTS-VEC) or, if it is
     nil, vector of characters in the composition range.  */
  if (FIXNUMP (components))
    key = make_vector (1, components);
  else if (STRINGP (components) || CONSP (components))
    key = Fvconcat (1, &components);
  else if (VECTORP (components))
    key = components;
  else if (NILP (components))
    {
      key = make_uninit_vector (nchars);
      if (STRINGP (string))
	for (ptrdiff_t i = 0; i < nchars; i++)
	  {
	    ch = fetch_string_char_advance (string, &charpos, &bytepos);
	    ASET (key, i, make_fixnum (ch));
	  }
      else
	for (ptrdiff_t i = 0; i < nchars; i++)
	  {
	    ch = fetch_char_advance (&charpos, &bytepos);
	    ASET (key, i, make_fixnum (ch));
	  }
    }
  else
    goto invalid_composition;

  hash_hash_t hash_code;
  hash_index = hash_lookup_get_hash (hash_table, key, &hash_code);
  if (hash_index >= 0)
    {
      /* We have already registered the same composition.  Change PROP
	 from Form-A above to Form-B while replacing COMPONENTS with
	 COMPONENTS-VEC stored in the hash table.  We can directly
	 modify the cons cell of PROP because it is not shared.  */
      key = HASH_KEY (hash_table, hash_index);
      id = HASH_VALUE (hash_table, hash_index);
      XSETCAR (prop, id);
      XSETCDR (prop, Fcons (make_fixnum (nchars), Fcons (key, XCDR (prop))));
      return XFIXNUM (id);
    }