Properly enumerate GX fonts of the same family with divergent styles

* doc/emacs/android.texi (Android Fonts): Revise explanation of
GX font replacement during font enumeration.

* src/sfntfont.c (sfnt_replace_fonts_p): If PREV and DESC are of
the same family and both are GX fonts, restrict actions taken to
removing duplicate styles from PREV.  Reported by a user who
tried to install GX versions of Cascadia Mono, which are
distributed as two files providing the bold and italic variation
axes respectively.

1 files changed, 53 insertions, 3 deletions

diff --git a/src/sfntfont.c b/src/sfntfont.c
index c48339b6b66..e2dfdaff3b6 100644
--- a/src/sfntfont.c
+++ b/src/sfntfont.c
@@ -834,6 +834,9 @@ sfnt_grok_registry (int fd, struct sfnt_font_desc *desc,
    newer font description DESC, and should be removed from the list of
    system fonts.
 
+   If both PREV and DESC are variable fonts, remove styles within PREV
+   that overlap with DESC and return false.
+
    If PREV is a variable font, potentially adjust its list of
    instances.  */
 
@@ -841,8 +844,8 @@ static bool
 sfnt_replace_fonts_p (struct sfnt_font_desc *prev,
                       struct sfnt_font_desc *desc)
 {
-  int i, width, weight, slant, count_instance;
-  Lisp_Object tem;
+  int i, j, width, weight, slant, count_instance;
+  Lisp_Object tem, tem1;
   bool family_equal_p;
 
   family_equal_p = !NILP (Fstring_equal (prev->family,
@@ -851,7 +854,54 @@ sfnt_replace_fonts_p (struct sfnt_font_desc *prev,
   if ((!NILP (desc->instances)
        || !NILP (Fstring_equal (prev->style, desc->style)))
       && family_equal_p)
-    return true;
+    {
+      /* If both inputs are GX fonts...  */
+      if (!NILP (desc->instances) && !NILP (prev->instances))
+        {
+          /* ...iterate over each of the styles provided by PREV.  If
+             they match any styles within DESC, remove the old style
+             from PREV.  */
+
+          count_instance = 0;
+          for (i = 0; i < ASIZE (prev->instances); ++i)
+            {
+              tem = AREF (prev->instances, i);
+
+              if (NILP (tem))
+                continue;
+
+              for (j = 0; j < ASIZE (desc->instances); ++j)
+                {
+                  tem1 = AREF (desc->instances, j);
+
+                  if (NILP (tem1))
+                    continue;
+
+                  if (!NILP (Fequal (tem1, tem)))
+                    {
+                      /* tem1 is identical to tem, so opt for it over
+                         tem.  */
+                      ASET (prev->instances, i, Qnil);
+                      goto next;
+                    }
+                }
+
+              /* Increment the number of instances remaining within
+                 PREV.  */
+              count_instance++;
+
+            next:
+              ;
+            }
+
+          /* Return true if no instances remain inside
+             PREV->instances, so that the now purposeless desc may be
+             removed.  */
+          return !count_instance;
+        }
+
+      return true;
+    }
 
   if (NILP (prev->instances) || !family_equal_p)
     return false;