emacs-30 3419e7ea522: Correct Android failure to open an old CJK font

[Po Lu]

branch: emacs-30
commit 3419e7ea522462c47a5ab4389a879d8b7c14452b
Author: Po Lu <luangruo@yahoo.com>
Commit: Po Lu <luangruo@yahoo.com>

    Correct Android failure to open an old CJK font
    
    * src/sfnt.c (sfnt_read_cmap_format_2): Properly compute
    subtable count, and append the empty table at position 0.
    (sfnt_lookup_glyph_2): Update commentary.
---
 src/sfnt.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/src/sfnt.c b/src/sfnt.c
index 1ed492b7506..02b8a33041c 100644
--- a/src/sfnt.c
+++ b/src/sfnt.c
@@ -383,15 +383,18 @@ sfnt_read_cmap_format_2 (int fd,
 
   for (i = 0; i < 256; ++i)
     {
+      /* Values in sub_header_keys are actually offsets from the end of
+        that array.  Since the language of the spec is such as to imply
+        that they must be divisible by eight, divide them by the
+        same.  */
       sfnt_swap16 (&format2->sub_header_keys[i]);
 
       if (format2->sub_header_keys[i] > nsub)
-       nsub = format2->sub_header_keys[i];
+       nsub = format2->sub_header_keys[i] / 8;
     }
 
-  if (!nsub)
-    /* If there are no subheaders, then things are finished.  */
-    return format2;
+  /* There always exists a subheader at index zero.  */
+  nsub ++;
 
   /* Otherwise, read the rest of the variable length data to the end
      of format2.  */
@@ -1108,9 +1111,11 @@ sfnt_lookup_glyph_2 (sfnt_char character,
          && j <= ((int) subheader->first_code
                   + (int) subheader->entry_count))
        {
-         /* id_range_offset is actually the number of bytes past
-            itself containing the uint16_t ``slice''.  It is possibly
-            unaligned.  */
+         /* id_range_offset is actually the number of bytes past itself
+            containing the uint16_t ``slice''.  Whether this may be
+            unaligned is not stated in the specification, but I doubt
+            it, for that would render values meaningless if the array
+            were byte swapped when read.  */
          slice = (unsigned char *) &subheader->id_range_offset;
          slice += subheader->id_range_offset;
          slice += (j - subheader->first_code) * sizeof (uint16_t);