font-patcher: Enable lowestRecPPEM fix in TTCs

[why]
The font flags and PPEM fix does not work with font collection files,
because it does not know how to handle them. It assumes a ttf or otf
font with the specified table structure.

The fix (for single font files) has been introduced with commit
  40138bee9  font-patcher: Handle lowestRecPPEM

[how]
Check if the file is of type 'ttcf', and if so fast forward to the given
single font index into the collection.

This can be rather slow...

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

font-patcher

@@ -88,9 +88,26 @@ class TableHEADWriter:
         checksum = (checksum + extra) & 0xFFFFFFFF
         return checksum
 
-    def find_head_table(self):
+    def find_head_table(self, idx):
         """ Search all tables for the HEAD table and store its metadata """
-        self.f.seek(4)
+        # Use font with index idx if this is a font collection file
+        self.f.seek(0, 0)
+        tag = self.f.read(4)
+        if tag == b'ttcf':
+            self.f.seek(2*2, 1)
+            self.num_fonts = self.getlong()
+            if (idx >= self.num_fonts):
+                raise Exception('Trying to access subfont index {} but have only {} fonts'.format(idx, num_fonts))
+            for _ in range(idx + 1):
+                offset = self.getlong()
+            self.f.seek(offset, 0)
+        elif idx != 0:
+            raise Exception('Trying to access subfont but file is no collection')
+        else:
+            self.f.seek(0, 0)
+            self.num_fonts = 1
+
+        self.f.seek(4, 1)
         numtables = self.getshort()
         self.f.seek(3*2, 1)
 
@@ -102,7 +119,7 @@ class TableHEADWriter:
             self.tab_length = self.getlong()
             if tab_name == b'head':
                 return
-        raise Exception('No HEAD table found')
+        raise Exception('No HEAD table found in font idx {}'.format(idx))
 
     def goto(self, where):
         """ Go to a named location in the file or to the specified index """
@@ -146,7 +163,7 @@ class TableHEADWriter:
         self.modified = False
         self.f = open(filename, 'r+b')
 
-        self.find_head_table()
+        self.find_head_table(0)
 
         self.flags = self.getshort('flags')
         self.lowppem = self.getshort('lowestRecPPEM')
@@ -266,15 +283,19 @@ class font_patcher:
         try:
             source_font = TableHEADWriter(self.args.font)
             dest_font = TableHEADWriter(outfile)
-            if source_font.flags & 0x08 == 0 and dest_font.flags & 0x08 != 0:
+            for idx in range(source_font.num_fonts):
-                print("Changing flags from 0x{:X} to 0x{:X}".format(dest_font.flags, dest_font.flags & ~0x08))
+                print("{}: Tweaking {}/{}".format(projectName, idx + 1, source_font.num_fonts))
-                dest_font.putshort(dest_font.flags & ~0x08, 'flags') # clear 'ppem_to_int'
+                source_font.find_head_table(idx)
-            if source_font.lowppem != dest_font.lowppem:
+                dest_font.find_head_table(idx)
-                print("Changing lowestRecPPEM from {} to {}".format(dest_font.lowppem, source_font.lowppem))
+                if source_font.flags & 0x08 == 0 and dest_font.flags & 0x08 != 0:
-                dest_font.putshort(source_font.lowppem, 'lowestRecPPEM')
+                    print("Changing flags from 0x{:X} to 0x{:X}".format(dest_font.flags, dest_font.flags & ~0x08))
-            if dest_font.modified:
+                    dest_font.putshort(dest_font.flags & ~0x08, 'flags') # clear 'ppem_to_int'
-                dest_font.reset_table_checksum()
+                if source_font.lowppem != dest_font.lowppem:
-                dest_font.reset_full_checksum()
+                    print("Changing lowestRecPPEM from {} to {}".format(dest_font.lowppem, source_font.lowppem))
+                    dest_font.putshort(source_font.lowppem, 'lowestRecPPEM')
+                if dest_font.modified:
+                    dest_font.reset_table_checksum()
+                    dest_font.reset_full_checksum()
         except Exception as error:
             print("Can not handle font flags ({})".format(repr(error)))
         finally:
@@ -1292,8 +1313,9 @@ def main():
     patcher = font_patcher(args)
 
     sourceFonts = []
-    for subfont in fontforge.fontsInFile(args.font):
+    all_fonts = fontforge.fontsInFile(args.font)
-        print("\n{}: Processing {}".format(projectName, subfont))
+    for i, subfont in enumerate(all_fonts):
+        print("\n{}: Processing {} ({}/{})".format(projectName, subfont, i + 1, len(all_fonts)))
         try:
             sourceFonts.append(fontforge.open("{}({})".format(args.font, subfont), 1)) # 1 = ("fstypepermitted",))
         except Exception: