font-patcher: Handle lowestRecPPEM

[why]
Some fonts set specific lower resolutions and handle the small rendering
very nicely. But fontforge does not copy that information when it opens
a font; rather some default values are always written. That can destroy
the small font rendering.

The responsible settings are the 'ppem_to_int' flag and the
'lowestRecPPEM' setting, both in the HEAD table.

[how]
After successfully patching and generating the new font we copy that
settings over from the source font.

Instead of using fontTools (and thereby requiring all users to pull that
dependency) we handle the raw font file changes ourselves.

Fixes: 612

Reported-by: nojus297
Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

font-patcher

@@ -37,6 +37,113 @@ except ImportError:
     )
 
 
+class TableHEADWriter:
+    """ Access to the HEAD table without external dependencies """
+    def getlong(self, pos = None):
+        """ Get four bytes from the font file as integer number """
+        if pos:
+            self.goto(pos)
+        return (ord(self.f.read(1)) << 24) + (ord(self.f.read(1)) << 16) + (ord(self.f.read(1)) << 8) + ord(self.f.read(1))
+
+    def getshort(self, pos = None):
+        """ Get two bytes from the font file as integer number """
+        if pos:
+            self.goto(pos)
+        return (ord(self.f.read(1)) << 8) + ord(self.f.read(1))
+
+    def putlong(self, num, pos = None):
+        """ Put number as four bytes into font file """
+        if pos:
+            self.goto(pos)
+        self.f.write(bytearray([(num >> 24) & 0xFF, (num >> 16) & 0xFF ,(num >> 8) & 0xFF, num & 0xFF]))
+        self.modified = True
+
+    def putshort(self, num, pos = None):
+        """ Put number as two bytes into font file """
+        if pos:
+            self.goto(pos)
+        self.f.write(bytearray([(num >> 8) & 0xFF, num & 0xFF]))
+        self.modified = True
+
+    def calc_checksum(self, start, end, checksum = 0):
+        """ Calculate a font table checksum, optionally ignoring another embedded checksum value (for table 'head') """
+        self.f.seek(start)
+        for i in range(start, end - 4, 4):
+            checksum += self.getlong()
+            checksum &= 0xFFFFFFFF
+        i += 4
+        extra = 0
+        for j in range(4):
+            if i + j <= end:
+                extra += ord(self.f.read(1))
+            extra = extra << 8
+        checksum = (checksum + extra) & 0xFFFFFFFF
+        return checksum
+
+    def find_head_table(self):
+        """ Search all tables for the HEAD table and store its metadata """
+        self.f.seek(4)
+        numtables = self.getshort()
+        self.f.seek(3*2, 1)
+
+        for i in range(numtables):
+            tab_name = self.f.read(4)
+            self.tab_check_offset = self.f.tell()
+            self.tab_check = self.getlong()
+            self.tab_offset = self.getlong()
+            self.tab_length = self.getlong()
+            if tab_name == b'head':
+                break
+
+    def goto(self, where):
+        """ Go to a named location in the file or to the specified index """
+        if type(where) is str:
+            positions = {'checksumAdjustment': 2+2+4,
+                         'flags': 2+2+4+4+4,
+                         'lowestRecPPEM': 2+2+4+4+4+2+2+8+8+2+2+2+2+2,
+                }
+            where = self.tab_offset + positions[where]
+        self.f.seek(where)
+
+
+    def calc_full_checksum(self, check = False):
+        """ Calculate the whole file's checksum """
+        self.f.seek(0, 2)
+        self.end = self.f.tell()
+        full_check = self.calc_checksum(0, self.end, (-self.checksum_adj) & 0xFFFFFFFF)
+        if check and (0xB1B0AFBA - full_check) & 0xFFFFFFFF != self.checksum_adj:
+            sys.exit("Checksum of whole font is bad")
+        return full_check
+
+    def calc_table_checksum(self, check = False):
+        tab_check_new = self.calc_checksum(self.tab_offset, self.tab_offset + self.tab_length - 1, (-self.checksum_adj) & 0xFFFFFFFF)
+        if check and tab_check_new != self.tab_check:
+            sys.exit("Checksum of 'head' in font is bad")
+        return tab_check_new
+
+    def reset_table_checksum(self):
+        new_check = self.calc_table_checksum()
+        self.putlong(new_check, self.tab_check_offset)
+
+    def reset_full_checksum(self):
+        new_adj = (0xB1B0AFBA - self.calc_full_checksum()) & 0xFFFFFFFF
+        self.putlong(new_adj, 'checksumAdjustment')
+
+    def close(self):
+        self.f.close()
+
+
+    def __init__(self, filename):
+        self.modified = False
+        self.f = open(filename, 'r+b')
+
+        self.find_head_table()
+
+        self.flags = self.getshort('flags')
+        self.lowppem = self.getshort('lowestRecPPEM')
+        self.checksum_adj = self.getlong('checksumAdjustment')
+
+
 class font_patcher:
     def __init__(self):
         self.args = None  # class 'argparse.Namespace'
@@ -137,11 +244,27 @@ class font_patcher:
         if self.sourceFont.fullname != None:
             outfile = self.args.outputdir + "/" + self.sourceFont.fullname + self.extension
             self.sourceFont.generate(outfile, flags=(str('opentype'), str('PfEd-comments')))
-            print("\nGenerated: {} in {}".format(self.sourceFont.fontname, outfile))
+            message = "\nGenerated: {} in '{}'".format(self.sourceFont.fontname, outfile)
         else:
-            outfile = self.sourceFont.generate(self.args.outputdir + "/" + self.sourceFont.cidfontname + self.extension
+            outfile = self.args.outputdir + "/" + self.sourceFont.cidfontname + self.extension
             self.sourceFont.generate(outfile, flags=(str('opentype'), str('PfEd-comments')))
-            print("\nGenerated: {} in {}".format(self.sourceFont.fullname, outfile))
+            message = "\nGenerated: {} in '{}'".format(self.sourceFont.fullname, outfile)
+
+        # Adjust flags that can not be changed via fontforge
+        source_font = TableHEADWriter(self.args.font)
+        source_font.close()
+        dest_font = TableHEADWriter(outfile)
+        if source_font.flags & 0x08 == 0 and dest_font.flags & 0x08 != 0:
+            print("Changing flags from 0x{:X} to 0x{:X}".format(dest_font.flags, dest_font.flags & ~0x08))
+            dest_font.putshort(dest_font.flags & ~0x08, 'flags') # clear 'ppem_to_int'
+        if source_font.lowppem != dest_font.lowppem:
+            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()
+        dest_font.close()
+        print(message)
 
         if self.args.postprocess:
             subprocess.call([self.args.postprocess, outfile])