Ubuntu: Fix wrong Condensed classification

[why]
The Ubuntu Condensed font is special because it is designed to be not
part of the `Ubuntu` font family but a self standing `Ubuntu Condensed`
family with just one member.

We want the `Ubuntu Condensed` to be part of `Ubuntu`, as it is usual
for fonts with different widths.

The problem then arises that the width class in the font metadata is set
to 'Normal', while it has to be 'Condensed'. That fact does not become
apparent when the font is in its own one member family, but even there
it is wrong.

[how]
To keep all the other information in the font untouched we just directly
patch that one byte where the width-class is stored - and repair the
checksums afterwards. Nothing more.

The code to do this is the Font Tweaker code in font-patcher, just
copied here for one time use.

Fixes: #1228

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

src/unpatched-fonts/Ubuntu/Condensed/make_condensed.py

@@ -0,0 +1,149 @@
+#!/usr/bin/env python
+# coding=utf8
+# Nerd Fonts Version: 3.0.0
+
+import sys
+
+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):
+            extra = extra << 8
+            if i + j <= end:
+                extra += ord(self.f.read(1))
+        checksum = (checksum + extra) & 0xFFFFFFFF
+        return checksum
+
+    def find_table(self, tablenames, idx):
+        """ Search all tables for one of the tables in tablenames and store its metadata """
+        # 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)
+
+        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 in tablenames:
+                return True
+        return False
+
+    def find_head_table(self, idx):
+        """ Search all tables for the HEAD table and store its metadata """
+        # Use font with index idx if this is a font collection file
+        found = self.find_table([ b'head' ], idx)
+        if not 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 """
+        if isinstance(where, 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,
+                         'avgWidth': 2,
+                         'widthClass': 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(0)
+
+        self.flags = self.getshort('flags')
+        self.lowppem = self.getshort('lowestRecPPEM')
+        self.checksum_adj = self.getlong('checksumAdjustment')
+
+
+print('Tweaking {}'.format(sys.argv[1]))
+font = TableHEADWriter(sys.argv[1])
+font.find_table([b'OS/2'], 0)
+width = font.getshort('widthClass')
+font.putshort(3, 'widthClass') # 3 == Condensed
+font.reset_table_checksum()
+font.reset_full_checksum()
+font.close()

src/unpatched-fonts/Ubuntu/README.md

@@ -14,3 +14,10 @@ you are expressly encouraged to experiment, modify, share and improve.
 http://font.ubuntu.com/
 
 Version: 0.83
+
+## Preprocessed Source Font
+
+For the _Ubuntu Condensed_ font Nerd Fonts corrected the `OS/2` `usWidthClass` to
+Condensed (3) by direct patching.
+
+The script used is in the `Condensed/` directory.