Subsetting of the kern table

src/calibre/utils/fonts/sfnt/container.py

@@ -21,6 +21,7 @@ from calibre.utils.fonts.sfnt.maxp import MaxpTable
 from calibre.utils.fonts.sfnt.loca import LocaTable
 from calibre.utils.fonts.sfnt.glyf import GlyfTable
 from calibre.utils.fonts.sfnt.cmap import CmapTable
+from calibre.utils.fonts.sfnt.kern import KernTable
 from calibre.utils.fonts.sfnt.cff.table import CFFTable
 
 # OpenType spec: http://www.microsoft.com/typography/otspec/otff.htm
@@ -44,6 +45,7 @@ class Sfnt(object):
                     b'glyf' : GlyfTable,
                     b'cmap' : CmapTable,
                     b'CFF ' : CFFTable,
+                    b'kern' : KernTable,
                     }.get(table_tag, UnknownTable)(table)
 
     def __getitem__(self, key):

src/calibre/utils/fonts/sfnt/kern.py

@@ -0,0 +1,89 @@
+#!/usr/bin/env python
+# vim:fileencoding=UTF-8:ts=4:sw=4:sta:et:sts=4:fdm=marker:ai
+from __future__ import (unicode_literals, division, absolute_import,
+                        print_function)
+
+__license__   = 'GPL v3'
+__copyright__ = '2012, Kovid Goyal <kovid at kovidgoyal.net>'
+__docformat__ = 'restructuredtext en'
+
+from struct import unpack_from, calcsize, pack, error as struct_error
+
+from calibre.utils.fonts.sfnt import (UnknownTable, FixedProperty,
+        max_power_of_two)
+from calibre.utils.fonts.sfnt.errors import UnsupportedFont
+
+class KernTable(UnknownTable):
+
+    version = FixedProperty('_version')
+
+    def __init__(self, *args, **kwargs):
+        super(KernTable, self).__init__(*args, **kwargs)
+        self._version, self.num_tables = unpack_from(b'>HH', self.raw)
+        if self._version == 1 and len(self.raw) >= 8:
+            self._version, self.num_tables = unpack_from(b'>LL', self.raw)
+        self.headerfmt = b'>HH' if self._version == 0 else b'>LL'
+
+    def restrict_to_glyphs(self, glyph_ids):
+        if self._version not in {0, 0x10000}:
+            raise UnsupportedFont('kern table has version: %x'%self._version)
+        offset = 4 if (self._version == 0) else 8
+        tables = []
+        for i in xrange(self.num_tables):
+            if self._version == 0:
+                version, length, coverage = unpack_from(b'>3H', self.raw, offset)
+                table_format = version
+            else:
+                length, coverage = unpack_from(b'>LH', self.raw, offset)
+                table_format = coverage & 0xff
+            raw = self.raw[offset:offset+length]
+            if table_format == 0:
+                raw = self.restrict_format_0(raw, glyph_ids)
+                if not raw:
+                    continue
+            tables.append(raw)
+            offset += length
+        self.raw = pack(self.headerfmt, self._version, len(tables)) + b''.join(tables)
+
+    def restrict_format_0(self, raw, glyph_ids):
+        if self._version == 0:
+            version, length, coverage, npairs = unpack_from(b'>4H', raw)
+            headerfmt = b'>3H'
+        else:
+            length, coverage, tuple_index, npairs = unpack_from(b'>L3H', raw)
+            headerfmt = b'>L2H'
+
+        offset = calcsize(headerfmt + b'4H')
+        entries = []
+        entrysz = calcsize(b'>2Hh')
+        for i in xrange(npairs):
+            try:
+                left, right, value = unpack_from(b'>2Hh', raw, offset)
+            except struct_error:
+                offset = len(raw)
+                break # Buggy kern table
+            if left in glyph_ids and right in glyph_ids:
+                entries.append(pack(b'>2Hh', left, right, value))
+            offset += entrysz
+
+        if offset != len(raw):
+            raise UnsupportedFont('This font has extra data at the end of'
+                    ' a Format 0 kern subtable')
+
+        npairs = len(entries)
+        if npairs == 0:
+            return b''
+
+		entry_selector = max_power_of_two(npairs)
+		search_range = (2 ** entry_selector) * 6
+		range_shift = (npairs - (2 ** entry_selector)) * 6
+
+        entries = b''.join(entries)
+        length = calcsize(headerfmt + b'4H') + len(entries)
+        if self._version == 0:
+            header = pack(headerfmt, version, length, coverage)
+        else:
+            header = pack(headerfmt, length, coverage, tuple_index)
+        return header + pack(b'>4H', npairs, search_range, entry_selector,
+                range_shift) + entries
+

src/calibre/utils/fonts/sfnt/subset.py

@@ -104,6 +104,16 @@ def subset(raw, individual_chars, ranges=()):
     # Restrict the cmap table to only contain entries for the resolved glyphs
     cmap.set_character_map(character_map)
 
+    if b'kern' in sfnt:
+        try:
+            sfnt[b'kern'].restrict_to_glyphs(frozenset(character_map.itervalues()))
+        except UnsupportedFont as e:
+            print ('Subsetting of kern table failed, ignoring: %s'%e)
+        except Exception as e:
+            print ('Subsetting of kern table failed, ignoring')
+            import traceback
+            traceback.print_exc()
+
     raw, new_sizes = sfnt()
     return raw, old_sizes, new_sizes