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"""ttLib/sfnt.py -- low-level module to deal with the sfnt file format.
Defines two public classes:
SFNTReader
SFNTWriter
(Normally you don't have to use these classes explicitly; they are
used automatically by ttLib.TTFont.)
The reading and writing of sfnt files is separated in two distinct
classes, since whenever to number of tables changes or whenever
a table's length chages you need to rewrite the whole file anyway.
"""
import struct
from kiva.fonttools import sstruct
import numpy
import os
# magic value corresponding to 0xb1b0afba; used in lieu of the hex
# constant to avoid signedness issues with python 2.4
CHECKSUM_MAGIC = -1313820742
class SFNTReader:
def __init__(self, file, checkChecksums=1):
self.file = file
self.checkChecksums = checkChecksums
data = self.file.read(sfntDirectorySize)
if len(data) <> sfntDirectorySize:
from kiva.fonttools.fontTools import ttLib
raise ttLib.TTLibError, "Not a TrueType or OpenType font (not enough data)"
sstruct.unpack(sfntDirectoryFormat, data, self)
if self.sfntVersion not in ("\000\001\000\000", "OTTO", "true"):
from kiva.fonttools.fontTools import ttLib
raise ttLib.TTLibError, "Not a TrueType or OpenType font (bad sfntVersion)"
self.tables = {}
for i in range(self.numTables):
entry = SFNTDirectoryEntry()
entry.fromFile(self.file)
if entry.length > 0:
self.tables[entry.tag] = entry
else:
# Ignore zero-length tables. This doesn't seem to be documented,
# yet it's apparently how the Windows TT rasterizer behaves.
# Besides, at least one font has been sighted which actually
# *has* a zero-length table.
pass
def has_key(self, tag):
return self.tables.has_key(tag)
def keys(self):
return self.tables.keys()
def __getitem__(self, tag):
"""Fetch the raw table data."""
entry = self.tables[tag]
self.file.seek(entry.offset)
data = self.file.read(entry.length)
if self.checkChecksums:
if tag == 'head':
# Beh: we have to special-case the 'head' table.
checksum = calcChecksum(data[:8] + '\0\0\0\0' + data[12:])
else:
checksum = calcChecksum(data)
if self.checkChecksums > 1:
# Be obnoxious, and barf when it's wrong
assert checksum == entry.checksum, "bad checksum for '%s' table" % tag
elif checksum <> entry.checkSum:
# Be friendly, and just print a warning.
print "bad checksum for '%s' table" % tag
return data
def __delitem__(self, tag):
del self.tables[tag]
def close(self):
self.file.close()
class SFNTWriter:
def __init__(self, file, numTables, sfntVersion="\000\001\000\000"):
self.file = file
self.numTables = numTables
self.sfntVersion = sfntVersion
self.searchRange, self.entrySelector, self.rangeShift = getSearchRange(numTables)
self.nextTableOffset = sfntDirectorySize + numTables * sfntDirectoryEntrySize
# clear out directory area
self.file.seek(self.nextTableOffset)
# make sure we're actually where we want to be. (XXX old cStringIO bug)
self.file.write('\0' * (self.nextTableOffset - self.file.tell()))
self.tables = {}
def __setitem__(self, tag, data):
"""Write raw table data to disk."""
if self.tables.has_key(tag):
# We've written this table to file before. If the length
# of the data is still the same, we allow overwriting it.
entry = self.tables[tag]
if len(data) <> entry.length:
from kiva.fonttools.fontTools import ttLib
raise ttLib.TTLibError, "cannot rewrite '%s' table: length does not match directory entry" % tag
else:
entry = SFNTDirectoryEntry()
entry.tag = tag
entry.offset = self.nextTableOffset
entry.length = len(data)
self.nextTableOffset = self.nextTableOffset + ((len(data) + 3) & ~3)
self.file.seek(entry.offset)
self.file.write(data)
self.file.seek(self.nextTableOffset)
# make sure we're actually where we want to be. (XXX old cStringIO bug)
self.file.write('\0' * (self.nextTableOffset - self.file.tell()))
if tag == 'head':
entry.checkSum = calcChecksum(data[:8] + '\0\0\0\0' + data[12:])
else:
entry.checkSum = calcChecksum(data)
self.tables[tag] = entry
def close(self, closeStream=1):
"""All tables must have been written to disk. Now write the
directory.
"""
tables = self.tables.items()
tables.sort()
if len(tables) <> self.numTables:
from kiva.fonttools.fontTools import ttLib
raise ttLib.TTLibError, "wrong number of tables; expected %d, found %d" % (self.numTables, len(tables))
directory = sstruct.pack(sfntDirectoryFormat, self)
self.file.seek(sfntDirectorySize)
for tag, entry in tables:
directory = directory + entry.toString()
self.calcMasterChecksum(directory)
self.file.seek(0)
self.file.write(directory)
if closeStream:
self.file.close()
def calcMasterChecksum(self, directory):
# calculate checkSumAdjustment
tags = self.tables.keys()
checksums = numpy.zeros(len(tags)+1)
for i in range(len(tags)):
checksums[i] = self.tables[tags[i]].checkSum
directory_end = sfntDirectorySize + len(self.tables) * sfntDirectoryEntrySize
assert directory_end == len(directory)
checksums[-1] = calcChecksum(directory)
checksum = numpy.add.reduce(checksums)
# BiboAfba!
checksumadjustment = numpy.array(CHECKSUM_MAGIC) - checksum
# write the checksum to the file
self.file.seek(self.tables['head'].offset + 8)
self.file.write(struct.pack(">l", checksumadjustment))
# -- sfnt directory helpers and cruft
sfntDirectoryFormat = """
> # big endian
sfntVersion: 4s
numTables: H # number of tables
searchRange: H # (max2 <= numTables)*16
entrySelector: H # log2(max2 <= numTables)
rangeShift: H # numTables*16-searchRange
"""
sfntDirectorySize = sstruct.calcsize(sfntDirectoryFormat)
sfntDirectoryEntryFormat = """
> # big endian
tag: 4s
checkSum: l
offset: l
length: l
"""
sfntDirectoryEntrySize = sstruct.calcsize(sfntDirectoryEntryFormat)
class SFNTDirectoryEntry:
def fromFile(self, file):
sstruct.unpack(sfntDirectoryEntryFormat,
file.read(sfntDirectoryEntrySize), self)
def fromString(self, str):
sstruct.unpack(sfntDirectoryEntryFormat, str, self)
def toString(self):
return sstruct.pack(sfntDirectoryEntryFormat, self)
def __repr__(self):
if hasattr(self, "tag"):
return "<SFNTDirectoryEntry '%s' at %x>" % (self.tag, id(self))
else:
return "<SFNTDirectoryEntry at %x>" % id(self)
def calcChecksum(data, start=0):
"""Calculate the checksum for an arbitrary block of data.
Optionally takes a 'start' argument, which allows you to
calculate a checksum in chunks by feeding it a previous
result.
If the data length is not a multiple of four, it assumes
it is to be padded with null byte.
"""
from kiva.fonttools.fontTools import ttLib
remainder = len(data) % 4
if remainder:
data = data + '\0' * (4-remainder)
a = numpy.fromstring(struct.pack(">l", start) + data, numpy.int32)
if ttLib.endian <> "big":
a = a.byteswapped()
return numpy.add.reduce(a)
def maxPowerOfTwo(x):
"""Return the highest exponent of two, so that
(2 ** exponent) <= x
"""
exponent = 0
while x:
x = x >> 1
exponent = exponent + 1
return max(exponent - 1, 0)
def getSearchRange(n):
"""Calculate searchRange, entrySelector, rangeShift for the
sfnt directory. 'n' is the number of tables.
"""
# This stuff needs to be stored in the file, because?
import math
exponent = maxPowerOfTwo(n)
searchRange = (2 ** exponent) * 16
entrySelector = exponent
rangeShift = n * 16 - searchRange
return searchRange, entrySelector, rangeShift
|
