fontTools/ttLib/sfnt.py

"""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, sstruct
import Numeric
import os


class SFNTReader:

	def __init__(self, file, checkChecksums=1):
		self.file = file
		self.checkChecksums = checkChecksums
		data = self.file.read(sfntDirectorySize)
		if len(data) <> sfntDirectorySize:
			from 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 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 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):
		"""All tables must have been written to disk. Now write the
		directory.
		"""
		tables = self.tables.items()
		tables.sort()
		if len(tables) <> self.numTables:
			from 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)
		seenHead = 0
		for tag, entry in tables:
			if tag == "head":
				seenHead = 1
			directory = directory + entry.toString()
		if seenHead:
			self.calcMasterChecksum(directory)
		self.file.seek(0)
		self.file.write(directory)

	def calcMasterChecksum(self, directory):
		# calculate checkSumAdjustment
		tags = self.tables.keys()
		checksums = Numeric.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 = Numeric.add.reduce(checksums)
		# BiboAfba!
		checksumadjustment = Numeric.array(0xb1b0afbaL - 0x100000000L,
				Numeric.Int32) - 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 fontTools import ttLib
	remainder = len(data) % 4
	if remainder:
		data = data + '\0' * (4-remainder)
	a = Numeric.fromstring(struct.pack(">l", start) + data, Numeric.Int32)
	if ttLib.endian <> "big":
		a = a.byteswapped()
	return Numeric.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