[reportlab-users] Marius' TTFontFile breaks in 2.2
Tue, 28 May 2002 14:18:11 +0200
On Mon, May 27, 2002 at 07:33:36PM +0100, Robin Becker wrote:
> Hi, we're trying to get a release out this week and I'm having problems
> with the Checksum for Marius' TTFont check sum. I suspect that the
> Python 2.2 changes to int/long in the following code are causing the
> checkSum = calcChecksum(self._data)
> checkSum = _add32(0xB1B0AFBA, -checkSum)
Yes, I just tried python 2.2 and it breaks TTFontFileTestCase.testAdd32
in my test suite.
If somebody know how to achieve efficient arithmetic modulo 2**32 in
Python, please tell me. This looks really gross:
> def _add32(x, y):
> "Calculate (x + y) modulo 2**32"
> return x + y
> except OverflowError:
> lo = (x & 0xFFFF) + (y & 0xFFFF)
> hi = (x >> 16) + (y >> 16) + (lo >> 16)
> return (hi << 16) | (lo & 0xFFFF)
and breaks under Python 2.2.
BTW calcChecksum could look like
> def calcChecksum(stream):
> """Calculates PDF-style checksums"""
> if len(stream) & 3:
> stream = stream + "\0\0\0"
> stream = stream[:len(stream)&~3]
> sum = 0
> for n in unpack(">%dl" % (len(stream)/4), stream):
sum = _add32(sum, n)
> return sum
I just was afraid that a function call for every 4 bytes of the output
(twice) was too much overhead. After profiling my first prototype I
noticed that 40% of the time was spent in calcChecksum (which was
implemented quite differently). I haven't yet found the time to profile
Perhaps the best way out of here would be to make calcChecksum a C
If "con" is the opposite of "pro", then what is the opposite of progress?