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Re: Inconsistent...

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.c
Subject Re: Inconsistent...
Date 2023-11-01 21:43 +0000
Organization A noiseless patient Spider
Message-ID <87msvxno8e.fsf@bsb.me.uk> (permalink)
References (8 earlier) <uhuacl$1omhf$3@dont-email.me> <875y2lp77l.fsf@bsb.me.uk> <uhubeg$1omhf$9@dont-email.me> <87zfzxnpoh.fsf@bsb.me.uk> <uhuf89$1pltd$3@dont-email.me>

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"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:

>>> Almost has to be a floating point issue? Or some undefined behavior I
>>> overlooked?
>> Well, in a post in reply to Bart, I gave some information about that so
>> now I'm going to mess up this sub thread by going back to it...
>> The code that calculates cpack mixes unsigned int and unsigned long int
>> arithmetic.  On Windows, unsigned long int is 32 bits, but it's usually
>> 64 bits on Linux.  There seems to be no logic behind this mixing, but
>> you won't get the same result with different sized integer types.
>> The situation is made worse by using floating point to halve an integer
>> quantity (in cantor_packing).  In effect, that introduces a 52-bit
>> integer type into the mix as well.
>> To cap it all, CT_FFE_ABET_LOOKUP is of type ptrdiff_t -- a /signed/
>> integer type whose size can vary between systems.
>> It would be almost impossible to state, mathematically, what cpack's
>> value "should" be.  The value is derived from a chaotic mix of wrapping
>> between 32 and sometimes 64 bit values as well as rounding to 52 bits of
>> accuracy.
>> Even the simple cantor_packing function relies on wrapping and/or
>> rounding to give a result that is not mathematically correct.
>> Since you rely on unsigned arithmetic being modular, the only way you'll
>> get the same result for cpack on all systems is to specify exactly what
>> arithmetic you want.  The crude way is to use integer types with
>> specified sizes, avoid signed integer types and replace "0.5*..." with
>> with ".../2".
>
> Agreed. However, that would take the funny part out of it...
>
> ;^)

Ah, sorry.  I thought you wanted to fix the code.  I'll stick to
answering your questions literally!

>>> Almost has to be a floating point issue?

No, not here.

>>> Or some undefined behavior I overlooked?

No, not here, though there is plenty of undefined behaviour elsewhere.

-- 
Ben.

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Thread

Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-30 14:01 -0700
  Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-30 14:11 -0700
  Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-30 14:30 -0700
    Re: Inconsistent... Richard Harnden <richard.nospam@gmail.invalid> - 2023-10-30 22:01 +0000
      Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-30 15:09 -0700
  Re: Inconsistent... bart <bc@freeuk.com> - 2023-10-30 23:22 +0000
    Re: Inconsistent... bart <bc@freeuk.com> - 2023-10-30 23:44 +0000
    Re: Inconsistent... bart <bc@freeuk.com> - 2023-10-31 02:05 +0000
      Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-31 11:32 +0000
        Re: Inconsistent... bart <bc@freeuk.com> - 2023-10-31 14:16 +0000
          Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-31 16:15 +0000
            Re: Inconsistent... bart <bc@freeuk.com> - 2023-10-31 18:35 +0000
              Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-31 14:05 -0700
                Re: Inconsistent... bart <bc@freeuk.com> - 2023-10-31 22:29 +0000
              Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 00:39 +0000
                Re: Inconsistent... bart <bc@freeuk.com> - 2023-11-01 10:35 +0000
                Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 13:29 +0000
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-01 12:52 -0700
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-01 14:30 -0700
        Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-31 13:37 -0700
          Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 00:44 +0000
            Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-31 20:26 -0700
              Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 11:05 +0000
            Re: Inconsistent... bart <bc@freeuk.com> - 2023-11-01 10:59 +0000
              Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-01 13:01 +0100
                Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 13:52 +0000
                Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-01 20:12 +0100
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-01 12:52 -0700
                Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-02 10:04 +0100
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-02 10:51 -0700
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-02 11:02 -0700
                Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-02 20:02 +0100
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-02 12:03 -0700
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-03 23:57 -0700
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-04 00:11 -0700
                Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-02 20:28 +0000
                Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-03 10:23 +0100
                [OT] Encryption (Was: Inconsistent...) Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-03 10:10 +0000
                Re: [OT] Encryption (Was: Inconsistent...) David Brown <david.brown@hesbynett.no> - 2023-11-03 15:01 +0100
                Re: [OT] Encryption (Was: Inconsistent...) scott@slp53.sl.home (Scott Lurndal) - 2023-11-03 16:52 +0000
                Re: [OT] Encryption (Was: Inconsistent...) "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-03 15:12 -0700
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-03 15:11 -0700
              Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 13:43 +0000
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-01 12:53 -0700
                Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 20:08 +0000
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-01 13:12 -0700
                Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 21:12 +0000
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-01 14:16 -0700
                Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 21:43 +0000
                Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-01 16:02 -0700
                Re: Inconsistent... bart <bc@freeuk.com> - 2023-11-01 22:59 +0000
  Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-10-30 23:52 +0000
    Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-31 13:45 -0700
    Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-10-31 14:29 -0700
    Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-02 10:10 +0100
      Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-02 09:52 +0000
        Re: Inconsistent... bart <bc@freeuk.com> - 2023-11-02 11:31 +0000
          Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-02 13:52 +0100
            Re: Inconsistent... bart <bc@freeuk.com> - 2023-11-02 14:07 +0000
              Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-02 16:53 +0100
                Re: Inconsistent... bart <bc@freeuk.com> - 2023-11-02 19:15 +0000
                Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-03 10:33 +0100
          Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-02 20:43 +0000
            Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-02 13:47 -0700
        Re: Inconsistent... David Brown <david.brown@hesbynett.no> - 2023-11-02 13:34 +0100
  Re: Inconsistent... Richard Harnden <richard.nospam@gmail.invalid> - 2023-11-01 20:32 +0000
    Re: Inconsistent... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-11-01 21:36 +0000
      Re: Inconsistent... Richard Harnden <richard.nospam@gmail.invalid> - 2023-11-01 21:54 +0000
      Re: Inconsistent... Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-11-09 15:54 -0800
  Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-09 01:16 -0800
  Re: Inconsistent... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2023-11-10 21:02 -0800

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