Test for overflow

From	jacob navia <jacob@jacob.remcomp.fr>
Newsgroups	comp.lang.c, sci.math.symbolic
Subject	Test for overflow
Date	2015-03-30 13:17 +0200
Organization	lccwin
Message-ID	<mfbbc6$9h9$3@speranza.aioe.org> (permalink)

Cross-posted to 2 groups.

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Hi the math experts!

When parsing a digit sequence I test for overflow not by calculating the 
maximum number of digits allowed but just with the test

tvalue = value * base;

if (tvalue < value) {
    // overflow detected
}

I am assuming then that

if value * base > 2^64 then
    value * base mod 2^64 < value

for base >= 2 and base <= 36.

Is this correct?

I do not have the proof of it. Maybe one of the math people can help me out?

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Test for overflow jacob navia <jacob@jacob.remcomp.fr> - 2015-03-30 13:17 +0200
  Re: Test for overflow Lőrinczy Zsigmond <zsiga@nospam.for.me> - 2015-03-30 13:34 +0200
  Re: Test for overflow Richard Damon <Richard@Damon-Family.org> - 2015-03-30 07:42 -0400
    Re: Test for overflow asetofsymbols@gmail.com - 2015-03-30 04:47 -0700
      Re: Test for overflow jacob navia <jacob@jacob.remcomp.fr> - 2015-03-30 13:57 +0200
        Re: Test for overflow Richard Damon <Richard@Damon-Family.org> - 2015-03-30 08:24 -0400
  Re: Test for overflow Richard Heathfield <rjh@cpax.org.uk> - 2015-03-30 13:03 +0100
  Re: Test for overflow Tim Rentsch <txr@alumni.caltech.edu> - 2015-03-30 06:54 -0700
  Re: Test for overflow James Kuyper <jameskuyper@verizon.net> - 2015-03-30 13:20 -0400
    Re: Test for overflow James Kuyper <jameskuyper@verizon.net> - 2015-03-30 13:32 -0400
    Re: Test for overflow James Kuyper <jameskuyper@verizon.net> - 2015-03-30 19:43 -0400
      Re: Test for overflow jacob navia <jacob@jacob.remcomp.fr> - 2015-03-31 07:11 +0200
        Re: Test for overflow James Kuyper <jameskuyper@verizon.net> - 2015-03-31 13:41 -0400
          Re: Test for overflow jacob navia <jacob@jacob.remcomp.fr> - 2015-03-31 19:48 +0200
            Re: Test for overflow James Kuyper <jameskuyper@verizon.net> - 2015-03-31 13:58 -0400
  Re: Test for overflow Thomas Jahns <jahns@idontlikespam.dkrz.de> - 2015-04-01 10:12 +0200
    Re: Test for overflow Richard Heathfield <rjh@cpax.org.uk> - 2015-04-01 09:43 +0100
      Re: Test for overflow Thomas Jahns <jahns@idontlikespam.dkrz.de> - 2015-04-01 10:59 +0200
        Re: Test for overflow Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-04-01 04:07 -0700
          Re: Test for overflow raltbos@xs4all.nl (Richard Bos) - 2015-04-01 13:52 +0000

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