Bug in Numeric#divmod

Hi all!

I submitted the following bug report to 
http://rubyforge.org/tracker/?func=detail&atid=1698&aid=14540&
group_id=426
and reported it on ruby-talk.

But Eric Hodel wrote:
> You may want to bring this up on ruby-core (maybe you have?  There  
> was also a documentation patch for divmod, so I may be confused).

I take his advice and report it here too together with all the 
additional comments sent.
(By the way, the documentation patch, which is also from me, is for the 
same method, but unrelated.)

===================================================
> There is a calculating bug in Numeric#divmod.
> The divisor is reproducibly calculated wrongly for certain numbers.
> 
> See the following example to illustrate the problem:
> 
> x=[0.58, 0.59, 0.63, 0.66, 0.67]
> y=1/12.0
> 
> z1=x.map {|a| a/y}
> z2=x.map {|a| a.divmod(y).inspect}
> 
> a=[RUBY_VERSION, RUBY_RELEASE_DATE, RUBY_PATCHLEVEL, RUBY_PLATFORM]
> puts "ruby #{a[0]} (#{a[1]} patchlevel #{a[2].to_s}) [#{a[3]}]"
> puts
> print 'x='
> p x
> puts 'y=1/12.0'
> puts
> puts "x/y\n---"
> puts z1
> puts
> puts "x.divmod(y)\n-----------"
> puts z2
> 
> 
> Output:
> -------------------------------------------------
> ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]
> 
> x=[0.58, 0.59, 0.63, 0.66, 0.67]
> y=1/12.0
> 
> x/y
> ---
> 6.96
> 7.08
> 7.56
> 7.92
> 8.04
> 
> x.divmod(y)
> -----------
> [6, 0.08]
> [6, 0.00666666666666667]
> [6, 0.0466666666666667]
> [6, 0.0766666666666667]
> [8, 0.00333333333333341]
> -------------------------------------------------
> 
> As you can see the divisor 7 is wrongly displayed as 6, while the 
> modulos are correct.
> 
> Dirk Traulsen
===================================================

Here are the additional comments:

By me:
===================================================
> I tested Numeric#divmod on two more systems: 
> SUSE 10.1 and Kubuntu 6.10.
> They both run Ruby 1.8.4 and under both systems Numeric#divmod
> works correctly! So this bug was either introduced between 
> Ruby 1.8.4 and 1.8.6 or it is a problem depending on the OS.
(...)
> SUSE 10.1:   ruby 1.8.4 (2005-12-24) [i586-linux]
> 
> x.divmod(y)
> -----------
> [6.0, 0.08]
> [7.0, 0.00666666666666667]
> [7.0, 0.0466666666666667]
> [7.0, 0.0766666666666667]
> [8.0, 0.00333333333333341]
> 
> 
> Kubuntu 6.10:   ruby 1.8.4 (2005-12-24) [i586-linux]
> 
> x.divmod(y)
> -----------
> [6.0, 0.08]
> [7.0, 0.00666666666666667]
> [7.0, 0.0466666666666667]
> [7.0, 0.0766666666666667]
> [8.0, 0.00333333333333341]
===================================================


On ruby-talk:
=================================================
Am 14 Oct 2007 um 17:03 hat Eric Hodel geschrieben:

> I get the same results for 1.8.6 and 1.9.0:
> 
> ruby 1.8.6 (2007-09-23) [powerpc-darwin8.10.0]
> ruby 1.9.0 (2007-10-13) [powerpc-darwin8.10.0]
(...) 
> x.divmod(y)
> -----------
> [6, 0.08]
> [6, 0.00666666666666667]
> [6, 0.0466666666666667]
> [6, 0.0766666666666667]
> [8, 0.00333333333333341]
=================================================
Am 14 Oct 2007 um 17:02 hat Konrad Meyer geschrieben:
 
> I can't confirm this (incorrect) behaviour.
> 
> 0.58.divmod(1/12.0) # => [6.0, 0.08]
> 0.59.divmod(1/12.0) # => [7.0, 0.00666666666666667]
> 
> Ruby 1.8.6, Fedora 7.
=================================================
Am 14 Oct 2007 um 18:15 hat Alex Shulgin geschrieben:

> It's correct on my system:
> 
> Debian lenny (testing)
> Linux 2.6.21-2-686 #1 SMP Wed Jul 11 03:53:02 UTC 2007 i686
> GNU/Linux 
> ruby 1.8.6 (2007-06-07) [i486-linux]
(...)
> x.divmod(y)
> -----------
> [6, 0.08]
> [7, 0.00666666666666667]
> [7, 0.0466666666666667]
> [7, 0.0766666666666667]
> [8, 0.00333333333333341]
==========================================
Am 14 Oct 2007 um 19:44 hat M. Edward (Ed) Borasky geschrieben:

> And on Gentoo Linux with ruby-1.8.6-p110-r1
> 
> ruby 1.8.6 (2007-09-24) [i686-linux]
(...)
> x.divmod(y)
> -----------
> [6, 0.08]
> [7, 0.00666666666666667]
> [7, 0.0466666666666667]
> [7, 0.0766666666666667]
> [8, 0.00333333333333341]
> 
> If it matters, the hardware is an Athlon Thunderbird and Ruby was
> compiled from source, as most of Gentoo is, using gcc 4.2.1 and CFLAGS
> "-march athlon-tbird -O2". So ... where is it broken again? 
> 
> Dirk says "ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32] is
> broken" Dirk says "Kubuntu 6.10 and SuSE 10.1 using Ruby 1.8.4 works."
> Konrad says "two test cases worked on Fedora 7 with 1.8.6" And I have
> it working on Ruby 1.8.6-p110 on Gentoo, compiled from source with GCC
> 4.2.1 And Eric has it broken on PowerPC Macs for both Ruby 1.8.6 and
> Ruby 1.9.0 
> 
> Eric, what compiler made the Ruby interpreters on your PowerPC?
> 
> And everybody -- recompile ruby-1.8.6-p111 and see whether it works or
> not. I say it's a compiler problem at this point -- maybe
> over-aggressive floating-point optimization?? I doubt if the Ruby
> source is broken. 
==========================================


Taken together:

incorrect = bug:
ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]
ruby 1.8.6 (2007-09-23) [powerpc-darwin8.10.0]
ruby 1.9.0 (2007-10-13) [powerpc-darwin8.10.0]
 
correct:
Fedora 7:  Ruby 1.8.6
Kubuntu 6.10:  ruby 1.8.4 (2005-12-24) [i586-linux]
SUSE 10.1:  ruby 1.8.4 (2005-12-24) [i586-linux]
Debian Lenny:  ruby 1.8.6 (2007-06-07) [i486-linux]
Gentoo Linux with ruby-1.8.6-p110-r1

Please help to confirm these findings. Especially with not tested 
system and Ruby version combinations this would be really helpful
to nail down this massive bug. A computer should really calculate 
correctly...

Dirk Traulsen