# Bug in BigDecimal#round?

A coworker discovered some disturbing behavior in BigDecimal#round.
The output I get from the program below is:

===== begin output

These values appended to ‘0.0000’ cause BigDecimal#round to return
nonzero:
[5, 6, 7, 8, 9, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,
63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,
80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,
97, 98, 99]

These numbers of zeroes between the decimal point and a 7 cause
BigDecimal#round to return nonzero:
[4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56, 60, 64, 68, 72,
76, 80, 84, 88, 92, 96, 100]

===== end output

===== begin code

require ‘bigdecimal’

vary_number_after_four_zeroes = (1…100).map { |i|
bd = BigDecimal.new(“0.0000#{i}”); [i, bd.round.to_s]
}
seed_values_triggering_misbehavior_with_four_zeroes =
vary_number_after_four_zeroes.reject {|i, bds| bds == ‘0.0’}.map {|
i, bds| i}

vary_number_of_zeroes = (1…100).map { |i|
bd = BigDecimal.new(“0.#{‘0’ * i}7”); [i, bd.round.to_s]
}
number_of_zeroes_triggering_misbehavior =
vary_number_of_zeroes.reject {|i, bds| bds == ‘0.0’}.map {|i, bds|
i}

puts “These values appended to ‘0.0000’ cause BigDecimal#round to
return nonzero:”
p seed_values_triggering_misbehavior_with_four_zeroes
puts
puts “These numbers of zeroes between the decimal point and a 7 cause
BigDecimal#round to return nonzero:”
p number_of_zeroes_triggering_misbehavior

===== end code

I get identical reports on Linux and Windows versions of Ruby:

ruby 1.8.6 (2007-03-13 patchlevel 0) [i686-linux]

ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]

We would be grateful for a pointer to a patch that fixes this issue if
one is available.

On 9/27/07, Ryan P. [email protected] wrote:

97, 98, 99]
require ‘bigdecimal’
}
p number_of_zeroes_triggering_misbehavior
one is available.
It seems to me as a bug. The code in trunk is the same, so I guess
it’s not fixed yet.
(rubyforge.org, project ruby)

The problem is most probably in ext/bigdecimal/bigdecimal.c,
VpMidRound().

As you have probably noticed, it happens when 1. the number of leading
zeros is divisible by 4 (BASE_FIG, the number of decimal digits stored
in one U_LONG), and the next digit is >= 5 (I guess due to selected
rounding mode)

That means, the problematic numbers have format 0.XE-Y where X in
[5…9] and Y%4 == 0

It’s even visible in
BigDecimal.new(“0.5”).round(-4).to_i # => 10000
BigDecimal.new(“0.000000005”).round(4) #=> 0.1E-3 == 0.0001
BigDecimal.new(“0.000000005”).round(-4) #=> 0.1E5 == 10000

On 9/27/07, Jano S. [email protected] wrote:

80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96,

bd = BigDecimal.new(“0.#{‘0’ * i}7”); [i, bd.round.to_s]
BigDecimal#round to return nonzero:"
We would be grateful for a pointer to a patch that fixes this issue if
As you have probably noticed, it happens when 1. the number of leading
BigDecimal.new(“0.000000005”).round(-4) #=> 0.1E5 == 10000

reported as #14271

On Sep 27, 6:25 am, “Jano S.” [email protected] wrote:

It seems to me as a bug. The code in trunk is the same, so I guess
it’s not fixed yet.

reported as #14271

Thank you very much for reporting the bug.

On 9/27/07, Ryan P. [email protected] wrote:

On Sep 27, 6:25 am, “Jano S.” [email protected] wrote:

It seems to me as a bug. The code in trunk is the same, so I guess
it’s not fixed yet.

reported as #14271

Thank you very much for reporting the bug.

I guess you can workaround the bug until it gets properly fixed - by
opening the BigDecimal class. Last night I could not come with
anything reasonable though

class BigDecimal
alias_method :old_round, :round
def round(*args)
if XXX
self.class.new(YYY)
else
old_round(*args)
end
end
end

the problematic parts are XXX and YYY…
args[0] and self.exponent might be usable…

I haven’t found any unit tests for BigDecimal, so if you create some,
please post them somewhere (I’m not sure where - possible choises are:
ruby tracker, http://rubyforge.org/projects/bfts/ tracker, ruby-core
mailing list)

Jano

On 9/27/07, Jano S. [email protected] wrote:

opening the BigDecimal class. Last night I could not come with
end
end

the problematic parts are XXX and YYY…
args[0] and self.exponent might be usable…

I haven’t found any unit tests for BigDecimal, so if you create some,
please post them somewhere (I’m not sure where - possible choises are:
ruby tracker, http://rubyforge.org/projects/bfts/ tracker, ruby-core
mailing list)

If you come up with some more, you could either submit them as a patch
there, or post them to ruby-core.

When we get around to BigDecimal in Rubinius, we will probably use the
JRuby test case as a starting point, and then fill in any gaps we
find. If you’ve posted some of them to ruby-core, that makes life
easier.

From: “Ryan P.” [email protected]

I haven’t seen the bug yet in detail.
But I will fix it anyway.
Thank you for reporting.

[email protected]

Subject: Re: Bug in BigDecimal#round ?

I attached the fix as a patch file.
Could anyone(perhaps Matz ?) apply the patch ?

With a change log like:
Round method bug pointed by Ryan P. fixed(Patch of the patch from
“NATORI Shin”).