From: Jano Svitok <jan.svitok@...>
Date: 2007-09-27T10:47:03+09:00
Subject: Re: Bug in BigDecimal#round ?

On 9/27/07, Ryan Platte <ryan.platte@gmail.com> wrote:
> 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.

It seems to me as a bug. The code in trunk is the same, so I guess
it's not fixed yet.
Please file a bug at
http://rubyforge.org/tracker/?func=add&group_id=426&atid=1698
(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