[ruby-core:59075] [ruby-trunk - Bug #7458][Closed] Depending on value of BigDecimal::limit, addition can give inaccurate results

Issue #7458 has been updated by mrkn (Kenta Murata).

Status changed from Assigned to Closed

I've fix this issue in r44153.
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Bug #7458: Depending on value of BigDecimal::limit, addition can give inaccurate results
https://bugs.ruby-lang.org/issues/7458#change-43628

Author: bpow (Bradford Powell)
Status: Closed
Priority: Normal
Assignee: mrkn (Kenta Murata)
Category: ext
Target version: next minor
ruby -v: 2.0.0dev
Backport: 


=begin
This bug is present in ruby-trunk and also in 1.8.7 and 1.9.3p125.

I apologize that I cannot figure out what exactly is going wrong and how to patch, but at least I have a small reproducible test case.

When adding very small numbers (like 7E-19) to 1, the result should be very close to 1. However, depending on the limit to BigDecimal's precision, this is not always what is resulted, as this script demonstrates:

  
 require 'bigdecimal'
 
 one = BigDecimal('1')
 smallnum = BigDecimal('7E-19')
 0.upto(20) do |lim|
   BigDecimal::limit(lim)
   result = one + smallnum
   puts "result is #{result}, as float is #{result.to_f} with limit #{lim}"
 end

When run in trunk (or 1.8.7), the results it gives are:

 result is 0.10000000000000000007E1, as float is 1.0 with limit 0
 result is 0.1E1, as float is 1.0 with limit 1
 result is 0.1E1, as float is 1.0 with limit 2
 result is 0.1E1, as float is 1.0 with limit 3
 result is 0.1E1, as float is 1.0 with limit 4
 result is 0.1E1, as float is 1.0 with limit 5
 result is 0.1E1, as float is 1.0 with limit 6
 result is 0.1E1, as float is 1.0 with limit 7
 result is 0.1E1, as float is 1.0 with limit 8
 result is 0.1E1, as float is 1.0 with limit 9
 result is 0.2E1, as float is 2.0 with limit 10
 result is 0.2E1, as float is 2.0 with limit 11
 result is 0.2E1, as float is 2.0 with limit 12
 result is 0.2E1, as float is 2.0 with limit 13
 result is 0.2E1, as float is 2.0 with limit 14
 result is 0.2E1, as float is 2.0 with limit 15
 result is 0.2E1, as float is 2.0 with limit 16
 result is 0.2E1, as float is 2.0 with limit 17
 result is 0.2E1, as float is 2.0 with limit 18
 result is 0.1000000000000000001E1, as float is 1.0 with limit 19
 result is 0.10000000000000000007E1, as float is 1.0 with limit 20

So the results with limit <=9 or >=19 are as expected, but in between the result returned is equal to 2.0

A test that demonstrates this (suitable for inclusion in (({test/bigdecimal/test_bigdecimal.rb})) is:

 def test_limited_one_plus_small
    one = BigDecimal('1')
    smallnum = BigDecimal('0.7E-18')
    BigDecimal::save_limit do
      1.upto(20) do |lim|
        BigDecimal::limit(lim)
        result = (one + smallnum).to_f.to_s
        assert_equal("1.0", result, "Error adding a small number to one with BigDecimal::limit(#{lim})")
      end
    end
 end

I think this is different from #7404 (from what I can tell by google translate), but I apologize for the duplicate if not.
=end


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