## Backport #9316

### BigDecimal division in Ruby 2.1

**Description**

=begin

When updating an app to Ruby 2.1, and I ran into the following difference between ruby 2.0.0-p353 and ruby 2.1.0 when dividing two BigDecimals:

((*Ruby 2.0.0p353:*))

2.0.0p353 :002 > (BigDecimal.new("1472.0") / BigDecimal.new("0.99")).to_f

=> 1486.868686869

((*Ruby 2.1.0p0:*))

2.1.0p0 :006 > (BigDecimal.new("1472.0") / BigDecimal.new("0.99")).to_f

=> 1487.0

=end

### Associated revisions

merge revision(s) 44588: [Backport #9316]

* ext/bigdecimal/bigdecimal.c (BigDecimal_divide): Add an additional digit for the quotient to be compatible with bigdecimal 1.2.1 and the former. [#9316] [#9305] * test/bigdecimal/test_bigdecimal.rb: tests for the above change. * ext/bigdecimal/bigdecimal.gemspec: bigdecimal version 1.2.4.

### History

#### #1 [ruby-core:59375] Updated by Dylan Markow almost 3 years ago

=begin

The to_f appears to be irrelevant:

2.0.0p353:

BigDecimal.new("1472.0") / BigDecimal.new("0.99")

=> 1486.868686869

2.1.0:

BigDecimal.new("1472.0") / BigDecimal.new("0.99")

=> 1487.0

=end

#### #2 [ruby-core:59376] Updated by Dylan Markow almost 3 years ago

=begin

Also, it appears to only happen when the right side number is less than 1:

BigDecimal.new("1472.0") / BigDecimal.new("0.99")

1487.0 # Should be 1486.868686869

BigDecimal.new("1472.0") / BigDecimal.new("1.0")

1472.0 # Correct

BigDecimal.new("1472.0") / BigDecimal.new("1.01")

1457.425742574257425743 # Correct

=end

#### #3 [ruby-core:59383] Updated by Zachary Scott almost 3 years ago

**Assignee**set to*Kenta Murata***Target version**set to*current: 2.2.0***Status**changed from*Open*to*Assigned*

Thanks for the report, I'll leave this one to murata-san since he knows the spec

#### #4 [ruby-core:59388] Updated by Matt Jones almost 3 years ago

Still fuzzy on the cause, but the behavior is interesting - it appears that something has changed regarding the way that non-terminating decimals are rounded.

There's also some magnitude-dependent behavior: 147.2 / 0.099 (and related variants shifting the decimals around) don't always return the same value.

Here's a little program to compare the behavior for different offsets:

https://gist.github.com/al2o3cr/8175722

Attached to that are also the results of running the script on 2.0.0 and 2.1.0.

#### #5 [ruby-core:59389] Updated by Matt Jones almost 3 years ago

Updated - I added a dump of the Prec + MaxPrec values to the output of the script. There definitely appears to be a pattern, where inputs with the same precision values produce the same results.

#### #6 [ruby-core:59397] Updated by Heesob Park almost 3 years ago

It seems that the size of exponenent affects the precison of division result, but that is a wrong assumption.

I think the mininum precison is required regardless of the size of exponent.

irb(main):001:0> puts BigDecimal.new('1') / BigDecimal.new('3')

0.333333333E0

irb(main):002:0> puts BigDecimal.new('0.1') / BigDecimal.new('0.3')

0.0

irb(main):003:0> puts BigDecimal.new('0.01') / BigDecimal.new('0.03')

0.0

irb(main):004:0> puts BigDecimal.new('0.001') / BigDecimal.new('0.003')

0.0

irb(main):005:0> puts BigDecimal.new('0.00000000001') / BigDecimal.new('0.00000000003')

0.333333333E0

Here is a simple patch to ensure the minimun precison of division result.

diff --git a/bigdecimal.c b/bigdecimal.c.new

index e0b7c01..615c15f 100644

--- a/bigdecimal.c

+++ b/bigdecimal.c.new

@@ -1222,6 +1222,7 @@ BigDecimal_divide(Real **c, Real **res, Real **div, VALUE self, VALUE r)
*div = b;
mx = a->Prec + vabs(a->exponent);
if (mxPrec + vabs(b->exponent)) mx = b->Prec + vabs(b->exponent);
+ if (mx<3) mx = 3;
mx =(mx + 1) * VpBaseFig();
GUARD_OBJ((*c), VpCreateRbObject(mx, "#0"));

GUARD_OBJ((

*res), VpCreateRbObject((mx+1) * 2 +(VpBaseFig() + 1), "#0"));*

@@ -1323,6 +1324,7 @@ BigDecimal_DoDivmod(VALUE self, VALUE r, Real *div, Real **mod)

@@ -1323,6 +1324,7 @@ BigDecimal_DoDivmod(VALUE self, VALUE r, Real *

mx = a->Prec + vabs(a->exponent); if (mx<b->Prec + vabs(b->exponent)) mx = b->Prec + vabs(b->exponent);

- if (mx<3) mx = 3;

mx = (mx + 1) * VpBaseFig(); GUARD_OBJ(c, VpCreateRbObject(mx, "0")); GUARD_OBJ(res, VpCreateRbObject((mx+1) * 2 +(VpBaseFig() + 1), "#0"));

After appling the patch, the results are all equal.

irb(main):001:0> puts BigDecimal.new('1') / BigDecimal.new('3')

0.333333333333333333E0

irb(main):002:0> puts BigDecimal.new('0.1') / BigDecimal.new('0.3')

0.333333333333333333E0

irb(main):003:0> puts BigDecimal.new('0.01') / BigDecimal.new('0.03')

0.333333333333333333E0

irb(main):004:0> puts BigDecimal.new('0.001') / BigDecimal.new('0.003')

0.333333333333333333E0

irb(main):005:0> puts BigDecimal.new('0.00000000001') / BigDecimal.new('0.00000000003')

0.333333333333333333E0

#### #7 [ruby-core:59411] Updated by Eric Hutzelman almost 3 years ago

=begin

I'm also seeing some different rounding/conversion in 2.1.0p0 when using BigDecimal and Floats:

((*Ruby 2.0.0p353:*))

0.25 / BigDecimal.new("0.5")

=> 0.5

_.class

=> Float

((*Ruby 2.1.0p0:*))

0.25 / BigDecimal.new("0.5")

=> 0.0

_.class

=> BigDecimal

=end

#### #8 [ruby-core:59475] Updated by Matt Jones almost 3 years ago

phasis68 (Heesob Park) wrote:

Here is a simple patch to ensure the minimun precison of division result.

diff --git a/bigdecimal.c b/bigdecimal.c.new

index e0b7c01..615c15f 100644

--- a/bigdecimal.c

+++ b/bigdecimal.c.new

@@ -1222,6 +1222,7 @@ BigDecimal_divide(Real *c, Real **res, Real **div, VALUE self, VALUE r)c), VpCreateRbObject(mx, "#0"));

*div = b;

mx = a->Prec + vabs(a->exponent);

if (mxPrec + vabs(b->exponent)) mx = b->Prec + vabs(b->exponent);

+ if (mx<3) mx = 3;

mx =(mx + 1) * VpBaseFig();

GUARD_OBJ((

GUARD_OBJ((res), VpCreateRbObject((mx+1) * 2 +(VpBaseFig() + 1), "#0"));div, Real **mod)

@@ -1323,6 +1324,7 @@ BigDecimal_DoDivmod(VALUE self, VALUE r, Real *mx = a->Prec + vabs(a->exponent); if (mx<b->Prec + vabs(b->exponent)) mx = b->Prec + vabs(b->exponent);

- if (mx<3) mx = 3;

mx = (mx + 1) * VpBaseFig(); GUARD_OBJ(c, VpCreateRbObject(mx, "0")); GUARD_OBJ(res, VpCreateRbObject((mx+1) * 2 +(VpBaseFig() + 1), "#0"));

Not a fan of this, since it's updating code that didn't change between the two releases (as far as I can tell). I'd prefer to fix the underlying reason why identical calculations in 2.0 and 2.1 aren't behaving the same way instead of just arbitrarily forcing more precision...

#### #9 [ruby-core:59492] Updated by Alexandre Riveira almost 3 years ago

When upgrading to ruby 2.1 calculate a percentage in our system broke

example

v1 = BigDecimal.new("0.94")

v2 = BigDecimal.new("0.97")

puts (((v2 - v1) * 100) / v1).to_f

ruby 2.0 => 3.191489362

ruby 2.1 => 3.0

#### #10 [ruby-core:59635] Updated by Kurt Werle almost 3 years ago

This looks like the same thing as https://bugs.ruby-lang.org/issues/9305

#### #11 [ruby-core:59669] Updated by Kurt Werle almost 3 years ago

It is worth noting that replacing the ruby 2.1 implementation of BigDecimal with the 2.0 implementation works and makes this issue go away. The downside is that there are a few BigDecimal tests that fail. At a glance, they all look like new BigDecimal functionality.

#### #12 Updated by Kenta Murata almost 3 years ago

This behavior change had been introduced from the version 1.2.2, which was released on 2013-11-22.

This change can be thought as a bug fix for the former behavior which calculates too many digits. I thought so when I implemented it.

For example, BigDecimal("1472.0") is 5-digit floating-point number and BigDecimal("0.99") is 2-digit floating-point number, so the result of BigDecimal("1472.0") / BigDecimal("0.99") has three exact digits and one ambiguous digit.

The reason why I implemented this behavior is that the "/" operator cannot determine the precision of the quotient generally because it cannot obtain the 2nd parameter to get the precision.

However, since the current precision system isn't strictly implemented, I think the current behavior is too strict for practical use.

For instance, BigDecimal("0.1200") cannot represent a 4-digit floating-point number in the current implementation.

So I'll fix it soon, but I'll put it back after I make the precision system more strict.

#### #13 Updated by Kenta Murata almost 3 years ago

I've fixed this and released the new bigdecimal version 1.2.4.

Please do `gem install bigdecimal`

and check it as the following:

gem 'bigdecimal', '>= 1.2.4' require 'bigdecimal' puts BigDecimal('1472.0') / BigDecimal('0.99')

#### #14 Updated by Kenta Murata almost 3 years ago

**Tracker**changed from*Bug*to*Backport***Project**changed from*Ruby trunk*to*Backport21***Category**deleted ()*lib***Assignee**changed from*Kenta Murata*to*Yui NARUSE***Target version**deleted ()*current: 2.2.0*

I think it needs to be backported to 2.1 and 2.0.

#### #15 Updated by Kenta Murata almost 3 years ago

bigdecimal 1.2.4 failed to support Ruby 1.9.3.

Please use the version 1.2.5 for Ruby 1.9.3.

#### #16 [ruby-core:60137] Updated by Yui NARUSE over 2 years ago

**Status**changed from*Assigned*to*Closed***% Done**changed from*0*to*100*

Applied in changeset r44711.

merge revision(s) 44588: [Backport #9316]

* ext/bigdecimal/bigdecimal.c (BigDecimal_divide): Add an additional digit for the quotient to be compatible with bigdecimal 1.2.1 and the former. [#9316] [#9305] * test/bigdecimal/test_bigdecimal.rb: tests for the above change. * ext/bigdecimal/bigdecimal.gemspec: bigdecimal version 1.2.4.