Bug #10979: Hash fails to distinguish 0.0 & -0.0

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin13]
• Backport: 2.0.0: REQUIRED, 2.1: DONE, 2.2: DONE

[avondrak (Alex Vondrak)]

According to the docs ( http://ruby-doc.org/core-2.2.1/Hash.html ):

Two objects refer to the same hash key when their hash value is identical and the two objects are eql? to each other.

However, that does not appear to be the case for 0.0 and -0.0:

$ irb
2.2.0 :001 > (0.0).hash
 => -204267982040607835 
2.2.0 :002 > (-0.0).hash
 => -204267982040607835 
2.2.0 :003 > (0.0).eql?(-0.0)
 => true 
2.2.0 :004 > (-0.0).eql?(0.0)
 => true 
2.2.0 :005 > { 0.0 => :positive }.has_key?(-0.0)
 => false 
2.2.0 :006 > { -0.0 => :negative }.has_key?(0.0)
 => false 
2.2.0 :007 > { 0.0 => :positive }[-0.0]
 => nil 
2.2.0 :008 > { -0.0 => :negative }[0.0]
 => nil 
2.2.0 :009 > { 0.0 => :positive }.keys.map(&:hash)
 => [-204267982040607835] 
2.2.0 :010 > { -0.0 => :negative }.keys.map(&:hash)
 => [-204267982040607835]

I realize that floating point numbers are weird, and that 0.0 & -0.0 aren't literally the same machine representation. However, they are #eql? to each other have the same hash codes. I figure either the docs are misleading or there's a bug in the Hash primitives.

This also has carry-over effects to Array differences, for example:

$ irb
2.2.0 :001 > [0.0] - [-0.0]
 => [0.0] 
2.2.0 :002 > [-0.0] - [0.0]
 => [-0.0]
2.2.0 :003 > [0.0, -0.0] - [-0.0]
 => [0.0]

This was the initial behavior I observed in Ruby 2.1.3 that led me to file this bug. I tried Ruby 2.2.0 and the same behavior still seems to persist.

[nobu (Nobuyoshi Nakada)]

• Backport changed from 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN to 2.0.0: REQUIRED, 2.1: REQUIRED, 2.2: REQUIRED

Seems a bug because of FLONUM since 2.0.0.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed
• % Done changed from 0 to 100

Applied in changeset r49999.

---

hash.c: same hash values with Float#hash

• hash.c (rb_any_hash): use same hash values with Float#hash so
  that -0.0 and +0.0 will be identical.
  [ruby-core:68541] [Bug #10979]

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.0.0: REQUIRED, 2.1: REQUIRED, 2.2: REQUIRED to 2.0.0: REQUIRED, 2.1: REQUIRED, 2.2: DONE

Backported into ruby_2_2 branch at rr50619.

[usa (Usaku NAKAMURA)]

• Backport changed from 2.0.0: REQUIRED, 2.1: REQUIRED, 2.2: DONE to 2.0.0: REQUIRED, 2.1: DONE, 2.2: DONE

ruby_2_1 r50655 merged revision(s) 49999,50000.