Range bug

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Wednesday, October 1. 2008

I appear to have found a bug in ruby - specifically in ruby 1.8.4 (2005-12-24) [i686-darwin8.7.1].

This one has been baffling me for a bit so I figured it was worth posting. Given the following code:

def test num  

  h = Hash.new

  h[1..num] = 'foo'  # set a value

  h[1..num] == 'foo' # check that same value

end

You would likely expect that this method would always return true but in fact, it does not. For values of num ranging from 1 to 536870911, it returns true. For values greater than 536870911, it returns false.

I thought perhaps that 536870911 is the largest possible value that can be put in a Fixnum and that larger values are converted to BigNum. That might explain a comparison failure but it turns out that a FixNum can hold 1073741823.

Now it's interesting that the failure point is exactly half of the biggest value that can be put in a Fixnum but I'm not sure what conclusions we can draw from this.

Posted by Mike Bowler in Technology at 13:10

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(1..536870912).hash == (1..536870912).hash

is false

That large Range seems to have a bad hash implementation.

#1 Dave Landers (http://dave.srednal.com) on Oct 1 2008, 15:11 Reply

This is an artifact of Ruby's object system. Each object has an #object_id (formerly also #id), which is limited by 2**32 values (interestingly, twice the size of a fixnum). For ease of comparisons, false, true, and nil have object ids of 0, 2, and 4, respectively, while small (positive and negative) fixnums have an object id of x*2+1. So 0.object_id == 1, 1.object_id == 3, -1.object_id == -1.

As such, 536870911.object_id == 1073741823, which is a Fixnum, but 536870912.object_id == 1073741825, which is a Bignum.

The confusion is that when Range#hash is called to find a bucket for it in the Hash, if the entire range is Fixnum based, it hashes consistently, but when the upper value has a Bignum object_id, it does not.

Also confusing is if you hop into the land of Floats, (0..0.3).object_id is non-constant, but (0..0.3).hash is.

So yeah, looks like a bug in Range#hash, also in 1.8.6.

#2 Jamie Macey (http://blog.tracefunc.com) on Oct 1 2008, 21:53 Reply

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