# Hash value inconsistencies **URL:** \ **Category:** Ruby\ **Created:** [February 5, 2008, 11:13pm UTC](https://www.ruby-forum.com/t/hash-value-inconsistencies/128216 "2008-02-05T23:13:48Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![timperrett](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/c5a1d2/32.png) [@timperrett](https://www.ruby-forum.com/u/timperrett)\ **Post date:** [February 5, 2008, 11:13pm UTC](https://www.ruby-forum.com/t/hash-value-inconsistencies/128216/1 "2008-02-05T23:13:48Z")
ruby -e “puts ‘test’.hash” Should this output the same integer value on all platforms where Ruby can run? - Windows ruby --version #=\> ruby 1.8.6 (2007-03-13 patchlevel 0) [i386- mswin32] ruby -e “puts ‘test’.hash” #=\> -914358341 - Mac 10.5 ruby --version #=\> ruby 1.8.6 (2007-03-13 patchlevel 0) [i686- darwin8.10.3] ruby -e “puts ‘test’.hash” #=\> -914358341 - Linux 2.6 Kernel ruby --version #=\> ruby 1.8.6 (2007-06-07 patchlevel 36) [x86\_64- linux] ruby -e “puts ‘test’.hash” #=\> 1233125307 ruby --version #=\> ruby 1.8.5 (2006-12-04 patchlevel 2) [x86\_64- linux] ruby -e “puts ‘test’.hash” #=\> 1233125307 It appears not! So, any suggestions on generating an ID number for an object that is unique yet consistent across different platforms? I’d like to have some method that I could call on an object that would return a reproducible value that would uniquely identify that object. Thoughts? TwP ---
**Author:** ![timperrett](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/c5a1d2/32.png) [@timperrett](https://www.ruby-forum.com/u/timperrett)\ **Post date:** [February 5, 2008, 11:33pm UTC](https://www.ruby-forum.com/t/hash-value-inconsistencies/128216/2 "2008-02-05T23:33:49Z")
Tim P. wrote: > ruby -e “puts ‘test’.hash” > > Should this output the same integer value on all platforms where Ruby > can run? > > - Windows > ruby --version #=\> ruby 1.8.6 (2007-03-13 patchlevel 0) [i386- mswin32] > ruby -e “puts ‘test’.hash” #=\> -914358341 I can’t speak to the larger problem you ask, but I did verify that this negative one million was the output for windows. Maybe we could alter the test slightly to figure out what ruby is doing. Cheers. ---
**Author:** ![timperrett](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/c5a1d2/32.png) [@timperrett](https://www.ruby-forum.com/u/timperrett)\ **Post date:** [February 6, 2008, 12:05am UTC](https://www.ruby-forum.com/t/hash-value-inconsistencies/128216/3 "2008-02-06T00:05:39Z")
Tim P. wrote: > Should this output the same integer value on all platforms where Ruby > can run? Perhaps, but if you read the below, you’ll see why you should never rely on it. > It appears not! So, any suggestions on generating an ID number for an > object that is unique yet consistent across different platforms? I’d > like to have some method that I could call on an object that would > return a reproducible value that would uniquely identify that object. That’s not possible. There is more entropy in an arbitrary object than can be represented in a FixNum. Basic coding theory stuff. If it was possible, then you could code all the data in all the databases in the world into a single Fixnum :-). If you want a fixed-length code that’s sufficiently likely to be unique that you can be almost certain that you’ll never see a false duplicate, you need to use a cryptographic hash function. I recommend SHA-256, but you might survive with a weaker one like MD5 or SHA-1. They take a lot more work to calculate than is justified for Ruby’s hash keys though! With these functions, the probability of a population containing a false duplicate is approximately 50% when the population contains sqrt(2^N), (or 2\*(N/2)) distinct items, where N is the number of bits in the checksum. For SHA-256, that means you need 2^128 items before you have a reasonable chance of a collision. All of the programs you’ll ever write, running for your entire life, will only create a tiny fraction of this many objects, so the chance of you ever seeing a collision is tiny. That might sound risky still, but all of e-commerce is built on the principle. If it’s good enough for that, it’s good enough for you 🙂 Clifford H… ---
**Author:** ![timperrett](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/c5a1d2/32.png) [@timperrett](https://www.ruby-forum.com/u/timperrett)\ **Post date:** [February 6, 2008, 12:16am UTC](https://www.ruby-forum.com/t/hash-value-inconsistencies/128216/4 "2008-02-06T00:16:16Z")
On Feb 5, 2008, at 4:04 PM, Clifford H. wrote: > > platforms? I’d like to have some method that I could call on an > > object that would return a reproducible value that would uniquely > > identify that object. > > That’s not possible. There is more entropy in an arbitrary object than > can be represented in a FixNum. Basic coding theory stuff. If it was > possible, then you could code all the data in all the databases in the > world into a single Fixnum :-). Darn information theory! I just need a fixnum. The number of objects we are creating is pretty tiny – maybe 100. I was quite surprised that the Ruby “hash” method is not consistent across platforms. The solution is to roll my own hash function that produces consistent results. Just wondering about the more general questions regarding the built in hash function. Blessings, TwP ---
**Author:** ![timperrett](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/c5a1d2/32.png) [@timperrett](https://www.ruby-forum.com/u/timperrett)\ **Post date:** [February 7, 2008, 12:49pm UTC](https://www.ruby-forum.com/t/hash-value-inconsistencies/128216/5 "2008-02-07T12:49:13Z")
2008/2/6, Tim P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > > > > Darn information theory! I just need a fixnum. The number of objects > we are creating is pretty tiny – maybe 100. > > I was quite surprised that the Ruby “hash” method is not consistent > across platforms. The solution is to roll my own hash function that > produces consistent results. A regular hash function is a bad candidate for a unique id anyway. I’d rather use a MD5 or something like that. If your strings are reasonably short you can as well convert them to Fixnums but then again: why bother and not directly use the string? > Just wondering about the more general > questions regarding the built in hash function. There is no need for a hash function to be consistent across platforms. Why should it? Kind regards robert ---
**Author:** ![timperrett](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/c5a1d2/32.png) [@timperrett](https://www.ruby-forum.com/u/timperrett)\ **Post date:** [February 6, 2008, 2:07am UTC](https://www.ruby-forum.com/t/hash-value-inconsistencies/128216/6 "2008-02-06T02:07:00Z")
Tim P. wrote: > On Feb 5, 2008, at 4:04 PM, Clifford H. wrote: > > > > platforms? I’d like to have some method that I could call on an > > > object that would return a reproducible value that would uniquely > > > identify that object. > > > > That’s not possible. There is more entropy in an arbitrary object than > > can be represented in a FixNum. Basic coding theory stuff. If it was > > possible, then you could code all the data in all the databases in the > > world into a single Fixnum :-). > > Darn information theory! I just need a fixnum. The number of objects > we are creating is pretty tiny – maybe 100. > > I was quite surprised that the Ruby “hash” method is not consistent > across platforms. The solution is to roll my own hash function that > produces consistent results. Just wondering about the more general > questions regarding the built in hash function. > What in particular are you going to hash? Under what circumstances do > you want to bomb out? – ---
**Author:** ![timperrett](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/c5a1d2/32.png) [@timperrett](https://www.ruby-forum.com/u/timperrett)\ **Post date:** [September 10, 2009, 8:48pm UTC](https://www.ruby-forum.com/t/hash-value-inconsistencies/128216/7 "2009-09-10T20:48:18Z")
Tim P. wrote: > ruby -e “puts ‘test’.hash” > > Should this output the same integer value on all platforms where Ruby > can run? > > - Windows > ruby --version #=\> ruby 1.8.6 (2007-03-13 patchlevel 0) [i386- mswin32] > ruby -e “puts ‘test’.hash” #=\> -914358341 > > - Mac 10.5 > ruby --version #=\> ruby 1.8.6 (2007-03-13 patchlevel 0) [i686- darwin8.10.3] > ruby -e “puts ‘test’.hash” #=\> -914358341 > > - Linux 2.6 Kernel > ruby --version #=\> ruby 1.8.6 (2007-06-07 patchlevel 36) [x86\_64- > linux] > ruby -e “puts ‘test’.hash” #=\> 1233125307 > > ruby --version #=\> ruby 1.8.5 (2006-12-04 patchlevel 2) [x86\_64- > linux] > ruby -e “puts ‘test’.hash” #=\> 1233125307 > > It appears not! So, any suggestions on generating an ID number for an > object that is unique yet consistent across different platforms? I’d > like to have some method that I could call on an object that would > return a reproducible value that would uniquely identify that object. > > Thoughts? > > TwP The problem is that some of your test machines are 64-bit and some are 32-bit. I Ran the same tests on some Macs running Snow Leopard(64-bit) and Leopard(32-bit) and Linux (64-bit) and Linux (32-bit) and all results were consistent over OSes with the same bits.