# String#hash changed in Ruby 1.9? **URL:** \ **Category:** Ruby\ **Created:** [May 4, 2009, 4:24pm UTC](https://www.ruby-forum.com/t/string-hash-changed-in-ruby-1-9/167138 "2009-05-04T16:24:45Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![dpitman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a9adbd/32.png) [@dpitman](https://www.ruby-forum.com/u/dpitman)\ **Post date:** [May 4, 2009, 4:24pm UTC](https://www.ruby-forum.com/t/string-hash-changed-in-ruby-1-9/167138/1 "2009-05-04T16:24:45Z")
Hi all, in ruby 1.8.7: david@trince ~$ ruby -e ‘puts “abc”.hash’ 833038373 david@trince ~$ ruby -e ‘puts “abc”.hash’ 833038373 david@trince ~$ ruby -e ‘puts “abc”.hash’ 833038373 [always the same number] in ruby 1.9.1: david@trince ~$ ruby -e ‘puts “abc”.hash’ 402929305 david@trince ~$ ruby -e ‘puts “abc”.hash’ -403532784 david@trince ~$ ruby -e ‘puts “abc”.hash’ -650364342 What happened? Is this intentional? Rationale? Any tips on how to replace it? ---
**Author:** ![dpitman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a9adbd/32.png) [@dpitman](https://www.ruby-forum.com/u/dpitman)\ **Post date:** [May 4, 2009, 4:41pm UTC](https://www.ruby-forum.com/t/string-hash-changed-in-ruby-1-9/167138/2 "2009-05-04T16:41:18Z")
Am Montag 04 Mai 2009 16:22:01 schrieb David P.: > in ruby 1.9.1: > david@trince ~$ ruby -e ‘puts “abc”.hash’ > 402929305 > david@trince ~$ ruby -e ‘puts “abc”.hash’ > -403532784 > david@trince ~$ ruby -e ‘puts “abc”.hash’ > -650364342 > > What happened? Is this intentional? 1.9 uses murmurhash([http://murmurhash.googlepages.com/](http://murmurhash.googlepages.com/)) with a random seed which is generated once per application-run. > Any tips on how to replace it? What does it hurt if the hash value of a string does not remain constant between runs of the application? HTH, Sebastian ---
**Author:** ![dpitman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a9adbd/32.png) [@dpitman](https://www.ruby-forum.com/u/dpitman)\ **Post date:** [May 4, 2009, 4:49pm UTC](https://www.ruby-forum.com/t/string-hash-changed-in-ruby-1-9/167138/3 "2009-05-04T16:49:33Z")
> > Any tips on how to replace it? > > What does it hurt if the hash value of a string does not remain constant > between runs of the application? In my case it’s pretty bad. I use it in a command line utility to cache rake tasks. I create one cachefile for each directory, naming them using the String#hash of the full path (Dir.pwd.hash). If the hash is different the next time the program runs the cache lookup fails (and I get a new cache file instead of the old one). So, I don’t need anything fancy, just an equivalent to Dir.pwd.hash that stay consistent. Do I need to MD5 it? Feels like overkill. Why was this changed in the first place? ---
**Author:** ![dpitman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a9adbd/32.png) [@dpitman](https://www.ruby-forum.com/u/dpitman)\ **Post date:** [May 4, 2009, 10:58pm UTC](https://www.ruby-forum.com/t/string-hash-changed-in-ruby-1-9/167138/4 "2009-05-04T22:58:10Z")
On Tue, 5 May 2009 02:15:09 +0900, Robert K. wrote: > Hm… But you do admit that this is a bit abusive, do you? > Especially since there are no guarantees that you won’t have any > collisions with a hash value like the one returned by #hash. Oh yes, it’s just a quick and convenient way of doing it. Dunno if I’d call it “abusive”, but it’s sure not military grade programming… > How about storing your cache files with a fixed name in the original > directory? Or have a file with metadata (mapping from path to cache > file name)? Fixed name won’t work; most directories are scm tracked so it’d be a mess to keep the cache files out of the way. One big(ish) cache file might work. Maybe even a sqlite db. Have to run some benchmark on that. > > that stay consistent. Do I need to MD5 it? Feels like overkill. Why > > was this changed in the first place? > > That’s an interesting question. I’m curious as well. Maybe the > changes are just a side effect of a new - supposedly better - hashing > algorithm. The link sebastian provided ([http://murmurhash.googlepages.com/](http://murmurhash.googlepages.com/)) was interesting but not exhaustive and I still don’t know when/how/why the behaviour was changed. Perhaps the ml archives will tell? ---
**Author:** ![dpitman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/a9adbd/32.png) [@dpitman](https://www.ruby-forum.com/u/dpitman)\ **Post date:** [May 4, 2009, 7:15pm UTC](https://www.ruby-forum.com/t/string-hash-changed-in-ruby-1-9/167138/5 "2009-05-04T19:15:46Z")
On 04.05.2009 16:48, David P. wrote: > > > Any tips on how to replace it? > > > What does it hurt if the hash value of a string does not remain constant > > > between runs of the application? > > In my case it’s pretty bad. I use it in a command line utility to cache rake tasks. I create one cachefile for each directory, naming them using the String#hash of the full path (Dir.pwd.hash). If the hash is different the next time the program runs the cache lookup fails (and I get a new cache file instead of the old one). Hm… But you do admit that this is a bit abusive, do you? Especially since there are no guarantees that you won’t have any collisions with a hash value like the one returned by #hash. How about storing your cache files with a fixed name in the original directory? Or have a file with metadata (mapping from path to cache file name)? > So, I don’t need anything fancy, just an equivalent to Dir.pwd.hash that stay consistent. Do I need to MD5 it? Feels like overkill. Why was this changed in the first place? That’s an interesting question. I’m curious as well. Maybe the changes are just a side effect of a new - supposedly better - hashing algorithm. Kind regards robert