# Problems with String#hash

**URL:** <https://www.ruby-forum.com/t/problems-with-string-hash/77111>\
**Category:** Ruby\
**Created:** [November 24, 2006, 11:19am UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111 "2006-11-24T11:19:23Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![gmichelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/db5fbb/32.png) [@gmichelson](https://www.ruby-forum.com/u/gmichelson)\
**Post date:** [November 24, 2006, 11:19am UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111/1 "2006-11-24T11:19:23Z")

</div>

Dear devs,

I am facing a very NASTY problem with String#hash

Here is the same code that I run on 3 different computers:

root@kosh:~ # irb  
irb(main):001:0\> “User”.hash  
=\> 690465448  
irb(main):002:0\> VERSION  
=\> “1.8.2”

C:\Documents and Settings\drak\>irb  
irb(main):001:0\> “User”.hash  
=\> 690465448  
irb(main):002:0\> VERSION  
=\> “1.8.4”

[http://tryruby.hobix.com/](http://tryruby.hobix.com/)  
Interactive ruby ready.

> > “User”.hash  
> > =\> 690465448  
> > VERSION  
> > =\> “1.8”

root@victory:~# irb  
irb(main):001:0\> “User”.hash  
=\> -1457018200  
irb(main):002:0\> VERSION  
=\> “1.8.4”

Please nottice that on the last case I get a different result! Any  
idea why this happens?

thanks in advance for any help!

George.

---

<div class="post-metadata">

**Author:** ![gmichelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/db5fbb/32.png) [@gmichelson](https://www.ruby-forum.com/u/gmichelson)\
**Post date:** [November 24, 2006, 11:51am UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111/2 "2006-11-24T11:51:26Z")

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On 11/24/06, George M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> =\> “1.8.2”  
> =\> 690465448  
> idea why this happens?
> 
> thanks in advance for any help!
> 
> George.

string.c:851  
int  
rb\_str\_hash(str)  
VALUE str;  
{  
register long len = RSTRING(str)-\>len;  
register char \*p = RSTRING(str)-\>ptr;  
register int key = 0;

#if defined(HASH\_ELFHASH)  
register unsigned int g;

```
while (len--) {
key = (key << 4) + *p++;
if (g = key & 0xF0000000)
    key ^= g >> 24;
key &= ~g;
}

```

#elif defined(HASH\_PERL)  
while (len–) {  
key += _p++;  
key += (key \<\< 10);  
key ^= (key \>\> 6);  
}  
key += (key \<\< 3);  
key ^= (key \>\> 11);  
key += (key \<\< 15);  
 #else  
while (len–) {  
key = key_65599 + \*p;  
p++;  
}  
key = key + (key\>\>5);  
#endif  
return key;  
}

I suspect those #ifdefs. See your config.h what’s compiled in.

---

<div class="post-metadata">

**Author:** ![gmichelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/db5fbb/32.png) [@gmichelson](https://www.ruby-forum.com/u/gmichelson)\
**Post date:** [November 24, 2006, 5:32pm UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111/3 "2006-11-24T17:32:18Z")

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## darshan@shadow ~ $ ri -T Object#hash ------------------------------------------------------------ Object#hash obj.hash =\> fixnum

```
 Generates a Fixnum hash value for this object. This function must
 have the property that a.eql?(b) implies a.hash == b.hash. The
 hash value is used by class Hash. Any hash value that exceeds the
 capacity of a Fixnum will be truncated before being used.

```

## darshan@shadow ~ $ ri -T “String#hash” ------------------------------------------------------------ String#hash str.hash =\> fixnum

```
 Return a hash based on the string's length and content.

```

I don’t see any reason you should assume the hash to be the same on  
different machines, or even across different instances of ruby. I’d  
only assume the hashes of eql? objects to be equal if they are computed  
within the same run of the script.

Your nasty problem seems to be that you were making an assumption not  
guaranteed to be true.

What are you actually trying to accomplish? Maybe we can help you find  
another way.

Darshan

BTW I get the same negative value as you do in your last (4th!)  
example. Could be a 64-bit thing.

---

<div class="post-metadata">

**Author:** ![gmichelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/db5fbb/32.png) [@gmichelson](https://www.ruby-forum.com/u/gmichelson)\
**Post date:** [November 24, 2006, 10:17pm UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111/4 "2006-11-24T22:17:16Z")

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On 24.11.2006 11:18, George M. wrote:

> =\> “1.8.2”  
> =\> 690465448  
> idea why this happens?  
> There is no guarantee that String#hash will behave the same in all  
> versions of Ruby. If this is a problem for you then you probably have  
> an application design problem. Why is this behavior a problem for you?

Kind regards

```
robert
```

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<div class="post-metadata">

**Author:** ![gmichelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/db5fbb/32.png) [@gmichelson](https://www.ruby-forum.com/u/gmichelson)\
**Post date:** [November 25, 2006, 7:01pm UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111/5 "2006-11-25T19:01:27Z")

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> There is no guarantee that String#hash will behave the same in all  
> versions of Ruby. If this is a problem for you then you probably have  
> an application design problem. Why is this behavior a problem for you?

Yeap, this is an application design problem. This is an old (legacy)  
application. It is strange that this problem has not shown up all  
these years (this is a 4 year old application). And it is really  
strange that has gives the exact same result on 4 different ruby  
installations and not on the target server.

-g.

---

<div class="post-metadata">

**Author:** ![gmichelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/db5fbb/32.png) [@gmichelson](https://www.ruby-forum.com/u/gmichelson)\
**Post date:** [November 25, 2006, 7:07pm UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111/6 "2006-11-25T19:07:28Z")

</div>

The offending version is the 1.8.4 version that comes with Ubuntu  
6.10, has anyone experienced similar problems?

-g.

---

<div class="post-metadata">

**Author:** ![gmichelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/db5fbb/32.png) [@gmichelson](https://www.ruby-forum.com/u/gmichelson)\
**Post date:** [November 25, 2006, 7:09pm UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111/7 "2006-11-25T19:09:28Z")

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> BTW I get the same negative value as you do in your last (4th!)  
> example. Could be a 64-bit thing.

Yeap, that’s what I am thinking. The offending machine has an AMD64  
processor.

-g.

---

<div class="post-metadata">

**Author:** ![gmichelson](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/db5fbb/32.png) [@gmichelson](https://www.ruby-forum.com/u/gmichelson)\
**Post date:** [November 25, 2006, 7:15pm UTC](https://www.ruby-forum.com/t/problems-with-string-hash/77111/8 "2006-11-25T19:15:30Z")

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> > > > > “G” == George M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

G\> The offending version is the 1.8.4 version that comes with Ubuntu  
G\> 6.10, has anyone experienced similar problems?

It’s an x86\_64 ?

Guy Decoux