[Pharo-project] Two objects with same hash

Denis Kudriashov dionisiydk at gmail.com
Fri Sep 23 10:32:01 EDT 2011


Yes, I know.

I just use generated object id to create unique file to archive object.
I generate id by current date and time "yyyyMMddHHmmssSSS + object hash".
But It is my mistake.
Now I use "yyyyMMddHHmmssSSS + UUID" (I need date parameter too).
And I think id like "yyyyMMddHHmmssSSS + object address" is good too for my
purpose.


2011/9/23 Mariano Martinez Peck <marianopeck at gmail.com>

>
>
> On Fri, Sep 23, 2011 at 3:22 PM, Denis Kudriashov <dionisiydk at gmail.com>wrote:
>
>> I am sure I see It in some block bost. But dont remember
>>
>>
> Ok, but GC can move objects arround. If you stick to that value as #hash
> then you dictionaries will be screw...you would need a rehash. But you don't
> know when to trigger it ;)
>
>
>
>>
>> 2011/9/23 Mariano Martinez Peck <marianopeck at gmail.com>
>>
>>>
>>>
>>> On Fri, Sep 23, 2011 at 3:02 PM, Denis Kudriashov <dionisiydk at gmail.com>wrote:
>>>
>>>> Thank you for all your responses. I will use UUID.
>>>>
>>>> Another assumption:
>>>> I know there is a trick to get object memory address.
>>>>
>>>
>>> which one?
>>>
>>>
>>>> Can it be used for object identity? (inside memory of objects)
>>>>
>>>>
>>>>
>>>> *
>>>> *
>>>> 2011/9/23 Marcus Denker <marcus.denker at inria.fr>
>>>>
>>>>>
>>>>> On Sep 23, 2011, at 1:08 PM, Denis Kudriashov wrote:
>>>>>
>>>>> > Hello,
>>>>> >
>>>>> > I use hash message to generate unique object id. (unique between
>>>>> current memory objects).
>>>>> > And I catch situation when two objects has same hash value. Objects
>>>>> has been created in loop almost at same time.
>>>>> >
>>>>> > So my question - is it really possible?
>>>>>
>>>>> Yes, easily, as there are only 12 bits for the hash, there are only
>>>>> 4096 different hash values.
>>>>>
>>>>> This means that a hash is *not* at all a UUID, and all hashing data
>>>>> structures need to be optimized
>>>>> for the case of hash collision, as it will happen a lot.
>>>>>
>>>>>        Marcus
>>>>>
>>>>>
>>>>> --
>>>>> Marcus Denker -- http://marcusdenker.de
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>>> --
>>> Mariano
>>> http://marianopeck.wordpress.com
>>>
>>>
>>
>
>
> --
> Mariano
> http://marianopeck.wordpress.com
>
>
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