Re: [boost] [guid] - vversion 1, 4 and in one class?

From: Andy (atompkins_at_[hidden])
Date: 2007-02-27 11:44:35

<Oliver.Kowalke_at_[hidden]> wrote in
news:B1EAFF2DAE7658488B631F25813CD91FD388B7_at_[hidden]: 

> Hello Andy,
> 
>> I am not very happy with the create_v1() function in 
>> guid_v3.zip.  Really it just pretends to implement the 
>> specification correctly.
> Because MAC-address of the  nic isn't used?

Yes, because the MAC-address is not used which increases the probability of 
duplicates.
Also my implementation does not use stable storage and this is the least desirable 
implementation, because it will increase the frequency of creation of new clock 
sequence numbers, which increases the probability of duplicates.

< snip >

> How is your implementation of uuid v4 unique in space (guids
> genereated on several computers at the same time) and time?

My implementation of version 4 (random-number-based) guids may not be unique in space 
and time.  It is based on boost::mt19937 to generate random numbers and it is seeded 
with std::time(0).  Cryptographic-quality random numbers would reduce the probability 
of repeated values.

Using version 5 (name-based) guid can be used to ensure that guids are unique in space 
(guids generated on several computers at the same time) and time.  Generate a guid for 
each computer as the namespace_guid and pass a counter as the name to the create 
function.

example:  (not tested or compiled)

const static guid computer_namespace_guid("{00000000-0000-0000-0000-000000000000}"); 
// or some other guid that never changes

guid generate_guid()
{
  // get unique string for computer
  // could be mac address
  const char* computer_name = get_unique_computer_name(); 
  const static guid namespace_guid
    = guid::create(computer_namespace_guid, computer_name,
        strlen(computer_name));

  static int count = 0;
  guid result = guid::create(namespace_guid,
    static_cast<char*>(&count), 4);

  ++count;

  return result;
}

> 
> Regards, Oliver
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> 

Andy.