[Boost-bugs] [Boost C++ Libraries] #4847: tweaking the memoized fibonnacci to produce > 40 number give strange results

Subject: [Boost-bugs] [Boost C++ Libraries] #4847: tweaking the memoized fibonnacci to produce > 40 number give strange results
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-11-12 15:58:36

#4847: tweaking the memoized fibonnacci to produce > 40 number give strange
results
---------------------------------------------------------+------------------
 Reporter:  sylvain foubert <sylvain.foubert@…>          |       Owner:  joaquin  
     Type:  Bugs                                         |      Status:  new      
Milestone:  To Be Determined                             |   Component:  flyweight
  Version:  Boost 1.44.0                                 |    Severity:  Problem  
 Keywords:  example fibonacci                            |  
---------------------------------------------------------+------------------
 I changed the integer type for numbers from int to unsigned long for
 higher precision and I got erroneous computed values after F(47) :

 F(46)=1836311903
 F(47)=2971215073
 F(48)=512559680
 F(49)=3483774753


 {{{
 typedef flyweight<key_value<unsigned long,compute_fibonacci>,no_tracking>
 fibonacci;

 struct compute_fibonacci:private boost::noncopyable
 {
   compute_fibonacci(unsigned long n):
     result(n==0?0:n==1?1:fibonacci(n-2).get()+fibonacci(n-1).get())
   {}

   operator unsigned long()const{return result;}
   unsigned long result;
 };

 }}}


 Internal bug or unprecautious use of the API ?

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/4847>
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