tweaking the memoized fibonnacci to produce > 40 number give strange results

[sylvain foubert <sylvain.foubert@…>]

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 ?

[Joaquín M López Muñoz]

Hi Sylvain,

Looks like in your platform unsigned long is 32 bits, and since F(48) > 2³² you're incurring overflow, hence the bogus results.