Re: [boost] A question about static_log2

[Vesa Karvonen]

Gennaro Prota:

Can someone who was subscribed when the Integer library was approved
explain me what were the reasons to choose the current implementation
of static_log2 against e.g.

I can't help in that, but

  template <unsigned long n>
  struct log2 {
      BOOST_STATIC_CONSTANT(unsigned long,
                            value = 1 + (log2<n/2>::value));
  };

  template <>
  struct log2<1> {
      BOOST_STATIC_CONSTANT(unsigned long, value = 0);
  };

the above algorithm is inefficient and should not be used. Consider the following algorithm instead:

#include <iostream>
#include <climits>

template<unsigned long x,
int n = sizeof(unsigned long) * CHAR_BIT / 2>
struct log2 {
private:
enum {c = (1 << n) <= x};
public:
enum {value = c*n + log2<(x>>(c*n)),(n/2)>::value};
};

template<>
struct log2<1,0> {enum {value = 0};};

int main() {
std::cerr << log2<1>::value << '\n';
std::cerr << log2<2>::value << '\n';
std::cerr << log2<3>::value << '\n';
std::cerr << log2<5>::value << '\n';
std::cerr << log2<6>::value << '\n';
std::cerr << log2<7>::value << '\n';
std::cerr << log2<8>::value << '\n';
std::cerr << log2<(1<<20)>::value << '\n';

return 0;
}

Regards,
Vesa Karvonen


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