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boost/integer/static_log2.hpp

//  Boost integer/static_log2.hpp header file  -------------------------------//



//  (C) Copyright Daryle Walker 2001.  Permission to copy, use, modify, sell and

//  distribute this software is granted provided this copyright notice appears 

//  in all copies.  This software is provided "as is" without express or

//  implied warranty, and with no claim as to its suitability for any purpose. 



//  See http://www.boost.org for updates, documentation, and revision history. 



#ifndef BOOST_INTEGER_STATIC_LOG2_HPP

#define BOOST_INTEGER_STATIC_LOG2_HPP



#include <boost/integer_fwd.hpp>  // self include



#include <boost/config.hpp>  // for BOOST_STATIC_CONSTANT, etc.

#include <boost/limits.hpp>  // for std::numeric_limits



#ifdef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION

#include <boost/pending/ct_if.hpp>  // for boost::ct_if<>

#endif





namespace boost

{





//  Implementation details  --------------------------------------------------//



namespace detail

{



// Forward declarations

template < unsigned long Val, int Place = 0, int Index

 = std::numeric_limits<unsigned long>::digits >

    struct static_log2_helper_t;



#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION



template < unsigned long Val, int Place >

    struct static_log2_helper_t< Val, Place, 1 >;



#else



template < int Place >

    struct static_log2_helper_final_step;



template < unsigned long Val, int Place = 0, int Index

 = std::numeric_limits<unsigned long>::digits >

    struct static_log2_helper_nopts_t;



#endif



// Recursively build the logarithm by examining the upper bits

template < unsigned long Val, int Place, int Index >

struct static_log2_helper_t

{

private:

    BOOST_STATIC_CONSTANT( int, half_place = Index / 2 );

    BOOST_STATIC_CONSTANT( unsigned long, lower_mask = (1ul << half_place)

     - 1ul );

    BOOST_STATIC_CONSTANT( unsigned long, upper_mask = ~lower_mask );

    BOOST_STATIC_CONSTANT( bool, do_shift = (Val & upper_mask) != 0ul );



    BOOST_STATIC_CONSTANT( unsigned long, new_val = do_shift ? (Val

     >> half_place) : Val );

    BOOST_STATIC_CONSTANT( int, new_place = do_shift ? (Place + half_place)

     : Place );

    BOOST_STATIC_CONSTANT( int, new_index = Index - half_place );



#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION

    typedef static_log2_helper_t<new_val, new_place, new_index>  next_step_type;

#else

    typedef static_log2_helper_nopts_t<new_val, new_place, new_index>  next_step_type;

#endif



public:

    BOOST_STATIC_CONSTANT( int, value = next_step_type::value );



};  // boost::detail::static_log2_helper_t



// Non-recursive case

#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION



template < unsigned long Val, int Place >

struct static_log2_helper_t< Val, Place, 1 >

{

public:

    BOOST_STATIC_CONSTANT( int, value = Place );



};  // boost::detail::static_log2_helper_t



#else



template < int Place >

struct static_log2_helper_final_step

{

public:

    BOOST_STATIC_CONSTANT( int, value = Place );



};  // boost::detail::static_log2_helper_final_step



template < unsigned long Val, int Place, int Index >

struct static_log2_helper_nopts_t

{

private:

    typedef static_log2_helper_t<Val, Place, Index>  recursive_step_type;

    typedef static_log2_helper_final_step<Place>     final_step_type;



    typedef typename ct_if<( Index != 1 ), recursive_step_type,

     final_step_type>::type  next_step_type;



public:

    BOOST_STATIC_CONSTANT( int, value = next_step_type::value );



};  // boost::detail::static_log2_helper_nopts_t



#endif



}  // namespace detail





//  Compile-time log-base-2 evaluator class declaration  ---------------------//



template < unsigned long Value >

struct static_log2

{

    BOOST_STATIC_CONSTANT( int, value

     = detail::static_log2_helper_t<Value>::value );

};



template < >

struct static_log2< 0ul >

{

    // The logarithm of zero is undefined.

};





}  // namespace boost





#endif  // BOOST_INTEGER_STATIC_LOG2_HPP