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new version - more efficient, and with no dependency on numeric_limits<>

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[SVN r23829]
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Gennaro Prota
2004-07-20 09:49:43 +00:00
parent 4862574bb1
commit 170a352574
1 changed files with 106 additions and 115 deletions
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221
include/boost/integer/static_log2.hpp
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@ -1,141 +1,132 @@
// Boost integer/static_log2.hpp header file -------------------------------//
// -------------- Boost static_log2.hpp header file ----------------------- //
//
// Copyright (C) 2001 Daryle Walker.
// Copyright (C) 2003 Vesa Karvonen.
// Copyright (C) 2003 Gennaro Prota.
//
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
//
// ---------------------------------------------------
// See http://www.boost.org/libs/integer for documentation.
// ------------------------------------------------------------------------- //
// (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
#include <boost/config.hpp> // for BOOST_STATIC_CONSTANT, etc.
#include <boost/limits.hpp> // for std::numeric_limits
namespace boost {
#ifdef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
#include <boost/pending/ct_if.hpp> // for boost::ct_if<>
#endif
namespace detail {
namespace static_log2_impl {
// choose_initial_n<>
//
// Recursively doubles its integer argument, until it
// becomes >= of the "width" (C99, 6.2.6.2p4) of
// static_log2_argument_type.
//
// Used to get the maximum power of two less then the width.
//
// Example: if on your platform argument_type has 48 value
// bits it yields n=32.
//
// It's easy to prove that, starting from such a value
// of n, the core algorithm works correctly for any width
// of static_log2_argument_type and that recursion always
// terminates with x = 1 and n = 0 (see the algorithm's
// invariant).
typedef unsigned long argument_type;
typedef int result_type;
namespace boost
{
template <result_type n>
struct choose_initial_n {
enum { c = (argument_type(1) << n << n) != 0 };
BOOST_STATIC_CONSTANT(
result_type,
value = !c*n + choose_initial_n<2*c*n>::value
);
};
template <>
struct choose_initial_n<0> {
BOOST_STATIC_CONSTANT(result_type, value = 0);
};
// Implementation details --------------------------------------------------//
namespace detail
{
// start computing from n_zero - must be a power of two
const result_type n_zero = 16;
const result_type initial_n = choose_initial_n<n_zero>::value;
// Forward declarations
template < unsigned long Val, int Place = 0, int Index
= std::numeric_limits<unsigned long>::digits >
struct static_log2_helper_t;
// static_log2_impl<>
//
// * Invariant:
// 2n
// 1 <= x && x < 2 at the start of each recursion
// (see also choose_initial_n<>)
//
// * Type requirements:
//
// argument_type maybe any unsigned type with at least n_zero + 1
// value bits. (Note: If larger types will be standardized -e.g
// . unsigned long long- then the argument_type typedef can be
// changed without affecting the rest of the code.)
//
#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
template <argument_type x, result_type n = initial_n>
struct static_log2_impl {
template < unsigned long Val, int Place >
struct static_log2_helper_t< Val, Place, 1 >;
enum { c = (x >> n) > 0 }; // x >= 2**n ?
BOOST_STATIC_CONSTANT(
result_type,
value = c*n + (static_log2_impl< (x>>c*n), n/2 >::value)
);
#else
};
template < int Place >
struct static_log2_helper_final_step;
template <>
struct static_log2_impl<1, 0> {
BOOST_STATIC_CONSTANT(result_type, value = 0);
};
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
}
} // 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 );
};
// --------------------------------------
// static_log2<x>
// ----------------------------------------
template < >
struct static_log2< 0ul >
{
// The logarithm of zero is undefined.
};
typedef detail::static_log2_impl::argument_type static_log2_argument_type;
typedef detail::static_log2_impl::result_type static_log2_result_type;
} // namespace boost
template <static_log2_argument_type x>
struct static_log2 {
BOOST_STATIC_CONSTANT(
static_log2_result_type,
value = detail::static_log2_impl::static_log2_impl<x>::value
);
};
#endif // BOOST_INTEGER_STATIC_LOG2_HPP
template <>
struct static_log2<0> { };
}
#endif // include guard