forked from boostorg/integer
92 lines
2.6 KiB
C++
92 lines
2.6 KiB
C++
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// Boost integer/static_log2.hpp header file -------------------------------//
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// (C) Copyright Daryle Walker 2001. Permission to copy, use, modify, sell and
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// distribute this software is granted provided this copyright notice appears
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// in all copies. This software is provided "as is" without express or
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// implied warranty, and with no claim as to its suitability for any purpose.
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// See http://www.boost.org for updates, documentation, and revision history.
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#ifndef BOOST_INTEGER_STATIC_LOG2_HPP
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#define BOOST_INTEGER_STATIC_LOG2_HPP
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#include <boost/integer_fwd.hpp> // self include
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#include <boost/config.hpp> // for BOOST_STATIC_CONSTANT
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#include <boost/limits.hpp> // for std::numeric_limits
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namespace boost
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{
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// Implementation details --------------------------------------------------//
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namespace detail
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{
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// Forward declarations
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template < unsigned long Val, int Place = 0, int Index
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= std::numeric_limits<unsigned long>::digits >
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struct static_log2_helper_t;
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template < unsigned long Val, int Place >
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struct static_log2_helper_t< Val, Place, 1 >;
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// Recursively build the logarithm by examining the upper bits
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template < unsigned long Val, int Place, int Index >
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struct static_log2_helper_t
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{
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private:
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BOOST_STATIC_CONSTANT( int, half_place = Index / 2 );
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BOOST_STATIC_CONSTANT( unsigned long, lower_mask = (1ul << half_place)
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- 1ul );
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BOOST_STATIC_CONSTANT( unsigned long, upper_mask = ~lower_mask );
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BOOST_STATIC_CONSTANT( bool, do_shift = (Val & upper_mask) != 0ul );
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BOOST_STATIC_CONSTANT( unsigned long, new_val = do_shift ? (Val
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>> half_place) : Val );
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BOOST_STATIC_CONSTANT( int, new_place = do_shift ? (Place + half_place)
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: Place );
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BOOST_STATIC_CONSTANT( int, new_index = Index - half_place );
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typedef static_log2_helper_t<new_val, new_place, new_index> next_step_type;
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public:
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BOOST_STATIC_CONSTANT( int, value = next_step_type::value );
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}; // boost::detail::static_log2_helper_t
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// Non-recursive case
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template < unsigned long Val, int Place >
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struct static_log2_helper_t< Val, Place, 1 >
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{
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public:
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BOOST_STATIC_CONSTANT( int, value = Place );
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}; // boost::detail::static_log2_helper_t
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} // namespace detail
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// Compile-time log-base-2 evaluator class declaration ---------------------//
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template < unsigned long Value >
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struct static_log2
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{
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BOOST_STATIC_CONSTANT( int, value
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= detail::static_log2_helper_t<Value>::value );
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};
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template < >
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struct static_log2< 0ul >
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{
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// The logarithm of zero is undefined.
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};
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} // namespace boost
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#endif // BOOST_INTEGER_STATIC_LOG2_HPP
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