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 Hu Chunming
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// -------------- 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
// https://www.boost.org/LICENSE_1_0.txt)
//
// ---------------------------------------------------
// See https://www.boost.org/libs/integer for documentation.
// ------------------------------------------------------------------------- //
#ifndef BOOST_INTEGER_STATIC_LOG2_HPP
#define BOOST_INTEGER_STATIC_LOG2_HPP
#include <boost/config.hpp>
#include <boost/integer_fwd.hpp>
namespace boost {
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 boost::static_log2_argument_type argument_type;
typedef boost::static_log2_result_type result_type;
template <result_type n>
struct choose_initial_n {
BOOST_STATIC_CONSTANT(bool, 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);
};
// 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;
// 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.)
//
template <argument_type x, result_type n = initial_n>
struct static_log2_impl {
BOOST_STATIC_CONSTANT(bool, c = (x >> n) > 0); // x >= 2**n ?
BOOST_STATIC_CONSTANT(
result_type,
value = c*n + (static_log2_impl< (x>>c*n), n/2 >::value)
);
};
template <>
struct static_log2_impl<1, 0> {
BOOST_STATIC_CONSTANT(result_type, value = 0);
};
}
} // detail
// --------------------------------------
// static_log2<x>
// ----------------------------------------
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
);
};
template <>
struct static_log2<0> { };
}
#endif // include guard
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