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File indexing completed on 2025-01-18 09:38:24
0001 // -------------- Boost static_log2.hpp header file ----------------------- // 0002 // 0003 // Copyright (C) 2001 Daryle Walker. 0004 // Copyright (C) 2003 Vesa Karvonen. 0005 // Copyright (C) 2003 Gennaro Prota. 0006 // 0007 // Distributed under the Boost Software License, Version 1.0. 0008 // (See accompanying file LICENSE_1_0.txt or copy at 0009 // https://www.boost.org/LICENSE_1_0.txt) 0010 // 0011 // --------------------------------------------------- 0012 // See https://www.boost.org/libs/integer for documentation. 0013 // ------------------------------------------------------------------------- // 0014 0015 0016 #ifndef BOOST_INTEGER_STATIC_LOG2_HPP 0017 #define BOOST_INTEGER_STATIC_LOG2_HPP 0018 0019 #include <boost/config.hpp> 0020 #include <boost/integer_fwd.hpp> 0021 0022 namespace boost { 0023 0024 namespace detail { 0025 0026 namespace static_log2_impl { 0027 0028 // choose_initial_n<> 0029 // 0030 // Recursively doubles its integer argument, until it 0031 // becomes >= of the "width" (C99, 6.2.6.2p4) of 0032 // static_log2_argument_type. 0033 // 0034 // Used to get the maximum power of two less then the width. 0035 // 0036 // Example: if on your platform argument_type has 48 value 0037 // bits it yields n=32. 0038 // 0039 // It's easy to prove that, starting from such a value 0040 // of n, the core algorithm works correctly for any width 0041 // of static_log2_argument_type and that recursion always 0042 // terminates with x = 1 and n = 0 (see the algorithm's 0043 // invariant). 0044 0045 typedef boost::static_log2_argument_type argument_type; 0046 typedef boost::static_log2_result_type result_type; 0047 0048 template <result_type n> 0049 struct choose_initial_n { 0050 0051 BOOST_STATIC_CONSTANT(bool, c = (argument_type(1) << n << n) != 0); 0052 BOOST_STATIC_CONSTANT( 0053 result_type, 0054 value = !c*n + choose_initial_n<2*c*n>::value 0055 ); 0056 0057 }; 0058 0059 template <> 0060 struct choose_initial_n<0> { 0061 BOOST_STATIC_CONSTANT(result_type, value = 0); 0062 }; 0063 0064 0065 0066 // start computing from n_zero - must be a power of two 0067 const result_type n_zero = 16; 0068 const result_type initial_n = choose_initial_n<n_zero>::value; 0069 0070 // static_log2_impl<> 0071 // 0072 // * Invariant: 0073 // 2n 0074 // 1 <= x && x < 2 at the start of each recursion 0075 // (see also choose_initial_n<>) 0076 // 0077 // * Type requirements: 0078 // 0079 // argument_type maybe any unsigned type with at least n_zero + 1 0080 // value bits. (Note: If larger types will be standardized -e.g. 0081 // unsigned long long- then the argument_type typedef can be 0082 // changed without affecting the rest of the code.) 0083 // 0084 0085 template <argument_type x, result_type n = initial_n> 0086 struct static_log2_impl { 0087 0088 BOOST_STATIC_CONSTANT(bool, c = (x >> n) > 0); // x >= 2**n ? 0089 BOOST_STATIC_CONSTANT( 0090 result_type, 0091 value = c*n + (static_log2_impl< (x>>c*n), n/2 >::value) 0092 ); 0093 0094 }; 0095 0096 template <> 0097 struct static_log2_impl<1, 0> { 0098 BOOST_STATIC_CONSTANT(result_type, value = 0); 0099 }; 0100 0101 } 0102 } // detail 0103 0104 0105 0106 // -------------------------------------- 0107 // static_log2<x> 0108 // ---------------------------------------- 0109 0110 template <static_log2_argument_type x> 0111 struct static_log2 { 0112 0113 BOOST_STATIC_CONSTANT( 0114 static_log2_result_type, 0115 value = detail::static_log2_impl::static_log2_impl<x>::value 0116 ); 0117 0118 }; 0119 0120 0121 template <> 0122 struct static_log2<0> { }; 0123 0124 } 0125 0126 #endif // include guard
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