// -------------- 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. // ------------------------------------------------------------------------- // #ifndef BOOST_INTEGER_STATIC_LOG2_HPP #define BOOST_INTEGER_STATIC_LOG2_HPP #include "boost/integer_fwd.hpp" // for boost::intmax_t 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 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::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 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 // ---------------------------------------- template struct static_log2 { BOOST_STATIC_CONSTANT( static_log2_result_type, value = detail::static_log2_impl::static_log2_impl::value ); }; template <> struct static_log2<0> { }; } #endif // include guard