// -------------- 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/config.hpp" // for BOOST_STATIC_CONSTANT

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 unsigned long argument_type;

     typedef          int  result_type;

     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);

     };

     // 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 {

         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)

         );

     };

     template <>

     struct static_log2_impl<1, 0> {

        BOOST_STATIC_CONSTANT(result_type, value = 0);

     };

     }

 } // detail

 // --------------------------------------

 // static_log2<x>

 // ----------------------------------------

 typedef detail::static_log2_impl::argument_type static_log2_argument_type;

 typedef detail::static_log2_impl::result_type   static_log2_result_type;

 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