1 // -------------- Boost static_log2.hpp header file  ----------------------- //
2 //
3 //                 Copyright (C) 2001 Daryle Walker.
4 //                 Copyright (C) 2003 Vesa Karvonen.
5 //                 Copyright (C) 2003 Gennaro Prota.
6 //
7 //     Distributed under the Boost Software License, Version 1.0.
8 //        (See accompanying file LICENSE_1_0.txt or copy at
9 //              http://www.boost.org/LICENSE_1_0.txt)
10 //
11 //         ---------------------------------------------------
12 //       See http://www.boost.org/libs/integer for documentation.
13 // ------------------------------------------------------------------------- //
14 
15 
16 #ifndef BOOST_INTEGER_STATIC_LOG2_HPP
17 #define BOOST_INTEGER_STATIC_LOG2_HPP
18 
19 #include "boost/integer_fwd.hpp" // for boost::intmax_t
20 
21 namespace boost {
22 
23  namespace detail {
24 
25      namespace static_log2_impl {
26 
27      // choose_initial_n<>
28      //
29      // Recursively doubles its integer argument, until it
30      // becomes >= of the "width" (C99, 6.2.6.2p4) of
31      // static_log2_argument_type.
32      //
33      // Used to get the maximum power of two less then the width.
34      //
35      // Example: if on your platform argument_type has 48 value
36      //          bits it yields n=32.
37      //
38      // It's easy to prove that, starting from such a value
39      // of n, the core algorithm works correctly for any width
40      // of static_log2_argument_type and that recursion always
41      // terminates with x = 1 and n = 0 (see the algorithm's
42      // invariant).
43 
44      typedef boost::static_log2_argument_type argument_type;
45      typedef boost::static_log2_result_type result_type;
46 
47      template <result_type n>
48      struct choose_initial_n {
49 
50          BOOST_STATIC_CONSTANT(bool, c = (argument_type(1) << n << n) != 0);
51          BOOST_STATIC_CONSTANT(
52              result_type,
53              value = !c*n + choose_initial_n<2*c*n>::value
54          );
55 
56      };
57 
58      template <>
59      struct choose_initial_n<0> {
60          BOOST_STATIC_CONSTANT(result_type, value = 0);
61      };
62 
63 
64 
65      // start computing from n_zero - must be a power of two
66      const result_type n_zero = 16;
67      const result_type initial_n = choose_initial_n<n_zero>::value;
68 
69      // static_log2_impl<>
70      //
71      // * Invariant:
72      //                 2n
73      //  1 <= x && x < 2    at the start of each recursion
74      //                     (see also choose_initial_n<>)
75      //
76      // * Type requirements:
77      //
78      //   argument_type maybe any unsigned type with at least n_zero + 1
79      //   value bits. (Note: If larger types will be standardized -e.g.
80      //   unsigned long long- then the argument_type typedef can be
81      //   changed without affecting the rest of the code.)
82      //
83 
84      template <argument_type x, result_type n = initial_n>
85      struct static_log2_impl {
86 
87          BOOST_STATIC_CONSTANT(bool, c = (x >> n) > 0); // x >= 2**n ?
88          BOOST_STATIC_CONSTANT(
89              result_type,
90              value = c*n + (static_log2_impl< (x>>c*n), n/2 >::value)
91          );
92 
93      };
94 
95      template <>
96      struct static_log2_impl<1, 0> {
97         BOOST_STATIC_CONSTANT(result_type, value = 0);
98      };
99 
100      }
101  } // detail
102 
103 
104 
105  // --------------------------------------
106  // static_log2<x>
107  // ----------------------------------------
108 
109  template <static_log2_argument_type x>
110  struct static_log2 {
111 
112      BOOST_STATIC_CONSTANT(
113          static_log2_result_type,
114          value = detail::static_log2_impl::static_log2_impl<x>::value
115      );
116 
117  };
118 
119 
120  template <>
121  struct static_log2<0> { };
122 
123 }
124 
125 
126 
127 #endif // include guard
128