libcxx/include/__random/log2.h

   1 //===----------------------------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #ifndef _LIBCPP___RANDOM_LOG2_H
  10 #define _LIBCPP___RANDOM_LOG2_H
  11
  12 #include <__config>
  13 #include <cstddef>
  14 #include <type_traits>
  15
  16 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  17 #pragma GCC system_header
  18 #endif
  19
  20 _LIBCPP_BEGIN_NAMESPACE_STD
  21
  22 template <class _UIntType, _UIntType _Xp, size_t _Rp>
  23 struct __log2_imp;
  24
  25 template <unsigned long long _Xp, size_t _Rp>
  26 struct __log2_imp<unsigned long long, _Xp, _Rp>
  27 {
  28     static const size_t value = _Xp & ((unsigned long long)(1) << _Rp) ? _Rp
  29                                            : __log2_imp<unsigned long long, _Xp, _Rp - 1>::value;
  30 };
  31
  32 template <unsigned long long _Xp>
  33 struct __log2_imp<unsigned long long, _Xp, 0>
  34 {
  35     static const size_t value = 0;
  36 };
  37
  38 template <size_t _Rp>
  39 struct __log2_imp<unsigned long long, 0, _Rp>
  40 {
  41     static const size_t value = _Rp + 1;
  42 };
  43
  44 #ifndef _LIBCPP_HAS_NO_INT128
  45
  46 template <__uint128_t _Xp, size_t _Rp>
  47 struct __log2_imp<__uint128_t, _Xp, _Rp>
  48 {
  49     static const size_t value = (_Xp >> 64)
  50         ? (64 + __log2_imp<unsigned long long, (_Xp >> 64), 63>::value)
  51         : __log2_imp<unsigned long long, _Xp, 63>::value;
  52 };
  53
  54 #endif // _LIBCPP_HAS_NO_INT128
  55
  56 template <class _UIntType, _UIntType _Xp>
  57 struct __log2
  58 {
  59     static const size_t value = __log2_imp<
  60 #ifndef _LIBCPP_HAS_NO_INT128
  61         typename conditional<
  62                 sizeof(_UIntType) <= sizeof(unsigned long long),
  63                     unsigned long long,
  64                     __uint128_t
  65             >::type,
  66 #else
  67         unsigned long long,
  68 #endif // _LIBCPP_HAS_NO_INT128
  69         _Xp, sizeof(_UIntType) * __CHAR_BIT__ - 1>::value;
  70 };
  71
  72 _LIBCPP_END_NAMESPACE_STD
  73
  74 #endif // _LIBCPP___RANDOM_LOG2_H