bit_ops.h

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/*
* Bit/Word Operations
* (C) 1999-2008 Jack Lloyd
* (C) Copyright Projet SECRET, INRIA, Rocquencourt
* (C) Bhaskar Biswas and  Nicolas Sendrier
* (C) 2014 cryptosource GmbH
* (C) 2014 Falko Strenzke fstrenzke@cryptosource.de
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#ifndef BOTAN_BIT_OPS_H_
#define BOTAN_BIT_OPS_H_

#include <botan/types.h>

namespace Botan {

/**
* Power of 2 test. T should be an unsigned integer type
* @param arg an integer value
* @return true iff arg is 2^n for some n > 0
*/
template<typename T>
inline bool is_power_of_2(T arg)
   {
   return ((arg != 0 && arg != 1) && ((arg & (arg-1)) == 0));
   }

/**
* Return the index of the highest set bit
* T is an unsigned integer type
* @param n an integer value
* @return index of the highest set bit in n
*/
template<typename T>
inline size_t high_bit(T n)
   {
   for(size_t i = 8*sizeof(T); i > 0; --i)
      if((n >> (i - 1)) & 0x01)
         return i;
   return 0;
   }

/**
* Return the index of the lowest set bit
* T is an unsigned integer type
* @param n an integer value
* @return index of the lowest set bit in n
*/
template<typename T>
inline size_t low_bit(T n)
   {
   for(size_t i = 0; i != 8*sizeof(T); ++i)
      if((n >> i) & 0x01)
         return (i + 1);
   return 0;
   }

/**
* Return the number of significant bytes in n
* @param n an integer value
* @return number of significant bytes in n
*/
template<typename T>
inline size_t significant_bytes(T n)
   {
   for(size_t i = 0; i != sizeof(T); ++i)
      if(get_byte(i, n))
         return sizeof(T)-i;
   return 0;
   }

/**
* Compute Hamming weights
* @param n an integer value
* @return number of bits in n set to 1
*/
template<typename T>
inline size_t hamming_weight(T n)
   {
   const uint8_t NIBBLE_WEIGHTS[] = {
      0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4 };

   size_t weight = 0;
   for(size_t i = 0; i != 2*sizeof(T); ++i)
      weight += NIBBLE_WEIGHTS[(n >> (4*i)) & 0x0F];
   return weight;
   }

/**
* Count the trailing zero bits in n
* @param n an integer value
* @return maximum x st 2^x divides n
*/
template<typename T>
inline size_t ctz(T n)
   {
   for(size_t i = 0; i != 8*sizeof(T); ++i)
      if((n >> i) & 0x01)
         return i;
   return 8*sizeof(T);
   }

#if defined(BOTAN_BUILD_COMPILER_IS_GCC) || defined(BOTAN_BUILD_COMPILER_IS_CLANG)

template<>
inline size_t ctz(uint32_t n)
   {
   if(n == 0)
      return 32;
   return __builtin_ctz(n);
   }

template<>
inline size_t ctz(uint64_t n)
   {
   if(n == 0)
      return 64;
   return __builtin_ctzll(n);
   }

template<>
inline size_t high_bit(uint32_t x)
   {
   if(x == 0)
      return 0;
   return (32 - __builtin_clz(x));
   }

template<>
inline size_t high_bit(uint64_t x)
   {
   if(x == 0)
      return 0;
   return (64 - __builtin_clzll(x));
   }

#endif

template<typename T>
size_t ceil_log2(T x)
   {
   if(x >> (sizeof(T)*8-1))
      return sizeof(T)*8;

   size_t result = 0;
   T compare = 1;

   while(compare < x)
      {
      compare <<= 1;
      result++;
      }

   return result;
   }

}

#endif