Line data    Source code

       1             : // Check whether a set intersection is empty or not
       2             : // Copyright (c) 2019  Made to Order Software Corp.  All Rights Reserved
       3             : //
       4             : // This program is free software; you can redistribute it and/or modify
       5             : // it under the terms of the GNU General Public License as published by
       6             : // the Free Software Foundation; either version 2 of the License, or
       7             : // (at your option) any later version.
       8             : //
       9             : // This program is distributed in the hope that it will be useful,
      10             : // but WITHOUT ANY WARRANTY; without even the implied warranty of
      11             : // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      12             : // GNU General Public License for more details.
      13             : //
      14             : // You should have received a copy of the GNU General Public License
      15             : // along with this program; if not, write to the Free Software
      16             : // Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
      17             : #pragma once
      18             : 
      19             : /** \file
      20             :  * \brief Calculate the log2() of an integer.
      21             :  *
      22             :  * This function calculates the number of zero bits before the first bit set
      23             :  * to 1. This is the floor of the log of a number in base 2 or:
      24             :  *
      25             :  *     floor(log(n) / log(2))
      26             :  *
      27             :  * The function makes use of an intrinsic so that way it goes as fast as
      28             :  * possible which should be one CPU cycle to find the bit and another couple
      29             :  * of cycles to transform the number in the correct `log2()` value.
      30             :  *
      31             :  * The result could be used to do:
      32             :  *
      33             :  *     1 << log2(v)
      34             :  *
      35             :  * to find v against assuming v was an exact power of 2 to start with.
      36             :  */
      37             : 
      38             : // C lib
      39             : //
      40             : #include <stdio.h>
      41             : 
      42             : 
      43             : namespace snap
      44             : {
      45             : 
      46             : 
      47             : /** \brief Calculate the log2() of an integer.
      48             :  *
      49             :  * This function uses an intrinsic instruction to calculate the log2()
      50             :  * of a 64 bit integer. The lower bits are ignored.
      51             :  *
      52             :  * This is equivalent to finding the position of the first bit set starting
      53             :  * at bit 63 and going down to bit 0.
      54             :  *
      55             :  * Note that `log2(0)` is undefined. The function returns -1 in that case.
      56             :  *
      57             :  * If your number is signed, make sure to pass the absolute value:
      58             :  * `log2(llabs(v))`.
      59             :  *
      60             :  * Source:
      61             :  * https://stackoverflow.com/questions/11376288/fast-computing-of-log2-for-64-bit-integers
      62             :  *
      63             :  * \param[in] v  The integer to calculate the log2() from.
      64             :  *
      65             :  * \return `floor(log(v) / log(2))` or -1 when v == 0.
      66             :  */
      67         128 : int log2(std::uint64_t v)
      68             : {
      69         128 :     if(v == 0)
      70             :     {
      71           0 :         return 0;
      72             :     }
      73             : 
      74         128 :     return 63 - __builtin_clzll(v);
      75             : }
      76             : 
      77             : 
      78             : 
      79             : } // namespace snap
      80             : // vim: ts=4 sw=4 et