Line data    Source code

       1             : // Copyright (c) 2019-2023  Made to Order Software Corp.  All Rights Reserved
       2             : //
       3             : // https://snapwebsites.org/project/snapdev
       4             : // contact@m2osw.com
       5             : //
       6             : // This program is free software; you can redistribute it and/or modify
       7             : // it under the terms of the GNU General Public License as published by
       8             : // the Free Software Foundation; either version 2 of the License, or
       9             : // (at your option) any later version.
      10             : //
      11             : // This program is distributed in the hope that it will be useful,
      12             : // but WITHOUT ANY WARRANTY; without even the implied warranty of
      13             : // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      14             : // GNU General Public License for more details.
      15             : //
      16             : // You should have received a copy of the GNU General Public License along
      17             : // with this program; if not, write to the Free Software Foundation, Inc.,
      18             : // 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
      19             : #pragma once
      20             : 
      21             : /** \file
      22             :  * \brief Calculate the log2() of an integer.
      23             :  *
      24             :  * This function calculates the number of zero bits before the first bit set
      25             :  * to 1. This is the floor of the log of a number in base 2 or:
      26             :  *
      27             :  *     floor(log(n) / log(2))
      28             :  *
      29             :  * The function makes use of an intrinsic so that way it goes as fast as
      30             :  * possible which should be one CPU cycle to find the bit and another couple
      31             :  * of cycles to transform the number in the correct `log2()` value.
      32             :  *
      33             :  * The result could be used to do:
      34             :  *
      35             :  *     1 << log2(v)
      36             :  *
      37             :  * to find v against assuming v was an exact power of 2 to start with.
      38             :  */
      39             : 
      40             : // C
      41             : //
      42             : #include <stdio.h>
      43             : 
      44             : 
      45             : namespace snapdev
      46             : {
      47             : 
      48             : 
      49             : /** \brief Calculate the log2() of an integer.
      50             :  *
      51             :  * This function uses an intrinsic instruction to calculate the log2()
      52             :  * of a 64 bit integer. The lower bits are ignored.
      53             :  *
      54             :  * This is equivalent to finding the position of the first bit set starting
      55             :  * at bit 63 and going down to bit 0.
      56             :  *
      57             :  * Note that `log2(0)` is undefined. The function returns -1 in that case.
      58             :  *
      59             :  * If your number is signed, make sure to pass the absolute value:
      60             :  * `log2(llabs(v))`.
      61             :  *
      62             :  * Source:
      63             :  * https://stackoverflow.com/questions/11376288/fast-computing-of-log2-for-64-bit-integers
      64             :  *
      65             :  * \param[in] v  The integer to calculate the log2() from.
      66             :  *
      67             :  * \return `floor(log(v) / log(2))` or -1 when v == 0.
      68             :  */
      69         129 : int log2(std::uint64_t v)
      70             : {
      71         129 :     if(v == 0)
      72             :     {
      73           1 :         return -1;
      74             :     }
      75             : 
      76         128 :     return 63 - __builtin_clzll(v);
      77             : }
      78             : 
      79             : 
      80             : 
      81             : } // namespace snapdev
      82             : // vim: ts=4 sw=4 et