1/* ffs.c -- find the first set bit in a word.
2   Copyright (C) 2011-2022 Free Software Foundation, Inc.
3
4   This file is free software: you can redistribute it and/or modify
5   it under the terms of the GNU Lesser General Public License as
6   published by the Free Software Foundation; either version 2.1 of the
7   License, or (at your option) any later version.
8
9   This file 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 Lesser General Public License for more details.
13
14   You should have received a copy of the GNU Lesser General Public License
15   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
16
17/* Written by Eric Blake.  */
18
19#include <config.h>
20
21/* Specification.  */
22#include <strings.h>
23
24#include <limits.h>
25
26#if defined _MSC_VER && !(__clang_major__ >= 4)
27# include <intrin.h>
28#endif
29
30int
31ffs (int i)
32{
33#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__clang_major__ >= 4)
34  return __builtin_ffs (i);
35#elif defined _MSC_VER
36  /* _BitScanForward
37     <https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
38  unsigned long bit;
39  if (_BitScanForward (&bit, i))
40    return bit + 1;
41  else
42    return 0;
43#else
44  /* <https://github.com/gibsjose/BitHacks>
45     gives this deBruijn constant for a branch-less computation, although
46     that table counted trailing zeros rather than bit position.  This
47     requires 32-bit int, we fall back to a naive algorithm on the rare
48     platforms where that assumption is not true.  */
49  if (CHAR_BIT * sizeof i == 32)
50    {
51      static unsigned int table[] = {
52        1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9,
53        32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10
54      };
55      unsigned int u = i;
56      unsigned int bit = u & -u;
57      return table[(bit * 0x077cb531U) >> 27] - !i;
58    }
59  else
60    {
61      unsigned int j;
62      for (j = 0; j < CHAR_BIT * sizeof i; j++)
63        if (i & (1U << j))
64          return j + 1;
65      return 0;
66    }
67#endif
68}
69