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author | Simon Rettberg | 2018-10-16 10:08:48 +0200 |
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committer | Simon Rettberg | 2018-10-16 10:08:48 +0200 |
commit | d3a98cf6cbc3bd0b9efc570f58e8812c03931c18 (patch) | |
tree | cbddf8e50f35a9c6e878a5bfe3c6d625d99e12ba /utils/pow2.c | |
download | xscreensaver-d3a98cf6cbc3bd0b9efc570f58e8812c03931c18.tar.gz xscreensaver-d3a98cf6cbc3bd0b9efc570f58e8812c03931c18.tar.xz xscreensaver-d3a98cf6cbc3bd0b9efc570f58e8812c03931c18.zip |
Original 5.40
Diffstat (limited to 'utils/pow2.c')
-rw-r--r-- | utils/pow2.c | 51 |
1 files changed, 51 insertions, 0 deletions
diff --git a/utils/pow2.c b/utils/pow2.c new file mode 100644 index 0000000..9cce3d8 --- /dev/null +++ b/utils/pow2.c @@ -0,0 +1,51 @@ +/* pow2, Copyright (c) 2016 Dave Odell <dmo2118@gmail.com> + * + * Permission to use, copy, modify, distribute, and sell this software and its + * documentation for any purpose is hereby granted without fee, provided that + * the above copyright notice appear in all copies and that both that + * copyright notice and this permission notice appear in supporting + * documentation. No representations are made about the suitability of this + * software for any purpose. It is provided "as is" without express or + * implied warranty. + */ + +#include "pow2.h" + +#include <limits.h> + +int +i_log2 (size_t x) +{ + /* -1 works best for to_pow2. */ + if (!x) + return -1; + + /* GCC 3.4 also has this. */ +# if defined __GNUC__ && __GNUC__ >= 4 || defined __clang__ + return sizeof(long) * CHAR_BIT - __builtin_clzl(x) - 1; +# else + { + unsigned bits = sizeof(x) * CHAR_BIT; + size_t mask = (size_t)-1; + unsigned result = bits - 1; + + while (bits) { + if (!(x & mask)) { + result -= bits; + x <<= bits; + } + + bits >>= 1; + mask <<= bits; + } + + return result; + } +# endif +} + +size_t +to_pow2 (size_t x) +{ + return !x ? 1 : 1 << (i_log2(x - 1) + 1); +} |