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util: New function log_2_floor().

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Calculates the position of the most significant bit in a 32 bit
word.
This commit is contained in:
Ben Pfaff
2011-07-22 10:20:52 -07:00
committed by Ethan Jackson
parent 897b9afc12
commit 711e0157cf
5 changed files with 96 additions and 0 deletions
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35
lib/util.c
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@@ -16,8 +16,11 @@
#include <config.h>
#include "util.h"
#include <assert.h>
#include <errno.h>
#include <limits.h>
#include <stdarg.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
@@ -607,3 +610,35 @@ english_list_delimiter(size_t index, size_t total)
: total > 2 ? ", and "
: " and ");
}
/* Given a 32 bit word 'n', calculates floor(log_2('n')). This is equivalent
* to finding the bit position of the most significant one bit in 'n'. It is
* an error to call this function with 'n' == 0. */
int
log_2_floor(uint32_t n)
{
assert(n);
#if !defined(UINT_MAX) || !defined(UINT32_MAX)
#error "Someone screwed up the #includes."
#elif __GNUC__ >= 4 && UINT_MAX == UINT32_MAX
return 31 - __builtin_clz(n);
#else
{
int log = 0;
#define BIN_SEARCH_STEP(BITS) \
if (n >= (1 << BITS)) { \
log += BITS; \
n >>= BITS; \
}
BIN_SEARCH_STEP(16);
BIN_SEARCH_STEP(8);
BIN_SEARCH_STEP(4);
BIN_SEARCH_STEP(2);
BIN_SEARCH_STEP(1);
#undef BIN_SEARCH_STEP
return log;
}
#endif
}