Browse Source Download (without any required ccan dependencies)

Module:

ilog

Summary:

Integer logarithm.

Author:

Timothy B. Terriberry <tterribe@xiph.org>

Dependencies:

Description:

ilog_32() and ilog_64() compute the minimum number of bits required to store an unsigned 32-bit or 64-bit value without any leading zero bits.

This can also be thought of as the location of the highest set bit, with counting starting from one (so that 0 returns 0, 1 returns 1, and 2**31 returns 32).

When the value is known to be non-zero ilog32_nz() and ilog64_nz() can compile into as few as two instructions, one of which may get optimized out later.

STATIC_ILOG_32 and STATIC_ILOG_64 allow computation on compile-time constants, so other compile-time constants can be derived from them.

Example:

#include <stdio.h>
#include <limits.h>
#include <ccan/ilog/ilog.h>

int main(void){
  int i;
  printf("ilog32(0x%08X)=%i\n",0,ilog32(0));
  for(i=1;i<=STATIC_ILOG_32(USHRT_MAX);i++){
    uint32_t v;
    v=(uint32_t)1U<<(i-1);
    //Here we know v is non-zero, so we can use ilog32_nz().
    printf("ilog32(0x%08X)=%i\n",v,ilog32_nz(v));
  }
  return 0;
}

License:

CC0 (Public domain)