Category : C Source Code
Archive   : E_C_STR.ZIP
Filename : FFS.C

/* File : ffs.c
Author : Richard A. O'Keefe.
Updated: 20 April 1984
Defines: ffs(), ffc()

ffs(i) returns the index of the least significant 1 bit in i,
where 1 means the least significant bit and 32 means the
most significant bit, or returns -1 if i is 0.

ffc(i) returns the index of the least significant 0 bit in i,
where 1 means the least significant bit and 32 means the
most significant bit, or returns -1 if i is ~0.

These functions mimic the VAX FFS and FFC instructions, except that
the latter return much more sensible values. This file only exists
to make it easier to move 4.2bsd programs to System III (which is
rather like moving up from a Rolls Royce to a model T Ford), and so
I haven't bother with assembly code versions.
*/

#include "strings.h"

int ffs(i)
register int i;
{
register int N;

for (N = 8*sizeof(int); --N >= 0; i >>= 1)
if (i&1) return 8*sizeof(int)-N;
return -1;
}

int ffc(i)
register int i;
{
register int N;

for (N = 8*sizeof(int); --N >= 0; i >>= 1)
if (!(i&1)) return 8*sizeof(int)-N;
return -1;
}