Re: 64-bit masks

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>> int first_free(uint64_t m)
>> {
>>  m = (~m) & ~((~m)-1);
>>  if (! m) return(-1);
>>  return( ((m & 0xffffffff00000000ULL) ? 32 : 0) +
>>       ((m & 0xffff0000ffff0000ULL) ? 16 : 0) +
>>       ((m & 0xff00ff00ff00ff00ULL) ?  8 : 0) +
>>       ((m & 0xf0f0f0f0f0f0f0f0ULL) ?  4 : 0) +
>>       ((m & 0xccccccccccccccccULL) ?  2 : 0) +
>>       ((m & 0x5555555555555555ULL) ?  1 : 0) );
>> }
> for m = 0x7 (binary 00000111), first_free() returns 2, which is
> wrong.

Doh!  My mistake.  0xaaaaaaaaaaaaaaaa, not 0x5555555555555555.
(Everything was working fine, except it was numbering the bits going
... 6 7 4 5 2 3 0 1 rather than ... 7 6 5 4 3 2 1 0, because I got that
mask complemented.)

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