http://www.perlmonks.org?node_id=994008

wirito has asked for the wisdom of the Perl Monks concerning the following question:

Hello fellow monks, I'm storing values in a bitwise manner for speed up comparations. I can have 0 or more values in the 0..8 range, so I can store it this way: 
$bits |= (1<<$val)

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When 1 single bit is set I want to retrive the original $val. So currently I have this implemented: 
if( unpack( "%16B*", pack("n", $bits)) == 1 ) {
  my $ret = 0;
  while (!($bits & 0x1)) {
    $ret++;
    $bits >>= 1;
  };
  return $ret;
};

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I don't know why, but the while loop seems ugly. Could you enligthen me with a more perlish way to do this? PS: I hope the title is not so much confusing.

Replies are listed 'Best First'.
Re: Which bit is set? (inverse of (1<<$value) operator)
by BrowserUk (Patriarch) on Sep 17, 2012 at 11:27 UTC

    1. If only 1-bit will ever be set:

      Use a lookup table:

      %lookup = map{ 2**$_ => $_ } 0 .. 63;;

$flags = 0;;

$flags |= 1<<(rand 64);;

print "The flag set is:", $lookup{ $flags };;
The flag set is: 12

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    2. If multiple flags can be set:

      Some form of loop is necessary to avoid an impossibly large lookup table:

      $flags = int rand( 2**64 );;

print for grep $flags & 1 << $_, 0 .. 63;;
49
50
51
54
55
57
58
59

printf "%u\n", $flags;;
1066790161733386240

$t = 0; print $t += 1<<$_ for  grep $flags & 1 << $_, 0 .. 63;;
562949953421312
1688849860263936
3940649673949184
21955048183431168
57983845202395136
202099033278251008
490329409429962752
1066790161733386240

      [download]
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[d/l]
[select]

      Many thanks!

      The lookup idea is nice and clean. I will use it as I only have 9 different bits.

      However, your second option is the one I was (foolish) looking for. In my code, this could be the line:

      # return the first less significant bit set.
return $_ for grep { $bit & 1<<$_ } (0..8)';

      [download]
[reply]
[d/l]
Re: Which bit is set? (inverse of (1<<$value) operator)
by grizzley (Chaplain) on Sep 17, 2012 at 11:17 UTC
    In description of unpack there is following method to count number of bits: 
    # Count the bits in a chunk of memory (e.g. a select vector)
unpack( '%32b*', $mask );

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     Update: sorry, I misunderstood question. If you have string of bits with one bit set, then you could try just $bitstring =~ /1(0*)$/; $originalvalue = length($1)+1;
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[d/l]
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      Nope, it's a two-byte int, not a strings filled with 0's and 1's.
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