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

Hi,
Given an unsigned 32 bit value (set initially to zero) and a 32 bit mask.
How would I increment just the masked bits through all their possible combinations, without trying the entire 32 bit range.
e.g. given a mask of 0x0f000007
it'll go through

0x00000000
0x00000001
..
0x00000007

[download]

then

0x01000000
0x01000001
0x01000002
..
0x01000007
..
0x02000000
0x02000001
..

[download]

and so on till 

0x0F000007

[download]

Is there a good and quick way of doing this in perl?

Thanks!

20051110 Janitored by Corion: Removed PRE tags, put in proper formatting

Replies are listed 'Best First'.
Re: Incrementing only the masked bits (sure)
by tye (Sage) on Nov 10, 2005 at 07:30 UTC 
    my $mask= 0x0f000007;
my $val= $mask;
do {
    printf "0x%08.8x\n", $val;
    --$val;
    $val &= $mask;
} while( $val != $mask );

    [download]

    - tye        

[reply]
[d/l]

      Use:

          printf "0x%08.8x\n", (~$val & $mask);

      [download]

      to count up.

      Perl is Huffman encoded by design.
[reply]
[d/l]
      Thanks, I didn't think of decrementing :).

      Definitely does what I need, though it's not what I asked for. hehe.

      p.s. Sorry about the pre thing.

[reply]
Re: Incrementing only the masked bits
by Roy Johnson (Monsignor) on Nov 10, 2005 at 16:42 UTC
    Here's an incrementing solution, inspired by tye's (but not simply doing the decrement and then subtracting from $mask, which would also work). 
    my ($val, $mask) = (-1, 0x01000007);
     
do {
    $val += ~$mask+1;
    $val &= $mask;
    printf "0x%08.8x\n", $val;
} while ($val < $mask);

    [download]
    Caution: Contents may have been coded under pressure.
[reply]
[d/l]

      My version actually started out in my head closer to: 

      my $mask= 0x0f000007;
my $val= 0;
do {
    printf "0x%08.8x\n", $val & $mask;
    $val |= ~$mask;
    ++$val;
} while( $val <= ~0 );

      [download]

      but in typing it I decided to simplify. (:

      - tye        

[reply]
[d/l]
Re: Incrementing only the masked bits
by gaal (Parson) on Nov 10, 2005 at 07:27 UTC
    I don't know of an immediate way to do this.

    If you have k bits sets in your mask, the whole iteration has 2**k values. So what you're looking for is a function n_to_masked($mask, $n) that maps a number from 0 .. 2**k-1 to a 32-bit value that fits in your mask. I don't have a really clever way of doing this other than shifting bits left and right, but with $m you know exactly what bits you need to shift how much.

    Update: tye's way is muuuuch more clever. Use that. :-)

[reply]
[d/l]

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