calculating the 3 MSBs from an integer

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

cheers brothers and sisters,

I have this sub to calculate the 3 most significant bits based on an integer. The integer I get is the integer value of the TOS byte, a value which is retreived over SNMP. I have to find the TOS Precedence, and that is the sum of the 3 MSBs.

The sub I have works, but laziness seems to strike everyone eventually, I was wondering if there are easier or better ways to do it? I'm just always curious about how someone else would do it ;-)

the code:

sub calc_tosprecedence {
    my $int_val  = shift;
    my $bit = 7;
    my @b = (0,0,0,0,0,0,0,0);
    while ($int_val > 0 ) {
        $rest = 1 if $int_val - 2**$bit >= 0 ;
        $rest = 0 if $int_val - 2**$bit < 0 ;
        $int_val = $int_val - 2**$bit if $rest;
        $b[$bit-- ] = $rest;
    }
    @b = reverse @b;
    my @prec = @b[0 .. 2];
    $sum = 0;
    @prec = reverse @prec;
    $sum += $prec[$_] * 2**$_ for (0 .. $#prec);
    return $sum
}
[download]

The code basically:

gets an integer as argument

converts it to a binary

takes the 3 last (MSB) bits from it as a binary

calculates the integer value of that binary

Thanks..

--
to ask a question is a moment of shame
to remain ignorant is a lifelong shame

Comment on calculating the 3 MSBs from an integer Download Code

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Re: calculating the 3 MSBs from an integer by borisz (Canon) on Feb 17, 2005 at 13:00 UTC
Here is some code, that does the same as yours. `sub calc_tosprecedence{ use integer; return ( $_[0] & 0xff ) >> 5; }` [download] Boris	[reply] [d/l]
Re^2: calculating the 3 MSBs from an integer by Random_Walk (Prior) on Feb 17, 2005 at 14:05 UTC
I think I have understood this correctly but please put me right if I am barking up the wrong tree `$_[0] & 0xff # A bitmask to gets the last (8 bit) byte from the intege +r. # It has exactly the same result as $_[0] % 256 >> 5 # bitshift the result of the above so all but # the 3 MSB drop off the end.` [download] the return automagically maps this to the decimal for the three remaining bits. Was use integer necessary for this or does it improve efficiency ? Here is a one liner to dump what is going on `perl -le'print join "\t", $_, (unpack "B", pack "I", $_),($_ & 0xFF) + >> 5 for (0..256)'` [download] and some of the output for the interesting regions 0 00000000000000000000000000000000 0 1 00000000000000000000000000000001 0 2 00000000000000000000000000000010 0 . . 30 00000000000000000000000000011110 0 31 00000000000000000000000000011111 0 32 00000000000000000000000000100000 1 33 00000000000000000000000000100001 1 . . 222 00000000000000000000000011011110 6 223 00000000000000000000000011011111 6 224 00000000000000000000000011100000 7 225 00000000000000000000000011100001 7 . . 255 00000000000000000000000011111111 7 256 00000000000000000000000100000000 0 [download] Cheers, R. Pereant, qui ante nos nostra dixerunt!	[reply] [d/l] [select]
Re^3: calculating the 3 MSBs from an integer by borisz (Canon) on Feb 17, 2005 at 14:13 UTC
You are right on everything. use integer is not needed, I do it always since shifts are done sign wo integer and unsigned with use integer. In this case it does not matter. Boris	[reply]
Re^3: calculating the 3 MSBs from an integer by insaniac (Friar) on Feb 17, 2005 at 14:40 UTC
thanks for the explanation, but my output is like this: `0 00000000000000000000000000000000 0 1 00000001000000000000000000000000 0 2 00000010000000000000000000000000 0 3 00000011000000000000000000000000 0 ..` [download] the binary forms are "a bit" akward, no? I just copy pasted your command line.. and why is the piece: `$_ & 0xFF` needed? without it, the code seems to work as well... or am i missing something mayor overhere? anyhow.. now i know i shouldn't have skipped the chapters about bitwise operators :-D -- to ask a question is a moment of shame to remain ignorant is a lifelong shame	[reply] [d/l] [select]
Re^4: calculating the 3 MSBs from an integer by Random_Walk (Prior) on Feb 17, 2005 at 15:27 UTC
Re^5: calculating the 3 MSBs from an integer by insaniac (Friar) on Feb 17, 2005 at 21:14 UTC
Some notes below your chosen depth have not been shown here
Re^2: calculating the 3 MSBs from an integer by insaniac (Friar) on Feb 17, 2005 at 13:03 UTC
wow... dr00l ... and thanx a bunch!!! this is truely superlazy... thanks for the insights -- to ask a question is a moment of shame to remain ignorant is a lifelong shame	[reply]
Re: calculating the 3 MSBs from an integer by spurperl (Priest) on Feb 18, 2005 at 10:26 UTC
In general, it may be useful to know what "MSB" means, because it depends on the machine. The following should be a simple and portable method of converting any unsigned integer to its binary representation: `sub dec2bin { return sprintf("%b", $_[0]); }` [download]	[reply] [d/l]
Re^2: calculating the 3 MSBs from an integer by insaniac (Friar) on Feb 18, 2005 at 11:57 UTC
yeah true... but it seems the bitshifting works correctly on whatever OS, so it's just the visual representation that can be a bit akward... but that was not a requirement ;-) thanks for the info.. -- to ask a question is a moment of shame to remain ignorant is a lifelong shame	[reply]