http://www.perlmonks.org?node_id=944804

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

I have been tasked to convert some C# code to Perl. I think I need pack and unpack and I am having some trouble understanding them well enough. I have read the tutorial and have worked though some of the examples, but still I am unable to get the right results.

The are where I have not been able to get right is with the byte casting of the result of the ^ operand. When I print out $mRc ^ $byte I get 4294965887 which I need to be 127.

Perl Code 
 $mPolyTable[(unpack "V", pack "U4", ($mCrc ^ $byte))] ;

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C# code 
mCrc = mPolyTable[(byte)(mCrc ^ iByteArray[x])];

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Any help would be greatly appreciated.

Replies are listed 'Best First'.
Re: pack and unpack trouble
by ikegami (Patriarch) on Dec 22, 2011 at 16:28 UTC 
    $mCrc = $mPolyTable[ ( $mCrc ^ $iByteArray[$x] ) & 0xFF ];

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    The closest pack equivalent to a (byte)val would be 

    unpack('C', substr(pack('J', $val), 0, 1))

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    or 

    unpack('C', substr(pack('J', $val), -1, 1))

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    depending on your machine. That could be made portable as 

    unpack('C', substr(pack('J<', $val), 0, 1))

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    which simplifies to 

    unpack('C', pack('J<', $val))

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    But what a waste.

    Update: Added the content underneath the bar.

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[d/l]
[select]
      Excellent! as a follow up, if I want to take 1365238130 to 515FE172? Upate: I would have thought it was hex(1365238130) but that gets me 83301204272. So clearly I am doing something wrong.
      In C# 
      uint temp = 1365238130;
System.Console.WriteLine(temp.ToString("X"));

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[d/l]
        Adapted from perlfaq4: How do I convert between numeric representations/bases/radixes? 
        my $hex_string = sprintf '%X', 1365238130;

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[d/l]

        hex converts *from* hex to a number.

        sprintf '%X', $n will represent a number using hex.

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