How can I replicate Perl's unpack functionality in C#?

Question

Hi,

I am trying to recreate a Perl script in C# but have a problem creating a checksum value that a target system needs.

In Perl this checksum is calculated using the unpack function:

while (<PACKAGE>) {
    $checksum += unpack("%32C*", $_);
}
$checksum %= 32767;
close(PACKAGE);

where PACKAGE is the .tar file input stream

I need to replicate this in C# but can't find a means of replicating that unpack function.

All help appreciated!

(I know there are much better checksum calculations available but can't change target system so can't change calculation)

Answer 1

There seems to be a library in Mono called DataConvert that was written to provide facilities similar to Perl's pack/unpack. Does this do what you need?

Answer 2

Perkl's unpack is described here and here. From that you should be able to write an equivalent in C#.

Answer 3

To supplement Mitch Wheat's comment, here's a Java implementation (which does a single block only). I'm sure you'll find a way to convert it into C#, and to do multiple blocks.

int sum = 0;
for (byte b : buffer) {
    sum += (int) b & 255;
}
return sum % 32767;

Hope this helps!

Answer 4

In my testing here, unpack with %32C appears to be the additive sum of the bytes, limited to 32 bits.

print unpack("%32C*", 'A');
65
print unpack("%32C*", 'AA');
130

It shouldn't be hard to reproduce that.