Cyclic redundancy check (CRC32) of a file


#1

Hello,

I try to calculate the CRC32 of a file with a script ruby, but I does
not obtain the good result by comparison with many of the other software
which already make this (in other programming language…), for example:
http://www.codeproject.com/KB/recipes/crc32.aspx

I use this script :


require ‘zlib’
filetest = File.read ‘image.jpg’
puts Zlib.crc32(filetest, 0).to_s(16).upcase


The result looks like well of an CRC32 but it is never good…

I also try a pure ruby implementation with this code but it makes
exactly the zlib.crc32 result… :


def crc32©
n = c.length
r = 0xFFFFFFFF
n.times do |i|
r ^= c[i]
8.times do
if (r & 1)!=0
r = (r>>1) ^ 0xEDB88320
else
r >>= 1
end
end
end
r ^ 0xFFFFFFFF
end

filetest = File.read ‘image.jpg’
puts crc32(filetest).to_s(16).upcase


Thank you very much to the person who will have the solution of my
problem.


#2

Le 23 février 2009 à 10:44, Paul Golea a écrit :

Thank you very much to the person who will have the solution of my
problem.

If you are under Windows, you need to open the file in binary mode.
(Well, arguably, you always need to open binary files in binary mode).

For instance :

Z:>ruby -v
ruby 1.8.6 (2007-09-24 patchlevel 111) [i386-mswin32]

Z:>irb

require “zlib”
=> true

f = File.read(‘bibi.jpg’) ; nil
=> nil

Zlib.crc32(f,0).to_s(16)
=> “332c7941”

f = nil
=> nil

File.open(‘bibi.jpg’, ‘rb’) { |h| f = h.read } ; nil
=> nil

Zlib.crc32(f,0).to_s(16)
=> “5ba68c3c”

And the second CRC corresponds to what other tools give me.

Fred


#3

F. Senault wrote:

And the second CRC corresponds to what other tools give me.

Fred

Excellent!
I didn’t know that File.read opened “badly” files under Windows…

Thank you very much.


#4

On Feb 23, 2009, at 5:31 AM, Paul Golea wrote:

F. Senault wrote:

And the second CRC corresponds to what other tools give me.

Fred

Excellent!
I didn’t know that File.read opened “badly” files under Windows…

Thank you very much.

Change this:
f = File.read(‘bibi.jpg’) ; nil

to this: (mode of ‘r’, not ‘rb’)
f = nil
File.open(‘bibi.jpg’, ‘r’) { |h| f = h.read } ; nil

And you’ll get the same result. File.read is not the problem, but the
use of text mode (which is the default). Under Unix, there is no
difference between text mode and binary mode, but the line endings
under Windows are \015\012.

-Rob

Rob B. http://agileconsultingllc.com
removed_email_address@domain.invalid