From: Pat Patterson <Andrew.Patterson@...>
Date: 2007-03-15T02:14:23+09:00
Subject: Re: Ruby Newbie Problems with deflate, base64...

Thanks, Eden - that works great and is much cleaner than cutting the 
header and checksum off of the deflated data.

Cheers,

Pat

eden li wrote:
> You can prevent deflate from generating the header by passing in -
> MAX_WBITS to the options for Deflate.new.  The following method
> emulates gzdeflate from php:
>
> def gzdeflate(s)
>   Zlib::Deflate.new(nil, -Zlib::MAX_WBITS).deflate(s, Zlib::FINISH)
> end
>
> puts gzdeflate("Hello World").unpack('H*').first
> # => f348cdc9c95708cf2fca490100
>
> On Mar 14, 8:16 am, Pat Patterson <Andrew.Patter...@Sun.COM> wrote:
>   
>> OK - so now I know what is happening...
>>
>> Zlib::Deflate.deflate implements ZLIB compression according to RFC 1950.
>> ZLIB defines a 2 byte header containing a variety of flags and a 4 byte
>> trailer containing an Adler-32 checksum. Just out of interest, the
>> header that I'm seeing translates as 'compression method = deflate,
>> windows size = 32k, no preset dictionary, default compression level',
>> which makes perfect sense. Deflate compression itself is defined by RFC
>> 1951.
>>
>> So - if you want 'raw' deflated data (which is called for in many
>> situations), cutting off the leading 2 and trailing 4 bytes is exactly
>> what you need to do.
>>
>> Cheers,
>>
>> Pat
>>
>>
>>
>> Pat Patterson wrote:
>>     
>>> Thanks, Brian! That revealed an obvious bug in the code I was using to
>>> examine the deflated data (should have been 0.upto(deflated.length-1)).
>>>       
>>> So Base64 /is/ working correctly. Deflate prepends 2 bytes (seems to
>>> be constant 0x789c for default deflate level) and appends 4 bytes
>>> (rather than 1 and 5 as I thought) to the deflated data. When I cut
>>> those off, I can get Ruby to work the same as PHP.
>>>       
>>> Still - it would be nice if deflate worked the same as on Java, PHP, ...
>>>       
>>> Cheers,
>>>       
>>> Pat
>>>       
>>> Brian Candler wrote:
>>>       
>>>> On Tue, Mar 13, 2007 at 01:21:52PM +0900, Pat Patterson wrote:
>>>>         
>>>>> While the Ruby
>>>>>           
>>>>> deflated = Zlib::Deflate.deflate( "Hello world" )
>>>>> myhex = ""
>>>>> 1.upto(deflated.length) { |i| myhex << "%02x" % deflated[i] }
>>>>> puts myhex
>>>>>           
>>>>> Shows
>>>>>           
>>>>> 9cf348cdc9c95728cf2fca49010018ab043d00
>>>>>           
>>>>> (BTW - if anyone knows a more succinct way to hex encode a string in
>>>>> Ruby, that would be useful)
>>>>>           
>>>> str = "\001\377"
>>>> puts str.unpack("H*")
>>>>         
>> --
>> Pat Patterson - pat.patter...@sun.com
>> Federation Architect,
>> Sun Microsystems, Inc.http://blogs.sun.com/superpat
>>     
>
>
>   

-- 
Pat Patterson - pat.patterson@sun.com
Federation Architect,
Sun Microsystems, Inc.
http://blogs.sun.com/superpat