From: Stefan Lang <langstefan@...>
Date: 2006-07-29T18:40:56+09:00
Subject: Re: How to md5 a file?

On Saturday 29 July 2006 10:31, Ben Johnson wrote:
> Ben Johnson wrote:
> > Basically I want to generate an md5 hash from considerably large
> > files to determine if they are exactly the same. Is there a
> > better way to do this besides comparing md5 hashes?
> >
> > Thanks for your help.
>
> I neglected to include some neccessary details, sorry about that.
> Basically the reason I want to do this is so I can store the md5 in
> the database and determine if I have come across this file before.
> So when I receive the file again I can md5 it, query my db, and if
> its in my db I know I've come across this file before.

There are basically two options.

1. Read in the whole file, generate hash:

    require 'digest/md5'

    Digest::MD5.digest(File.read("data")) => string with binary hash
    Digest::MD5.hexdigest(File.read("data")) => string with 
hexadecimal digits

2. Read block-wise, save memory ;)

    require 'digest/md5'

    md5 = Digest::MD5.new
    md5.update("chunk of data")
    md5.update("another chunk of data")

    md5.digest # => string with binary hash
    md5.hexdigest # => string with hexadecimal digits

Hope that helps,
  Stefan