From: Joel VanderWerf <vjoel@...>
Date: 2009-05-15T06:53:17+09:00
Subject: Re: BitStruct technique

Chuck Remes wrote:
> While working on a problem this morning I came up with an interesting 
> technique for using the BitStruct gem to produce binary strings. After 
> some hacking around I discovered I didn't need to use BitStruct at all. 
> Even so I thought I would share what I learned so the technique doesn't 
> get lost.
> 
> Imagine you need to construct a binary string from some arbitrary 
> object. BitStruct allows you to do this in 3 ways: pass a binary string 
> directly, pass in a hash, or yield a block. It's easy to extend this 
> mechanism for parsing your own classes by building on the block yield 
> initialization mechanism.
> 
> hsh = {:a => 1, :b => "foo", :c => "bar"}
> 
> class C < BitStruct
>   signed :context, 32, "c id"
>   char :password, 12*8, "pw"
>   char :usr_name, 12*8, "name"
> 
>   # important to initialize these values so the
>   # block passed to #super is evaluated correctly
>   initial_value.context = 0
>   initial_value.password = ''
>   initial_value.usr_name = ''
> 
>   def initialize message
>    # parens on #super are important so no args
>    # are passed up to the parent; if this part fails
>    # then you missed
>    super() do |struct|
>     struct.context = message[:a]
>     struct.password = message[:b]
>     struct.usr_name = message[:c]
>    end
>   end
> end
> 
> c = C.new hsh  # very clean!
> c.inspect
> 
> In this example I passed in a hash as my message, but the +message+ 
> variable could have been any object that could be interrogated to 
> retrieve values for setting the bitstruct fields. I like this technique 
> because it delegates the responsibility of proper bitstruct 
> initialization to the class under construction. It nicely encapsulates 
> that operation which I believe demonstrates the Single Responsibility 
> principle.
> 
> I hope this is of use to someone someday.
> 
> cr

I think I see where you're going with that, but just so others know, the 
hash-based initialization is simple (though it does require that the 
hash keys match the field names--and avoiding this is probably the point 
of your code):

require 'bit-struct'

hsh = { :context => 1, :password => "foo", :usr_name => "bar" }

class C < BitStruct
   signed :context, 32, "c id"
   char :password, 12*8, "pw"
   char :usr_name, 12*8, "name"
end

c = C.new hsh

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407