BitStruct technique


#1

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, 128, “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


#2

Chuck R. wrote:

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]

If you add this line here:

   yield struct if block_given?

then the block initialization can still be used by subclasses or by the
caller of #new:

c = C.new hsh do |struct|
struct.context = 2
end


#3

Chuck R. wrote:

initialization mechanism.
initial_value.context = 0
struct.usr_name = message[:c]
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, 128, “pw”
char :usr_name, 12
8, “name”
end

c = C.new hsh


#4

On May 14, 2009, at 4:53 PM, Joel VanderWerf wrote:

Chuck R. wrote:

I think I see where you’re going with that, but just so others know,
char :password, 128, “pw”
char :usr_name, 12
8, “name”
end

c = C.new hsh

Joel,

that’s right. I probably shouldn’t have used a hash as my example
since you already have hash-based initialization built in. I meant for
this to be a nice way to pass an arbitrary object to the constructor
so the logic of getting data from that object would be encapsulated in
one spot. I also could have built a temporary hash from the object
argument and passed that to the superclass’ constructor but I prefer
the block-based initialization for readability.

And thanks for creating such a neat library. I don’t have a need for
it now that I understand my problem domain better, but rest assured it
is now a member of my toolbox for some future problem when I do need it.

cr