From: Robert Klemme <shortcutter@...>
Date: 2007-06-08T01:50:09+09:00
Subject: Re: best DRY solution for this task

On 06.06.2007 22:12, Ball, Donald A Jr (Library) wrote:
>> so how can I do this check and outputting better? Because 
>> there are many 
>> bytes with many different meanings and as said sometimes I 
>> need to read 
>> in 4 bytes then just one then again 2 bytes... is this at all 
>> possible 
>> to do dry-like?
> 
> Without knowing the details of the file you're trying to parse it's hard
> to say, but you might be well-served to write a simple finite state
> machine. E.g. something like:
> 
> mode = :expect_header
> while !(mode == :done || mode == :fail)
>   case mode
>     when :expect_header
>       if target.read(3) == $startflag
>         mode = :expect_debug
>       else
>         mode = :fail
>       end
>     when :expect_debug
>       case target.read(1)
>         when "\x1"
>           debug = true
>           mode = :expect_body
>         when "\x255"
>           debug = nil
>           mode = :expect_body
>         else
>           mode = :fail
>       end
>   end
> end

You can even nest cases

mode = :expect_header
while !(mode == :done || mode == :fail)
   case mode
     when :expect_header
       mode = case target.read(3)
         when "\x1\x3d\x0"
           :expect_debug
         else
           :fail
         end
     when :expect_debug
       mode = case target.read(1)
         when "\x1"
           debug = true
           :expect_body
         when "\xff"
           debug = nil
           :expect_body
         else
           :fail
         end
       end
   end
end

If you use nil as :fail then it becomes even simpler because you can 
omit "else" branches in "case".  Another alternative is to throw 
immediately when an error occurs.

Just some more food for experimening.

Kind regards

	robert