From: Brian Candler <b.candler@...>
Date: 2009-07-22T16:46:19+09:00
Subject: Re: Byte–stream parsing in Ruby

Elliott Cable wrote:
> Hrm, #valid_encoding? is very helpful. But how can I stuff numerical 
> (`Fixnum`) bytes onto the string?

If you're doing STDIN.read(1), you get a String. Just use << to 
concatenate, or the 2-argument form of read() where you supply a buffer 
to append to.

If you are forced to use Fixnum, then try Integer#chr:

>> str = ""
=> ""
>> str << 255.chr
=> "\xFF"

Warning: you have to deal with all the (undocumented) ruby-1.9 encoding 
stupidity. Have fun guessing the behaviour of each of the methods. e.g.

>> str = "hello"
=> "hello"
>> str.encoding
=> #<Encoding:UTF-8>
>> str << 255.chr
=> "hello\xFF"
>> str.encoding
=> #<Encoding:ASCII-8BIT>

Surprised that the encoding changed? This means that:

>> str.valid_encoding?
=> true

until you do:

>> str.force_encoding("UTF-8")
=> "hello\xFF"
>> str.valid_encoding?
=> false

Now have a guess what happens if you try to append another byte. Go on.

>> str.encoding
=> #<Encoding:UTF-8>
>> str << 250.chr
Encoding::CompatibilityError: incompatible character encodings: UTF-8 
and ASCII-8BIT
  from (irb):24
  from /usr/local/bin/irb19:12:in `<main>'
>> 

Haha, fooled you. You thought it was safe to append a non-UTF8 character 
to a UTF8 string (after all, you did before quite happily), but this 
time you get an exception. So now you have to do:

>> str.force_encoding("ASCII-8BIT")
=> "hello\xFF"
>> str << 250.chr
=> "hello\xFF\xFA"
>> str.force_encoding("UTF-8")
=> "hello\xFF\xFA"
>> str.valid_encoding?
=> false

This is why I hate ruby 1.9.

Regards,

Brian.

P.S. The above example was with ruby 1.9.2 r23158 under Linux with UTF8 
locale. Behaviour may or may not be different with other 1.9.x versions 
and/or under different locale settings.
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