encoding message

关于practicingruby

practicingruby是Gregory Brown定期撰写的关于ruby编程的文章。很有深度，欢迎大家订阅。

The anatomy of information in software systems

9月份的文章是关于如何把消息编码。内容如下的IRC消息，应该如何解释

PRIVMSG #practicing ruby testing :Seasons greetings to you all!\r\n

在ruby里可以表示为

["PRIVMSG", "#practicing ruby testing", "Seasons greetings to you all!"]

序列化为字符串

使用Msgpack序列化上面的消息,得到的HEX是

93 a7 50 52 49 56 4d 53 47 b8 23 70 72 61 63 74 69 63 69 6e 67 2d 72 75 62 
79 2d 74 65 73 74 69 6e 67 bd 53 65 61 73 6f 6e 73 20 67 72 65 65 74 69 6e 
67 73 20 74 6f 20 79 6f 75 20 61 6c 6c 21

In Ruby:

"\x93\xA7PRIVMSG\xB8#practicing-ruby-testing\xBDSeasons greetings to you all!"

分解它的结构得到：

其中93, A7, B8, BD有特定含义, 实际上A0到BF表示的是0到31 byte的string

>> 0xA7-0xA0
=> 7
>> "PRIVMSG".size
=> 7

>> 0xB8-0xA0
=> 24
>> "#practicing-ruby-testing".size
=> 24

>> 0xBD-0xA0
=> 29
>> "Seasons greetings to you all!".size
=> 29

结构化

不仅是得到字符串数组，我们还要明确每个部分的意义，可以用MessagePack’s application-specific type

分解后得到的是

ruby decode 代码

data_types = { 1 => CommandName, 2 => Parameter, 3 => MessageBody }

command = MessagePackDecoder.unpack(raw_bytes, data_types)
#  [ CommandName <"PRIVMSG">, 
#    Parameter   <"#practicing-ruby-testing">, 
#    MessageBody <"Season greetings to you all!"> ]

handler decode的例子代码

class Parameter
  def initialize(byte_array)
    @text = byte_array.pack("C*")

    raise ArgumentError if @text.include?(" ")
  end

  attr_reader :text
end

使用语法规则定义结构

上面的方法虽然让计算机容易理解消息的结构，但让人去理解就困难了。

如果使用Augmented Backus–Naur Form语法规则，可以表示如下

message    =  command [ params ] crlf
command    =  1*letter / 3digit
params     =  *14( SPACE middle ) [ SPACE ":" trailing ]
           =/ 14( SPACE middle ) [ SPACE [ ":" ] trailing ]

nospcrlfcl =  %x01-09 / %x0B-0C / %x0E-1F / %x21-39 / %x3B-FF
                ; any octet except NUL, CR, LF, " " and ":"

middle     =  nospcrlfcl *( ":" / nospcrlfcl )
trailing   =  *( ":" / " " / nospcrlfcl )

SPACE      =  %x20        ; space character
crlf       =  %x0D %x0A   ; "carriage return" "linefeed"
letter     =  %x41-5A / %x61-7A       ; A-Z / a-z
digit      =  %x30-39                 ; 0-9

在ruby，我们用citrus

grammar IRC
  rule message
    (command params? endline) {
      { :command => capture(:command).value,
        :params  => capture(:params).value }
    }
  end

  rule command
    letters | three_digit_code 
  end

  rule params
    ( ((space middle)14*14 (space ":"? trailing)?) |
      ((space middle)*14 (space ":" trailing)?) ) {
      captures.fetch(:middle, []) + captures.fetch(:trailing, [])
    }
  end

  rule middle
    non_special (non_special | ":")*
  end

  rule trailing
    (non_special | space | ":")+
  end

  rule letters
    [a-zA-Z]+
  end

  rule three_digit_code
    /\d{3}/ { to_str.to_i }
  end

  rule non_special
    [^\0:\r\n ]
  end

  rule space
    " "
  end

  rule endline
    "\r\n"
  end
end


require 'citrus'
Citrus.load('irc')

msg = "PRIVMSG #practicing-ruby-testing :Seasons greetings to you all!\r\n"

data = IRC.parse(msg).value

p data[:command] 
#=> "PRIVMSG"

p data[:params]
#=> ["#practicing-ruby-testing", "Seasons greetings to you all!"]