= Packable Library - Intro

If you need to do read and write binary data, there is of course <tt>Array::pack</tt> and <tt>String::unpack</tt>.
The packable library makes (un)packing nicer, smarter and more powerful.
In case you are wondering why on earth someone would want to do serious (un)packing when YAML & XML are built-in:
I wrote this library to read and write FLV files...

== Feature summary:

=== Explicit forms
Strings, integers & floats have long forms instead of the cryptic letter notation. For example:
    ["answer", 42].pack("C3n")
can be written as:
    ["answer", 42].pack({:bytes => 3}, {:bytes => 2, :endian => :big})
This can look a bit too verbose, so let's introduce shortcuts right away:
=== Shortcuts
Most commonly used options have shortcuts and you can define your own. For example:
    :unsigned_long  <===> {:bytes => 4, :signed => false, :endian => :big}
=== IO
IO classes (File & StringIO) can use (un)packing routines.
For example:
    signature, block_len, temperature = my_file >> [String, :bytes=>3] >> Integer >> :float
The method +each+ also accepts packing options:
    StringIO.new("\000\001\000\002\000\003").each(:short).to_a  ===>  [1,2,3]
=== Custom classes
It's easy to make you own classes (un)packable. All the previous goodies are thus available:
		File.open("great_flick.flv") do |f|
			head = f.read(FLV::Header)
			f.each(FLV::Tag) do |tag|
				# do something meaningful with each tag...
			end
		end

=== Filters
It's also easy to define special shortcuts that will call blocks to (un)pack any classe.
As an example, this could be useful to add special packing features to String (without monkey patching String::pack).

== Installation

First, ensure that you're running at least RubyGems 1.2 (check <tt>gem --version</tt> if you're not sure -- to update: <tt>sudo gem update --system</tt>).

Add GitHub to your gem sources (if you haven't already):

  sudo gem sources -a http://gems.github.com

Get the gem:

  sudo gem install marcandre-packable
  
That's it! Simply <tt>require 'packable'</tt> in your code to use it.

Compatibility: Ruby 1.8, 1.9

= Documentation

== Packing and unpacking

The library was designed to be backward compatible, so the usual packing and unpacking methods still work as before.
All packable objects can also be packed directly (no need to use an array). For example:

  42.pack("n")  ===>  "\000*"

In a similar fashion, unpacking can done using class methods:

  Integer.unpack("\000*", "n")  ===>  42

== Formats

Although the standard string formats can still be used, it is possible to pass a list of options (see example in feature summary).
These are the options for core types:

=== Integer
[+bytes+]     Number of bytes (default is 4) to use.
[+endian+]    Either <tt>:big</tt> (or :network, default) or <tt>:little</tt>.
[+signed+]    Either +true+ (default) or not. This will make a difference only when unpacking.

=== Float
[+precision+] Either <tt>:single</tt> (default) or <tt>:double</tt>.
[+endian+]    Either <tt>:big</tt> (or :network, default) or <tt>:little</tt>.

=== String
[+bytes+]     Total length (default is the full length)
[+fill+]      The string to use for filling when packing a string shorter than the specified bytes option. Default is a space.

=== Array
[+repeat+]    This option can be used (when packing only) to repeat the current option. A value of <tt>:all</tt> will mean for all remaining elements of the array.

When unpacking, it is necessary to specify the class in addition to any option, like so:

  "AB".unpack(Integer, :bytes => 2, :endian => :big, :signed => false)  ===>  0x3132

== Shortcuts and default values

It's easy to add shortcuts for easier (un)packing:

	String.packers.set :flv_signature, :bytes => 3, :fill => "FLV"

  "x".pack(:flv_signature)  ===>  "xFL"

Two shortcut names have special meanings: +default+ and +merge_all+. +default+ specifies the options to use when
nothing is specified, while +merge_all+ will be merged with all options. For example:

  String.packers do |p|
    p.set :merge_all, :fill => "*"	# Unless explicitly specified, :fill will now be "*"
    p.set :default, :bytes => 8     # If no option is given, this will act as default
  end

  "ab".pack  ===>  "ab******"
  "ab".pack(:bytes=>4)  ===>  "ab**"
  "ab".pack(:fill => "!")  ===>  "ab"     # Not "ab!!"

A shortcut can refer to another shortcut, as so:

  String.packers do |p|
		p.set :creator, :bytes => 4
		p.set :app_type, :creator
	end
  "hello".pack(:app_type)  ===>  "hell"

The following shortcuts and defaults are built-in the library:

=== Integer
  :merge_all      =>   :bytes=>4, :signed=>true, :endian=>:big
  :default        =>   :long
  :long           =>   {}
  :short          =>   :bytes=>2
  :byte           =>   :bytes=>1
  :unsigned_long  =>   :bytes=>4, :signed=>false
  :unsigned_short =>   :bytes=>2, :signed=>false
                       
=== Float              
  :merge_all      =>   :precision => :single, :endian => :big
  :default        =>   :float
  :double         =>   :precision => :double
  :float          =>   {}
                       
=== String             
  :merge_all      =>   :fill => " "

== Files and StringIO

All IO objects (in particular files) can deal with packing easily. These examples will all return an array with 3 elements (a string, an integer and another string):

  io >> :flv_signature >> Integer >> [String, {:bytes => 8}]
  io.read(:flv_signature, Integer, [String, {:bytes => 8}])
  io.read(:flv_signature, Integer, String, {:bytes => 8})
  [io.read(:flv_signature), io.read(Integer), io.read(String, :bytes => 8)]

In a similar fashion, these have the same effect although the return value is different

  io << "x".pack(:flv_signature) << 66.pack << "Hello".pack(:bytes => 8)  # returns io
  io << ["x", 66, "Hello"].pack(:flv_signature, {} , {:bytes => 8})       # returns io
  io.write("x", :flv_signature, 66, "Hello", {:bytes => 8})               # returns the # of bytes written
  io.packed << ["x",:flv_signature] << 66 << ["Hello", {:bytes => 8}]     # returns a "packed io"

The last example shows how <tt>io.packed</tt> returns a special IO object (a packing IO) that will pack arguments before writing it.
This is to insure compatibility with the usual behavior of IO objects:
	io << 66  ==> appends "66"
	io.packed << 66  ==> appends "\000\000\000B"

We "cheated" in the previous example; instead of writing <tt>io.packed.write(...)</tt> we used the shorter form.
This works because we're passing more than one argument; for only one argument we must call <tt>io.packed.write(66)</tt>
less the usual +write+ method is called.

Since the standard library desn't define the <tt>>></tt> operator for IO objects, we are free to use either <tt>io.packed</tt> or <tt>io</tt> directly.
Note that reading one value only will return that value directly, not an array containing that value:

  io.read(Integer)  ===>  42, not [42]
  io.read(Integer,Integer)  ===>  [42,43]
  io << Integer     ===>  [42]

== Custom classes

Including the mixin +Packable+ will make a class (un)packable. Packable relies on +write_packed+
and unpacking on +read_packed+. For example:

  class MyHeader < Struct.new(:signature, :nb_blocks)
    include Packable
  
    def write_packed(packedio, options)
      packedio << [signature, {:bytes=>3}] << [nb_blocks, :short]
    end

    def self.read_packed(packedio, options)
      h = MyHeader.new
      h.signature, h.nb_blocks = packedio >> [String, {:bytes => 3}] >> :short
      h
    end
  end

We used the argument name +packedio+ to remind us that these are packed IO objects, i.e.
they will write their arguments after packing them instead of converting them to string like normal IO objects.
With this definition, +MyHeader+ can be both packed and unpacked:

  h = MyHeader.new("FLV", 65)
  h.pack  ===>  "FLV\000A"
  StringIO.new("FLV\000A") >> Signature  ===>  [a copy of h]

A default <tt>self.read_packed</tt> is provided by the +Packable+ mixin, which allows you to define +read_packed+ as
an instance method instead of a class method. In that case, +read_packed+ instance method is called with
the same arguments and should modify +self+ accordingly (instead of returning a new object).
It is not necessary to return +self+. The previous example can thus be shortened:

  class MyHeader
    #...
    def read_packed(packedio, options)
      self.signature, self.nb_blocks = packedio >> [String, {:bytes => 3}] >> :short
    end
  end

== Filter

Instead of writing a full-fledge class, sometimes it can be convenient to define a sort of wrapper we'll call filter. Here's an example:

  String.packers.set :length_encoded do |packer|
    packer.write  { |packedio| packedio << length << self }
    packer.read   { |packedio| packedio.read(packedio.read(Integer)) }
  end

	"hello!".pack(:length_encoded)  ===>  "\000\000\000\006hello!"
	["this", "is", "great!"].pack(*[:length_encoded]*3).unpack(*[:length_encoded]*3)  ===>  ["this", "is", "great!"]

Note that the +write+ block will be executed as an instance method (which is why we could use +length+ & +self+),
while +read+ is a normal block that must return the newly read object.

== Inheritance

A final note to say that packers are inherited in some way. For instance one could define a filter for all objects:

  Object.packers.set :with_class do |packer|
    packer.write { |io| io << [self.class.name, :length_encoded] << self }
    packer.read  do |io|
      klass = eval(io.read(:length_encoded))
      io.read(klass)
    end
  end
  
  [42, MyHeader.new("Wow", 1)].pack(:with_class, :with_class).unpack(:with_class, :with_class) ===> [42, MyHeader.new("Wow", 1)]

= License

packable is licensed under the terms of the (modified) BSD License, see the included LICENSE file.

Author::                 Marc-André Lafortune