GIF 画像解析するやつ（書きかけ）

gif.rb

# -*- coding: utf-8 -*-

class GIF
  class ImplementationError < StandardError
  end

  def self.parse(arg)
    if arg.is_a? File
      new(arg).parse
    elsif File.exists?(arg)
      new(open(arg, 'rb')).parse
    else
      raise Errno::ENOENT
    end
  end

  HEADER = [
            :signature,
            :version,
            :logical_screen_width,
            :logical_screen_height,
            :global_color_table_flag,
            :color_resolution,
            :sort_flag,
            :size_of_global_color_table,
            :background_color_index,
            :pixel_aspect_ratio,
            :global_color_table,
           ]
  attr_accessor *HEADER

  def initialize(file)
    @file = file
    @file.singleton_class.send(:include, IOExtension)
  end

  def parse
    parse_header
    self
  end

  def parse_header
    @file.seek(0, IO::SEEK_SET)

    @signature,
    @version,
    @logical_screen_width,
    @logical_screen_height,
    bit,
    @background_color_index,
    @pixel_aspect_ratio = @file.unpack('a3a3vvCCC')

    @global_color_table_flag    = bit & 0b10000000 != 0
    @color_resolution           = ((bit >> 4) & 0b0111) + 1
    @sort_flag                  = bit & 0b1000 != 0
    @size_of_global_color_table = ((bit & 0b00000111) + 1) ** 2

    if @global_color_table_flag
      @global_color_table = (0..@size_of_global_color_table).map { @file.unpack('CCC') }
    end
  end

  module IOExtension
    def unpack(template)
      read(unpack_size(template)).unpack(template)
    end

    private
    def unpack_size(template)
      template.scan(/\w\d+|\w/).inject(0) do |total, value|
        format = value.slice(0, 1)
        size = (value.slice(1) || 1).to_i
        case format.to_sym
        when :a, :C
          total + 1 * size
        when :v
          total + 2 * size
        else
          raise ImplementationError
        end
      end
    end
  end
end

if $0 == __FILE__
  file = ARGV.shift
  gif = GIF.parse(file)
  p gif
end

nyan.gif