#!/usr/bin/ruby
def bin(str)
	ret = 0
	str.length.times { |i| ret += str[i] << (8*i) }
	ret
end

class Color
	attr_reader :r, :g, :b
	
	def initialize(fd)
		@r = bin(fd.read(1))
		@g = bin(fd.read(1))
		@b = bin(fd.read(1))
	end

	def ==(other)
		(@r == other.r) and (@g == other.g) and (@b == other.b)
	end

	def inspect
		"<#@r, #@g, #@b>"
	end
	alias :to_s :inspect
end

class GIFBlock
	attr_reader :type, :subblocks
	
	def initialize(fd)
		typeid = bin(fd.read(1))
		@type = case typeid
		when 0x01
			'plain text extension'
		when 0x2C
			'image descriptor??'
		when 0xF9
			'graphic control block'
		when 0xFE
			'comment'
		when 0xFF
			'application extension'
		else
			'0x%.2X' % typeid
		end
		
		@blocks = Array.new
		loop do
			bsz = bin(fd.read(1))
			break if bsz == 0
			@blocks << fd.read(bsz)
		end
		
	end
end

class GIFImage
	attr_reader :left, :top, :height, :width
	attr_reader :loc_colortable_flag, :interlaced, :reserved, :loc_colortable_size
	attr_reader :colortable, :extensions, :lzw_raw_data, :lzw_initsz
	
	def initialize(fd)
		@left = bin(fd.read(2))
		@top = bin(fd.read(2))
		@height = bin(fd.read(2))
		@width = bin(fd.read(2))

		tmp = bin(fd.read(1))
		@loc_colortable_flag = tmp >> 7
		@interlaced = (tmp >> 6) & 1
		@sort = (tmp >> 5) & 1
		@reserved = (tmp >> 4) & 3
		@loc_colortable_size = 2 ** ((tmp & 7)+1)
		
		if @loc_colortable_flag == 1
			@colortable = Array.new
			@loc_colortable_size.times {
				@colortable << Color.new(fd)
			}
		end
		
		@lzw_initsz = bin(fd.read(1))

		# table-based image
		@lzw_raw_data = ''
		loop do
			bsz = bin(fd.read(1))
			break if bsz == 0
			@lzw_raw_data << fd.read(bsz)
		end

		# @pixels = lzw_inflate(@lzw_raw_data, @lzw_initsz)
	end
end

class GIF
	attr_reader :screenwidth, :screenheight, :bgcoloridx, :pixelration
	attr_reader :glob_colortable_flag, :color_res, :sort_flag, :glob_colortable_size
	attr_reader :colortable, :images, :blocks, :trailing_bytes
	
	alias :palette :colortable
	
	def initialize(filename = 'Georges_Painvin_jj.gif')
		fd = File.open(filename)

		# signature
		sig = fd.read 6
		raise RuntimeError.new('Not a GIF file') if sig != "GIF89a"
		
		# logical screen descriptor
		@screenwidth = bin(fd.read(2))
		@screenheight = bin(fd.read(2))
		
		tmp = bin(fd.read(1))
		@glob_colortable_flag = tmp >> 7
		@color_res = (tmp >> 4) & 0x7
		@sort_flag = (tmp & 0x8) >> 3
		@glob_colortable_size = 2 ** ((tmp & 0x7)+1)
		
		@bgcoloridx = bin(fd.read(1))
		@pixelratio = bin(fd.read(1))
		
		# global color table
		if @glob_colortable_flag == 1
			@colortable = Array.new
			@glob_colortable_size.times {
				@colortable << Color.new(fd)
			}
		end
		
		# blocks
		@images = Array.new
		@blocks = Array.new
		loop do
			c = bin(fd.read(1))
			case c
			when 0x3b
				# terminator
				break
				
			when 0x21
				# extension
				@blocks << GIFBlock.new(fd)
				
			when 0x2C
				# image
				@blocks << GIFImage.new(fd)

			when nil
				@blocks << 'eof'
				break
				
			else
				@blocks << "invalid gif block: id #{c}"
			end
		end
		
		@trailing_bytes = fd.read
		fd.close
	end
end

if ARGV[0]
	gif = GIF.new(ARGV[0])
	puts gif.inspect
end