class Bmp
	def self.load(filename)
		new File.read(filename)
	end

	attr_reader :width, :height, :bpp, :numcolors, :numsigcolors
	def initialize(data)
		sig, sz, res1, res2, off, hlen = data.unpack('a2LSSLL')
		raise 'invalid BMP signature' if sig != 'BM'
		raise 'unsupported BMP header' if hlen != 40
		@width, @height, planes, @bpp, compr, size, xppm, yppm, @numcolors, @numsigcolors = data[18...54].unpack('LLSSLLLLLL')
		raise 'unsupported data format' if @bpp != 24
		@data = data[off..(sz-off)]

		if $DEBUG
			puts "size r: #{data.length}, hdr: #{sz}, hsz: #{hlen}, pxoff: #{off}"
			puts "pxlen: #{size}, w #@width h #@height w*h*bpp: #{@width*@height*@bpp/8}"
		end
	end

	def pix(x, y)
		linelen = ((@width * @bpp/8) + 3) & 0xfffffffc
		off = x*@bpp/8 + linelen*y
		[@data[off+2], @data[off+1], @data[off]]
	end
end