read jpeg till width and height are known

wxh.rb

def examine(io)
  class << io
    def readbyte; readchar; end unless method_defined?(:readbyte)
    def readint; (readbyte << 8) + readbyte; end
    def readframe; read(readint - 2); end
    def readsof; [readint, readbyte, readint, readint, readbyte]; end
    def next
      c = readbyte while c != 0xFF
      c = readbyte while c == 0xFF
      c
    end
  end
  raise 'malformed JPEG' unless io.readbyte == 0xFF && io.readbyte == 0xD8 # SOI

  height = width = nil
  while marker = io.next
    case marker
    when 0xC0..0xC3, 0xC5..0xC7, 0xC9..0xCB, 0xCD..0xCF # SOF markers
      length, _, height, width, components = io.readsof
      break
    when 0xD9, 0xDA
      break # EOI, SOS
    else
      io.readframe # ignore frame
    end
  end
  
  [width, height]
end

ARGV.each do |fname|
  File.open(fname) do |f|
    puts "#{fname}: #{examine(f).join('x')} (after reading #{f.pos} bytes)"
  end
end