gistfile1.rb

desc "build pngs from svgs"
task :build do 
  Dir.glob("sources/*.svg").each do |file|
    if modified?(file)
      basename = File.basename(file, '.svg')
      slide = "slides/#{basename}.png"
      system("inkscape -e #{slide} -f #{file}")
    end
  end
end

def modified?(filename)
  index_filename = File.join(File.dirname(__FILE__), "cache", "index")
  # read or initialise index
  if File.exists?(index_filename)
    @index = File.open(index_filename, 'r') { |f| Marshal.load(f) }
  else
    @index = {}
  end

  # check if modified
  if @index[filename]
    modified = File.mtime(filename) > @index[filename][:mtime]
  else
    modified = true
  end
 
  # update index
  @index[filename] = {:mtime => File.mtime(filename)}
  File.open(index_filename, 'w') { |f| Marshal.dump(@index, f) }

  return modified
end