prepare_images.rb

#!/usr/bin/env ruby

require "open-uri"

class Converter
  RE_MD_IMAGE = /\!\[.*\]\((http.+)\)/
  RE_TAG_IMAGE = /<img .*src="(http.+?)".+?>/

  def initialize(file, conf)
    @conf = conf
    @source_text = File.read(file)
  end

  def output
    urls = image_urls
    image_paths = urls.map do |url|
      download(url)
    end

    text = @source_text
    urls.each_with_index do |url, i|
      text.gsub!(url, image_paths[i])
    end
    text
  end

  def download(url)
    resource = open(url)
    path = "#{@conf[:download_path]}/#{url.split('/').last}"
    File.open(path, 'wb') do |f|
      f.write(resource.read)
    end
    path
  end

  def image_urls
    image_tags = @source_text.scan(RE_TAG_IMAGE)
    image_tags.zip(@source_text.scan(RE_MD_IMAGE)).flatten
  end
end

if __FILE__==$0
  source_file = ARGV[0] || "slide.md"
  conv = Converter.new(source_file, {download_path: 'images'})
  puts conv.output
end