convert_asset_to_inline.rb

require 'open-uri'
require 'image_size'

class ConvertAssetInline
  def initialize
    @orig = nil
    @cache = {}
  end

  def convert(url)
    body = crawl(url)
    convert_img(body, url)
  end

  private
  def crawl(url, options = {})
    open(url) {|f| f.read }.freeze
  end

  def url_to_data(url)
    url = url.to_s
    return @cache[url] if @cache[url]

    target = open(url) {|f| f.read }.freeze
    target_is = ImageSize.new(target)

    content_type = "image/#{target_is.get_type.downcase}"
    target_b64 = [target].pack('m')

    result = "data:#{content_type};base64,#{target_b64}"
    @cache[url] = result

    result
  end

  def convert_img(body, base_url)
    body.gsub(/<img[^>]*?src=["'](.*?)["']/) { # don't care there are ["'] in the URL
      url = URI.join(base_url, $1)
      $&.sub($1, url_to_data(url))
    }
  end

  # def convert_favicon ...
  # def convert_stylesheets ...
end