module Preview
  class ThumbnailFetcher
    class GenerationFailed < StandardError; end

    def self.fetch(raw_url)
      uri, = UrlSafety.validate(raw_url)
      page = nil

      known_url = KnownSiteExtractor.thumbnail_url(uri)
      image = fetch_image(known_url) if known_url

      unless image
        page = HttpFetcher.fetch(uri.to_s)
        metadata = HtmlMetadataExtractor.extract(page)
        image = fetch_image(metadata[:image_url]) if metadata[:image_url]
      end

      image || Screenshot.capture(uri.to_s)
    end

    def self.title(raw_url)
      uri, = UrlSafety.validate(raw_url)
      HtmlMetadataExtractor.extract(HttpFetcher.fetch(uri.to_s))[:title]
    end

    def self.fetch_image(url)
      response = HttpFetcher.fetch(url)
      return unless response.content_type.downcase.start_with?('image/')

      response.body
    rescue HttpFetcher::FetchFailed
      nil
    end

    private_class_method :fetch_image
  end
end