module Preview
  class ThumbnailFetcher
    class GenerationFailed < StandardError; end

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

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

      page = HttpFetcher.fetch(uri.to_s)
      metadata = HtmlMetadataExtractor.extract(page)
      raise GenerationFailed, 'サムネール画像が見つかりませんでした．' if metadata[:image_url].blank?

      fetch_image(metadata[:image_url])
    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)
      unless response.content_type.downcase.start_with?('image/')
        raise GenerationFailed, 'サムネール画像が見つかりませんでした．'
      end

      response.body
    rescue HttpFetcher::FetchTimeout
      raise
    rescue HttpFetcher::FetchFailed
      raise GenerationFailed, 'サムネール画像を取得できませんでした．'
    end

    private_class_method :fetch_image
  end
end