#398

backend/app/services/preview/known_site_extractor.rb

@@ -0,0 +1,32 @@
+module Preview
+  class KnownSiteExtractor
+    def self.thumbnail_url(uri)
+      youtube_thumbnail(uri) || niconico_thumbnail(uri)
+    end
+
+    def self.youtube_thumbnail(uri)
+      id =
+          case uri.host&.downcase
+          when 'youtu.be'
+            uri.path.split('/').reject(&:blank?).first
+          when 'www.youtube.com', 'youtube.com', 'm.youtube.com'
+            uri.path == '/watch' ? URI.decode_www_form(uri.query.to_s).to_h['v'] : nil
+          end
+      return unless id&.match?(/\A[A-Za-z0-9_-]{6,20}\z/)
+
+      "https://i.ytimg.com/vi/#{ id }/hqdefault.jpg"
+    end
+
+    def self.niconico_thumbnail(uri)
+      return unless ['www.nicovideo.jp', 'nicovideo.jp', 'nico.ms'].include?(uri.host&.downcase)
+
+      id = uri.path[%r{/(?:watch/)?(sm\d+)\z}, 1]
+      return unless id
+
+      numeric_id = id.delete_prefix('sm')
+      "https://nicovideo.cdn.nimg.jp/thumbnails/#{ numeric_id }/#{ numeric_id }.L"
+    end
+
+    private_class_method :youtube_thumbnail, :niconico_thumbnail
+  end
+end