Add API endpoint for fetching transcripts from YouTube (#4788)

src/invidious/routes/api/v1/videos.cr

@@ -429,4 +429,90 @@ module Invidious::Routes::API::V1::Videos
       end
     end
   end
+
+  # Fetches transcripts from YouTube
+  #
+  # Use the `lang` and `autogen` query parameter to select which transcript to fetch
+  # Request without any URL parameters to see all the available transcripts.
+  def self.transcripts(env)
+    env.response.content_type = "application/json"
+
+    id = env.params.url["id"]
+    lang = env.params.query["lang"]?
+    label = env.params.query["label"]?
+    auto_generated = env.params.query["autogen"]? ? true : false
+
+    # Return all available transcript options when none is given
+    if !label && !lang
+      begin
+        video = get_video(id)
+      rescue ex : NotFoundException
+        return error_json(404, ex)
+      rescue ex
+        return error_json(500, ex)
+      end
+
+      response = JSON.build do |json|
+        # The amount of transcripts available to fetch is the
+        # same as the amount of captions available.
+        available_transcripts = video.captions
+
+        json.object do
+          json.field "transcripts" do
+            json.array do
+              available_transcripts.each do |transcript|
+                json.object do
+                  json.field "label", transcript.name
+                  json.field "languageCode", transcript.language_code
+                  json.field "autoGenerated", transcript.auto_generated
+
+                  if transcript.auto_generated
+                    json.field "url", "/api/v1/transcripts/#{id}?lang=#{URI.encode_www_form(transcript.language_code)}&autogen"
+                  else
+                    json.field "url", "/api/v1/transcripts/#{id}?lang=#{URI.encode_www_form(transcript.language_code)}"
+                  end
+                end
+              end
+            end
+          end
+        end
+      end
+
+      return response
+    end
+
+    # If lang is not given then we attempt to fetch
+    # the transcript through the given label
+    if lang.nil?
+      begin
+        video = get_video(id)
+      rescue ex : NotFoundException
+        return error_json(404, ex)
+      rescue ex
+        return error_json(500, ex)
+      end
+
+      target_transcript = video.captions.select(&.name.== label)
+      if target_transcript.empty?
+        return error_json(404, NotFoundException.new("Requested transcript does not exist"))
+      else
+        target_transcript = target_transcript[0]
+        lang, auto_generated = target_transcript.language_code, target_transcript.auto_generated
+      end
+    end
+
+    params = Invidious::Videos::Transcript.generate_param(id, lang, auto_generated)
+
+    begin
+      transcript = Invidious::Videos::Transcript.from_raw(
+        YoutubeAPI.get_transcript(params), lang, auto_generated
+      )
+    rescue ex : NotFoundException
+      return error_json(404, ex)
+    rescue ex
+      return error_json(500, ex)
+    end
+
+    return transcript.to_json
+  end
 end