Rewrite transcript logic to be more generic

The transcript logic in Invidious was written specifically
as a workaround for captions, and not transcripts as a feature.

This commit genericises the logic a bit as so it can be used for
implementing transcripts within Invidious' API and UI as well.

The most notable change is the added parsing of section headings
when it was previously skipped over in favor of regular lines.

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

@@ -89,9 +89,14 @@ module Invidious::Routes::API::V1::Videos
 
     if CONFIG.use_innertube_for_captions
       params = Invidious::Videos::Transcript.generate_param(id, caption.language_code, caption.auto_generated)
-      initial_data = YoutubeAPI.get_transcript(params)
 
-      webvtt = Invidious::Videos::Transcript.convert_transcripts_to_vtt(initial_data, caption.language_code)
+      transcript = Invidious::Videos::Transcript.from_raw(
+        YoutubeAPI.get_transcript(params),
+        caption.language_code,
+        caption.auto_generated
+      )
+
+      webvtt = transcript.to_vtt
     else
       # Timedtext API handling
       url = URI.parse("#{caption.base_url}&tlang=#{tlang}").request_target

src/invidious/videos/transcript.cr

@@ -1,8 +1,21 @@
 module Invidious::Videos
-  # Namespace for methods primarily relating to Transcripts
-  module Transcript
-    record TranscriptLine, start_ms : Time::Span, end_ms : Time::Span, line : String
+  # A `Transcripts` struct encapsulates a sequence of lines that together forms the whole transcript for a given YouTube video.
+  # These lines can be categorized into two types: section headings and regular lines representing content from the video.
+  struct Transcript
+    # Types
+    record HeadingLine, start_ms : Time::Span, end_ms : Time::Span, line : String
+    record RegularLine, start_ms : Time::Span, end_ms : Time::Span, line : String
+    alias TranscriptLine = HeadingLine | RegularLine
+
+    property lines : Array(TranscriptLine)
+    property language_code : String
+    property auto_generated : Bool
+
+    # Initializes a new Transcript struct with the contents and associated metadata describing it
+    def initialize(@lines : Array(TranscriptLine), @language_code : String, @auto_generated : Bool)
+    end
 
+    # Generates a protobuf string to fetch the requested transcript from YouTube
     def self.generate_param(video_id : String, language_code : String, auto_generated : Bool) : String
       kind = auto_generated ? "asr" : ""
 
@@ -30,48 +43,57 @@ module Invidious::Videos
       return params
     end
 
-    def self.convert_transcripts_to_vtt(initial_data : Hash(String, JSON::Any), target_language : String) : String
-      # Convert into array of TranscriptLine
-      lines = self.parse(initial_data)
-
-      settings_field = {
-        "Kind"     => "captions",
-        "Language" => target_language,
-      }
-
-      # Taken from Invidious::Videos::Captions::Metadata.timedtext_to_vtt()
-      vtt = WebVTT.build(settings_field) do |vtt|
-        lines.each do |line|
-          vtt.cue(line.start_ms, line.end_ms, line.line)
-        end
-      end
-
-      return vtt
-    end
-
-    private def self.parse(initial_data : Hash(String, JSON::Any))
+    # Constructs a Transcripts struct from the initial YouTube response
+    def self.from_raw(initial_data : Hash(String, JSON::Any), language_code : String, auto_generated : Bool)
       body = initial_data.dig("actions", 0, "updateEngagementPanelAction", "content", "transcriptRenderer",
         "content", "transcriptSearchPanelRenderer", "body", "transcriptSegmentListRenderer",
         "initialSegments").as_a
 
       lines = [] of TranscriptLine
+
       body.each do |line|
-        # Transcript section headers. They are not apart of the captions and as such we can safely skip them.
-        if line.as_h.has_key?("transcriptSectionHeaderRenderer")
-          next
+        if unpacked_line = line["transcriptSectionHeaderRenderer"]?
+          line_type = HeadingLine
+        else
+          unpacked_line = line["transcriptSegmentRenderer"]
+          line_type = RegularLine
         end
 
-        line = line["transcriptSegmentRenderer"]
+        start_ms = unpacked_line["startMs"].as_s.to_i.millisecond
+        end_ms = unpacked_line["endMs"].as_s.to_i.millisecond
+        text = extract_text(unpacked_line["snippet"]) || ""
 
-        start_ms = line["startMs"].as_s.to_i.millisecond
-        end_ms = line["endMs"].as_s.to_i.millisecond
+        lines << line_type.new(start_ms, end_ms, text)
+      end
 
-        text = extract_text(line["snippet"]) || ""
+      return Transcript.new(
+        lines: lines,
+        language_code: language_code,
+        auto_generated: auto_generated,
+      )
+    end
 
-        lines << TranscriptLine.new(start_ms, end_ms, text)
+    # Converts transcript lines to a WebVTT file
+    #
+    # This is used within Invidious to replace subtitles
+    # as to workaround YouTube's rate-limited timedtext endpoint.
+    def to_vtt
+      settings_field = {
+        "Kind"     => "captions",
+        "Language" => @language_code,
+      }
+
+      vtt = WebVTT.build(settings_field) do |vtt|
+        @lines.each do |line|
+          # Section headers are excluded from the VTT conversion as to
+          # match the regular captions returned from YouTube as much as possible
+          next if line.is_a? HeadingLine
+
+          vtt.cue(line.start_ms, line.end_ms, line.line)
+        end
       end
 
-      return lines
+      return vtt
     end
   end
 end