pass the api/v1/playlists with videos before the offset

src/invidious/playlists.cr

@@ -439,7 +439,8 @@ def get_playlist_videos(db, playlist, offset, locale = nil, continuation = nil)
   else
     videos = [] of PlaylistVideo
 
-    until videos.size >= 50 || videos.size == playlist.video_count || offset >= playlist.video_count
+    original_offset = offset
+    until videos.size >= 100 || videos.size == playlist.video_count || offset >= playlist.video_count
       if offset >= 100
         # Normalize offset to match youtube's behavior (100 videos chunck per request)
         normalized_offset = (offset / 100).to_i64 * 100_i64
@@ -459,7 +460,7 @@ def get_playlist_videos(db, playlist, offset, locale = nil, continuation = nil)
           end
         end
       else
-        until videos[0].index == offset
+        until videos[0].index == original_offset
           videos.shift
           if videos.size == 0
             break
@@ -550,7 +551,7 @@ def template_playlist(playlist)
 
   playlist["videos"].as_a.each do |video|
     html += <<-END_HTML
-      <li class="pure-menu-item">
+      <li class="pure-menu-item" id="#{video["videoId"]}">
         <a href="/watch?v=#{video["videoId"]}&list=#{playlist["playlistId"]}&index=#{video["index"]}">
           <div class="thumbnail">
               <img class="thumbnail" src="/vi/#{video["videoId"]}/mqdefault.jpg">

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

@@ -24,7 +24,7 @@ module Invidious::Routes::API::V1::Misc
     offset ||= env.params.query["page"]?.try &.to_i?.try { |page| (page - 1) * 100 }
     offset ||= 0
 
-    continuation = env.params.query["continuation"]?
+    video_id = env.params.query["continuation"]?
 
     format = env.params.query["format"]?
     format ||= "json"
@@ -46,12 +46,32 @@ module Invidious::Routes::API::V1::Misc
       return error_json(404, "Playlist does not exist.")
     end
 
-    response = playlist.to_json(offset, locale, continuation: continuation)
+    # includes into the playlist a maximum of 20 videos, before the offset
+    lookback = 20
+    if offset > 0
+      lookback = offset < lookback ? offset : lookback
+      response = playlist.to_json(offset - lookback, locale)
+      json_response = JSON.parse(response)
+    else
+      #  Unless the continuation is really the offset 0, it becomes expensive.
+      #  It happens when the offset is not set.
+      #  First we find the actual offset, and then we lookback
+      #  it shouldn't happen often though
+
+      response = playlist.to_json(offset, locale, continuation: continuation)
+      json_response = JSON.parse(response)
+
+      if json_response["videos"].as_a[0]["index"] != offset
+        offset = json_response["videos"].as_a[0]["index"].as_i
+        lookback = offset < 50 ? offset : 50
+        response = playlist.to_json(offset - lookback, locale)
+        json_response = JSON.parse(response)
+      end
+    end
 
     if format == "html"
-      response = JSON.parse(response)
-      playlist_html = template_playlist(response)
-      index, next_video = response["videos"].as_a.skip(1).select { |video| !video["author"].as_s.empty? }[0]?.try { |v| {v["index"], v["videoId"]} } || {nil, nil}
+      playlist_html = template_playlist(json_response)
+      index, next_video = json_response["videos"].as_a.skip(1 + lookback).select { |video| !video["author"].as_s.empty? }[0]?.try { |v| {v["index"], v["videoId"]} } || {nil, nil}
 
       response = {
         "playlistHtml" => playlist_html,