Extract visitor data from YouTube API

YoutubeExplode/Bridge/VisitorData.cs

@@ -1,6 +1,9 @@
 using System;
 using System.Linq;
-using System.Text;
+using System.Net.Http;
+using System.Net.Http.Headers;
+using System.Text.Json;
+using System.Threading.Tasks;
 using YoutubeExplode.Utils;
 using YoutubeExplode.Utils.Extensions;
 
@@ -11,6 +14,7 @@ internal static class VisitorData
     private static readonly Random Random = new();
     private static readonly char[] SessionIdAllowedChars =
         "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_".ToCharArray();
+    private static string _visitorData = string.Empty;
 
     public static string Generate() =>
         new ProtoBuilder()
@@ -36,4 +40,53 @@ public static string Generate() =>
                     .ToBytes()
             )
             .ToUrlEncodedBase64();
+
+    public static async Task<string> ExtractFromYoutube(HttpClient http)
+    {
+        //avoid fetching visitor data for each request (?) could slow down the app
+        if (!string.IsNullOrEmpty(_visitorData))
+            return _visitorData;
+
+        //use the same iOS user agent (?) idk if it's necessary
+        http.DefaultRequestHeaders.UserAgent.ParseAdd(
+            "com.google.ios.youtube/19.45.4 (iPhone16,2; U; CPU iOS 18_1_0 like Mac OS X; US)"
+        );
+
+        //request JSON format
+        http.DefaultRequestHeaders.Accept.Add(
+            new MediaTypeWithQualityHeaderValue("application/json")
+        );
+
+        var url = "https://www.youtube.com/sw.js_data";
+        HttpResponseMessage response = await http.GetAsync(url);
+        response.EnsureSuccessStatusCode();
+        string jsonString = await response.Content.ReadAsStringAsync();
+
+        //Remove the prefix ")]}'"
+        if (jsonString.StartsWith(")]}'"))
+            jsonString = jsonString.Substring(4);
+
+        using (JsonDocument doc = JsonDocument.Parse(jsonString))
+        {
+            JsonElement root = doc.RootElement;
+
+            //jsonArray[0][2][0][0][13]
+            var value = root[0]
+                .EnumerateArray()
+                .ElementAt(2)
+                .EnumerateArray()
+                .ElementAt(0)
+                .EnumerateArray()
+                .ElementAt(0)
+                .EnumerateArray()
+                .ElementAt(13)
+                .GetString();
+
+            if (value == null)
+                throw new Exception("Failed to fetch visitor data");
+
+            _visitorData = value;
+        }
+        return _visitorData;
+    }
 }

YoutubeExplode/Videos/VideoController.cs

@@ -76,7 +76,7 @@ public async ValueTask<PlayerResponse> GetPlayerResponseAsync(
                   "platform": "MOBILE",
                   "osName": "IOS",
                   "osVersion": "18.1.0.22B83",
-                  "visitorData": {{Json.Serialize(VisitorData.Generate())}},
+                  "visitorData": {{Json.Serialize(await VisitorData.ExtractFromYoutube(Http))}},
                   "hl": "en",
                   "gl": "US",
                   "utcOffsetMinutes": 0