fix(MediaStream.swift): adjust external subtitle indexes to correct ordering in VLC player

ref: jellyfin#1311
Signed-off-by: ddrccw <[email protected]>

Author: ddrccw

Shared/Extensions/JellyfinAPI/MediaStream.swift

@@ -220,14 +220,41 @@ extension [MediaStream] {
     func adjustExternalSubtitleIndexes(audioStreamCount: Int) -> [MediaStream] {
         guard allSatisfy({ $0.type == .subtitle }) else { return self }
         let embeddedSubtitleCount = filter { !($0.isExternal ?? false) }.count
+        let externalSubtitleCount = filter { $0.isExternal ?? false }.count
 
         var mediaStreams = self
 
+        // TODO: how to fix when transcoding
+        // case：鱿鱼游戏第二季，第五集
+        // display order
+        // - external, ch
+        // - korean
+        // - ...
+        // - viet
+
+        // vlc player subtitle track order
+        // - 2 ==> viet
+        // - 3 ==> korean
+        // - 37 ==> same as displayed subtitle
+        // - 38 ==> external, ch while displaying viet
+
+        // display order
+        // - extern subtitles
+        // - embeded subtitles
+
+        // ??? weird order
+        // vlc player subtitle track order
+        // - embeded subtitles n
+        // - embeded subtitles (1, n-1)
+        // - extern subtitles
+
         for (i, mediaStream) in mediaStreams.enumerated() {
-            guard mediaStream.isExternal ?? false else { continue }
             var copy = mediaStream
-            copy.index = (copy.index ?? 0) + 1 + embeddedSubtitleCount + audioStreamCount
-
+            if copy.isExternal ?? false {
+                copy.index = (copy.index ?? 0) + 1 + embeddedSubtitleCount + audioStreamCount
+            } else {
+                copy.index = (copy.index ?? 0) - externalSubtitleCount
+            }
             mediaStreams[i] = copy
         }