Match and mux in dropped external subtitles

Drag in subtitle files alongside .mkv files and they're matched to
the right video by episode code (S04E06, zero-padding and separator
insensitive, plus 4x06 as a fallback) and muxed in as extra subtitle
tracks. Any subtitle format mkvmerge accepts is recognized: SRT,
(S)SA, VobSub, WebVTT, PGS/SUP, USF, SAMI, TTML/DFXP, STL, Kate.

Matching works regardless of drop order — subtitles dropped before
their video are held pending until a matching video shows up, and
vice versa. Language, forced, and SDH flags are guessed from common
filename tokens (e.g. "eng", "forced", "sdh") to set the muxed
track's language/name/flags; unmatched subtitles are reported in the
status bar.

MainWindow.xaml.cs

@@ -20,6 +20,7 @@ public partial class MainWindow : FluentWindow
 
     private AppSettings _settings = new();
     private readonly ObservableCollection<QueuedFileItem> _fileQueue = new();
+    private readonly List<string> _pendingSubtitles = new();
     private readonly SemaphoreSlim _scanSemaphore = new(MaxConcurrentScans);
     private bool _isProcessing;
     private bool _propagatingSelection;
@@ -196,10 +197,9 @@ public partial class MainWindow : FluentWindow
         if (e.Data.GetDataPresent(DataFormats.FileDrop))
         {
             var paths = (string[])e.Data.GetData(DataFormats.FileDrop);
-            bool hasMkv = paths.Any(p =>
-                string.Equals(Path.GetExtension(p), ".mkv", StringComparison.OrdinalIgnoreCase)
-                && File.Exists(p));
-            e.Effects = hasMkv ? DragDropEffects.Copy : DragDropEffects.None;
+            bool hasAcceptedFile = paths.Any(p =>
+                File.Exists(p) && (IsMkvFile(p) || SubtitleMatcher.IsSubtitleFile(p)));
+            e.Effects = hasAcceptedFile ? DragDropEffects.Copy : DragDropEffects.None;
         }
         else
         {
@@ -208,6 +208,9 @@ public partial class MainWindow : FluentWindow
         e.Handled = true;
     }
 
+    private static bool IsMkvFile(string path)
+        => string.Equals(Path.GetExtension(path), ".mkv", StringComparison.OrdinalIgnoreCase);
+
     private void Window_Drop(object sender, DragEventArgs e)
     {
         if (!e.Data.GetDataPresent(DataFormats.FileDrop)) return;
@@ -215,28 +218,158 @@ public partial class MainWindow : FluentWindow
         Activate();
 
         var paths = (string[])e.Data.GetData(DataFormats.FileDrop);
-        var newFiles = paths
-            .Where(p => string.Equals(Path.GetExtension(p), ".mkv", StringComparison.OrdinalIgnoreCase)
+
+        var newVideoPaths = paths
+            .Where(p => IsMkvFile(p)
                         && File.Exists(p)
                         && !_fileQueue.Any(q => string.Equals(q.FilePath, p, StringComparison.OrdinalIgnoreCase)))
             .ToList();
 
-        foreach (var path in newFiles)
+        var subtitlePaths = paths
+            .Where(p => SubtitleMatcher.IsSubtitleFile(p) && File.Exists(p))
+            .ToList();
+
+        var newItems = new List<QueuedFileItem>();
+        foreach (var path in newVideoPaths)
         {
             var item = new QueuedFileItem(path);
             item.PropertyChanged += QueuedFileItem_PropertyChanged;
             _fileQueue.Add(item);
+            newItems.Add(item);
             _ = ScanFileItemAsync(item);
         }
 
+        // Newly-added videos might satisfy subtitles dropped in earlier (or
+        // in this same batch, if the subtitle sorts before its video).
+        foreach (var item in newItems)
+            MatchPendingSubtitlesToFile(item);
+
+        int matchedSubtitles = 0;
+        foreach (var subPath in subtitlePaths)
+        {
+            if (MatchSubtitleToQueue(subPath))
+                matchedSubtitles++;
+            else if (!_pendingSubtitles.Contains(subPath, StringComparer.OrdinalIgnoreCase))
+                _pendingSubtitles.Add(subPath);
+        }
+
         UpdateLoadingOverlay();
 
-        if (newFiles.Count > 0)
+        if (newItems.Count > 0)
         {
             var first = _fileQueue.FirstOrDefault(q =>
-                string.Equals(q.FilePath, newFiles[0], StringComparison.OrdinalIgnoreCase));
+                string.Equals(q.FilePath, newVideoPaths[0], StringComparison.OrdinalIgnoreCase));
             if (first != null) FileQueueList.SelectedItem = first;
         }
+
+        int unmatchedSubtitles = subtitlePaths.Count - matchedSubtitles;
+        if (matchedSubtitles > 0 || unmatchedSubtitles > 0)
+        {
+            StatusText.Text = unmatchedSubtitles > 0
+                ? $"Matched {matchedSubtitles} subtitle(s); {unmatchedSubtitles} couldn't be matched to an episode code (S04E06 style)."
+                : $"Matched {matchedSubtitles} subtitle(s) to their episode.";
+        }
+    }
+
+    // ── Subtitle matching ───────────────────────────────────────────────────
+
+    /// <summary>
+    /// Attempts to attach a dropped subtitle file to whichever queued video(s)
+    /// share the same episode code (e.g. S04E06). Falls back to the single
+    /// queued file when there's exactly one and no episode code could be
+    /// extracted from the subtitle's filename. Returns false if no target
+    /// could be determined, in which case the file is kept pending until a
+    /// matching video is dropped later.
+    /// </summary>
+    private bool MatchSubtitleToQueue(string subtitlePath)
+    {
+        string subFileName = Path.GetFileName(subtitlePath);
+        string? code = SubtitleMatcher.ExtractEpisodeCode(subFileName);
+
+        List<QueuedFileItem> targets;
+        if (code != null)
+        {
+            targets = _fileQueue
+                .Where(q => string.Equals(SubtitleMatcher.ExtractEpisodeCode(q.FileName), code, StringComparison.OrdinalIgnoreCase))
+                .ToList();
+        }
+        else if (_fileQueue.Count == 1)
+        {
+            targets = new List<QueuedFileItem> { _fileQueue[0] };
+        }
+        else
+        {
+            targets = new List<QueuedFileItem>();
+        }
+
+        if (targets.Count == 0) return false;
+
+        foreach (var target in targets)
+            AttachExternalSubtitle(target, subtitlePath);
+
+        return true;
+    }
+
+    /// <summary>
+    /// Called whenever a new video is added to the queue: checks the pending
+    /// (previously unmatched) subtitle files for one sharing this video's
+    /// episode code, and attaches + removes any matches from the pending list.
+    /// </summary>
+    private void MatchPendingSubtitlesToFile(QueuedFileItem item)
+    {
+        if (_pendingSubtitles.Count == 0) return;
+
+        string? code = SubtitleMatcher.ExtractEpisodeCode(item.FileName);
+        if (code == null) return;
+
+        var matches = _pendingSubtitles
+            .Where(p => string.Equals(SubtitleMatcher.ExtractEpisodeCode(Path.GetFileName(p)), code, StringComparison.OrdinalIgnoreCase))
+            .ToList();
+
+        foreach (var match in matches)
+        {
+            AttachExternalSubtitle(item, match);
+            _pendingSubtitles.Remove(match);
+        }
+    }
+
+    private static void AttachExternalSubtitle(QueuedFileItem target, string subtitlePath)
+    {
+        if (target.ExternalSubtitles.Any(s => string.Equals(s.FilePath, subtitlePath, StringComparison.OrdinalIgnoreCase)))
+            return;
+
+        var info = SubtitleLanguageHelper.Detect(Path.GetFileName(subtitlePath));
+        var sub = new ExternalSubtitleTrack(subtitlePath)
+        {
+            Language = info.Language,
+            TrackName = info.TrackName,
+            Forced = info.Forced,
+            HearingImpaired = info.HearingImpaired,
+            IsDefault = target.ExternalSubtitles.Count == 0,
+        };
+        target.ExternalSubtitles.Add(sub);
+    }
+
+    private void RemoveExternalSubtitle_Click(object sender, RoutedEventArgs e)
+    {
+        if (sender is not System.Windows.Controls.Button btn || btn.Tag is not ExternalSubtitleTrack sub) return;
+
+        var owner = _fileQueue.FirstOrDefault(f => f.ExternalSubtitles.Contains(sub));
+        if (owner == null) return;
+
+        bool wasDefault = sub.IsDefault;
+        owner.ExternalSubtitles.Remove(sub);
+
+        if (wasDefault)
+        {
+            var next = owner.ExternalSubtitles.FirstOrDefault();
+            if (next != null) next.IsDefault = true;
+        }
+
+        if (ReferenceEquals(FileQueueList.SelectedItem, owner))
+            ExternalSubsPanel.Visibility = owner.HasExternalSubtitles ? Visibility.Visible : Visibility.Collapsed;
+
+        e.Handled = true;
     }
 
     // ── File scanning ─────────────────────────────────────────────────────────
@@ -457,12 +590,16 @@ public partial class MainWindow : FluentWindow
             TrackGrid.ItemsSource = file.Tracks;
             TrackGrid.Visibility = Visibility.Visible;
             NoSelectionHint.Visibility = Visibility.Collapsed;
+            ExternalSubsList.ItemsSource = file.ExternalSubtitles;
+            ExternalSubsPanel.Visibility = file.HasExternalSubtitles ? Visibility.Visible : Visibility.Collapsed;
         }
         else
         {
             TrackGrid.ItemsSource = null;
             TrackGrid.Visibility = Visibility.Collapsed;
             NoSelectionHint.Visibility = Visibility.Visible;
+            ExternalSubsList.ItemsSource = null;
+            ExternalSubsPanel.Visibility = Visibility.Collapsed;
         }
     }