fix(mpv): auto-select subtitle and audio tracks on file open (#12)

Closes #7

mpv only turns on a subtitle that is flagged default or matches `slang`, so releases like VisionQuest (Russian dub by default, ru/ru/en subtitles, none flagged) open with subtitles off, which also disabled **Translate subtitle**.

- New `TrackSelection` helpers in `qtpotato_core` with unit tests.
- On file load: audio follows *Preferred languages* (Playback settings); subtitle follows *Preferred languages* (Subtitles settings), else keeps mpv's choice, else (new *Turn on a subtitle when none matches*, on by default) picks a full text subtitle in the system language, then English, then the audio language, then the default/first track.
- Translate is enabled whenever the file has a text subtitle; with none or a bitmap subtitle shown it translates the best text track (English first).

Verified in the real app on VisionQuest S01E01: subtitle 3 (en) is selected on open; with audio set to `en` and auto-select off, audio switches to track 7, subtitle stays off and Translate stays enabled with the English source.

Reviewed-on: #12
Co-authored-by: Tri Vo Khac <git@trivk.dev>

src/mpv/mpvitem.cpp

@@ -1,11 +1,13 @@
 #include "mpvitem.h"
 
 #include "mpvuserconfig.h"
+#include "trackselection.h"
 
 #include <MpvController>
 
 #include <QDir>
 #include <QFile>
+#include <QLocale>
 #include <QSaveFile>
 #include <QStandardPaths>
 
@@ -25,6 +27,33 @@ QVariant trackIdValue(int id)
 {
     return id > 0 ? QVariant(id) : QVariant(QStringLiteral("no"));
 }
+
+QList<TrackSelection::Track> selectionTracks(const QVariantList &tracks)
+{
+    QList<TrackSelection::Track> result;
+    for (const QVariant &entry : tracks) {
+        const QVariantMap track = entry.toMap();
+        result.append({
+            .id = track.value(QStringLiteral("id")).toInt(),
+            .lang = track.value(QStringLiteral("lang")).toString(),
+            .codec = track.value(QStringLiteral("codec")).toString(),
+            .forced = track.value(QStringLiteral("forced")).toBool(),
+            .isDefault = track.value(QStringLiteral("default")).toBool(),
+        });
+    }
+    return result;
+}
+
+QString trackLanguage(const QVariantList &tracks, int id)
+{
+    for (const QVariant &entry : tracks) {
+        const QVariantMap track = entry.toMap();
+        if (track.value(QStringLiteral("id")).toInt() == id) {
+            return track.value(QStringLiteral("lang")).toString();
+        }
+    }
+    return {};
+}
 }
 
 MpvItem::MpvItem(QQuickItem *parent)
@@ -64,6 +93,7 @@ MpvItem::MpvItem(QQuickItem *parent)
         this,
         [this] {
             setFileLoaded(true);
+            selectInitialTracks();
         },
         Qt::QueuedConnection);
     connect(
@@ -338,6 +368,37 @@ void MpvItem::showTranslatedSubtitleFile(const QUrl &url, const QString &title)
     commandAsync(QStringList{QStringLiteral("sub-add"), path, QStringLiteral("auto"), title});
 }
 
+int MpvItem::translationSourceId() const
+{
+    // English first: it is the usual original, and dubbed releases often default to
+    // subtitles translated from it.
+    return TrackSelection::pickTranslationSource(selectionTracks(m_subtitleTracks),
+                                                 {QStringLiteral("en"), trackLanguage(m_audioTracks, m_audioId)},
+                                                 m_subtitleId);
+}
+
+void MpvItem::selectInitialTracks()
+{
+    // The observed track-list may still describe the previous file; read it directly.
+    updateTracks(getProperty(QStringLiteral("track-list")).toList());
+
+    const int audioId = TrackSelection::pickAudio(selectionTracks(m_audioTracks), m_preferredAudioLanguages, m_audioId);
+    if (audioId != m_audioId) {
+        setAudioId(audioId);
+    }
+    // Subtitles are for reading: the user's own language, then English, before the
+    // audio language. Dubbed releases often default to a dub with matching subtitles.
+    const QStringList fallbackLanguages{QLocale::languageToCode(QLocale::system().language()), QStringLiteral("en"), trackLanguage(m_audioTracks, audioId)};
+    const int subtitleId = TrackSelection::pickSubtitle(selectionTracks(m_subtitleTracks),
+                                                        m_preferredSubtitleLanguages,
+                                                        fallbackLanguages,
+                                                        m_subtitleId,
+                                                        m_autoSelectSubtitle);
+    if (subtitleId != m_subtitleId) {
+        setSubtitleId(subtitleId);
+    }
+}
+
 void MpvItem::showOnlySubtitle(int id)
 {
     setSubtitleId(id);
@@ -484,6 +545,8 @@ void MpvItem::updateTracks(const QVariantList &trackList)
                 {QStringLiteral("title"), track.value(QStringLiteral("title")).toString()},
                 {QStringLiteral("lang"), track.value(QStringLiteral("lang")).toString()},
                 {QStringLiteral("codec"), track.value(QStringLiteral("codec")).toString()},
+                {QStringLiteral("forced"), track.value(QStringLiteral("forced")).toBool()},
+                {QStringLiteral("default"), track.value(QStringLiteral("default")).toBool()},
             });
             continue;
         }
@@ -505,6 +568,9 @@ void MpvItem::updateTracks(const QVariantList &trackList)
             {QStringLiteral("title"), track.value(QStringLiteral("title")).toString()},
             {QStringLiteral("lang"), track.value(QStringLiteral("lang")).toString()},
             {QStringLiteral("codec"), track.value(QStringLiteral("codec")).toString()},
+            {QStringLiteral("bitmap"), TrackSelection::isBitmapSubtitle(track.value(QStringLiteral("codec")).toString())},
+            {QStringLiteral("forced"), track.value(QStringLiteral("forced")).toBool()},
+            {QStringLiteral("default"), track.value(QStringLiteral("default")).toBool()},
             {QStringLiteral("external"), track.value(QStringLiteral("external")).toBool()},
             {QStringLiteral("externalFilename"), track.value(QStringLiteral("external-filename")).toString()},
             {QStringLiteral("ffIndex"), track.value(QStringLiteral("ff-index"), -1).toInt()},