feat(translate): translate a whole playlist so the next episodes are ready (#16)

Co-authored-by: Tri Vo Khac <git@trivk.dev>

src/subtitles/subtitleextractor.cpp

@@ -1,5 +1,7 @@
 #include "subtitleextractor.h"
 
+#include "trackselection.h"
+
 #include <KLocalizedString>
 
 #include <memory>
@@ -138,4 +140,39 @@ ExtractResult extractEmbedded(const QString &path, int streamIndex, const std::a
     }
     return result;
 }
+
+SourceResult findTranslationSource(const QString &path)
+{
+    AVFormatContext *rawFormat = nullptr;
+    if (const int ret = avformat_open_input(&rawFormat, path.toUtf8().constData(), nullptr, nullptr); ret < 0) {
+        return {-1, i18n("Could not open %1: %2", path, ffmpegError(ret))};
+    }
+    std::unique_ptr<AVFormatContext, FormatContextDeleter> format(rawFormat);
+
+    const auto language = [](const AVStream *stream) {
+        const AVDictionaryEntry *tag = av_dict_get(stream->metadata, "language", nullptr, 0);
+        return tag ? QString::fromUtf8(tag->value) : QString();
+    };
+    // Track ids start at 1, since 0 means "none"; id - 1 is the stream index.
+    QList<TrackSelection::Track> subtitles;
+    QString audioLanguage;
+    bool audioIsDefault = false;
+    for (unsigned i = 0; i < format->nb_streams; ++i) {
+        const AVStream *stream = format->streams[i];
+        const bool isDefault = stream->disposition & AV_DISPOSITION_DEFAULT;
+        if (stream->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
+            if (audioLanguage.isEmpty() || (isDefault && !audioIsDefault)) {
+                audioLanguage = language(stream);
+                audioIsDefault = isDefault;
+            }
+        } else if (stream->codecpar->codec_type == AVMEDIA_TYPE_SUBTITLE) {
+            subtitles.append({static_cast<int>(i) + 1,
+                              language(stream),
+                              QString::fromUtf8(avcodec_get_name(stream->codecpar->codec_id)),
+                              bool(stream->disposition & AV_DISPOSITION_FORCED),
+                              isDefault});
+        }
+    }
+    return {TrackSelection::pickTranslationSource(subtitles, {QStringLiteral("en"), audioLanguage}, 0) - 1, {}};
+}
 }