jellytau/src-tauri/src/player/track_switch.rs

//! Audio-track switch strategy decision logic.
//!
//! Pure logic, extracted from the command layer so it can be unit-tested in the
//! player core — the sibling of [`super::seek`]. `player_switch_audio_track`
//! turns the resulting [`AudioTrackSwitchStrategy`] into a concrete action.
//!
//! The rule this module exists to state: **an engine can only select a track
//! the stream in front of it actually carries.** A Jellyfin transcode is built
//! around one `AudioStreamIndex`, so the alternate tracks are not in the stream
//! at all — the switch has to re-open it. Only a direct play/stream hands the
//! engine the source file with every track present.

/// How a request to change audio track has to be carried out.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AudioTrackSwitchStrategy {
    /// Re-open the stream pinned to the chosen track; the frontend reloads its
    /// `<video>` element. An HTML5 element cannot select an audio track at all,
    /// so this holds whether or not the current stream is a transcode.
    Html5ReloadStream,
    /// Re-open the stream pinned to the chosen track; the backend reloads
    /// itself and restores the position.
    BackendReloadStream,
    /// The engine already holds every track — select in place, no reload.
    BackendSelectInPlace,
}

/// Decide how to honour an audio-track change.
///
/// # Arguments
/// * `needs_transcoding` - Whether the stream now playing is a server-side
///   transcode, which carries exactly the one audio track it was built around.
/// * `use_html5` - Whether the frontend `<video>` element is rendering.
///
/// TRACES: UR-021 | IR-019, DR-024, DR-258 | UT-232
pub fn determine_audio_track_switch_strategy(
    needs_transcoding: bool,
    use_html5: bool,
) -> AudioTrackSwitchStrategy {
    if use_html5 {
        return AudioTrackSwitchStrategy::Html5ReloadStream;
    }

    if needs_transcoding {
        AudioTrackSwitchStrategy::BackendReloadStream
    } else {
        AudioTrackSwitchStrategy::BackendSelectInPlace
    }
}

/// Where to resume after re-opening the stream for a track change.
///
/// `requested` is what the caller supplied; `engine_position` is where the
/// engine itself says it is. The caller wins when it has something real to say,
/// and the engine answers otherwise — which is the whole point: **position is
/// the player's to know**, not the UI's to remember.
///
/// The native path proved why. It has no `<video>` element, so the frontend
/// sent `null`, the command defaulted to `0.0`, and switching audio track
/// re-opened the stream at the beginning of the film — the track changed and
/// the viewer lost their place. A non-finite or negative value is treated the
/// same as absent rather than passed through to a backend that would reject it.
///
/// TRACES: UR-021, UR-005 | IR-019, DR-024, DR-258 | UT-233
pub fn resume_position(requested: Option<f64>, engine_position: f64) -> f64 {
    let usable = requested.filter(|p| p.is_finite() && *p > 0.0);
    let fallback = if engine_position.is_finite() && engine_position > 0.0 {
        engine_position
    } else {
        0.0
    };

    usable.unwrap_or(fallback)
}

#[cfg(test)]
mod tests {
    use super::*;

    /// The reported bug, seen on a device: switching audio track changed the
    /// track but "restarts from zero". The native path has no `<video>`
    /// element, so the frontend passed `null` and the re-opened stream began at
    /// the start of the film — logcat: `Re-opened the stream on audio stream 2
    /// and resumed at 0` while playback was 22 minutes in.
    #[test]
    fn a_caller_with_no_position_resumes_where_the_engine_is() {
        assert_eq!(resume_position(None, 1337.5), 1337.5);
    }

    /// The HTML5 path does have an element and its clock is the honest answer
    /// there, so what the caller supplies wins.
    #[test]
    fn a_caller_that_knows_its_position_is_believed() {
        assert_eq!(resume_position(Some(42.0), 1337.5), 42.0);
    }

    /// A position that is not a position — NaN from an element with no
    /// metadata, or a negative from a clock read mid-teardown — is treated as
    /// absent. Passing it through re-opens at a place no backend accepts.
    #[test]
    fn a_nonsense_position_falls_back_to_the_engine() {
        assert_eq!(resume_position(Some(f64::NAN), 90.0), 90.0);
        assert_eq!(resume_position(Some(-5.0), 90.0), 90.0);
        assert_eq!(resume_position(None, f64::NAN), 0.0);
    }

    /// Switching track in the first moments of playback resumes at the start,
    /// which is where the viewer actually is.
    #[test]
    fn the_very_beginning_stays_the_very_beginning() {
        assert_eq!(resume_position(None, 0.0), 0.0);
    }

    /// The reported bug: on Android the audio-track menu did nothing and the
    /// default track kept playing.
    ///
    /// Jellyfin had negotiated a transcode (`TranscodeReasons=AudioCodecNot
    /// Supported`) whose URL pins `AudioStreamIndex=1`, so ExoPlayer was handed
    /// a stream with exactly one audio track — logcat: `Audio tracks: 1`. The
    /// native path nonetheless only ever called `setAudioTrack(n)`, which
    /// indexes ExoPlayer's audio track *groups* and so found nothing to select:
    /// `Invalid audio track index: 1 (available: 1)`, warned and dropped. The
    /// track the viewer asked for is not in the stream; it has to be re-opened.
    #[test]
    fn a_transcode_is_re_opened_because_it_carries_only_one_track() {
        assert_eq!(
            determine_audio_track_switch_strategy(true, false),
            AudioTrackSwitchStrategy::BackendReloadStream
        );
    }

    /// A direct play hands the engine the source file, every track included, so
    /// ExoPlayer selects in place — no reload, no re-buffer, no lost position.
    #[test]
    fn a_direct_play_switches_in_place() {
        assert_eq!(
            determine_audio_track_switch_strategy(false, false),
            AudioTrackSwitchStrategy::BackendSelectInPlace
        );
    }

    /// An HTML5 `<video>` element has no track-selection API, so it reloads
    /// either way. This is the path that already worked, and it must keep
    /// working: the fix is about the native side only.
    #[test]
    fn html5_always_reloads_because_the_element_cannot_select() {
        assert_eq!(
            determine_audio_track_switch_strategy(true, true),
            AudioTrackSwitchStrategy::Html5ReloadStream
        );
        assert_eq!(
            determine_audio_track_switch_strategy(false, true),
            AudioTrackSwitchStrategy::Html5ReloadStream
        );
    }
}