jellytau/src-tauri/src/repository/device_profile.rs

//! Device-profile policy: turning what a device *reports* about its audio
//! output into the constraints we send Jellyfin.
//!
//! The platform layer reports raw facts (what `MediaCodecList` enumerates, how
//! many channels the current audio route accepts); deciding what those facts
//! mean for a `DeviceProfile` is domain logic and lives here, on the Rust side
//! of the boundary, where it is testable without a device.

/// Channel count assumed when the platform cannot tell us — every audio route
/// can voice stereo, so it is the only safe floor.
const FALLBACK_AUDIO_CHANNELS: u32 = 2;

/// Upper bound we are willing to claim. Jellyfin profiles top out at 7.1, and a
/// nonsense reading from a driver should not become a nonsense profile.
const MAX_SUPPORTED_AUDIO_CHANNELS: u32 = 8;

/// Decide the `MaxAudioChannels` to advertise, given what the current audio
/// route reported.
///
/// Without this constraint Jellyfin is free to direct-play a 5.1 or 7.1 track to
/// a sink that only has two channels. What the user hears then is device
/// dependent and rarely correct — a failed `AudioSink` configuration (silence),
/// or centre-channel dialogue folded away to near-inaudibility. Naming the real
/// channel count makes the server downmix instead, which is always audible.
///
/// A missing or zero reading means "route not established yet", not "no audio":
/// fall back to stereo rather than claiming a capability we have not seen.
///
/// TRACES: UR-004 | DR-141 | UT-141
pub fn clamp_max_audio_channels(reported: Option<u32>) -> u32 {
    match reported {
        Some(channels) if channels >= 1 => channels.min(MAX_SUPPORTED_AUDIO_CHANNELS),
        _ => FALLBACK_AUDIO_CHANNELS,
    }
}

/// The channel cap for this device, reading the platform's report where one
/// exists.
///
/// TRACES: UR-004 | DR-141 | UT-141
pub fn max_audio_channels() -> u32 {
    #[cfg(target_os = "android")]
    let reported = crate::player::get_detected_codecs().and_then(|(_, _, channels)| channels);

    // Desktop plays video through the WebKitGTK HTML5 <video> element, which we
    // do not interrogate for a channel count; stereo is the safe assumption.
    #[cfg(not(target_os = "android"))]
    let reported: Option<u32> = None;

    clamp_max_audio_channels(reported)
}

/// Audio codecs the webview's `<video>` element can decode.
///
/// Deliberately narrower than what the platform reports: see
/// [`video_audio_codecs`].
const WEBVIEW_AUDIO_CODECS: &[&str] = &["aac", "mp3", "opus", "vorbis", "flac"];

/// The codec claimed when a device reports nothing we can use. Every renderer
/// decodes AAC, and claiming *something* is what makes the server transcode to
/// it rather than give up.
const FALLBACK_AUDIO_CODEC: &str = "aac";

/// Jellyfin's sentinel for "negotiate no subtitle stream at all".
///
/// Omitting `SubtitleStreamIndex` does **not** mean this: the server then applies
/// the source's default/forced flags and picks a track itself. See
/// [`playback_subtitle_stream_index`] for why that is never what we want.
pub const NO_SUBTITLE_STREAM: i32 = -1;

/// Subtitle formats we can render ourselves, delivered as an external sidecar
/// track rather than painted into the video.
///
/// Every entry here is *text*. Image-based subtitles (PGS, DVD, DVB) are
/// deliberately absent: they are bitmaps, so the only way a server can show them
/// on a client that cannot composite them is to burn them into the picture.
const EXTERNAL_SUBTITLE_FORMATS: &[&str] = &["srt", "subrip", "ass", "ssa", "vtt"];

/// The `SubtitleProfile` entries to advertise, as `(format, method)`.
///
/// All `External`: the app fetches subtitle tracks itself and renders them over
/// the video (UR-020), so it never needs the server to composite them.
///
/// TRACES: UR-020 | DR-176 | UT-168
pub fn subtitle_profiles() -> Vec<(&'static str, &'static str)> {
    EXTERNAL_SUBTITLE_FORMATS
        .iter()
        .map(|format| (*format, "External"))
        .collect()
}

/// Whether asking the server to serve this subtitle codec forces it to burn the
/// subtitle into the picture.
///
/// Burn-in is not a subtitle cost — it is a *video* cost. It rules out remuxing
/// the video stream, so a source we would otherwise have passed through untouched
/// gets fully re-encoded frame by frame.
///
/// TRACES: UR-020 | DR-176 | UT-168
pub fn subtitle_forces_burn_in(codec: &str) -> bool {
    !EXTERNAL_SUBTITLE_FORMATS
        .iter()
        .any(|format| format.eq_ignore_ascii_case(codec.trim()))
}

/// The `SubtitleStreamIndex` to negotiate with: always "none".
///
/// The reported bug: a source with an E-AC-3 track and a **PGSSUB** default
/// subtitle track. Sending no index let the server honour that default, and since
/// PGS cannot go out as a sidecar it chose `SubtitleMethod=Encode` — burn-in.
/// That turned an audio-only transcode (the HEVC video was directly supported)
/// into a full HEVC→h264 re-encode, which the server could not sustain in real
/// time: the buffer never grew beyond one segment and playback stalled every few
/// seconds, taking seeking down with it.
///
/// Asking for no subtitle stream costs nothing, because the app never wanted the
/// server's composited version — it fetches the text tracks separately and
/// renders them itself (UR-020).
///
/// TRACES: UR-020, UR-004 | DR-176 | UT-168
pub fn playback_subtitle_stream_index() -> i32 {
    NO_SUBTITLE_STREAM
}

/// Query keys through which a stream URL can carry a subtitle decision.
///
/// Jellyfin binds query keys case-insensitively, so the match has to be too —
/// the server itself mixes casing (`SubtitleStreamIndex` but
/// `alwaysBurnInSubtitleWhenTranscoding`).
const SUBTITLE_QUERY_KEYS: &[&str] = &[
    "subtitlestreamindex",
    "subtitlemethod",
    "subtitlecodec",
    "alwaysburninsubtitlewhentranscoding",
];

/// Rewrite a stream URL so it asks for no subtitle, whoever built it.
///
/// [`playback_subtitle_stream_index`] only governs the URLs *this app* builds.
/// When `PlaybackInfo` answers with a `TranscodingUrl`, the URL was built by the
/// server from its own subtitle verdict, and we play it verbatim — so a server
/// that picked a track anyway (a live channel opened without an index, a source
/// whose default is image-based) hands us `SubtitleMethod=Encode`, and the
/// burn-in the negotiation just declined comes back through the URL. Burn-in is
/// a *video* cost: it rules out remuxing and forces a full re-encode.
///
/// Stripping the keys is not enough on its own — an absent index is not "none",
/// it is "you choose" — so the sentinel is always appended.
///
/// TRACES: UR-020, UR-004 | DR-176 | UT-168
pub fn without_server_chosen_subtitle(url: &str) -> String {
    let (path, query) = match url.split_once('?') {
        Some((path, query)) => (path, query),
        None => (url, ""),
    };

    let mut kept: Vec<&str> = query
        .split('&')
        .filter(|param| !param.is_empty())
        .filter(|param| {
            let key = param.split_once('=').map_or(*param, |(key, _)| key);
            !SUBTITLE_QUERY_KEYS
                .iter()
                .any(|subtitle_key| key.eq_ignore_ascii_case(subtitle_key))
        })
        .collect();

    let sentinel = format!("SubtitleStreamIndex={}", NO_SUBTITLE_STREAM);
    kept.push(&sentinel);

    format!("{}?{}", path, kept.join("&"))
}

/// Whether a subtitle in this format can reach the app as a sidecar it draws
/// itself — the same verdict as [`subtitle_forces_burn_in`], from the reader's
/// side, and the one a subtitle picker needs.
///
/// Since the app asks for burn-in nowhere (see
/// [`playback_subtitle_stream_index`]), a format that only burn-in could deliver
/// is one it can never display. An unnamed format is treated as undeliverable
/// rather than guessed at: offering a track and drawing nothing is worse than
/// not offering it.
///
/// TRACES: UR-020 | DR-176 | UT-168
pub fn subtitle_supports_external_delivery(codec: Option<&str>) -> bool {
    codec.is_some_and(|codec| !subtitle_forces_burn_in(codec))
}

/// Narrow a detected audio-codec list to what the renderer that will actually
/// play the **video** can decode.
///
/// The platform list comes from `MediaCodecList`, which describes ExoPlayer —
/// but the webview `<video>` element may be what renders the video, and
/// Chromium/WebKit decode a much smaller set than the platform does. Advertising
/// the raw list makes Jellyfin direct-play a track the webview cannot decode, and
/// the user gets picture with no sound.
///
/// Which renderer gets it is not fixed: Linux is always the element, and Android
/// follows `experimentalNativeVideo`, which took ExoPlayer as its default in
/// DR-161 but is a user setting either way. So the *narrow* list is the only one
/// that holds on both sides of that switch. The cost is a Dolby-licensed Android
/// device transcoding an E-AC-3 track its ExoPlayer could have direct-played;
/// the alternative is silence for everyone the switch lands the other way, which
/// is the bug this exists to prevent.
///
/// The gap is widest on devices whose vendor licenses Dolby: a phone with
/// `c2.dolby.eac3.decoder` reports `eac3`, so it — and only it — gets a silent
/// direct play where a leaner device is transcoded to AAC and plays fine.
///
/// This applies to the *video* direct-play profile only. Audio-only playback
/// really is ExoPlayer's, so its profile keeps the full platform list.
///
/// TRACES: UR-004 | DR-148 | UT-142
pub fn video_audio_codecs(detected: &str) -> String {
    // Where the video renderer decodes the audio itself (ExoPlayer), the
    // platform list *is* the answer and narrowing it to the webview's throws
    // away codecs the device genuinely plays — dts, on the tablet this was
    // found on. TRACES: UR-004, UR-080 | DR-234
    #[cfg(target_os = "android")]
    {
        return detected.to_string();
    }

    #[allow(unreachable_code)]
    let kept: Vec<&str> = detected
        .split(',')
        .filter_map(|codec| {
            let codec = codec.trim();
            // Match case-insensitively but emit our own spelling: the platform
            // list is assembled from MIME strings and its casing is not ours to
            // forward to the server.
            WEBVIEW_AUDIO_CODECS
                .iter()
                .copied()
                .find(|supported| supported.eq_ignore_ascii_case(codec))
        })
        .collect();

    if kept.is_empty() {
        FALLBACK_AUDIO_CODEC.to_string()
    } else {
        kept.join(",")
    }
}

/// What the renderer that will actually decode video on this platform can play.
///
/// Returns `(video_codecs, audio_codecs)` as Jellyfin-style comma lists.
///
/// This exists because the answer was previously derived in four places and
/// hardcoded in a fifth, each of them assuming the *webview* was decoding:
/// the device profile, the transcoding targets, the direct-play audio
/// narrowing, the client-side audio override, and `get_video_stream_url`'s
/// `VideoCodec`. On Android the decoder is ExoPlayer, so every one of those was
/// wrong there — the observed cost being an hevc source re-encoded to h264
/// because its *audio* was eac3, and dts forced to transcode though the device
/// decodes it.
///
/// One source, so the copies cannot disagree again.
///
/// TRACES: UR-004, UR-080 | DR-234
pub fn renderer_codecs() -> (String, String) {
    #[cfg(target_os = "android")]
    {
        // ExoPlayer, and the device itself answers via MediaCodecList.
        crate::player::get_detected_codecs()
            .map(|(video, audio, _channels)| (video, audio))
            .unwrap_or_else(|| {
                log::warn!(
                    "[DeviceProfile] Codec detection not complete, using conservative defaults"
                );
                ("h264,hevc".to_string(), "aac,mp3".to_string())
            })
    }

    // Linux desktop draws video in the WebKitGTK HTML5 <video> element, which
    // cannot reliably decode HEVC/AV1/VP9. Claim only what it decodes, so
    // Jellyfin transcodes the rest to h264 HLS. (Audio-only playback goes
    // through MPV and is unaffected — that is a different renderer and a
    // different profile.)
    //
    // When mpv draws the picture here this stops being a platform constant and
    // becomes a question about the active renderer — which is the whole point of
    // returning it from a function rather than a `cfg` block.
    #[cfg(all(not(target_os = "android"), target_os = "linux"))]
    {
        ("h264".to_string(), "aac,mp3,opus,vorbis,flac".to_string())
    }

    #[cfg(all(not(target_os = "android"), not(target_os = "linux")))]
    {
        (
            "h264,hevc,vp8,vp9,av1,mpeg4".to_string(),
            "aac,mp3,opus,vorbis,flac".to_string(),
        )
    }
}

/// Whether the renderer that decodes *video* on this platform can also decode
/// this audio codec.
///
/// On a webview platform this is the webview's narrow list, because the element
/// decodes both halves. On Android it is the device's own list: ExoPlayer plays
/// the audio, so judging it against the webview's capabilities transcodes files
/// that would have played.
///
/// TRACES: UR-004, UR-080 | DR-234
pub fn renderer_can_decode_audio(codec: &str) -> bool {
    let codec = codec.trim();
    #[cfg(target_os = "android")]
    {
        let (_video, audio) = renderer_codecs();
        return audio
            .split(',')
            .any(|supported| supported.trim().eq_ignore_ascii_case(codec));
    }
    #[cfg(not(target_os = "android"))]
    {
        webview_can_decode_audio(codec)
    }
}

/// Whether the webview `<video>` element can decode this audio codec.
///
/// TRACES: UR-004 | DR-149 | UT-148
pub fn webview_can_decode_audio(codec: &str) -> bool {
    WEBVIEW_AUDIO_CODECS
        .iter()
        .any(|supported| supported.eq_ignore_ascii_case(codec.trim()))
}

/// Decide whether we must transcode *regardless of what the server negotiated*,
/// given the source's audio streams as `(codec, is_default)` in source order.
///
/// Advertising a narrow profile ([`video_audio_codecs`]) is necessary but not
/// sufficient: Jellyfin 10.11.5 enforces a `DirectPlayProfile`'s container and
/// video codec but **ignores its audio codec** — an E-AC-3 track is offered for
/// direct play even when the profile lists only AAC, and neither a `VideoAudio`
/// `CodecProfile` nor `MaxAudioChannels` changes that. So the client cannot
/// delegate this decision; it knows what its own renderer can decode and must
/// apply that itself.
///
/// The track that matters is the one the server will actually serve (see
/// [`served_audio_codec`]). An unknown codec is left alone — forcing a transcode
/// on a guess would burn server CPU for files that play.
///
/// TRACES: UR-004 | DR-149 | UT-148
pub fn audio_forces_transcode(streams: &[(Option<&str>, bool)]) -> bool {
    match served_audio_codec(streams) {
        // The renderer that will decode it, not always the webview — see
        // `renderer_can_decode_audio`. TRACES: UR-004, UR-080 | DR-234
        Some(codec) => !renderer_can_decode_audio(codec),
        // No audio at all, or a codec the server did not name: leave it alone.
        None => false,
    }
}

/// The codec of the audio track the server will actually serve, given the
/// source's audio streams as `(codec, is_default)` in source order: the default,
/// or the first when none is marked.
///
/// `None` means "nothing to judge" — no audio streams, or the server named no
/// codec for the one it would serve. Both callers of this rule treat that as
/// leave-well-alone, never as a licence to assume compatibility.
///
/// TRACES: UR-004, UR-071 | DR-149, DR-171 | UT-148, UT-166
pub fn served_audio_codec<'a>(streams: &[(Option<&'a str>, bool)]) -> Option<&'a str> {
    streams
        .iter()
        .find(|(_, is_default)| *is_default)
        .or_else(|| streams.first())
        .and_then(|(codec, _)| *codec)
}

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

    /// The reported bug, at the level it was decided: a source whose default
    /// subtitle track is PGSSUB must not drag the video into a re-encode.
    ///
    /// TRACES: UR-020 | DR-176 | UT-168
    #[test]
    fn no_subtitle_stream_is_negotiated_so_the_server_never_burns_one_in() {
        assert_eq!(playback_subtitle_stream_index(), NO_SUBTITLE_STREAM);
        // Not `None`/omitted: that is what let the server pick the PGS track.
        assert_eq!(playback_subtitle_stream_index(), -1);
    }

    /// A transcode URL the *server* built carries the server's own subtitle
    /// verdict. Adopting it verbatim re-introduces the burn-in
    /// [`playback_subtitle_stream_index`] exists to prevent — the negotiation
    /// asks for no subtitle, and then we play a URL that asks for one anyway.
    ///
    /// TRACES: UR-020, UR-004 | DR-176 | UT-168
    #[test]
    fn a_server_built_transcode_url_has_its_burn_in_stripped() {
        // Shape taken from Jellyfin's `StreamInfo.ToUrl`: it appends
        // `SubtitleStreamIndex` and `SubtitleMethod` whenever it picked a track.
        let served = "/videos/abc/master.m3u8?DeviceId=jt&MediaSourceId=src1\
             &VideoCodec=h264&SubtitleMethod=Encode&SubtitleStreamIndex=2\
             &PlaySessionId=xyz";

        let url = without_server_chosen_subtitle(served);

        assert!(
            url.contains("SubtitleStreamIndex=-1"),
            "the adopted URL must ask for no subtitle: {url}"
        );
        assert!(
            !url.contains("SubtitleStreamIndex=2"),
            "the server's chosen track must not survive: {url}"
        );
        assert!(
            !url.contains("SubtitleMethod"),
            "burn-in must not be requested: {url}"
        );
        // Everything else identifies the job and must survive untouched.
        for kept in [
            "DeviceId=jt",
            "MediaSourceId=src1",
            "VideoCodec=h264",
            "PlaySessionId=xyz",
        ] {
            assert!(url.contains(kept), "{kept} must survive: {url}");
        }
    }

    /// The server may also be told to burn in unconditionally
    /// (`alwaysBurnInSubtitleWhenTranscoding`), which is appended to the URL
    /// rather than expressed as a method — and its keys are not PascalCase.
    ///
    /// TRACES: UR-020, UR-004 | DR-176 | UT-168
    #[test]
    fn an_unconditional_burn_in_flag_is_stripped_whatever_its_casing() {
        let url = without_server_chosen_subtitle(
            "/videos/abc/master.m3u8?api_key=k&alwaysBurnInSubtitleWhenTranscoding=true\
             &subtitlestreamindex=3&SubtitleCodec=ass",
        );

        assert!(!url.to_lowercase().contains("alwaysburnin"), "{url}");
        assert!(!url.to_lowercase().contains("subtitlecodec"), "{url}");
        assert!(!url.contains("subtitlestreamindex=3"), "{url}");
        assert!(url.contains("SubtitleStreamIndex=-1"), "{url}");
        assert!(url.contains("api_key=k"), "{url}");
    }

    /// A URL the server built without any subtitle in it still has to *say* so:
    /// omitting the index is what makes the server apply the source's default.
    ///
    /// TRACES: UR-020, UR-004 | DR-176 | UT-168
    #[test]
    fn a_url_with_no_subtitle_params_is_still_made_to_ask_for_none() {
        let url = without_server_chosen_subtitle("/videos/abc/master.m3u8?api_key=k");
        assert_eq!(
            url,
            "/videos/abc/master.m3u8?api_key=k&SubtitleStreamIndex=-1"
        );

        // A bare URL is rare but must not come out malformed.
        let bare = without_server_chosen_subtitle("/videos/abc/master.m3u8");
        assert_eq!(bare, "/videos/abc/master.m3u8?SubtitleStreamIndex=-1");
    }

    /// TRACES: UR-020 | DR-176 | UT-168
    #[test]
    fn text_subtitles_are_advertised_as_external_sidecars() {
        let profiles = subtitle_profiles();
        for format in ["srt", "subrip", "ass", "ssa", "vtt"] {
            let entry = profiles.iter().find(|(f, _)| *f == format);
            assert!(
                entry.is_some(),
                "{format} must be advertised or the server burns it into the picture"
            );
            assert_eq!(entry.unwrap().1, "External");
        }
    }

    /// TRACES: UR-020 | DR-176 | UT-168
    #[test]
    fn text_subtitles_never_force_burn_in_but_image_ones_do() {
        // Text: deliverable as a sidecar, so the video can still be remuxed.
        assert!(!subtitle_forces_burn_in("subrip"));
        assert!(!subtitle_forces_burn_in("ASS"));
        assert!(!subtitle_forces_burn_in("ssa"));
        // Image formats are bitmaps — the server can only composite them.
        assert!(subtitle_forces_burn_in("PGSSUB"));
        assert!(subtitle_forces_burn_in("dvdsub"));
    }

    #[test]
    fn an_undecodable_default_track_forces_a_transcode() {
        // The reported bug: one E-AC-3 track, which the webview cannot decode.
        assert!(audio_forces_transcode(&[(Some("eac3"), false)]));
        assert!(audio_forces_transcode(&[(Some("ac3"), true)]));
    }

    #[test]
    fn a_decodable_track_is_left_to_direct_play() {
        // Never spend server CPU on a file that already plays.
        assert!(!audio_forces_transcode(&[(Some("aac"), true)]));
        assert!(!audio_forces_transcode(&[(Some("mp3"), false)]));
    }

    #[test]
    fn the_default_track_decides_not_the_first() {
        // The webview plays the default track, so that is the one that has to be
        // decodable — a supported track further down does not save us.
        assert!(audio_forces_transcode(&[
            (Some("aac"), false),
            (Some("eac3"), true)
        ]));
        assert!(!audio_forces_transcode(&[
            (Some("eac3"), false),
            (Some("aac"), true)
        ]));
    }

    #[test]
    fn with_no_default_marked_the_first_track_decides() {
        // Jellyfin leaves IsDefault false on every stream for some files; the
        // server then serves the first, so judge that one.
        assert!(audio_forces_transcode(&[
            (Some("eac3"), false),
            (Some("aac"), false)
        ]));
    }

    #[test]
    fn a_source_with_no_audio_is_not_transcoded() {
        // Nothing to rescue, and a transcode would not create audio.
        assert!(!audio_forces_transcode(&[]));
    }

    #[test]
    fn an_unknown_codec_is_not_second_guessed() {
        // The server did not tell us the codec; assuming the worst would
        // transcode files that play perfectly.
        assert!(!audio_forces_transcode(&[(None, true)]));
    }

    /// The download path needs the codec itself, not just the verdict, so it can
    /// tell the server what to re-encode. It picks the same track the streaming
    /// verdict is formed from — one rule, one place.
    ///
    /// TRACES: UR-071 | DR-171 | UT-166
    #[test]
    fn the_served_codec_is_the_one_the_verdict_is_formed_from() {
        assert_eq!(
            served_audio_codec(&[(Some("aac"), false), (Some("eac3"), true)]),
            Some("eac3")
        );
        assert_eq!(
            served_audio_codec(&[(Some("eac3"), false), (Some("aac"), false)]),
            Some("eac3")
        );
        assert_eq!(served_audio_codec(&[]), None);
        assert_eq!(served_audio_codec(&[(None, true)]), None);
    }

    #[test]
    fn a_dolby_device_does_not_advertise_dolby_for_video() {
        // The bug: a Motorola reporting c2.dolby.eac3.decoder direct-played
        // E-AC-3 into a webview that cannot decode it — silent video, on that
        // device only.
        let codecs = video_audio_codecs("aac,ac3,amrnb,amrwb,eac3,flac,mp3,opus,pcm,vorbis");
        assert_eq!(codecs, "aac,flac,mp3,opus,vorbis");
    }

    #[test]
    fn codecs_the_webview_cannot_decode_are_dropped() {
        // AMR and raw PCM come from the AOSP set, so this is not a Dolby-only
        // problem — it is just rarer content.
        assert_eq!(video_audio_codecs("amrnb,amrwb,pcm,aac"), "aac");
        assert_eq!(video_audio_codecs("dts,truehd,mp3"), "mp3");
    }

    #[test]
    fn a_list_the_webview_fully_supports_is_untouched() {
        assert_eq!(
            video_audio_codecs("aac,mp3,opus,vorbis,flac"),
            "aac,mp3,opus,vorbis,flac"
        );
    }

    #[test]
    fn nothing_decodable_still_claims_aac() {
        // Claiming an empty list invites the server to give up rather than
        // transcode. AAC is universally decodable, so ask for it.
        assert_eq!(video_audio_codecs("eac3,dts"), "aac");
        assert_eq!(video_audio_codecs(""), "aac");
    }

    #[test]
    fn spacing_and_case_in_the_platform_list_are_tolerated() {
        // The list is assembled from MediaCodecList strings; do not let
        // whitespace decide whether the user gets sound.
        assert_eq!(video_audio_codecs("aac, EAC3 , Mp3"), "aac,mp3");
    }

    #[test]
    fn an_unknown_route_falls_back_to_stereo() {
        // Codec detection has not run yet, or the platform has no answer. Never
        // claim surround we have not seen — every sink can do stereo.
        assert_eq!(clamp_max_audio_channels(None), 2);
    }

    #[test]
    fn a_zero_reading_is_not_a_capability() {
        // A route that has not been established reports 0; taking that literally
        // would advertise a device with no audio at all.
        assert_eq!(clamp_max_audio_channels(Some(0)), 2);
    }

    #[test]
    fn a_stereo_sink_is_reported_as_stereo() {
        // The phone speaker / Bluetooth headset case: the server must downmix
        // 5.1 rather than direct-play it.
        assert_eq!(clamp_max_audio_channels(Some(2)), 2);
    }

    #[test]
    fn a_surround_route_keeps_its_channels() {
        // HDMI to an AVR: 5.1 and 7.1 direct play stay available.
        assert_eq!(clamp_max_audio_channels(Some(6)), 6);
        assert_eq!(clamp_max_audio_channels(Some(8)), 8);
    }

    #[test]
    fn an_absurd_reading_is_capped_rather_than_forwarded() {
        // Some drivers report the AudioTrack maximum rather than the route's.
        assert_eq!(clamp_max_audio_channels(Some(32)), 8);
    }

    #[test]
    fn a_mono_route_is_taken_at_its_word() {
        assert_eq!(clamp_max_audio_channels(Some(1)), 1);
    }
}