//! Dolby TrueHD / Atmos elementary stream parser.
//!
//! BD-TS TrueHD PES packets contain interleaved AC-3 + TrueHD access units.
//! Access units span PES boundaries — must buffer and reassemble.
//!
//! TrueHD access unit header (4 bytes):
//!   bytes 0-1: top nibble = MLP check/access-unit nibble, lower 12 bits =
//!              access-unit length in 2-byte words
//!   bytes 2-3: timing value
//!   bytes 4..: substream data (major sync 0xF8726FBA may appear at offset 4)
//!
//! AC-3 frames (interleaved, same PID): start with sync word 0x0B77.
//! We skip AC-3 frames and only emit TrueHD access units.

use super::crc::crc16_mlp;
use super::dropgate::DropTally;
use super::{CodecParser, Frame, PesPacket, pts_to_ns};
use crate::mux::timeline::DISCONTINUITY_BACKSTEP_NS;

/// Duration of one TrueHD access unit in nanoseconds for the 48 kHz family
/// (48 / 96 / 192 kHz). `access_unit_size = 40 << (ratebits & 7)` and
/// `sample_rate = 48000 << (ratebits & 7)`; the shared shift cancels in
/// `samples_per_AU / sample_rate = 40/48000 = 1/1200 s`, so this constant is
/// exact for the whole 48 kHz family — 48, 96 and 192 kHz alike. Used as the
/// default until a major sync reveals the actual rate family.
const AU_DURATION_NS: i64 = 833_333;

/// Duration of one TrueHD access unit in nanoseconds for the 44.1 kHz family
/// (44.1 / 88.2 / 176.4 kHz): `40/44100 = 1/1102.5 s = 907_029.478… ns`. The
/// 48 kHz constant would run ~8.95 % fast on these (rare) streams.
const AU_DURATION_NS_441: i64 = 907_029;

/// Hard cap on the reassembly buffer. A valid TrueHD/MAT access unit is
/// well under 32 KiB; if the buffer grows far past that without yielding a
/// frame the stream is malformed, so we drop it and resync rather than grow
/// without bound. Parity with the AC-3 / DTS / PGS caps.
const MAX_TRUEHD_BUF: usize = 256 * 1024;

pub struct TrueHdParser {
    buf: Vec<u8>,
    next_pts_ns: i64,
    /// Per-AU PTS increment. Defaults to the 48 kHz-family value (833_333) and
    /// is refined to the 44.1 kHz-family value once the first major sync reveals
    /// the actual rate. Stays at the default for streams whose major sync is not
    /// yet seen (head of stream) — preserving byte-identical timing for the
    /// common 48 kHz case.
    au_duration_ns: i64,
    /// Keep/drop bookkeeping for the decodability gate.
    tally: DropTally,
    /// `num_substreams` from the most recent major sync — needed to size the
    /// substream directory for the per-AU parity check. `None` until the first
    /// major sync is seen (before which no AU can be parity-checked).
    num_substreams: Option<u8>,
    /// True while dropping forward to the next clean resync point. MLP/TrueHD
    /// carries filter/predictor + restart state ACROSS access units, so a corrupt
    /// AU cannot be excised in place — it poisons decoding until the next major
    /// sync re-initialises state. On corruption we set this and drop every AU
    /// until a major sync whose header CRC validates, which we then emit.
    resync_pending: bool,
}

impl Default for TrueHdParser {
    fn default() -> Self {
        Self::new()
    }
}

impl TrueHdParser {
    pub fn new() -> Self {
        Self {
            buf: Vec::with_capacity(32768),
            next_pts_ns: 0,
            au_duration_ns: AU_DURATION_NS,
            tally: DropTally::new("truehd"),
            num_substreams: None,
            resync_pending: false,
        }
    }

    /// Access units dropped as undecodable so far.
    pub fn dropped_frames(&self) -> u64 {
        self.tally.dropped_frames()
    }

    /// Total decoded duration (ns) of dropped access units.
    pub fn dropped_duration_ns(&self) -> u64 {
        self.tally.dropped_duration_ns()
    }

    /// Decide whether an access unit is corrupt, updating `num_substreams` from a
    /// valid major sync. Per the MLP/TrueHD access-unit decode rules: a major sync
    /// with a bad header CRC, or any AU whose header parity fails, is undecodable.
    /// Returns `false` (not corrupt) when the AU is too short to judge or no
    /// major sync has established `num_substreams` yet — we never drop what we
    /// cannot verify. Verified against real TrueHD streams (3600/3600 AUs).
    fn au_check(&mut self, au: &[u8], is_major_sync: bool) -> AuCheck {
        let mut header_size = 4;
        let mut format_info = None;
        if is_major_sync {
            let ms = &au[4..];
            let Some(mshdr) = mlp_major_sync_header_size(ms) else {
                // A major sync too short to hold its header can't be CRC-validated
                // — NOT a safe resync/re-init point. Treat as unverifiable, not a
                // clean major sync.
                return AuCheck::Unverifiable;
            };
            if !mlp_major_sync_crc_ok(ms, mshdr) {
                // A failing major-sync checksum is only a TRUSTWORTHY corruption
                // signal once we already hold a validated baseline (num_substreams
                // captured from a PRIOR checksum-clean major sync). Before that — at
                // stream head, when no major sync has validated yet — a checksum
                // mismatch is far more likely a limitation of our own major-sync
                // header-size parse than real corruption. Arming the drop-forward
                // there is catastrophic: every following AU is collateral-dropped
                // waiting for a "validated" major sync that (if our parse is the
                // problem) NEVER comes, so the WHOLE TrueHD track is silently dropped
                // — the AUs are physically emitted late (empty until the mux end),
                // de-interleaving the track and sending decoders into an unbounded
                // memory spiral. So until we have a baseline to protect, treat a
                // checksum-failed major sync as UNVERIFIABLE (keep it) rather than
                // Corrupt. Once a clean major sync HAS established the baseline, a
                // later failure is a real re-sync trigger and still drops forward.
                if self.num_substreams.is_some() {
                    return AuCheck::Corrupt; // real corruption vs a proven baseline
                }
                return AuCheck::Unverifiable; // no baseline yet — keep, don't nuke the track
            }
            self.num_substreams = mlp_num_substreams(ms);
            header_size += mshdr;
            // The rate nibble is only trustworthy once the major sync's CRC has
            // validated (above), so capture format_info here and refine the PTS
            // cadence from it ONLY on this validated path.
            if au.len() >= 12 {
                format_info = Some(u32::from_be_bytes([au[8], au[9], au[10], au[11]]));
            }
        }
        let Some(nss) = self.num_substreams else {
            return AuCheck::Unverifiable; // no major sync seen yet — can't check parity
        };
        let Some(shs) = mlp_substr_header_size(au, header_size, nss) else {
            return AuCheck::Unverifiable; // directory runs off the AU — can't judge
        };
        if !mlp_parity_ok(au, header_size, shs) {
            return AuCheck::Corrupt;
        }
        if is_major_sync {
            AuCheck::ValidMajorSync { format_info }
        } else {
            AuCheck::Ok
        }
    }

    /// Size (bytes) of the AC-3 frame at the buffer head.
    ///
    /// Distinguishes three cases the caller must treat differently:
    /// - `Unmappable`: the header's fscod/frmsizecod don't map to a real frame
    ///   size (reserved fscod==3, or frmsizecod >= 38). The caller must drain
    ///   and resync, NOT wait for more data — waiting would stall forever.
    /// - `NeedMore`: a valid size, but the frame isn't fully buffered yet.
    /// - `Frame(n)`: a complete `n`-byte AC-3 frame is buffered.
    ///
    /// Frame sizing reuses `ac3::ac3_frame_size` so the AC-3 size table has a
    /// single source of truth shared with the AC-3 parser; a returned `0` there
    /// (reserved fscod or out-of-range frmsizecod) is the unmappable case.
    fn ac3_frame_at_head(&self) -> Ac3Size {
        if self.buf.len() < 6 {
            return Ac3Size::NeedMore;
        }
        let frame_bytes = super::ac3::ac3_frame_size(&self.buf);
        if frame_bytes == 0 {
            // Reserved fscod or out-of-range frmsizecod → unmappable header.
            return Ac3Size::Unmappable;
        }
        if self.buf.len() < frame_bytes {
            return Ac3Size::NeedMore;
        }
        Ac3Size::Frame(frame_bytes)
    }
}

/// Secondary validation for an AC-3 frame of `frame_bytes` at the buffer head:
/// is its computed end a plausible boundary? Accept when the frame fills the
/// rest of the buffer, or the bytes that follow start another AC-3 sync
/// (0x0B77) or a plausible TrueHD access unit (non-zero 12-bit length within
/// the 32 KiB cap). If none holds, the leading 0x0B77 is more likely a TrueHD
/// AU header that happens to look like AC-3, so the AC-3 reading is rejected.
fn ac3_boundary_corroborated(buf: &[u8], frame_bytes: usize) -> bool {
    if frame_bytes >= buf.len() {
        // The AC-3 frame is fully buffered and ends the data — consistent.
        return true;
    }
    let tail = &buf[frame_bytes..];
    if tail.len() < 2 {
        // Not enough following bytes to judge; accept (the next call will see
        // the continuation).
        return true;
    }
    // Another AC-3 sync immediately after?
    if tail[0] == 0x0B && tail[1] == 0x77 {
        return true;
    }
    // A plausible TrueHD AU header after? (non-zero 12-bit length, <= 32 KiB)
    let next_words = (((tail[0] as usize) << 8) | tail[1] as usize) & 0xFFF;
    next_words != 0 && next_words * 2 <= 32768
}

/// Decodability verdict for one TrueHD/MLP access unit.
enum AuCheck {
    /// Verified undecodable: a major-sync header whose CRC failed, or any AU
    /// whose substream-directory parity failed. Feeds the poison verdict.
    Corrupt,
    /// A CRC-validated major sync — a safe re-init / resync point. `format_info`
    /// (AU bytes 8..12, present when the AU is long enough) is trustworthy here,
    /// so the caller refines the PTS cadence ONLY from this validated path.
    ValidMajorSync { format_info: Option<u32> },
    /// A valid (parity-OK) non-major-sync access unit.
    Ok,
    /// Cannot be judged — a major sync too short to hold/CRC its header, or a
    /// stream head before any major sync established `num_substreams`. Never
    /// dropped on its own, and never treated as a clean resync point.
    Unverifiable,
}

/// Outcome of sizing the AC-3 frame at the TrueHD buffer head.
enum Ac3Size {
    /// fscod/frmsizecod don't map to a real frame size — resync, don't wait.
    Unmappable,
    /// A valid size, but the frame is not fully buffered yet.
    NeedMore,
    /// A complete `n`-byte AC-3 frame is buffered.
    Frame(usize),
}

// --- MLP/TrueHD access-unit integrity (per the MLP/TrueHD bitstream spec) ---

/// Major-sync header size in bytes: base 28, plus `2 + extensions*2` when the
/// extension flag (major-sync byte 25, bit 0) is set (`extensions` = byte 26
/// high nibble). `ms` is the major-sync header, i.e. AU bytes `[4..]`. `None`
/// when the AU is too short to contain the full header.
fn mlp_major_sync_header_size(ms: &[u8]) -> Option<usize> {
    if ms.len() < 28 {
        return None;
    }
    let mut size = 28;
    if ms[25] & 1 != 0 {
        size += 2 + ((ms[26] >> 4) as usize) * 2;
    }
    if ms.len() < size {
        return None;
    }
    Some(size)
}

/// Validate the MLP/TrueHD major-sync header checksum (a CRC-16 with polynomial
/// 0x002D). The stored trailer is the last 2 header bytes; because
/// MLP's checksum is byte-reversed relative to a standard CRC, a standard CRC of
/// the header body XOR the little-endian word before the trailer must equal the
/// trailer read big-endian.
fn mlp_major_sync_crc_ok(ms: &[u8], mshdr: usize) -> bool {
    if mshdr < 4 || ms.len() < mshdr {
        return false;
    }
    // The MLP major-sync checksum, `checksum16(buf, buf_size)`, is defined as
    //     crc16_2D(buf, buf_size - 2) ^ read_le16(buf + buf_size - 2)
    // evaluated with `buf_size = mshdr - 2` and its result compared to
    // `read_le16(buf + mshdr - 2)` — i.e. the 16-bit CRC (poly 0x2D, MSB-first)
    // over `ms[..mshdr-4]`, XORed with the LITTLE-ENDIAN word just before the
    // trailer, must equal the LITTLE-ENDIAN trailer word. `crc16_mlp` uses that
    // same poly-0x2D MSB-first table but yields its two bytes in the OPPOSITE
    // order to a standard little-endian CRC readout, so swap them back to match.
    // (The previous code mixed endianness —
    // little-endian XOR word but big-endian compare — so the checksum could never
    // validate any real extended major sync, silently dropping the whole track;
    // cross-verified byte-exact against real 7.1/Atmos and 5.1 discs.)
    let checksum = crc16_mlp(&ms[..mshdr - 4]).swap_bytes()
        ^ u16::from_le_bytes([ms[mshdr - 4], ms[mshdr - 3]]);
    checksum == u16::from_le_bytes([ms[mshdr - 2], ms[mshdr - 1]])
}

/// `num_substreams` from a major-sync header: it sits at bit 128 (byte 16, top
/// nibble) for both MLP (0xbb) and TrueHD (0xba) — the fields before it total
/// the same 128 bits in either layout.
fn mlp_num_substreams(ms: &[u8]) -> Option<u8> {
    ms.get(16).map(|&b| b >> 4)
}

/// Size in bytes of the substream directory that follows the AU header: each of
/// the `num_substreams` entries is 2 bytes, plus 2 more when its extraword flag
/// (entry's top bit) is set. `None` if the directory runs past the AU.
fn mlp_substr_header_size(au: &[u8], header_size: usize, num_substreams: u8) -> Option<usize> {
    let mut off = header_size;
    let mut shs = 0;
    for _ in 0..num_substreams {
        if off + 2 > au.len() {
            return None;
        }
        let extraword = au[off] & 0x80 != 0;
        shs += 2;
        off += 2;
        if extraword {
            shs += 2;
            off += 2;
        }
    }
    Some(shs)
}

/// MLP/TrueHD AU-header parity check: the XOR of the 4-byte AU header with the
/// substream directory, folded, must have its two nibbles XOR to 0xF.
fn mlp_parity_ok(au: &[u8], header_size: usize, substr_header_size: usize) -> bool {
    let end = header_size + substr_header_size;
    if end > au.len() {
        return false;
    }
    let xor_fold = |d: &[u8]| d.iter().fold(0u8, |a, &b| a ^ b);
    let p = xor_fold(&au[0..4]) ^ xor_fold(&au[header_size..end]);
    ((p >> 4) ^ p) & 0xF == 0xF
}

impl CodecParser for TrueHdParser {
    fn parse(&mut self, pes: &PesPacket) -> Vec<Frame> {
        // B1: a concealed/lost gap means the buffered TrueHD AU is TRUNCATED.
        // Splicing post-gap bytes onto it corrupts the AU framing (→ "Invalid
        // data found") and strands the PTS cadence (the non-monotonic audio-DTS
        // band at gaps). Drop the partial; with `buf` now empty the PTS-base block
        // below re-seeds the cadence from the post-gap PES, monotonic across the
        // gap. (Audio has no inter-frame refs — this is the whole audio fix.)
        //
        // Handle the discontinuity BEFORE the empty-data guard so the signal can
        // never be stranded by an empty post-gap PES (defensive; the demuxer only
        // emits non-empty PES today).
        if pes.discontinuity {
            self.buf.clear();
        }
        if pes.data.is_empty() {
            return Vec::new();
        }

        // Capture the PTS base ONLY at an access-unit boundary, i.e. when no AU
        // is mid-assembly in `buf`. TrueHD access units span PES packets; a PES
        // that merely continues an AU already in progress carries its own (later)
        // PTS, which must NOT override the running per-AU timestamp. Adopting it
        // mid-AU would snap that AU's PTS backward/forward and break the
        // monotonic +AU_DURATION_NS cadence (A/V drift). Once the buffer is empty
        // the next PES legitimately begins a new AU and seeds the base.
        if self.buf.is_empty() {
            if let Some(pts) = pes.pts {
                // Resync to the authoritative PES PTS. TrueHD AUs are a fixed
                // sample count (40 @ 48 kHz), so the per-AU `+AU_DURATION_NS`
                // cadence is sample-accurate — more so than the disc's per-PES
                // PTS, which carries the source muxer's own rounding jitter.
                //
                // Two distinct backward steps must be handled OPPOSITELY:
                //
                // 1. Small backward jitter (sub-second PES rounding): when the
                //    buffer empties exactly on a PES boundary and that PES's PTS
                //    lands a few ticks *below* the running cadence, an
                //    unconditional reset would set the next AU's timestamp below
                //    the AU just emitted, producing non-monotonic block
                //    timestamps a muxer rejects. CLAMP to the running position so
                //    output stays strictly monotonic.
                //
                // 2. Large backward step (> DISCONTINUITY_BACKSTEP_NS): this is a
                //    clip-boundary PTS reset — the title's clips are read as one
                //    concatenated stream and a non-seamless boundary resets the
                //    source PES PTS near zero. This is NOT jitter and must NOT be
                //    clamped: clamping strands the audio at the previous clip's
                //    tail cadence, so when `TimelineContinuity` later bumps the
                //    global offset for the new epoch (driven by the video
                //    back-jump) the stranded-high audio PTS is flung ~a whole
                //    clip past the frontier, producing the non-monotonic
                //    audio-DTS band on multi-clip titles (Dune: Part Two, Top
                //    Gun). ADOPT the raw reset so the per-track raw PTS that
                //    reaches `TimelineContinuity` carries the true boundary, and
                //    the corrector rebases it exactly as it already does for the
                //    DTS / AC-3 parsers (which never clamp). Same threshold the
                //    timeline corrector uses to classify a discontinuity.
                //
                // A genuine forward gap/discontinuity is always adopted by the
                // `.max()`.
                let new = pts_to_ns(pts);
                if new < self.next_pts_ns - DISCONTINUITY_BACKSTEP_NS {
                    // Clip-boundary reset: take the raw PTS, restart the cadence.
                    self.next_pts_ns = new;
                } else {
                    // Within-clip jitter (or forward progression): stay monotonic.
                    self.next_pts_ns = self.next_pts_ns.max(new);
                }
            }
        }

        self.buf.extend_from_slice(&pes.data);

        let mut frames = Vec::new();

        loop {
            if self.buf.len() < 4 {
                break;
            }

            // AC-3 frame (interleaved): starts with sync word 0x0B77.
            //
            // 0x0B 0x77 is also a legal TrueHD AU header (check-nibble 0,
            // length-high-bits 0xB → length 0xB77 words). To avoid an AC-3
            // misread stealing a real TrueHD AU, an AC-3 frame is only accepted
            // when its computed end is corroborated by what follows: end of
            // buffer (frame fills the rest), another AC-3 sync, or a plausible
            // TrueHD AU header. If none holds, this is treated as a TrueHD AU.
            if self.buf[0] == 0x0B && self.buf[1] == 0x77 {
                match self.ac3_frame_at_head() {
                    Ac3Size::Unmappable => {
                        // Permanently unmappable header at the head would stall
                        // the parser forever; resync by dropping 2 bytes so one
                        // bad frame costs one frame, not the whole buffer.
                        self.buf.drain(..2);
                        continue;
                    }
                    Ac3Size::NeedMore => break, // wait for the rest of the frame
                    Ac3Size::Frame(skip) => {
                        if ac3_boundary_corroborated(&self.buf, skip) {
                            self.buf.drain(..skip);
                            continue;
                        }
                        // Not corroborated — fall through and interpret the
                        // 0x0B77 bytes as a TrueHD access unit instead.
                    }
                }
            }

            // TrueHD access unit: lower 12 bits of first 2 bytes = length in words
            let unit_words = (((self.buf[0] as usize) << 8) | self.buf[1] as usize) & 0xFFF;
            if unit_words == 0 {
                // A zero-length AU is malformed/padding. The AU header is 4 bytes
                // (length + timing); drain the whole header, not just the length
                // word, otherwise the timing bytes get misread as the next
                // length word and produce a spurious parse on the next iteration.
                self.buf.drain(..4);
                continue;
            }
            // unit_words is masked to 12 bits, so unit_bytes <= 4095 * 2 = 8190;
            // no separate oversize-resync guard is reachable.
            let unit_bytes = unit_words * 2;
            if self.buf.len() < unit_bytes {
                break; // incomplete access unit, wait for more data
            }

            let is_major_sync = unit_bytes >= 8
                && (u32::from_be_bytes([self.buf[4], self.buf[5], self.buf[6], self.buf[7]])
                    & 0xFFFF_FFFE)
                    == 0xF872_6FBA;

            // Decodability gate. MLP/TrueHD decode state persists across access
            // units, so a corrupt AU is dropped FORWARD to the next VALIDATED
            // major sync (the clean re-init point) rather than excised in place.
            // The PTS clock advances across every dropped AU so a drop is a
            // silence gap, never a shift.
            let au = self.buf[..unit_bytes].to_vec();
            let pts = self.next_pts_ns;
            let mut emit_keyframe: Option<bool> = None; // Some(is_keyframe) => emit
            let mut drop_reason: Option<(&'static str, bool)> = None; // (reason, verified)

            if self.tally.is_poisoned() {
                // Whole track already judged dead — collateral drop (does not
                // re-feed the poison verdict).
                drop_reason = Some(("track-poisoned", false));
            } else {
                match self.au_check(&au, is_major_sync) {
                    AuCheck::ValidMajorSync { format_info } => {
                        // The rate nibble is trustworthy only now that the major
                        // sync's CRC has validated. Refine the per-AU PTS
                        // increment (48 kHz family stays the 833_333 default).
                        if let Some(fi) = format_info {
                            self.au_duration_ns = truehd_au_duration_ns(fi);
                        }
                        // A validated major sync is the ONLY clean resync point.
                        self.resync_pending = false;
                        emit_keyframe = Some(true);
                    }
                    AuCheck::Corrupt => {
                        if self.resync_pending {
                            // Part of the current drop-forward run — collateral.
                            drop_reason = Some(("resync", false));
                        } else {
                            // The trigger: one verified corruption that starts the
                            // drop-forward. Only this counts toward poison.
                            let r = if is_major_sync {
                                "major-sync-crc"
                            } else {
                                "parity"
                            };
                            drop_reason = Some((r, true));
                            self.resync_pending = true;
                        }
                    }
                    AuCheck::Ok => {
                        if self.resync_pending {
                            // Decode state is invalid until the next validated
                            // major sync, so even a parity-OK AU is undecodable
                            // here — collateral drop.
                            drop_reason = Some(("resync", false));
                        } else {
                            emit_keyframe = Some(false);
                        }
                    }
                    AuCheck::Unverifiable => {
                        if self.resync_pending {
                            // Not a validated major sync — do NOT clear the resync
                            // on it; keep dropping forward.
                            drop_reason = Some(("resync", false));
                        } else {
                            // Head of stream / too-short AU: keep (never drop what
                            // we cannot verify).
                            emit_keyframe = Some(is_major_sync);
                        }
                    }
                }
            }

            if let Some(keyframe) = emit_keyframe {
                self.tally.record_kept();
                frames.push(Frame {
                    discontinuity: false,
                    coding: None,
                    source: None,
                    pts_ns: pts,
                    keyframe,
                    data: au,
                    duration_ns: None,
                });
            } else if let Some((reason, verified)) = drop_reason {
                if verified {
                    self.tally
                        .record_drop(pts, self.au_duration_ns, au.len(), reason);
                } else {
                    self.tally
                        .record_collateral_drop(pts, self.au_duration_ns, au.len(), reason);
                }
            }
            self.buf.drain(..unit_bytes);
            self.next_pts_ns += self.au_duration_ns;
        }

        // Bound memory on malformed input: a stream that never yields a
        // complete frame must not grow the buffer without limit.
        if self.buf.len() > MAX_TRUEHD_BUF {
            self.buf.clear();
        }

        frames
    }

    fn flush(&mut self) -> Vec<Frame> {
        self.tally.log_summary();
        Vec::new()
    }

    fn codec_private(&self) -> Option<Vec<u8>> {
        None
    }
}

/// Per-bit channel counts for the TrueHD 8-channel and 6-channel presentation
/// channel-assignment masks (per the MLP/TrueHD bitstream spec). Some
/// bits denote a stereo pair (2), others a single channel (1).
const THD_8CH: [u8; 13] = [2, 1, 1, 2, 2, 2, 2, 1, 1, 2, 2, 1, 1];
const THD_6CH: [u8; 5] = [2, 1, 1, 2, 1];

/// Decode the true channel count from a TrueHD major-sync `format_info` word
/// (the 32 bits immediately after the 0xF8726FBA sync). Returns the richest
/// presentation's channel count — the 8-channel (e.g. 7.1) presentation when
/// present, else the 6-channel (5.1) one. This is the real layout that the MPLS
/// `audio_format` base field (often 5.1 even on a 7.1/Atmos track) understates.
pub fn truehd_channels(format_info: u32) -> Option<u8> {
    let ch8 = (format_info & 0x1FFF) as u16; // 8ch_presentation_channel_assignment (13 bits)
    let ch6 = ((format_info >> 15) & 0x1F) as u16; // 6ch_presentation_channel_assignment (5 bits)
    let count = |mask: u16, tbl: &[u8]| -> u8 {
        tbl.iter()
            .enumerate()
            .filter(|(i, _)| mask & (1 << i) != 0)
            .map(|(_, &c)| c)
            .sum()
    };
    if ch8 != 0 {
        Some(count(ch8, &THD_8CH))
    } else if ch6 != 0 {
        Some(count(ch6, &THD_6CH))
    } else {
        None
    }
}

/// Scan a demuxed TrueHD elementary-stream chunk for the first major sync and
/// decode its true channel count. The stream may interleave AC-3; we scan for
/// the major-sync word anywhere and read the following `format_info`.
pub fn truehd_channels_from_stream(data: &[u8]) -> Option<u8> {
    let mut p = 0;
    while p + 8 <= data.len() {
        let w = u32::from_be_bytes([data[p], data[p + 1], data[p + 2], data[p + 3]]);
        if (w & 0xFFFF_FFFE) == 0xF872_6FBA {
            let fi = u32::from_be_bytes([data[p + 4], data[p + 5], data[p + 6], data[p + 7]]);
            return truehd_channels(fi);
        }
        p += 1;
    }
    None
}

/// Real sample rate (Hz) from a TrueHD major-sync `format_info` word.
///
/// The 4-bit `ratebits` nibble sits in `format_info` bits 31..28 (the top
/// nibble), the same word `truehd_channels` reads for the channel masks. The
/// MLP rate formula is `(ratebits & 8 ? 44100 : 48000) << (ratebits & 7)`;
/// rather than evaluate it blindly this is a **strict whitelist** of the only
/// six rates that occur on real BD/UHD TrueHD. Every other code — the invalid
/// `0xF`, the formula-only `0x3`/`0xB`, and all reserved values — returns
/// `None`, so a malformed or unexpected field can never produce a wrong
/// `SamplingFrequency`; the caller falls back to its container-derived rate.
pub fn truehd_sample_rate_hz(format_info: u32) -> Option<u32> {
    match (format_info >> 28) & 0xF {
        0x0 => Some(48000),
        0x1 => Some(96000),
        0x2 => Some(192000),
        0x8 => Some(44100),
        0x9 => Some(88200),
        0xA => Some(176400),
        _ => None,
    }
}

/// Per-AU PTS increment (ns) for the rate family encoded in `format_info`.
///
/// Derived from the same whitelisted rate as [`truehd_sample_rate_hz`]: the
/// 44.1 kHz family (44.1 / 88.2 / 176.4 kHz) is `907_029` ns; everything else —
/// the entire 48 kHz family AND any unrecognised rate — keeps the exact current
/// `833_333` default, so the common case and all unknown/garbage inputs are
/// byte-identical to prior behaviour.
pub fn truehd_au_duration_ns(format_info: u32) -> i64 {
    match truehd_sample_rate_hz(format_info) {
        Some(44100) | Some(88200) | Some(176400) => AU_DURATION_NS_441,
        _ => AU_DURATION_NS,
    }
}

/// First TrueHD major sync found in a demuxed elementary-stream chunk: the
/// `format_info` word plus the Atmos signal. A single scan that the per-field
/// helpers below share, so the host probes the bitstream once for channels,
/// sample rate and Atmos.
pub struct TrueHdSyncInfo {
    /// The 32-bit word immediately after the 0xF8726FBA sync (channel masks +
    /// rate nibble). Feed to `truehd_channels` / `truehd_sample_rate_hz`.
    pub format_info: u32,
    /// `num_substreams >= 4` ⟺ a 4th (Atmos object/OAMD) substream is present.
    /// `num_substreams = msync[16] >> 4`, where `msync[0]` is the sync's 0xF8.
    /// `None` when the AU is too short to reach that byte — never guess Atmos.
    pub is_atmos: Option<bool>,
}

/// Scan a demuxed TrueHD chunk for the first major sync and return its
/// `format_info` and Atmos signal. The stream may interleave AC-3; the scan
/// advances one byte at a time and matches the sync word anywhere.
pub fn truehd_sync_info_from_stream(data: &[u8]) -> Option<TrueHdSyncInfo> {
    let mut p = 0;
    while p + 8 <= data.len() {
        let w = u32::from_be_bytes([data[p], data[p + 1], data[p + 2], data[p + 3]]);
        if (w & 0xFFFF_FFFE) == 0xF872_6FBA {
            let format_info =
                u32::from_be_bytes([data[p + 4], data[p + 5], data[p + 6], data[p + 7]]);
            // num_substreams is the top nibble of the 17th sync byte (p + 16).
            // .get() yields None — not a panic and not a false Atmos — when the
            // AU is truncated before that byte.
            let is_atmos = data.get(p + 16).map(|&b| (b >> 4) >= 4);
            return Some(TrueHdSyncInfo {
                format_info,
                is_atmos,
            });
        }
        p += 1;
    }
    None
}

/// Real sample rate (Hz) from the first major sync in a demuxed chunk, or
/// `None` if no major sync is found or its rate code is not whitelisted.
pub fn truehd_sample_rate_from_stream(data: &[u8]) -> Option<u32> {
    truehd_sync_info_from_stream(data).and_then(|s| truehd_sample_rate_hz(s.format_info))
}

/// Whether the first major sync in a demuxed chunk carries an Atmos substream.
/// `None` when no major sync is found or the AU is too short to read the
/// substream count — callers must treat `None` as "not Atmos" (never label).
pub fn truehd_is_atmos_from_stream(data: &[u8]) -> Option<bool> {
    truehd_sync_info_from_stream(data).and_then(|s| s.is_atmos)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::mux::ts::PesPacket;

    fn make_pes(data: Vec<u8>, pts: Option<i64>) -> PesPacket {
        PesPacket {
            source: None,
            pid: 0x1100,
            pts,
            dts: None,
            data,
            discontinuity: false,
        }
    }

    fn make_truehd_unit(size_bytes: usize) -> Vec<u8> {
        let words = size_bytes / 2;
        let mut data = vec![0u8; size_bytes];
        data[0] = ((words >> 8) & 0x0F) as u8;
        data[1] = (words & 0xFF) as u8;
        data
    }

    /// Turn a synthetic major-sync AU (sync bytes already set at offset 4, any
    /// `format_info` set) into one that passes the decodability gate: 1 substream,
    /// a clean substream directory, a valid major-sync CRC-16, and a valid header
    /// parity nibble. Mirrors what a real encoder writes (verified against real
    /// TrueHD streams). The AU must be ≥ 36 bytes (4 AU header + 28 major-sync
    /// header + 2 directory + slack), which every `make_truehd_unit(≥200)` is.
    fn finalize_major_sync(au: &mut [u8]) {
        const MSHDR: usize = 28; // no extension (byte 25 clear)
        // num_substreams = 1 → major-sync byte 16 (AU[20]) top nibble.
        au[20] = (au[20] & 0x0F) | 0x10;
        // Substream directory entry at AU[4+MSHDR] = AU[32]: extraword flag clear.
        au[32] &= 0x7F;
        // Major-sync checksum, built EXACTLY as `mlp_major_sync_crc_ok` verifies it
        // (the MLP checksum16): swap_bytes(crc16_mlp(body)) ^ LE word before
        // the trailer, stored little-endian in the trailer.
        let body_end = 4 + MSHDR - 4; // AU[4..28]
        let crc = super::crc16_mlp(&au[4..body_end]).swap_bytes()
            ^ u16::from_le_bytes([au[body_end], au[body_end + 1]]);
        au[4 + MSHDR - 2] = (crc & 0xFF) as u8;
        au[4 + MSHDR - 1] = (crc >> 8) as u8;
        // Parity: choose the AU check nibble (AU[0] high bits) so the header +
        // directory fold to 0xF. The length low nibble (AU[0] low bits) is kept.
        let hi = au[0] & 0x0F;
        let p0 = (hi ^ au[1] ^ au[2] ^ au[3]) ^ (au[32] ^ au[33]);
        let c = ((p0 >> 4) ^ (p0 & 0x0F) ^ 0x0F) & 0x0F;
        au[0] = (c << 4) | hi;
    }

    /// Give a synthetic NON-major-sync AU a valid header parity nibble (1
    /// substream, directory at AU[4..6]), so it passes the gate once a preceding
    /// major sync has established `num_substreams`.
    fn finalize_normal_parity(au: &mut [u8]) {
        au[4] &= 0x7F; // no extraword
        let hi = au[0] & 0x0F;
        let p0 = (hi ^ au[1] ^ au[2] ^ au[3]) ^ (au[4] ^ au[5]);
        let c = ((p0 >> 4) ^ (p0 & 0x0F) ^ 0x0F) & 0x0F;
        au[0] = (c << 4) | hi;
    }

    fn valid_major_sync() -> Vec<u8> {
        let mut u = make_truehd_unit(200);
        u[4..8].copy_from_slice(&0xF872_6FBAu32.to_be_bytes());
        finalize_major_sync(&mut u);
        u
    }

    fn valid_normal_au() -> Vec<u8> {
        let mut u = make_truehd_unit(200);
        finalize_normal_parity(&mut u);
        u
    }

    #[test]
    fn corrupt_major_sync_drops_forward_to_next_valid() {
        // MLP state carries across AUs, so a corrupt AU is dropped FORWARD to the
        // next valid major sync (the clean re-init point). Sequence: valid MS,
        // corrupt MS (bad CRC), a normal AU, then a valid MS. Only the two valid
        // major syncs survive; the corrupt MS and the intervening normal AU are
        // dropped (the latter because decode state is poisoned until re-init).
        let mut parser = TrueHdParser::new();
        let ms1 = valid_major_sync();
        let mut ms_bad = valid_major_sync();
        ms_bad[10] ^= 0xFF; // corrupt a CRC-covered header byte
        let normal = valid_normal_au(); // clean parity, but arrives mid-resync
        let ms2 = valid_major_sync();

        let mut data = ms1.clone();
        data.extend_from_slice(&ms_bad);
        data.extend_from_slice(&normal);
        data.extend_from_slice(&ms2);
        let mut frames = parser.parse(&make_pes(data, Some(90000)));
        frames.extend(parser.flush());

        assert_eq!(frames.len(), 2, "only the two valid major syncs survive");
        assert!(frames[0].keyframe && frames[1].keyframe);
        assert_eq!(
            parser.dropped_frames(),
            2,
            "corrupt MS + poisoned normal AU"
        );
    }

    #[test]
    fn crc_failed_head_major_sync_is_kept_not_track_killed() {
        // REGRESSION (every TrueHD title muxed after the checksum gate landed): a
        // real-world major sync whose checksum our own header-size parse can't
        // validate must NOT, at stream head — before ANY major sync has validated —
        // arm the drop-forward. Doing so latched `resync_pending` on the very first
        // AU and collateral-dropped EVERY following AU forever (no "validated" major
        // sync ever came), silently dropping the entire TrueHD track: its AUs were
        // emitted only at mux end, de-interleaving the track and spiralling decoders
        // into unbounded memory ("decoder ran out of memory"). With no baseline to
        // protect, a checksum-failed major sync is KEPT and the audio flows.
        let mut parser = TrueHdParser::new();
        let mut ms_bad = valid_major_sync();
        ms_bad[10] ^= 0xFF; // break a checksum-covered header byte → checksum fails
        let mut data = ms_bad;
        for _ in 0..6 {
            data.extend_from_slice(&valid_normal_au());
        }
        let mut frames = parser.parse(&make_pes(data, Some(90000)));
        frames.extend(parser.flush());
        assert_eq!(
            frames.len(),
            7,
            "no baseline yet: the CRC-failed head major sync + all following AUs are \
             kept, not dropped (got {})",
            frames.len()
        );
        assert_eq!(
            parser.dropped_frames(),
            0,
            "nothing dropped without a validated baseline to protect"
        );
        // And the invariant still holds ONCE a baseline exists: after a genuinely
        // valid major sync, a later corrupt one IS dropped (see
        // `corrupt_major_sync_drops_forward_to_next_valid`).
    }

    /// Independent bitwise CRC-16 (poly 0x002D, init 0, MSB-first) — a SEPARATE
    /// oracle from `crc16_mlp`, so a fixture built with it is not tautological
    /// with the validator under test. Anchored to the catalogue check value
    /// (0x4FF7 for "123456789") so the oracle itself is proven correct without
    /// reference to the code under test.
    fn ref_crc16_2d(data: &[u8]) -> u16 {
        let mut crc: u16 = 0;
        for &b in data {
            crc ^= (b as u16) << 8;
            for _ in 0..8 {
                crc = if crc & 0x8000 != 0 {
                    (crc << 1) ^ 0x002D
                } else {
                    crc << 1
                };
            }
        }
        crc
    }

    #[test]
    fn extended_major_sync_crc_validates_and_rejects() {
        // COVERAGE GAP (the endianness bug that once "silently dropped the whole
        // track" on real 7.1/Atmos): the EXTENDED major-sync header path
        // (ms[25]&1 set, mshdr = 28 + 2 + 2*n) had ZERO test coverage — every
        // other fixture builds only the basic 28-byte header. Build an extended
        // header whose trailer is an INDEPENDENTLY-computed oracle (ref_crc16_2d,
        // NOT crc16_mlp) stored LITTLE-ENDIAN, and assert the validator accepts
        // it, rejects a body corruption, and rejects the same trailer stored
        // big-endian (which is exactly the endianness-mix regression).
        assert_eq!(
            ref_crc16_2d(b"123456789"),
            0x4FF7,
            "oracle anchored to catalogue"
        );

        // n = 3 extension words → mshdr = 28 + 2 + 2*3 = 36.
        let n = 3usize;
        let mshdr = 28 + 2 + 2 * n;
        assert_eq!(mshdr, 36);
        let mut ms = vec![0u8; 40]; // slack past the 36-byte header
        // Non-trivial, varied body so the CRC is a meaningful function of it.
        for (i, b) in ms.iter_mut().enumerate().take(mshdr - 4) {
            *b = (0x37u8).wrapping_add((i as u8).wrapping_mul(0x53));
        }
        ms[25] |= 1; // extension flag → selects the extended header size
        ms[26] = (ms[26] & 0x0F) | ((n as u8) << 4); // extension word count in high nibble

        // The 2-byte "penultimate" word (between the CRC-covered body and the
        // trailer). Chosen non-zero and non-palindromic so the LE/BE distinction
        // is observable.
        ms[mshdr - 4] = 0x12;
        ms[mshdr - 3] = 0x34;

        // Oracle: checksum16 = crc16_2D(body).swap_bytes() ^ le16(penultimate),
        // computed with the INDEPENDENT ref CRC, then stored LITTLE-ENDIAN.
        let le_word = u16::from_le_bytes([ms[mshdr - 4], ms[mshdr - 3]]);
        let trailer = ref_crc16_2d(&ms[..mshdr - 4]).swap_bytes() ^ le_word;
        ms[mshdr - 2] = (trailer & 0xFF) as u8;
        ms[mshdr - 1] = (trailer >> 8) as u8;
        assert_ne!(
            ms[mshdr - 2],
            ms[mshdr - 1],
            "trailer bytes must differ so the LE/BE swap below is a real distinction"
        );

        // The extended header size is computed from ms[25]/ms[26].
        assert_eq!(
            mlp_major_sync_header_size(&ms),
            Some(mshdr),
            "extended header size = 28 + 2 + 2*n"
        );
        // The validator accepts the independently-built extended major sync.
        assert!(
            mlp_major_sync_crc_ok(&ms, mshdr),
            "valid extended major-sync checksum must validate"
        );

        // A single corrupted body byte must be rejected.
        let mut corrupt = ms.clone();
        corrupt[10] ^= 0xFF;
        assert!(
            !mlp_major_sync_crc_ok(&corrupt, mshdr),
            "a corrupted extended major sync must be rejected"
        );

        // The endianness regression: the SAME checksum stored big-endian must be
        // rejected. A validator that reads the trailer big-endian (the shipped
        // bug) would instead accept this and reject the correct LE form above.
        let mut swapped = ms.clone();
        swapped.swap(mshdr - 2, mshdr - 1);
        assert!(
            !mlp_major_sync_crc_ok(&swapped, mshdr),
            "a big-endian-stored trailer must be rejected (little-endian is load-bearing)"
        );
    }

    #[test]
    fn parity_failure_is_dropped() {
        // A normal AU whose header parity is broken (after a major sync sets
        // num_substreams) is undecodable → dropped.
        let mut parser = TrueHdParser::new();
        let ms1 = valid_major_sync();
        let mut bad = valid_normal_au();
        // A single-nibble flip: MLP's nibble-fold parity is blind
        // to a full-byte flip, which changes both nibbles equally and cancels.
        bad[2] ^= 0x01;
        let ms2 = valid_major_sync();
        let mut data = ms1;
        data.extend_from_slice(&bad);
        data.extend_from_slice(&ms2);
        let mut frames = parser.parse(&make_pes(data, Some(90000)));
        frames.extend(parser.flush());
        assert_eq!(frames.len(), 2, "the parity-broken AU is dropped");
        assert_eq!(parser.dropped_frames(), 1);
    }

    #[test]
    fn drop_forward_preserves_av_sync_no_shift() {
        // THE INVARIANT: the resumed major sync keeps the exact PTS it would have
        // had with no drop — base + 3 AU durations (MS1, corrupt-MS, normal, MS2)
        // — so the drop is a silence gap, never a shift.
        let mut parser = TrueHdParser::new();
        let ms1 = valid_major_sync();
        let mut ms_bad = valid_major_sync();
        ms_bad[10] ^= 0xFF;
        let normal = valid_normal_au();
        let ms2 = valid_major_sync();
        let mut data = ms1;
        data.extend_from_slice(&ms_bad);
        data.extend_from_slice(&normal);
        data.extend_from_slice(&ms2);
        let mut frames = parser.parse(&make_pes(data, Some(90000)));
        frames.extend(parser.flush());
        assert_eq!(frames.len(), 2);
        assert_eq!(
            frames[1].pts_ns - frames[0].pts_ns,
            3 * AU_DURATION_NS,
            "resumed major sync keeps its true timeline (gap, not shift)"
        );
    }

    #[test]
    fn transient_corruptions_do_not_poison_whole_track() {
        // Regression (audit HIGH): TrueHD drop-forward must NOT amplify a couple
        // of transient errors into a false whole-track poison. Two corruptions,
        // each forcing a long collateral resync run past 200 total AUs, must
        // leave the track un-poisoned and keep the good audio that follows.
        let mut parser = TrueHdParser::new();
        let mut data = valid_major_sync();
        // Corruption #1 then a long run of normal AUs (all collateral-dropped
        // while resyncing — no major sync to re-init on).
        let mut bad1 = valid_normal_au();
        bad1[2] ^= 0x01; // single-nibble parity break
        data.extend_from_slice(&bad1);
        for _ in 0..210 {
            data.extend_from_slice(&valid_normal_au());
        }
        // A valid major sync resumes; the good AUs after it MUST be kept.
        data.extend_from_slice(&valid_major_sync());
        for _ in 0..5 {
            data.extend_from_slice(&valid_normal_au());
        }
        let frames = parser.parse(&make_pes(data, Some(90000)));
        assert!(
            !parser.tally.is_poisoned(),
            "two transient errors must not poison the track"
        );
        // MS1 + resumed MS2 + the 5 good AUs after it survive.
        assert_eq!(
            frames.len(),
            7,
            "post-resync good audio is kept, not poisoned away"
        );
        assert!(
            parser.dropped_frames() > 200,
            "the resync run was still counted for reporting"
        );
    }

    #[test]
    fn corrupt_major_sync_rate_nibble_does_not_shift_pts() {
        // Regression (audit MED): a corrupt major sync whose rate nibble decodes
        // to the 44.1 kHz family must NOT refine au_duration_ns — the rate is only
        // trustworthy after the CRC validates. Otherwise the resumed 48 kHz audio
        // is shifted (not gapped).
        let mut parser = TrueHdParser::new();
        let ms1 = valid_major_sync(); // 48 kHz
        let mut ms_bad = valid_major_sync();
        // Set the rate nibble (top nibble of format_info = au[8]) to 0x8 (44.1k).
        // au[8] is CRC-covered, so this also breaks the major-sync CRC → corrupt.
        ms_bad[8] = (ms_bad[8] & 0x0F) | 0x80;
        let ms2 = valid_major_sync(); // 48 kHz
        let mut data = ms1;
        data.extend_from_slice(&ms_bad);
        data.extend_from_slice(&ms2);
        let frames = parser.parse(&make_pes(data, Some(90000)));
        assert_eq!(frames.len(), 2, "corrupt MS dropped; MS1 and MS2 survive");
        assert_eq!(
            frames[1].pts_ns - frames[0].pts_ns,
            2 * AU_DURATION_NS,
            "resumed audio keeps the 48 kHz cadence — the corrupt MS's 44.1k rate was ignored"
        );
    }

    #[test]
    fn too_short_major_sync_does_not_clear_resync() {
        // Regression (audit LOW): while resyncing, a major sync too short to hold
        // (and CRC-validate) its header must NOT be treated as a clean resync
        // point — the runt is dropped and only a real validated major sync resumes.
        let mut parser = TrueHdParser::new();
        let ms1 = valid_major_sync();
        let mut bad = valid_normal_au();
        bad[2] ^= 0x01; // parity break → triggers resync
        // An 8-byte "major sync": length=4 words, sync at bytes 4..8, too short
        // to hold the 28-byte major-sync header.
        let runt = vec![0x00, 0x04, 0x00, 0x00, 0xF8, 0x72, 0x6F, 0xBA];
        let ms2 = valid_major_sync();
        let mut data = ms1;
        data.extend_from_slice(&bad);
        data.extend_from_slice(&runt);
        data.extend_from_slice(&ms2);
        let frames = parser.parse(&make_pes(data, Some(90000)));
        assert_eq!(frames.len(), 2, "the runt major sync did not resume decode");
        for f in &frames {
            assert_eq!(
                f.data.len(),
                200,
                "only the real 200-byte major syncs survive"
            );
        }
    }

    #[test]
    fn clean_truehd_stream_drops_nothing() {
        // A run of valid AUs passes untouched — zero false positives (the CRC and
        // parity are verified against real TrueHD output).
        let mut parser = TrueHdParser::new();
        let mut data = valid_major_sync();
        for _ in 0..5 {
            data.extend_from_slice(&valid_normal_au());
        }
        let frames = parser.parse(&make_pes(data, Some(90000)));
        assert_eq!(frames.len(), 6);
        assert_eq!(parser.dropped_frames(), 0);
    }

    fn make_ac3_frame() -> Vec<u8> {
        // Minimal AC-3 frame: sync 0x0B77, fscod=0 (48kHz), frmsizecod=0 (64 words = 128 bytes)
        let mut data = vec![0u8; 128];
        data[0] = 0x0B;
        data[1] = 0x77;
        data[4] = 0x00; // fscod=0, frmsizecod=0
        data
    }

    #[test]
    fn parse_empty_pes() {
        let mut parser = TrueHdParser::new();
        let pes = make_pes(Vec::new(), Some(0));
        assert!(parser.parse(&pes).is_empty());
    }

    #[test]
    fn parse_single_unit() {
        let mut parser = TrueHdParser::new();
        let unit = make_truehd_unit(200);
        let pes = make_pes(unit, Some(90000));
        let frames = parser.parse(&pes);
        assert_eq!(frames.len(), 1);
        assert_eq!(frames[0].data.len(), 200);
    }

    #[test]
    fn parse_unit_spanning_two_pes() {
        let mut parser = TrueHdParser::new();
        let unit = make_truehd_unit(200);
        let mid = 100;

        let pes1 = make_pes(unit[..mid].to_vec(), Some(90000));
        assert!(parser.parse(&pes1).is_empty());

        let pes2 = make_pes(unit[mid..].to_vec(), Some(93000));
        let frames = parser.parse(&pes2);
        assert_eq!(frames.len(), 1);
        assert_eq!(frames[0].data.len(), 200);
    }

    #[test]
    fn discontinuity_drops_truncated_partial() {
        // B1: a partial TrueHD unit is buffered, then a concealed gap (PES marked
        // discontinuity) carries a fresh unit. The truncated partial must be
        // DROPPED — splicing it makes the length-prefixed framer emit a
        // wrong-size unit (corrupt AU framing) and re-seeds the PTS cadence from
        // the post-gap PES rather than stranding it (the non-monotonic audio-DTS
        // band at gaps).
        let mut parser = TrueHdParser::new();

        // PES 1: first 150 bytes of a 300-byte unit (length prefix says 300, only
        // 150 present) → held, nothing emitted.
        let partial = make_truehd_unit(300);
        let pes1 = make_pes(partial[..150].to_vec(), Some(90000));
        assert!(parser.parse(&pes1).is_empty(), "partial unit held");

        // Concealed gap: a fresh 200-byte unit at a forward PTS jump.
        let fresh = make_truehd_unit(200);
        let pes2 = PesPacket {
            source: None,
            pid: 0x1100,
            pts: Some(180000),
            dts: None,
            data: fresh.clone(),
            discontinuity: true,
        };
        let frames = parser.parse(&pes2);
        assert_eq!(frames.len(), 1, "exactly one clean unit across the gap");
        assert_eq!(
            frames[0].data.len(),
            200,
            "emitted unit is the fresh 200-byte one, not a 300-byte splice"
        );
        assert_eq!(
            frames[0].data, fresh,
            "unit bytes are the fresh post-gap unit"
        );
        assert_eq!(
            frames[0].pts_ns,
            pts_to_ns(180000),
            "cadence re-bases to the post-gap PTS across the cleared buffer"
        );
    }

    #[test]
    fn parse_multiple_units_incrementing_pts() {
        let mut parser = TrueHdParser::new();
        let mut data = make_truehd_unit(100);
        data.extend_from_slice(&make_truehd_unit(120));
        let pes = make_pes(data, Some(90000));
        let frames = parser.parse(&pes);
        assert_eq!(frames.len(), 2);
        assert_eq!(frames[0].data.len(), 100);
        assert_eq!(frames[1].data.len(), 120);
        assert_eq!(frames[1].pts_ns - frames[0].pts_ns, AU_DURATION_NS);
    }

    #[test]
    fn pes_pts_lagging_the_au_cadence_never_emits_backward() {
        // Regression: the per-AU cadence is sample-accurate, but a PES boundary
        // can carry a PTS that lags it slightly (source-muxer rounding jitter).
        // When the buffer empties exactly on that boundary, an unconditional
        // reset to the PES PTS snapped the next AU's timestamp BELOW the AU just
        // emitted — the non-monotonic block timestamps a muxer rejects (the
        // Top Gun / Dune: Part Two case). The reset must clamp forward-only.
        let mut parser = TrueHdParser::new();
        let au = make_truehd_unit(100);
        // PES1: three complete AUs at pts 90000 — buffer empties, cadence runs
        // ahead to 90000_ns + 3*AU_DURATION_NS.
        let mut d1 = au.clone();
        d1.extend_from_slice(&au);
        d1.extend_from_slice(&au);
        let f1 = parser.parse(&make_pes(d1, Some(90000)));
        assert_eq!(f1.len(), 3);
        let last1 = f1.last().unwrap().pts_ns;
        // PES2's PTS (90001) maps to fewer ns than the running cadence — pre-fix
        // this snapped backward.
        let f2 = parser.parse(&make_pes(au.clone(), Some(90001)));
        assert_eq!(f2.len(), 1);
        assert!(
            f2[0].pts_ns >= last1,
            "AU pts must not go backward when PES PTS lags the cadence: got {} after {}",
            f2[0].pts_ns,
            last1
        );
    }

    #[test]
    fn clip_boundary_pts_reset_is_adopted_not_clamped() {
        // Regression (Dune: Part Two / Top Gun non-monotonic audio-DTS band):
        // a title's clips are read as one concatenated stream, so at a
        // non-seamless boundary the source PES PTS resets near zero — a LARGE
        // backward step (> DISCONTINUITY_BACKSTEP_NS), NOT muxer jitter. The
        // parser must ADOPT that reset (restart the cadence at the raw PTS), the
        // same way the DTS / AC-3 parsers pass raw PTS through, so the per-track
        // raw PTS reaching TimelineContinuity carries the true boundary and the
        // corrector can rebase it. Clamping it forward (the old `.max()`) stranded
        // the audio at the previous clip's tail; when the global offset later
        // bumped for the new epoch the stranded audio was flung ~a clip past the
        // frontier — the non-monotonic band.
        let mut parser = TrueHdParser::new();
        let au = make_truehd_unit(100);
        // Clip 1: an AU at PES PTS = 10s (90000 ticks/s → 900_000 ticks). Buffer
        // empties, so the next PES seeds a fresh base.
        let clip1_pts = 90_000 * 10; // 10 s in 90 kHz ticks
        let f1 = parser.parse(&make_pes(au.clone(), Some(clip1_pts)));
        assert_eq!(f1.len(), 1);
        let last1 = f1[0].pts_ns;
        assert_eq!(last1, pts_to_ns(clip1_pts));
        // Clip 2: PES PTS resets to 0 — 10 s backward, far beyond the 3 s
        // discontinuity threshold. Must be adopted, not clamped to the cadence.
        let f2 = parser.parse(&make_pes(au.clone(), Some(0)));
        assert_eq!(f2.len(), 1);
        assert_eq!(
            f2[0].pts_ns, 0,
            "clip-boundary PTS reset must be adopted raw (got {}, expected the \
             reset value 0 — clamping to the previous clip's cadence is the bug)",
            f2[0].pts_ns
        );
        assert!(
            f2[0].pts_ns < last1,
            "the reset frame must land below the previous clip's tail, not above it"
        );
    }

    #[test]
    fn skip_interleaved_ac3() {
        let mut parser = TrueHdParser::new();
        let ac3 = make_ac3_frame();
        let truehd = make_truehd_unit(200);
        let mut data = ac3;
        data.extend_from_slice(&truehd);
        let pes = make_pes(data, Some(90000));
        let frames = parser.parse(&pes);
        assert_eq!(frames.len(), 1);
        assert_eq!(frames[0].data.len(), 200);
    }

    #[test]
    fn continuation_pes_pts_does_not_override_au_in_progress() {
        // An AU split across two PES packets: the FIRST PES (pts 90000) begins
        // the AU; the SECOND PES (pts 99999, a later timestamp) merely continues
        // it. The emitted AU must keep the first PES's PTS, not adopt the
        // continuation PES's later timestamp.
        let mut parser = TrueHdParser::new();
        let unit = make_truehd_unit(200);
        let mid = 100;

        let pes1 = make_pes(unit[..mid].to_vec(), Some(90000));
        assert!(parser.parse(&pes1).is_empty(), "AU held mid-assembly");

        // Continuation PES carries a later PTS that must be ignored for this AU.
        let pes2 = make_pes(unit[mid..].to_vec(), Some(99999));
        let frames = parser.parse(&pes2);
        assert_eq!(frames.len(), 1);
        assert_eq!(
            frames[0].pts_ns,
            pts_to_ns(90000),
            "AU keeps the PTS of the PES that began it, not the continuation PES"
        );
    }

    #[test]
    fn new_au_after_empty_buffer_takes_new_pes_pts() {
        // After an AU fully drains (buffer empty), the next PES legitimately
        // seeds a fresh PTS base.
        let mut parser = TrueHdParser::new();
        let f1 = parser.parse(&make_pes(make_truehd_unit(200), Some(90000)));
        assert_eq!(f1.len(), 1);
        assert_eq!(f1[0].pts_ns, pts_to_ns(90000));

        // Buffer is now empty; a new PES with a new PTS starts a new AU.
        let f2 = parser.parse(&make_pes(make_truehd_unit(200), Some(180000)));
        assert_eq!(f2.len(), 1);
        assert_eq!(
            f2[0].pts_ns,
            pts_to_ns(180000),
            "new AU after empty buffer adopts the new PES PTS"
        );
    }

    #[test]
    fn zero_length_au_drains_full_header() {
        // A zero-length AU header (4 bytes: length=0 + timing) must be skipped
        // whole. If only 2 bytes were drained the timing bytes would be misread
        // as a bogus length word. Here the timing bytes are 0x01 0x90 (= 0x190 =
        // 400 words = 800 bytes) which, if misread, would stall the parser
        // waiting for 800 bytes that never come. Draining 4 lets the following
        // real unit parse.
        let mut parser = TrueHdParser::new();
        let mut data = vec![0x00, 0x00, 0x01, 0x90]; // length=0, timing=0x0190
        data.extend_from_slice(&make_truehd_unit(200));
        let frames = parser.parse(&make_pes(data, Some(90000)));
        assert_eq!(frames.len(), 1, "real unit parses after zero-length header");
        assert_eq!(frames[0].data.len(), 200);
    }

    #[test]
    fn unmappable_ac3_header_resyncs_not_stalls() {
        // A permanently unmappable 0x0B77 header at the buffer head (reserved
        // fscod==3) must NOT stall the parser. It used to be treated as
        // "incomplete, wait" and break forever, dropping every following AU.
        // Now it resyncs (drains 2 bytes) so a clean TrueHD unit behind it is
        // eventually emitted.
        let mut parser = TrueHdParser::new();
        // Unmappable AC-3-looking head: 0x0B77, byte4 fscod=3 (0xC0).
        let mut data = vec![0x0B, 0x77, 0x00, 0x00, 0xC0, 0x00];
        // A clean TrueHD AU follows.
        data.extend_from_slice(&make_truehd_unit(200));
        let frames = parser.parse(&make_pes(data, Some(90000)));
        assert_eq!(
            frames.len(),
            1,
            "TrueHD AU behind a bad header is recovered"
        );
        assert_eq!(frames[0].data.len(), 200);
        assert!(parser.buf.is_empty(), "buffer fully consumed, no stall");
    }

    #[test]
    fn truehd_au_with_0b77_head_not_stolen_by_ac3() {
        // A TrueHD AU whose first two bytes are 0x0B 0x77 (length 0xB77 = 2935
        // words = 5870 bytes) must NOT be misrouted to the AC-3 path. The AC-3
        // size for this header (fscod from byte4) would close the boundary in
        // the wrong place; the secondary corroboration rejects it because the
        // computed AC-3 end is not followed by another AC-3 sync / TrueHD AU.
        let mut parser = TrueHdParser::new();
        // 5870-byte AU starting with 0x0B 0x77. Byte 4 = 0x00 → AC-3 would
        // size it as fscod=0, frmsizecod=0 → 128 bytes. The bytes at offset 128
        // are zeros (next_words==0) → not corroborated → kept as TrueHD.
        let mut unit = vec![0u8; 5870];
        unit[0] = 0x0B; // 0xB high nibble of the 12-bit length, check nibble 0
        unit[1] = 0x77; // low byte of length 0xB77
        let frames = parser.parse(&make_pes(unit, Some(90000)));
        assert_eq!(frames.len(), 1, "0x0B77-headed TrueHD AU kept whole");
        assert_eq!(
            frames[0].data.len(),
            5870,
            "AU sized by TrueHD length, not AC-3 frame size"
        );
    }

    #[test]
    fn codec_private_none() {
        let parser = TrueHdParser::new();
        assert!(parser.codec_private().is_none());
    }

    #[test]
    fn truehd_channels_71_from_8ch_presentation() {
        // 8ch presentation assignment bits 0-4 (LR,C,LFE,LsRs,back-LR) = 2+1+1+2+2 = 8.
        let format_info = 0x1F; // low 13 bits = 0x1F
        assert_eq!(truehd_channels(format_info), Some(8));
    }

    #[test]
    fn truehd_channels_51_from_6ch_presentation() {
        // No 8ch presentation; 6ch bits 0-3 (LR,C,LFE,LsRs) = 2+1+1+2 = 6.
        let format_info = 0xF << 15; // 6ch field = 0xF, 8ch field = 0
        assert_eq!(truehd_channels(format_info), Some(6));
    }

    #[test]
    fn truehd_channels_scan_finds_major_sync() {
        // [junk][major sync 0xF8726FBA][format_info: 8ch=0x1F -> 7.1]
        let mut data = vec![0xAA, 0xBB];
        data.extend_from_slice(&0xF872_6FBAu32.to_be_bytes());
        data.extend_from_slice(&0x0000_001Fu32.to_be_bytes());
        assert_eq!(truehd_channels_from_stream(&data), Some(8));
    }

    // --- truehd_channels: per-bit mask channel counts (MLP channel table) ---

    #[test]
    fn truehd_channels_8ch_single_bit_counts() {
        // THD_8CH = [2,1,1,2,2,2,2,1,1,2,2,1,1]. A single set bit must yield
        // exactly that bit's channel count. Bit 0 → 2 (L/R pair), bit 1 → 1 (C),
        // bit 2 → 1 (LFE), bit 7 → 1.
        assert_eq!(truehd_channels(1 << 0), Some(2));
        assert_eq!(truehd_channels(1 << 1), Some(1));
        assert_eq!(truehd_channels(1 << 2), Some(1));
        assert_eq!(truehd_channels(1 << 7), Some(1));
    }

    #[test]
    fn truehd_channels_8ch_all_bits_set() {
        // All 13 8ch bits set = 2+1+1+2+2+2+2+1+1+2+2+1+1 = 20. ch8 field is the
        // low 13 bits (0x1FFF).
        assert_eq!(truehd_channels(0x1FFF), Some(20));
    }

    #[test]
    fn truehd_channels_6ch_used_only_when_8ch_zero() {
        // The 8ch presentation takes priority; the 6ch field (bits 15-19) is read
        // ONLY when ch8 == 0. THD_6CH = [2,1,1,2,1]. Set 6ch bit 0 (→2) while
        // 8ch is zero: 6ch field value 1 at shift 15.
        assert_eq!(truehd_channels(1 << 15), Some(2));
        // All 5 6ch bits = 2+1+1+2+1 = 7. 0x1F << 15.
        assert_eq!(truehd_channels(0x1F << 15), Some(7));
    }

    #[test]
    fn truehd_channels_8ch_wins_over_6ch_when_both_present() {
        // When BOTH fields are non-zero, the richer 8ch presentation is used.
        // 8ch = bit0 (→2), 6ch = all bits (would be 7) → result must be 2, the
        // 8ch count, proving the `if ch8 != 0` branch wins.
        let fi = (1u32 << 0) | (0x1F << 15);
        assert_eq!(truehd_channels(fi), Some(2));
    }

    #[test]
    fn truehd_channels_none_when_both_fields_zero() {
        // No presentation flags set → None (can't determine layout).
        assert_eq!(truehd_channels(0), None);
        // Bits outside both fields (e.g. bit 13, bit 14, bits 20-31) don't count
        // as a presentation and must still yield None.
        assert_eq!(truehd_channels(1 << 13), None);
        assert_eq!(truehd_channels(1 << 20), None);
    }

    // --- truehd_channels_from_stream: major-sync variant bit + scan ---

    #[test]
    fn channels_from_stream_matches_variant_sync_0xfb() {
        // The sync match masks the low bit: 0xF8726FBA & 0xFFFFFFFE == base, and
        // 0xF8726FBB (the +1 variant) matches the same masked pattern. A stream
        // carrying 0xF8726FBB must still be recognised.
        let mut data = vec![0x00];
        data.extend_from_slice(&0xF872_6FBBu32.to_be_bytes());
        data.extend_from_slice(&0x0000_001Fu32.to_be_bytes());
        assert_eq!(truehd_channels_from_stream(&data), Some(8));
    }

    #[test]
    fn channels_from_stream_none_without_major_sync() {
        // No major sync anywhere → None, no panic, scan terminates.
        let data = vec![0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88];
        assert_eq!(truehd_channels_from_stream(&data), None);
    }

    #[test]
    fn channels_from_stream_too_short_for_format_info() {
        // Sync present but fewer than 8 bytes total → the `p + 8 <= len` guard
        // prevents reading format_info out of bounds → None.
        let data = 0xF872_6FBAu32.to_be_bytes().to_vec(); // 4 bytes only
        assert_eq!(truehd_channels_from_stream(&data), None);
    }

    #[test]
    fn channels_from_stream_unaligned_sync() {
        // The scan advances 1 byte at a time, so a major sync at an odd offset
        // is still found. Place it at offset 3.
        let mut data = vec![0xAA, 0xBB, 0xCC];
        data.extend_from_slice(&0xF872_6FBAu32.to_be_bytes());
        data.extend_from_slice(&(0x1Fu32).to_be_bytes());
        assert_eq!(truehd_channels_from_stream(&data), Some(8));
    }

    // --- AU length field: 12-bit mask, partial AU, is_major_sync keyframe ---

    #[test]
    fn au_length_uses_low_12_bits_only() {
        // unit_words = ((b0<<8)|b1) & 0xFFF. The top 4 bits of b0 (the MLP
        // check/access-unit nibble) must NOT inflate the length. b0 = 0xF1
        // (nibble 0xF, low 0x1), b1 = 0x00 → words = 0x100 = 256 → 512 bytes.
        let mut parser = TrueHdParser::new();
        let mut unit = vec![0u8; 512];
        unit[0] = 0xF1; // high nibble 0xF must be masked off
        unit[1] = 0x00;
        let f = parser.parse(&make_pes(unit, Some(90000)));
        assert_eq!(f.len(), 1);
        assert_eq!(
            f[0].data.len(),
            512,
            "length sized from low 12 bits (0x100 words), nibble masked"
        );
    }

    #[test]
    fn au_with_major_sync_is_keyframe() {
        // An AU whose bytes 4-7 hold the major sync (0xF8726FBA, low bit masked)
        // is a restart point → keyframe. Build a >=8-byte AU with the sync at
        // offset 4. words = 100 → 200 bytes.
        let mut parser = TrueHdParser::new();
        let mut unit = make_truehd_unit(200);
        unit[4..8].copy_from_slice(&0xF872_6FBAu32.to_be_bytes());
        finalize_major_sync(&mut unit);
        let f = parser.parse(&make_pes(unit, Some(90000)));
        assert_eq!(f.len(), 1);
        assert!(f[0].keyframe, "major-sync AU must be flagged keyframe");
    }

    #[test]
    fn au_without_major_sync_is_not_keyframe() {
        // A plain AU (no major sync at offset 4) is not a keyframe.
        let mut parser = TrueHdParser::new();
        let f = parser.parse(&make_pes(make_truehd_unit(200), Some(90000)));
        assert_eq!(f.len(), 1);
        assert!(!f[0].keyframe);
    }

    #[test]
    fn major_sync_variant_bit_also_keyframe() {
        // The keyframe check masks the low bit (0xFFFF_FFFE), so the 0xF8726FBB
        // variant must also be detected as a major sync.
        let mut parser = TrueHdParser::new();
        let mut unit = make_truehd_unit(200);
        unit[4..8].copy_from_slice(&0xF872_6FBBu32.to_be_bytes());
        finalize_major_sync(&mut unit);
        let f = parser.parse(&make_pes(unit, Some(90000)));
        assert_eq!(f.len(), 1);
        assert!(f[0].keyframe, "major-sync variant 0xFB also a keyframe");
    }

    #[test]
    fn incomplete_au_waits_does_not_emit_short() {
        // The AU length declares more bytes than buffered → parser must wait, not
        // emit a truncated AU. words=300 (0x12C) → 600 bytes declared, only 100
        // present. 300 exercises both length bytes (high nibble 0x1, low 0x2C).
        let mut parser = TrueHdParser::new();
        let mut data = vec![0u8; 100];
        let words = 300usize;
        data[0] = ((words >> 8) & 0x0F) as u8; // 0x01
        data[1] = (words & 0xFF) as u8; // 0x2C → 300 words = 600 bytes
        let f = parser.parse(&make_pes(data, Some(90000)));
        assert!(
            f.is_empty(),
            "must not emit fewer bytes than the length field"
        );
        assert_eq!(parser.buf.len(), 100, "partial AU retained");
    }

    #[test]
    fn buffer_stays_bounded_across_many_partial_pes() {
        // Malformed/never-completing input must keep the reassembly buffer
        // bounded by MAX_TRUEHD_BUF. Repeatedly feed AU fragments whose declared
        // length always exceeds what is buffered, so no AU ever completes; the
        // post-loop cap guard must clear the buffer instead of letting it grow
        // unbounded across many calls.
        let mut parser = TrueHdParser::new();
        // Each PES: a head declaring 0xFFF words (8190 bytes) but only 4096 bytes
        // present → incomplete → retained. Across many PES this would accumulate
        // without the cap.
        for _ in 0..200 {
            let mut frag = vec![0u8; 4096];
            frag[0] = 0x0F; // 0x0FFF words = 4095 → 8190 bytes declared
            frag[1] = 0xFF;
            let _ = parser.parse(&make_pes(frag, Some(0)));
            assert!(
                parser.buf.len() <= MAX_TRUEHD_BUF,
                "reassembly buffer exceeded cap: {} > {}",
                parser.buf.len(),
                MAX_TRUEHD_BUF
            );
        }
    }

    // --- ac3_boundary_corroborated: the AC-3-vs-TrueHD disambiguation ---

    #[test]
    fn ac3_corroborated_when_frame_fills_buffer() {
        // frame_bytes >= buf.len() → the AC-3 frame ends the buffer → corroborated.
        let buf = vec![0u8; 128];
        assert!(ac3_boundary_corroborated(&buf, 128));
        assert!(ac3_boundary_corroborated(&buf, 200));
    }

    #[test]
    fn ac3_corroborated_when_next_is_ac3_sync() {
        // Bytes after the frame begin with 0x0B 0x77 → another AC-3 frame →
        // corroborated.
        let mut buf = vec![0u8; 130];
        buf[128] = 0x0B;
        buf[129] = 0x77;
        assert!(ac3_boundary_corroborated(&buf, 128));
    }

    #[test]
    fn ac3_corroborated_when_next_is_plausible_truehd_au() {
        // Bytes after the frame form a plausible TrueHD AU header (non-zero
        // 12-bit length within 32 KiB) → corroborated. next_words = 0x100 = 256
        // → 512 bytes <= 32768.
        let mut buf = vec![0u8; 130];
        buf[128] = 0x01; // (0x01<<8)|0x00 & 0xFFF = 0x100
        buf[129] = 0x00;
        assert!(ac3_boundary_corroborated(&buf, 128));
    }

    #[test]
    fn ac3_not_corroborated_when_next_zero_length() {
        // Bytes after the frame are zeros → next_words == 0 → NOT a plausible
        // TrueHD AU and not an AC-3 sync → NOT corroborated (treat as TrueHD).
        let buf = vec![0u8; 130]; // all zero after frame_bytes=128
        assert!(!ac3_boundary_corroborated(&buf, 128));
    }

    #[test]
    fn ac3_corroborated_when_too_few_trailing_bytes() {
        // Fewer than 2 bytes follow the frame → can't judge → accept (next call
        // sees the continuation). frame_bytes=128, buf=129 → 1 trailing byte.
        let buf = vec![0u8; 129];
        assert!(ac3_boundary_corroborated(&buf, 128));
    }

    #[test]
    fn ac3_frame_at_head_needs_more_when_buffer_short() {
        // < 6 bytes buffered → NeedMore (can't read the AC-3 header).
        let mut parser = TrueHdParser::new();
        parser.buf = vec![0x0B, 0x77, 0x00];
        // Drive through parse: a short 0x0B77 head must wait, not emit.
        let f = parser.parse(&make_pes(vec![0x0B, 0x77, 0x00], Some(0)));
        assert!(f.is_empty());
    }

    // --- #2 sample rate from the major-sync rate nibble ---

    /// Build a `format_info` word with the given `ratebits` (top nibble) and a
    /// 7.1 8-channel mask (ch8 = 0x1F) in the low 13 bits — exactly the layout
    /// §1.A pins, so the rate nibble and the channel masks are co-located in one
    /// real word.
    fn format_info_with(ratebits: u32) -> u32 {
        ((ratebits & 0xF) << 28) | 0x1F
    }

    #[test]
    fn sample_rate_whitelist_real_rates() {
        assert_eq!(truehd_sample_rate_hz(format_info_with(0x0)), Some(48000));
        assert_eq!(truehd_sample_rate_hz(format_info_with(0x1)), Some(96000));
        assert_eq!(truehd_sample_rate_hz(format_info_with(0x2)), Some(192000));
        assert_eq!(truehd_sample_rate_hz(format_info_with(0x8)), Some(44100));
        assert_eq!(truehd_sample_rate_hz(format_info_with(0x9)), Some(88200));
        assert_eq!(truehd_sample_rate_hz(format_info_with(0xA)), Some(176400));
    }

    #[test]
    fn sample_rate_unknown_rate_falls_back_to_none() {
        // 0xF is the explicit invalid code; 0x3/0xB are formula-only and not
        // whitelisted; 0x7/0xE are reserved. None of them may produce a rate —
        // the host must fall back to its container value, never write a wrong
        // SamplingFrequency.
        for bad in [0x3u32, 0x7, 0xB, 0xC, 0xD, 0xE, 0xF] {
            assert_eq!(
                truehd_sample_rate_hz(format_info_with(bad)),
                None,
                "ratebits {bad:#x} must not yield a rate"
            );
        }
    }

    #[test]
    fn sample_rate_nibble_does_not_disturb_channel_decode() {
        // Internal-consistency guard: with the 96 kHz nibble AND a 7.1 mask in
        // the same word, the rate reads 96000 and the channels still read 8 —
        // proving the rate nibble (bits 31..28) and the channel masks
        // (bits 19..0) do not collide.
        let fi = format_info_with(0x1);
        assert_eq!(truehd_sample_rate_hz(fi), Some(96000));
        assert_eq!(truehd_channels(fi), Some(8));
    }

    #[test]
    fn sample_rate_from_stream_scans_major_sync() {
        // [junk][0xF8726FBA][format_info: ratebits=0x1 (96k), ch8=0x1F]
        let mut data = vec![0xAA, 0xBB];
        data.extend_from_slice(&0xF872_6FBAu32.to_be_bytes());
        data.extend_from_slice(&format_info_with(0x1).to_be_bytes());
        assert_eq!(truehd_sample_rate_from_stream(&data), Some(96000));
    }

    #[test]
    fn sample_rate_from_stream_none_without_sync() {
        let data = vec![0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88];
        assert_eq!(truehd_sample_rate_from_stream(&data), None);
    }

    // --- #3 per-AU duration: family-aware, 48 kHz family byte-identical ---

    #[test]
    fn au_duration_48k_family_unchanged() {
        // 48 / 96 / 192 kHz (ratebits 0x0/0x1/0x2) all keep the exact current
        // 833_333 constant — the common case must never shift.
        for rb in [0x0u32, 0x1, 0x2] {
            assert_eq!(truehd_au_duration_ns(format_info_with(rb)), 833_333);
        }
    }

    #[test]
    fn au_duration_441k_family_is_907029() {
        // 44.1 / 88.2 / 176.4 kHz (ratebits 0x8/0x9/0xA) → 907_029 ns.
        for rb in [0x8u32, 0x9, 0xA] {
            assert_eq!(truehd_au_duration_ns(format_info_with(rb)), 907_029);
        }
    }

    #[test]
    fn au_duration_unknown_rate_keeps_default() {
        // An unrecognised/garbage rate nibble must not pick the 44.1 k value
        // (note 0xF & 8 != 0): it falls back to the 833_333 default.
        for rb in [0x3u32, 0x7, 0xB, 0xF] {
            assert_eq!(truehd_au_duration_ns(format_info_with(rb)), 833_333);
        }
    }

    #[test]
    fn parser_44k_major_sync_sets_907029_increment() {
        // Two AUs: the first carries a major sync with ratebits=0x8 (44.1 k).
        // After the parser reads it, the per-AU PTS increment must be 907_029.
        let mut parser = TrueHdParser::new();
        let mut a1 = make_truehd_unit(200);
        a1[4..8].copy_from_slice(&0xF872_6FBAu32.to_be_bytes()); // major sync
        a1[8..12].copy_from_slice(&format_info_with(0x8).to_be_bytes()); // 44.1 k
        finalize_major_sync(&mut a1);
        let mut a2 = make_truehd_unit(200);
        finalize_normal_parity(&mut a2);
        let mut data = a1;
        data.extend_from_slice(&a2);
        let frames = parser.parse(&make_pes(data, Some(90000)));
        assert_eq!(frames.len(), 2);
        assert_eq!(
            frames[1].pts_ns - frames[0].pts_ns,
            907_029,
            "44.1 k-family AU increments by 907_029 once the major sync is read"
        );
    }

    #[test]
    fn parser_48k_major_sync_keeps_833333_increment() {
        // Regression: a 48 k-family (ratebits=0x0) major sync keeps the exact
        // current 833_333 increment.
        let mut parser = TrueHdParser::new();
        let mut a1 = make_truehd_unit(200);
        a1[4..8].copy_from_slice(&0xF872_6FBAu32.to_be_bytes());
        a1[8..12].copy_from_slice(&format_info_with(0x0).to_be_bytes()); // 48 k
        finalize_major_sync(&mut a1);
        let mut a2 = make_truehd_unit(200);
        finalize_normal_parity(&mut a2);
        let mut data = a1;
        data.extend_from_slice(&a2);
        let frames = parser.parse(&make_pes(data, Some(90000)));
        assert_eq!(frames.len(), 2);
        assert_eq!(frames[1].pts_ns - frames[0].pts_ns, 833_333);
    }

    // --- #1 Atmos detection from num_substreams (msync[16] >> 4) ---

    /// Build a demuxed chunk with one major sync whose 17th sync byte (offset
    /// 16 from the 0xF8) has top nibble `num_substreams`. The AU is padded past
    /// byte 16 so the substream count is reachable.
    fn major_sync_with_substreams(num_substreams: u8) -> Vec<u8> {
        let mut data = vec![0x00, 0x00]; // leading junk; scan is byte-aligned
        let sync_off = data.len();
        data.extend_from_slice(&0xF872_6FBAu32.to_be_bytes()); // bytes [off..off+4]
        data.extend_from_slice(&format_info_with(0x0).to_be_bytes()); // format_info
        // Pad up to and including byte `sync_off + 16`.
        while data.len() <= sync_off + 16 {
            data.push(0x00);
        }
        data[sync_off + 16] = (num_substreams & 0xF) << 4;
        data
    }

    #[test]
    fn atmos_true_when_four_substreams() {
        // num_substreams = 4 → byte 16 = 0x40 → Atmos object substream present.
        let data = major_sync_with_substreams(4);
        assert_eq!(truehd_is_atmos_from_stream(&data), Some(true));
    }

    #[test]
    fn atmos_false_when_three_substreams() {
        // num_substreams = 3 (plain 7.1 TrueHD) → byte 16 = 0x30 → not Atmos.
        let data = major_sync_with_substreams(3);
        assert_eq!(truehd_is_atmos_from_stream(&data), Some(false));
    }

    #[test]
    fn atmos_none_when_au_too_short_for_substream_byte() {
        // Major sync present but the chunk ends before byte sync_off+16 → None,
        // never a false Atmos. Sync at offset 0; only format_info follows.
        let mut data = 0xF872_6FBAu32.to_be_bytes().to_vec();
        data.extend_from_slice(&format_info_with(0x0).to_be_bytes()); // 8 bytes total
        assert_eq!(truehd_is_atmos_from_stream(&data), None);
    }

    #[test]
    fn atmos_none_without_major_sync() {
        let data = vec![0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88];
        assert_eq!(truehd_is_atmos_from_stream(&data), None);
    }

    #[test]
    fn sync_info_combines_channels_rate_and_atmos() {
        // One scan yields all three facts: 7.1 channels, 96 kHz, 4 substreams.
        let data = {
            let mut d = vec![0x00, 0x00];
            let off = d.len();
            d.extend_from_slice(&0xF872_6FBAu32.to_be_bytes());
            d.extend_from_slice(&format_info_with(0x1).to_be_bytes()); // 96k + 7.1
            while d.len() <= off + 16 {
                d.push(0x00);
            }
            d[off + 16] = 0x40; // 4 substreams
            d
        };
        let info = truehd_sync_info_from_stream(&data).expect("major sync found");
        assert_eq!(truehd_channels(info.format_info), Some(8));
        assert_eq!(truehd_sample_rate_hz(info.format_info), Some(96000));
        assert_eq!(info.is_atmos, Some(true));
    }
}