//! AACS Media Key Variant chain.
//!
//! On AACS 2.1 the Media Key derivation gains a second stage on top of
//! the classical subset-difference walk. The classical walk yields a
//! Media Key Precursor (Kmp) rather than the final Media Key; the
//! Precursor combines with disc-supplied Variant Key Data (VKD) and an
//! integrator-supplied Key Correction Data (KCD) constant to produce
//! the Media Key.
//!
//! This module is wiring only — `resolve_keys` is not aware of it. The
//! entry point is [`derive_media_key_variant`]. The Variant scheme is
//! detected via the new MKB record types `0x82` (Encrypted Media Key
//! Variant Data + Variant Key Data) and `0x83` (Variant Number). When
//! a disc carries neither, callers should fall back to the classical
//! single-stage derivation in [`super::keys`].
//!
//! The chain follows the published spec:
//!
//! ```text
//! Kmp     = AES-128D(Kp, C) XOR uv
//! Kpnew   = Kmp XOR KCD
//! Kvn     = AES-G(Kp, Nonce) & 0xFFFF   (low 16 bits, BE)
//! VKD_idx = Kvn XOR VARIANTS[uv]
//! VKD     = vkd_table[VKD_idx * 16 .. +16]
//! Km      = AES-128D(Kpnew, VKD) XOR uv
//! ```
//!
//! Two condition bits on `Kmp[15]` route off the hardcoded-KCD path
//! (Soft Correction and Online Challenge). The chain refuses to run in
//! either case — callers must handle those modes out of band.

use super::decrypt::aes_ecb_decrypt;
use super::keydb::DeviceKey;

// ── Public constants ──────────────────────────────────────────────────────

/// Placeholder Key Correction Data. Sixteen zero bytes.
///
/// Integrators MUST supply a non-placeholder KCD via the `kcd` argument
/// to [`derive_media_key_variant`]; the chain refuses to operate when
/// the supplied KCD compares equal to this placeholder.
pub const KEY_CORRECTION_DATA_PLACEHOLDER: [u8; 16] = [0u8; 16];

// ── MKB record walking ────────────────────────────────────────────────────

/// A single MKB record produced by [`walk_mkb`].
#[derive(Debug, Clone)]
pub struct MkbRecord {
    /// Byte offset of the record within the MKB.
    pub offset: usize,
    /// Record type byte.
    pub rec_type: u8,
    /// Record length in bytes (includes the 4-byte header).
    pub rec_len: usize,
    /// Record body (the bytes after the 4-byte header).
    pub body: Vec<u8>,
}

/// Walk an MKB into a flat list of records.
///
/// MKB record framing per AACS: 1 byte type, 3 bytes BE length
/// INCLUDING the 4-byte header, followed by payload. The walker stops
/// at the first `(type=0, len=0)` end marker or at end of buffer.
pub fn walk_mkb(mkb: &[u8]) -> Vec<MkbRecord> {
    let mut out = Vec::new();
    let mut pos = 0;
    while pos + 4 <= mkb.len() {
        let rec_type = mkb[pos];
        let rec_len = ((mkb[pos + 1] as usize) << 16)
            | ((mkb[pos + 2] as usize) << 8)
            | (mkb[pos + 3] as usize);
        if rec_type == 0 && rec_len == 0 {
            break;
        }
        if rec_len < 4 || pos + rec_len > mkb.len() {
            break;
        }
        let body = mkb[pos + 4..pos + rec_len].to_vec();
        out.push(MkbRecord {
            offset: pos,
            rec_type,
            rec_len,
            body,
        });
        pos += rec_len;
    }
    out
}

/// True iff `records` contains at least one Media Key Variant record
/// (type `0x82` or `0x83`).
pub fn is_variant_mkb(records: &[MkbRecord]) -> bool {
    records.iter().any(|r| matches!(r.rec_type, 0x82 | 0x83))
}

/// Body of the Encrypted Media Key Variant Data record (type `0x82`).
pub fn variant_data_record(records: &[MkbRecord]) -> Option<&[u8]> {
    records
        .iter()
        .find(|r| r.rec_type == 0x82)
        .map(|r| r.body.as_slice())
}

/// 16-byte Nonce from the Variant Number record (type `0x83`). Returns
/// the first 16 bytes of the body.
pub fn variant_nonce(records: &[MkbRecord]) -> Option<[u8; 16]> {
    let r = records.iter().find(|r| r.rec_type == 0x83)?;
    if r.body.len() < 16 {
        return None;
    }
    let mut out = [0u8; 16];
    out.copy_from_slice(&r.body[..16]);
    Some(out)
}

/// Body of the Variant Key Data record. Returns the first `0x82` body
/// that is a non-empty multiple of 16 bytes.
pub fn variant_key_data(records: &[MkbRecord]) -> Option<&[u8]> {
    records
        .iter()
        .find(|r| r.rec_type == 0x82 && !r.body.is_empty() && r.body.len() % 16 == 0)
        .map(|r| r.body.as_slice())
}

// ── AES-G ────────────────────────────────────────────────────────────────

/// AES-G(x1, x2) = AES-128D(x1, x2) XOR x2.
///
/// The Media Key Variant chain uses AES-G to derive both the variant
/// number (`Kvn = AES-G(Kp, Nonce)`) and the Volume Unique Key
/// (`Kvu = AES-G(Km, VID)`). See [`super::keys::derive_vuk`] for the
/// classical VUK form — the math is identical, this exposes it as a
/// neutral primitive for the variant chain.
fn aes_g(x1: &[u8; 16], x2: &[u8; 16]) -> [u8; 16] {
    let mut out = aes_ecb_decrypt(x1, x2);
    for i in 0..16 {
        out[i] ^= x2[i];
    }
    out
}

// ── Subset-difference walk that exposes (Kp, uv) ──────────────────────────

/// AES-G3 seed register initial value.
const AESG3_SEED: [u8; 16] = [
    0x7B, 0x10, 0x3C, 0x5D, 0xCB, 0x08, 0xC4, 0xE5, 0x1A, 0x27, 0xB0, 0x17, 0x99, 0x05, 0x3B, 0xD9,
];

/// AES-G3 single step: AES-G against the seed register at offset `inc`.
fn aesg3_step(key: &[u8; 16], inc: u8) -> [u8; 16] {
    let mut seed = AESG3_SEED;
    seed[15] = seed[15].wrapping_add(inc);
    aes_g(key, &seed)
}

fn calc_v_mask(uv: u32) -> u32 {
    let mut v_mask: u32 = 0xFFFF_FFFF;
    while (uv & !v_mask) == 0 && v_mask != 0 {
        v_mask <<= 1;
    }
    v_mask
}

fn calc_pk_from_dk(dk: &[u8; 16], uv: u32, v_mask: u32, dev_key_v_mask: u32) -> [u8; 16] {
    let mut left_child = aesg3_step(dk, 0);
    let mut pk = aesg3_step(dk, 1);
    let mut right_child = aesg3_step(dk, 2);
    let mut current_v_mask = dev_key_v_mask;

    while current_v_mask != v_mask {
        let mut bit_pos: i32 = -1;
        for i in (0..32).rev() {
            if (current_v_mask & (1u32 << i)) == 0 {
                bit_pos = i;
                break;
            }
        }

        let curr_key = if bit_pos < 0 || (uv & (1u32 << bit_pos as u32)) == 0 {
            left_child
        } else {
            right_child
        };

        left_child = aesg3_step(&curr_key, 0);
        pk = aesg3_step(&curr_key, 1);
        right_child = aesg3_step(&curr_key, 2);

        current_v_mask = ((current_v_mask as i32) >> 1) as u32;
    }

    pk
}

/// Outcome of a subset-difference walk against an MKB. Carries the
/// processing key and the matching `uv` slot — both needed as inputs
/// to the variant chain.
#[derive(Debug, Clone, Copy)]
pub struct ProcessingKeyMatch {
    /// Processing Key.
    pub kp: [u8; 16],
    /// Subset-difference node number that matched.
    pub uv: u32,
    /// 16-byte cvalue that the matched uv selected.
    pub cvalue: [u8; 16],
    /// Index of the matching cvalue within the cvalues record.
    pub cvalue_index: usize,
}

fn mkb_find_body(records: &[MkbRecord], rec_type: u8) -> Option<&[u8]> {
    records
        .iter()
        .find(|r| r.rec_type == rec_type && !r.body.is_empty())
        .map(|r| r.body.as_slice())
}

fn mkb_find_mk_dv(records: &[MkbRecord]) -> Option<[u8; 16]> {
    let r = records
        .iter()
        .find(|r| (r.rec_type == 0x81 || r.rec_type == 0x86) && r.body.len() >= 16)?;
    let mut out = [0u8; 16];
    out.copy_from_slice(&r.body[..16]);
    Some(out)
}

/// Walk an MKB and return the first `(Kp, uv, cvalue)` that
/// `device_keys` covers. Returns `None` if no DK walks any uv.
pub fn walk_processing_key(
    records: &[MkbRecord],
    device_keys: &[DeviceKey],
) -> Option<ProcessingKeyMatch> {
    let mk_dv = mkb_find_mk_dv(records)?;
    let uvs = mkb_find_body(records, 0x04)?;
    let cvalues = mkb_find_body(records, 0x07).or_else(|| mkb_find_body(records, 0x05))?;

    let num_uvs = uvs
        .chunks(5)
        .take_while(|c| c.len() == 5 && (c[0] & 0xC0) == 0)
        .count();

    for dk in device_keys {
        let device_number = dk.node as u32;

        for uvs_idx in 0..num_uvs {
            let p_uv = &uvs[1 + 5 * uvs_idx..];
            let u_mask_shift = uvs[5 * uvs_idx];

            if u_mask_shift & 0xC0 != 0 {
                break;
            }

            let uv = u32::from_be_bytes([p_uv[0], p_uv[1], p_uv[2], p_uv[3]]);
            if uv == 0 {
                continue;
            }

            let u_mask: u32 = 0xFFFF_FFFFu32.wrapping_shl(u_mask_shift as u32);
            let v_mask = calc_v_mask(uv);

            if ((device_number & u_mask) == (uv & u_mask))
                && ((device_number & v_mask) != (uv & v_mask))
            {
                let dev_key_v_mask = calc_v_mask(dk.uv);
                let dev_key_u_mask: u32 = 0xFFFF_FFFFu32.wrapping_shl(dk.u_mask_shift as u32);

                if u_mask == dev_key_u_mask && (uv & dev_key_v_mask) == (dk.uv & dev_key_v_mask) {
                    let pk = calc_pk_from_dk(&dk.key, uv, v_mask, dev_key_v_mask);

                    if uvs_idx >= cvalues.len() / 16 {
                        continue;
                    }
                    let mut cv = [0u8; 16];
                    cv.copy_from_slice(&cvalues[uvs_idx * 16..(uvs_idx + 1) * 16]);

                    // Validate: AES-D(Kp, cv), XOR uv into low 4 bytes,
                    // then AES-D(.., mk_dv) must reveal the verify magic.
                    let mut km_candidate = aes_ecb_decrypt(&pk, &cv);
                    let uv_bytes = uv.to_be_bytes();
                    for i in 0..4 {
                        km_candidate[12 + i] ^= uv_bytes[i];
                    }
                    let dec_vd = aes_ecb_decrypt(&km_candidate, &mk_dv);
                    const VERIFY_MAGIC: [u8; 8] = [0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF];
                    // On a classical (non-variant) MKB this magic must
                    // match. On a variant MKB it won't — `km_candidate`
                    // is really Kmp and the magic check is moot. We
                    // still gate the walk on cvalue indexing being
                    // sane; the chain itself enforces the variant
                    // semantics downstream.
                    let classical_ok = dec_vd[..8] == VERIFY_MAGIC;
                    let variant_present = is_variant_mkb(records);
                    if !(classical_ok || variant_present) {
                        continue;
                    }

                    return Some(ProcessingKeyMatch {
                        kp: pk,
                        uv,
                        cvalue: cv,
                        cvalue_index: uvs_idx,
                    });
                }
            }
        }
    }
    None
}

// ── Error reporting ───────────────────────────────────────────────────────

/// Outcome of [`derive_media_key_variant`] when the chain cannot
/// produce a Media Key. Every variant is a classification only — no
/// strings, no Display impl beyond the error code.
#[derive(Debug, PartialEq, Eq, Clone, Copy)]
pub enum MediaKeyVariantError {
    /// MKB carries no Variant records. Caller should fall back to the
    /// classical single-stage derivation.
    NotVariantMkb,
    /// MKB is missing a required record (mk_dv, subset-difference,
    /// cvalues, variant data, or variant nonce).
    MkbIncomplete,
    /// `device_keys` did not cover any uv slot in this MKB.
    ProcessingKeyUnavailable,
    /// `Kmp[15]` carries bit `0x02`: the soft-correction path applies
    /// for this Precursor. Out of scope for the hardcoded-KCD chain.
    SoftCorrectionRequired,
    /// `Kmp[15]` carries bit `0x04`: the online-challenge path applies
    /// for this Precursor. Out of scope for the hardcoded-KCD chain.
    OnlineChallengeRequired,
    /// Supplied KCD equals [`KEY_CORRECTION_DATA_PLACEHOLDER`]. The
    /// derivation refuses to run with the all-zero placeholder.
    KcdNotProvided,
    /// `VARIANTS[uv]` lookup for the matched uv is not implemented.
    VariantsTableUnavailable,
    /// VKD index resolved out of the supplied `vkd_table`.
    VkdIndexOutOfRange,
}

impl std::fmt::Display for MediaKeyVariantError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let code: u16 = match self {
            MediaKeyVariantError::NotVariantMkb => 7100,
            MediaKeyVariantError::MkbIncomplete => 7101,
            MediaKeyVariantError::ProcessingKeyUnavailable => 7102,
            MediaKeyVariantError::SoftCorrectionRequired => 7103,
            MediaKeyVariantError::OnlineChallengeRequired => 7104,
            MediaKeyVariantError::KcdNotProvided => 7105,
            MediaKeyVariantError::VariantsTableUnavailable => 7106,
            MediaKeyVariantError::VkdIndexOutOfRange => 7107,
        };
        write!(f, "E{code}")
    }
}

impl std::error::Error for MediaKeyVariantError {}

// ── Chain ─────────────────────────────────────────────────────────────────

/// Look up `VARIANTS[uv]` for the matched uv. The byte layout of the
/// per-uv slot in the Variant Number record is undocumented and is
/// disc-specific; this helper returns `None` until a Variant disc is
/// available to fix the layout against.
fn variants_for_uv(_records: &[MkbRecord], _uv_index: usize) -> Option<u16> {
    None
}

/// Run the Media Key Variant chain on an MKB.
///
/// Inputs:
///
/// - `mkb_records`  : MKB pre-walked via [`walk_mkb`].
/// - `device_keys`  : pool of device keys; the chain runs against the
///   first uv slot any DK covers.
/// - `kcd`          : integrator-supplied Key Correction Data. Must not
///   equal [`KEY_CORRECTION_DATA_PLACEHOLDER`].
/// - `vid`          : 16-byte Volume ID for the disc. Used to derive
///   the final VUK alongside the Media Key.
///
/// Returns `(Km, Kvu)` on success.
pub fn derive_media_key_variant(
    mkb_records: &[MkbRecord],
    device_keys: &[DeviceKey],
    kcd: &[u8; 16],
    vid: &[u8; 16],
) -> Result<([u8; 16], [u8; 16]), MediaKeyVariantError> {
    if !is_variant_mkb(mkb_records) {
        return Err(MediaKeyVariantError::NotVariantMkb);
    }

    let pkm = walk_processing_key(mkb_records, device_keys)
        .ok_or(MediaKeyVariantError::ProcessingKeyUnavailable)?;

    let nonce = variant_nonce(mkb_records).ok_or(MediaKeyVariantError::MkbIncomplete)?;
    let vkd_table = variant_key_data(mkb_records).ok_or(MediaKeyVariantError::MkbIncomplete)?;
    let c_value = variant_data_record(mkb_records).ok_or(MediaKeyVariantError::MkbIncomplete)?;
    if c_value.len() < 16 {
        return Err(MediaKeyVariantError::MkbIncomplete);
    }
    let mut c_block = [0u8; 16];
    c_block.copy_from_slice(&c_value[..16]);

    // Step: Kmp = AES-128D(Kp, C) XOR uv  (uv into low 4 bytes).
    let mut kmp = aes_ecb_decrypt(&pkm.kp, &c_block);
    let uv_bytes = pkm.uv.to_be_bytes();
    for i in 0..4 {
        kmp[12 + i] ^= uv_bytes[i];
    }

    // Condition bits on Kmp[15] route off the hardcoded-KCD path.
    if kmp[15] & 0b0000_0010 != 0 {
        return Err(MediaKeyVariantError::SoftCorrectionRequired);
    }
    if kmp[15] & 0b0000_0100 != 0 {
        return Err(MediaKeyVariantError::OnlineChallengeRequired);
    }
    if kcd == &KEY_CORRECTION_DATA_PLACEHOLDER {
        return Err(MediaKeyVariantError::KcdNotProvided);
    }

    // Step: Kpnew = Kmp XOR KCD.
    let mut kpnew = [0u8; 16];
    for i in 0..16 {
        kpnew[i] = kmp[i] ^ kcd[i];
    }

    // Step: Kvn = AES-G(Kp, Nonce) & 0xFFFF  (low 16 bits, BE).
    let kvn_block = aes_g(&pkm.kp, &nonce);
    let kvn = u16::from_be_bytes([kvn_block[14], kvn_block[15]]);

    // Step: VKD_idx = Kvn XOR VARIANTS[uv].
    let v_for_uv = variants_for_uv(mkb_records, pkm.cvalue_index)
        .ok_or(MediaKeyVariantError::VariantsTableUnavailable)?;
    let vkd_idx = kvn ^ v_for_uv;

    // Step: VKD = vkd_table[VKD_idx * 16 .. +16].
    let off = (vkd_idx as usize) * 16;
    if off + 16 > vkd_table.len() {
        return Err(MediaKeyVariantError::VkdIndexOutOfRange);
    }
    let mut vkd = [0u8; 16];
    vkd.copy_from_slice(&vkd_table[off..off + 16]);

    // Step: Km = AES-128D(Kpnew, VKD) XOR uv.
    let mut km = aes_ecb_decrypt(&kpnew, &vkd);
    for i in 0..4 {
        km[12 + i] ^= uv_bytes[i];
    }

    // Step: Kvu = AES-G(Km, VID).
    let kvu = aes_g(&km, vid);

    Ok((km, kvu))
}

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

    // ── Helpers ──

    fn synthetic_mkb_classical() -> Vec<u8> {
        // Minimal MKB: type/version record + cvalues + mk_dv. No variant
        // records.
        let mut mkb = vec![
            0x10, 0x00, 0x00, 0x0C, 0x48, 0x14, 0x10, 0x03, 0x00, 0x00, 0x00, 0x4D,
        ];
        mkb.extend_from_slice(&[0x07, 0x00, 0x00, 0x14]);
        mkb.extend_from_slice(&[0xAB; 16]);
        mkb.extend_from_slice(&[0x86, 0x00, 0x00, 0x14]);
        mkb.extend_from_slice(&[0xCD; 16]);
        mkb
    }

    fn synthetic_mkb_with_variant() -> Vec<u8> {
        let mut mkb = synthetic_mkb_classical();
        // 0x82 — 16-byte body (Variant data / VKD slot).
        mkb.extend_from_slice(&[0x82, 0x00, 0x00, 0x14]);
        mkb.extend_from_slice(&[0xEE; 16]);
        // 0x83 — 16-byte body (Variant Nonce).
        mkb.extend_from_slice(&[0x83, 0x00, 0x00, 0x14]);
        mkb.extend_from_slice(&[0x55; 16]);
        mkb
    }

    // ── Walker / record detection ──

    #[test]
    fn walker_parses_synthetic_mkb() {
        let mkb = synthetic_mkb_classical();
        let recs = walk_mkb(&mkb);
        assert_eq!(recs.len(), 3);
        assert_eq!(recs[0].rec_type, 0x10);
        assert_eq!(recs[1].rec_type, 0x07);
        assert_eq!(recs[2].rec_type, 0x86);
    }

    #[test]
    fn variant_detection_negative_on_classical() {
        let recs = walk_mkb(&synthetic_mkb_classical());
        assert!(!is_variant_mkb(&recs));
        assert!(variant_nonce(&recs).is_none());
        assert!(variant_key_data(&recs).is_none());
        assert!(variant_data_record(&recs).is_none());
    }

    #[test]
    fn variant_detection_positive_on_variant() {
        let recs = walk_mkb(&synthetic_mkb_with_variant());
        assert!(is_variant_mkb(&recs));
        assert_eq!(variant_nonce(&recs), Some([0x55; 16]));
        assert_eq!(variant_key_data(&recs), Some(&[0xEE; 16][..]));
        assert_eq!(variant_data_record(&recs), Some(&[0xEE; 16][..]));
    }

    // ── Chain entry-point classification ──

    #[test]
    fn chain_rejects_non_variant_mkb() {
        let recs = walk_mkb(&synthetic_mkb_classical());
        let err = derive_media_key_variant(&recs, &[], &[0xAA; 16], &[0u8; 16])
            .expect_err("classical MKB must be rejected");
        assert_eq!(err, MediaKeyVariantError::NotVariantMkb);
    }

    #[test]
    fn chain_rejects_placeholder_kcd() {
        // To reach the KCD check we need a complete variant MKB AND a
        // DK that walks it. We construct both via the synthetic
        // fixture below.
        let (recs, dk, _kp, _expected_kmp) = synthetic_variant_setup(/*kmp15*/ 0x00);
        let err =
            derive_media_key_variant(&recs, &[dk], &KEY_CORRECTION_DATA_PLACEHOLDER, &[0u8; 16])
                .expect_err("placeholder KCD must be rejected");
        assert_eq!(err, MediaKeyVariantError::KcdNotProvided);
    }

    #[test]
    fn chain_detects_soft_correction_bit() {
        let (recs, dk, _, _) = synthetic_variant_setup(/*kmp15*/ 0x02);
        let err = derive_media_key_variant(&recs, &[dk], &[0xAA; 16], &[0u8; 16])
            .expect_err("bit 0x02 must surface SoftCorrectionRequired");
        assert_eq!(err, MediaKeyVariantError::SoftCorrectionRequired);
    }

    #[test]
    fn chain_detects_online_challenge_bit() {
        let (recs, dk, _, _) = synthetic_variant_setup(/*kmp15*/ 0x04);
        let err = derive_media_key_variant(&recs, &[dk], &[0xAA; 16], &[0u8; 16])
            .expect_err("bit 0x04 must surface OnlineChallengeRequired");
        assert_eq!(err, MediaKeyVariantError::OnlineChallengeRequired);
    }

    #[test]
    fn chain_surfaces_variants_table_gap_on_clean_kmp() {
        // With both condition bits clear and a non-placeholder KCD, the
        // chain advances to the per-uv VARIANTS[uv] lookup, which is
        // not yet wired. That returns VariantsTableUnavailable —
        // proving the bit checks and KCD check all passed.
        let (recs, dk, _, _) = synthetic_variant_setup(/*kmp15*/ 0x00);
        let err = derive_media_key_variant(&recs, &[dk], &[0xAA; 16], &[0u8; 16])
            .expect_err("expected VariantsTableUnavailable at the per-uv lookup");
        assert_eq!(err, MediaKeyVariantError::VariantsTableUnavailable);
    }

    #[test]
    fn error_display_is_code_only() {
        // No English in Display — every variant emits "E7xxx" and
        // nothing else.
        let cases = [
            MediaKeyVariantError::NotVariantMkb,
            MediaKeyVariantError::MkbIncomplete,
            MediaKeyVariantError::ProcessingKeyUnavailable,
            MediaKeyVariantError::SoftCorrectionRequired,
            MediaKeyVariantError::OnlineChallengeRequired,
            MediaKeyVariantError::KcdNotProvided,
            MediaKeyVariantError::VariantsTableUnavailable,
            MediaKeyVariantError::VkdIndexOutOfRange,
        ];
        for e in cases {
            let s = e.to_string();
            assert!(
                s.starts_with('E') && s.len() == 5,
                "error display must be E#### only, got {s:?}"
            );
            assert!(
                s.chars().skip(1).all(|c| c.is_ascii_digit()),
                "error display must be E + digits, got {s:?}"
            );
        }
    }

    // ── Fixture construction ──

    /// Build a synthetic variant MKB plus a DK that walks the single
    /// subset-difference slot it carries. `kmp15` is the value of the
    /// low byte of `Kmp[15]` that the chain will land on — pick `0x02`
    /// to exercise the SoftCorrection bit, `0x04` to exercise
    /// OnlineChallenge, `0x00` otherwise.
    ///
    /// The fixture pins:
    /// - MKB subset-difference: `u_mask_shift=3, uv=2`. With these
    ///   masks the discriminator bit (u_mask=1, v_mask=0) is bit 2.
    /// - one DK at `node=4, uv=2, u_mask_shift=3`. node 4 has bit 2 set
    ///   (differs from uv=2 on bit 2 → disagrees on v_mask) while
    ///   agreeing with uv on bits 3+ (the u_mask=1 region). dk.uv ==
    ///   MKB.uv and dk.u_mask_shift == MKB.u_mask_shift make
    ///   `dev_key_v_mask == v_mask`, so `calc_pk_from_dk` loops zero
    ///   times — Kp = aesg3_step(dk, 1).
    /// - one cvalue in record 0x07 chosen so AES-D(Kp, C) ⊕ uv produces a
    ///   Kmp whose byte-15 is exactly `kmp15`.
    /// - record 0x82 with a 16-byte body (acts as both Variant Data
    ///   and Variant Key Data; satisfies the parser heuristics).
    /// - record 0x83 with a 16-byte Nonce.
    ///
    /// Returns (records, dk, planted_kp, planted_kmp).
    fn synthetic_variant_setup(kmp15: u8) -> (Vec<MkbRecord>, DeviceKey, [u8; 16], [u8; 16]) {
        use crate::aacs::decrypt::aes_ecb_encrypt;

        // Build header.
        let mut mkb = vec![
            0x10, 0x00, 0x00, 0x0C, 0x48, 0x14, 0x10, 0x03, 0x00, 0x00, 0x00, 0x4D,
        ];

        // Subset-difference (0x04): u_mask_shift=3, uv=00 00 00 02.
        mkb.extend_from_slice(&[0x04, 0x00, 0x00, 0x09]);
        mkb.extend_from_slice(&[0x03, 0x00, 0x00, 0x00, 0x02]);

        // Pick a known DK; with dk.uv == MKB.uv (==2) and
        // dk.u_mask_shift == MKB.u_mask_shift (==1), dev_key_v_mask
        // equals the MKB's v_mask and the calc_pk_from_dk loop is a
        // no-op — Kp = aesg3_step(dk, 1).
        let dk_bytes: [u8; 16] = [
            0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xAA, 0xBB, 0xCC, 0xDD, 0xEE,
            0xFF, 0x00,
        ];
        let kp = aesg3_step(&dk_bytes, 1);

        // Plant Kmp with chosen byte-15, then compute C such that
        // AES-D(Kp, C) ⊕ uv == Kmp. uv=2 → low-4 bytes XOR is 00 00 00 02.
        let mut kmp = [0x42u8; 16];
        kmp[15] = kmp15;
        let mut aes_d_result = kmp;
        aes_d_result[15] ^= 0x02;
        let c_block = aes_ecb_encrypt(&kp, &aes_d_result);

        // cvalues record (0x07): one 16-byte cvalue. The walker
        // indexes it for the magic-check step; on a variant MKB the
        // magic check fails but `variant_present` is true so the
        // walker still returns the match. Content is don't-care.
        mkb.extend_from_slice(&[0x07, 0x00, 0x00, 0x14]);
        mkb.extend_from_slice(&[0xAB; 16]);

        // Verify Media Key (0x86): body content is don't-care.
        mkb.extend_from_slice(&[0x86, 0x00, 0x00, 0x14]);
        mkb.extend_from_slice(&[0xCD; 16]);

        // 0x82 record: holds C (Encrypted Media Key Variant Data) AND
        // doubles as the VKD table (single 16-byte entry → VKDidx must
        // resolve to 0 for `chain_surfaces_variants_table_gap` test —
        // but the test never reaches the VKD lookup since the
        // VARIANTS[uv] helper is not yet wired).
        mkb.extend_from_slice(&[0x82, 0x00, 0x00, 0x14]);
        mkb.extend_from_slice(&c_block);

        // 0x83 record: 16-byte Nonce.
        mkb.extend_from_slice(&[0x83, 0x00, 0x00, 0x14]);
        mkb.extend_from_slice(&[0x77; 16]);

        let recs = walk_mkb(&mkb);

        let dk = DeviceKey {
            key: dk_bytes,
            node: 4,
            uv: 2,
            u_mask_shift: 3,
        };
        (recs, dk, kp, kmp)
    }
}