oxideav-av1 0.1.21

//! Spec-faithful frame decode driver — §7.4 `decode_frame_wrapup` order
//! over the §5.11 `PartitionWalker` syntax walk.
//!
//! This is the decode path for **real encoder-produced bitstreams** (as
//! opposed to the encoder-mirror drivers retired in r428, which
//! accepted only this crate's own
//! constrained encoder output). It wires the crate's spec modules end to
//! end for one frame:
//!
//!   1. §5.9 frame-header derived state → [`TileDecodeParams`] +
//!      [`QuantizerParams`] (including the §5.9.2 `CodedLossless` /
//!      `LosslessArray` derivations and the §5.9.14 segmentation
//!      feature plumbing).
//!   2. §8.2.2 `init_symbol` over the tile bytes + §8.3.1 default CDF
//!      load (`new_from_defaults` + the q-context-selected
//!      `init_coeff_cdfs( base_q_idx )` copy).
//!   3. §5.11.2 [`PartitionWalker::decode_tile_syntax`] — the
//!      superblock loop over §5.11.4 `decode_partition` → §5.11.5
//!      `decode_block` → §5.11.34 `residual`, reconstructing every
//!      intra transform block into `CurrFrame[ plane ]`.
//!   4. The §7.4 in-loop / post passes in decode order over the
//!      mi-grid-padded planes: §7.14 deblock (only when a luma filter
//!      level is nonzero) → §7.15 CDEF → §7.16 superres (both the CDEF
//!      output and the retained post-deblock frame) → §7.17 loop
//!      restoration → the §7.18.2 crop → §7.18.3 film grain.
//!
//! ## Scope
//!
//! * Intra-only frames (KEY / INTRA_ONLY), 8-bit output, single- and
//!   multi-tile layouts.
//! * 4:2:0 / 4:2:2 / 4:4:4 and monochrome layouts (the walker threads
//!   `subsampling_x/y` + `mono_chrome`; only 8-bit output is surfaced).
//! * Inter frames, `show_existing_frame`, and quantizer-matrix streams
//!   return [`Error::PartitionWalkOutOfRange`] (follow-ups).
//!
//! Spec provenance: `docs/video/av1/av1-spec.txt` §5.9, §5.10, §5.11,
//! §7.4, §7.12.2, §7.14, §7.15, §7.16, §7.17, §7.18.

use crate::cdf::{
    FrameInterOrderHints, InterFrameContext, InterWalkPixels, MotionFieldMvs, PartitionWalker,
    QuantizerParams, TileCdfContext, TileDecodeParams, TileGeometry,
};
use crate::encoder::ivf::IvfReader;
use crate::encoder::tile_group_obu::{parse_tile_group_obu_body, TilePayload};
use crate::film_grain::film_grain_synthesis;
use crate::frame_header::{
    parse_frame_header_with_refs, FrameHeader, RefInfo, NUM_REF_FRAMES, PRIMARY_REF_NONE,
};
use crate::inter_pred::get_relative_dist;
use crate::loop_filter::PlaneBuffer;
use crate::obu::{ObuIter, ObuType};
use crate::sequence_header::{parse_sequence_header, SequenceHeader};
use crate::symbol_decoder::SymbolDecoder;
use crate::uncompressed_header_tail::{
    SegmentationParams, ALTREF_FRAME, LAST_FRAME, MAX_SEGMENTS, SEG_LVL_ALT_Q, SEG_LVL_SKIP,
};
use crate::Error;
use crate::{PlaneRefSpec, RefFrameStoreEntry};
use std::sync::Arc;
use std::time::{Duration, Instant};

/// r475 — wall time per §7.4 decode stage of the LAST temporal unit
/// a [`SpecDecodeSession`] decoded (summed over its frames). A
/// measurement hook for the decode-budget harness
/// (`examples/decode_bench`); every field is a plain [`Duration`].
#[doc(hidden)]
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
#[non_exhaustive]
pub struct DecodeStageTimings {
    /// OBU framing + §5.5 / §5.9 header parse + §5.11.1 tile-group
    /// splitting.
    pub parse: Duration,
    /// §5.11 tile syntax walk + §7.11 / §7.12 / §7.13 reconstruction
    /// (every tile).
    pub tiles: Duration,
    /// §7.14 deblocking.
    pub deblock: Duration,
    /// §7.15 CDEF.
    pub cdef: Duration,
    /// §7.16 superres upscaling.
    pub superres: Duration,
    /// §7.17 loop restoration.
    pub restoration: Duration,
    /// §7.18.2 crop + the §7.20 reference-store payload (`u16`).
    pub ref_store: Duration,
    /// §7.18.3 film-grain synthesis.
    pub grain: Duration,
    /// Narrowing to the output byte layout + the §7.19 motion-field
    /// payload.
    pub output: Duration,
}

impl DecodeStageTimings {
    /// The all-zero record.
    #[must_use]
    pub fn new() -> Self {
        Self::default()
    }

    /// Sum of every stage.
    #[must_use]
    pub fn total(&self) -> Duration {
        self.parse
            + self.tiles
            + self.deblock
            + self.cdef
            + self.superres
            + self.restoration
            + self.ref_store
            + self.grain
            + self.output
    }
}

/// One frame decoded by the spec-faithful driver. Planes are surfaced
/// at their §5.9.8 cropped extents (`FrameWidth` × `FrameHeight` for
/// luma, the §5.5.2 subsampled extent for chroma), row-major, 8-bit.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct SpecFrame {
    /// Luma width in pixels (`FrameWidth`, post-superres if any).
    pub width: u32,
    /// Luma height in pixels (`FrameHeight`).
    pub height: u32,
    /// Decoded planes: `[Y]` (monochrome) or `[Y, U, V]`. For
    /// `bit_depth == 8` one byte per sample; for 10/12-bit output two
    /// bytes per sample, little-endian (the `yuv4xxp1{0,2}le` layout).
    pub planes: Vec<Vec<u8>>,
    /// `(width, height)` per surfaced plane (in samples).
    pub plane_dims: Vec<(u32, u32)>,
    /// §5.5.2 `BitDepth` (8, 10 or 12).
    pub bit_depth: u8,
}

/// §7.20 per-slot reference store — the decoded frame a later inter
/// frame motion-compensates against, plus the per-mi grids the §7.9
/// temporal projection and the §7.21 `show_existing_frame` output
/// path consume.
#[derive(Debug, Clone)]
struct SpecRefSlot {
    /// §7.20 `FrameStore[ i ][ plane ]` — the post-§7.17 (pre-§7.18.3
    /// film-grain) planes at their §7.18.2 cropped extents
    /// (`UpscaledWidth × FrameHeight`, per-plane subsampled), `u16`
    /// (post-`Clip1`, so the widening from the walker's `i32` is
    /// lossless).
    planes: Vec<Vec<u16>>,
    /// `(width, height)` per stored plane.
    plane_dims: Vec<(u32, u32)>,
    /// §7.20 `SavedMvs[ i ]` — the §7.19 `MfMvs[ row ][ col ][ 0..2 ]`
    /// grid snapshot (2 `i16` per mi cell), the §7.9.2 projection
    /// source.
    mf_mvs: Vec<i16>,
    /// §7.20 `SavedRefFrames[ i ]` — the §7.19 `MfRefFrames[ row ][
    /// col ]` grid snapshot (1 `i8` per mi cell).
    mf_ref_frames: Vec<i8>,
    /// §7.20 `SavedOrderHints[ i ][ ref ]` — the stored frame's
    /// `OrderHints[]` array (what ITS references' output order was),
    /// consumed by the §7.9.1/§7.9.2 `SavedOrderHints[ srcIdx ][ .. ]`
    /// reads.
    saved_order_hints: [i32; ALTREF_FRAME + 1],
    /// The stored frame's mi extent.
    mi_rows: u32,
    mi_cols: u32,
    /// §7.20 `RefFrameType[ i ]` — `FrameIsIntra` of the stored frame
    /// (the §7.9.2 projection skips KEY / INTRA_ONLY sources).
    frame_is_intra: bool,
    /// §7.20 `RefFrameType[ i ] == KEY_FRAME` — the §7.21 trigger.
    frame_type_is_key: bool,
    /// §7.20 `RefBitDepth[ i ]` — the stored frame's `BitDepth`.
    bit_depth: u8,
    /// §7.20 `save_cdfs( i )` — the frame-end CDF state (§8.4
    /// `frame_end_update_cdf`), loaded back by §8.3.1 `load_cdfs()`
    /// when a later frame names this slot as its primary reference.
    cdfs: Box<TileCdfContext>,
    /// §7.20 `SavedGmParams[ i ]` — this frame's `gm_params`.
    gm_params: [[i32; 6]; 8],
    /// §7.20 `save_loop_filter_params( i )` — the §5.9.11 running
    /// delta state at the end of this frame's header parse.
    lf_ref_deltas: [i8; 8],
    /// Mode-delta half of `save_loop_filter_params( i )`.
    lf_mode_deltas: [i8; 2],
    /// §7.20 `SavedSegmentIds[ i ]` — the decoded frame's
    /// `SegmentIds[ row ][ col ]` grid (row-major, one `i32` per mi
    /// cell), loaded back by `load_previous_segment_ids()` as the
    /// §5.11.21 `PrevSegmentIds` prediction source.
    segment_ids: Vec<i32>,
    /// §7.20 `save_grain_params( i )` — the frame's resolved §5.9.30
    /// film-grain state (post-`load_grain_params` on the
    /// `update_grain == 0` predicted path). `None` when the frame
    /// carries no grain (`reset_grain_params()`). Loaded back by the
    /// §5.9.30 `update_grain == 0` arm and by the §5.9.2
    /// `show_existing_frame` `load_grain_params(
    /// frame_to_show_map_idx )` output path.
    grain_params: Option<crate::uncompressed_header_tail::FilmGrainParams>,
}

/// Cross-frame decoder session state: the §5.9.2 `RefInfo` arrays the
/// inter `uncompressed_header()` parse consumes plus the §7.20
/// per-slot pixel/grid stores.
#[derive(Debug, Clone)]
struct SpecRefState {
    info: RefInfo,
    /// r453 — `Arc`-shared: §7.20 stores the SAME immutable payload
    /// into every slot `refresh_frame_flags` selects (and the §7.21
    /// show-existing re-store copies a stored one wholesale), so an
    /// all-refresh KEY frame keeps ONE payload alive instead of
    /// eight deep copies of planes + grids + CDFs. Nothing mutates a
    /// stored payload in place.
    slots: [Option<Arc<SpecRefSlot>>; NUM_REF_FRAMES as usize],
}

impl SpecRefState {
    fn new() -> Self {
        Self {
            info: RefInfo::default(),
            slots: Default::default(),
        }
    }
}

/// One frame decoded by [`decode_frame_spec_full`] — the surfaced
/// [`SpecFrame`] plus the §7.20 reference-update payload.
struct DecodedFrameInternal {
    /// The §7.18 output frame (post-film-grain).
    frame: SpecFrame,
    /// The §7.20 store payload: pre-grain cropped planes (`u16`).
    ref_planes: Vec<Vec<u16>>,
    /// `(width, height)` per `ref_planes` entry.
    ref_plane_dims: Vec<(u32, u32)>,
    /// §7.19 `MfMvs[]` grid (2 `i16` per mi cell).
    mf_mvs: Vec<i16>,
    /// §7.19 `MfRefFrames[]` grid (1 `i8` per mi cell).
    mf_ref_frames: Vec<i8>,
    /// This frame's `OrderHints[]` array (§5.9.2), the §7.20
    /// `SavedOrderHints` payload.
    order_hints_by_ref: [i32; ALTREF_FRAME + 1],
    /// §8.4 frame-end CDF state (`frame_end_update_cdf` output — the
    /// `context_update_tile_id` tile's adapted CDFs, or the frame-start
    /// state under `disable_frame_end_update_cdf == 1`).
    end_cdfs: Box<TileCdfContext>,
    /// The decoded frame's mi extent.
    mi_rows: u32,
    mi_cols: u32,
    /// §7.20 `SegmentIds[][]` snapshot (unwritten `-1` cells clamped
    /// to `0`, matching the spec's always-written map).
    segment_ids: Vec<i32>,
    /// The frame's RESOLVED film-grain state (the §5.9.30
    /// `update_grain == 0` predicted path replaced by the referenced
    /// slot's saved params with this frame's `grain_seed`), the §7.20
    /// `save_grain_params` payload.
    grain_params: Option<crate::uncompressed_header_tail::FilmGrainParams>,
}

/// §5.9.2 `LosslessArray[ segmentId ]` — `get_qindex( 1, segmentId ) ==
/// 0 && DeltaQYDc == 0 && DeltaQ{U,V}{Ac,Dc} == 0` for every segment.
pub(crate) fn lossless_array(
    qp: &crate::uncompressed_header_tail::QuantizationParams,
    sp: &SegmentationParams,
) -> [bool; MAX_SEGMENTS] {
    let deltas_all_zero = qp.delta_q_y_dc == 0
        && qp.delta_q_u_dc == 0
        && qp.delta_q_u_ac == 0
        && qp.delta_q_v_dc == 0
        && qp.delta_q_v_ac == 0;
    let mut out = [false; MAX_SEGMENTS];
    for (segment_id, slot) in out.iter_mut().enumerate() {
        // §7.12.2 get_qindex( ignoreDeltaQ = 1, segmentId ).
        let qindex = if sp.enabled && sp.segment_feature_active[segment_id][SEG_LVL_ALT_Q] {
            let data = i32::from(sp.segment_feature_data[segment_id][SEG_LVL_ALT_Q]);
            (i32::from(qp.base_q_idx) + data).clamp(0, 255)
        } else {
            i32::from(qp.base_q_idx)
        };
        *slot = qindex == 0 && deltas_all_zero;
    }
    out
}

/// Decode one intra frame through the §5.11 syntax walker + the §7.4
/// post-pass chain, given its already-parsed sequence header, frame
/// header, and the §5.11.1 tile-group OBU body.
pub fn decode_frame_spec(
    seq: &SequenceHeader,
    fh: &FrameHeader,
    tile_group_body: &[u8],
) -> Result<SpecFrame, Error> {
    // The historical intra-only entry: no cross-frame reference state,
    // so inter frames are rejected inside the full driver.
    let mut stages = DecodeStageTimings::default();
    Ok(decode_frame_spec_full(seq, fh, tile_group_body, None, &mut stages, 1)?.frame)
}

/// Parse one §5.11.1 tile-group OBU body against the frame's
/// [`TileInfo`] — the shared derivation (`NumTiles`, `tileBits`,
/// `TileSizeBytes`) both the whole-frame path and the r433
/// multi-tile-group accumulation ride.
fn parse_tile_group_body_with_ti(
    ti: &crate::tile_info::TileInfo,
    body: &[u8],
) -> Result<crate::encoder::tile_group_obu::ParsedTileGroup, Error> {
    let ceil_log2 = |v: u32| -> u32 {
        if v <= 1 {
            0
        } else {
            32 - (v - 1).leading_zeros()
        }
    };
    if ti.mi_col_starts.len() != (ti.tile_cols as usize) + 1
        || ti.mi_row_starts.len() != (ti.tile_rows as usize) + 1
    {
        return Err(Error::PartitionWalkOutOfRange);
    }
    parse_tile_group_obu_body(
        body,
        ti.tile_cols * ti.tile_rows,
        ceil_log2(ti.tile_cols),
        ceil_log2(ti.tile_rows),
        u32::from(ti.tile_size_bytes),
    )
}

/// Parse a tile-group body that must carry the WHOLE frame — the
/// §5.10 `frame_obu` shape (§6.10.1 requires
/// `tile_start_and_end_present_flag == 0` there, which forces
/// `tg_start = 0` / `tg_end = NumTiles - 1`) and the historical
/// single-tile-group packing the standalone entries accept.
fn parse_whole_frame_tile_group(
    ti: &crate::tile_info::TileInfo,
    body: &[u8],
    require_flag_zero: bool,
) -> Result<Vec<TilePayload>, Error> {
    let parsed = parse_tile_group_body_with_ti(ti, body)?;
    let num_tiles = ti.tile_cols * ti.tile_rows;
    if parsed.tg_start != 0 || parsed.tg_end + 1 != num_tiles {
        return Err(Error::TileGroupInvalid);
    }
    if require_flag_zero && parsed.tile_start_and_end_present_flag {
        return Err(Error::TileGroupInvalid);
    }
    Ok(parsed.tiles)
}

/// [`decode_frame_spec`] with the cross-frame reference state — the
/// full §5.11 + §7.4 decode for one KEY / INTRA_ONLY / INTER frame
/// whose tile-group body carries every tile, returning both the
/// output frame and the §7.20 reference-update payload.
fn decode_frame_spec_full(
    seq: &SequenceHeader,
    fh: &FrameHeader,
    tile_group_body: &[u8],
    refs: Option<&SpecRefState>,
    stages: &mut DecodeStageTimings,
    threads: usize,
) -> Result<DecodedFrameInternal, Error> {
    let ti = fh
        .tile_info
        .as_ref()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    let t0 = Instant::now();
    let tiles = parse_whole_frame_tile_group(ti, tile_group_body, false)?;
    stages.parse += t0.elapsed();
    decode_frame_spec_tiles(seq, fh, &tiles, refs, stages, threads)
}

/// The frame decode proper, over already-parsed per-tile §8.2
/// payloads — `tiles` must hold exactly `NumTiles` entries in tile
/// raster order (§5.11.1 `TileNum` order; the r433 multi-tile-group
/// accumulation concatenates conformant `tg_start..=tg_end` slices to
/// get here).
fn decode_frame_spec_tiles(
    seq: &SequenceHeader,
    fh: &FrameHeader,
    tiles: &[TilePayload],
    refs: Option<&SpecRefState>,
    stages: &mut DecodeStageTimings,
    threads: usize,
) -> Result<DecodedFrameInternal, Error> {
    if fh.show_existing_frame {
        // §7.21 output-existing path is the caller's (no tile group).
        return Err(Error::PartitionWalkOutOfRange);
    }
    if !fh.frame_is_intra && refs.is_none() {
        // Inter frames are undecodable without the §7.20 store.
        return Err(Error::PartitionWalkOutOfRange);
    }
    let cc = &seq.color_config;
    if !matches!(cc.bit_depth, 8 | 10 | 12) {
        return Err(Error::PartitionWalkOutOfRange);
    }
    let fs = fh
        .frame_size
        .as_ref()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    if fs.use_superres && fs.upscaled_width <= fs.frame_width {
        // §5.9.8 conformance: superres only ever widens.
        return Err(Error::PartitionWalkOutOfRange);
    }
    let ti = fh
        .tile_info
        .as_ref()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    let qp = fh
        .quantization_params
        .as_ref()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    let default_seg = SegmentationParams::disabled();
    let sp = fh.segmentation_params.as_ref().unwrap_or(&default_seg);
    let dq = fh.delta_q_params.unwrap_or_default();
    let dlf = fh.delta_lf_params.unwrap_or_default();
    let lr = fh
        .lr_params
        .as_ref()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    // §5.11.57 loop-restoration state for the per-superblock `read_lr`
    // interleave (walker-facing shape).
    let lr_walk = crate::cdf::LrParams {
        num_planes: cc.num_planes as usize,
        frame_restoration_type: [
            lr.frame_restoration_type[0] as u8,
            lr.frame_restoration_type[1] as u8,
            lr.frame_restoration_type[2] as u8,
        ],
        loop_restoration_size: lr.loop_restoration_size,
        subsampling_x: u8::from(cc.subsampling_x),
        subsampling_y: u8::from(cc.subsampling_y),
        frame_height: fs.frame_height,
        upscaled_width: fs.upscaled_width,
        use_superres: fs.use_superres,
        superres_denom: fs.superres_denom,
        allow_intrabc: fh.allow_intrabc,
    };

    // §5.9.2 CodedLossless / LosslessArray.
    let lossless = lossless_array(qp, sp);
    let coded_lossless = lossless.iter().all(|&l| l);

    // §7.12.2 quantizer state for the walk. `CurrentQIndex` starts at
    // `base_q_idx` (§5.9.18); the walker's §5.11.13 read_delta_qindex
    // maintains the running value internally.
    let seg_alt_q_active = {
        let mut a = [false; MAX_SEGMENTS];
        for (i, slot) in a.iter_mut().enumerate() {
            *slot = sp.segment_feature_active[i][SEG_LVL_ALT_Q];
        }
        a
    };
    let seg_alt_q_data = {
        let mut a = [0i16; MAX_SEGMENTS];
        for (i, slot) in a.iter_mut().enumerate() {
            *slot = sp.segment_feature_data[i][SEG_LVL_ALT_Q];
        }
        a
    };
    // §5.9.2 `SegQMLevel[ plane ][ segmentId ]`: when `using_qmatrix`,
    // a lossless segment takes the no-QM sentinel `15`, every other
    // segment `[ qm_y, qm_u, qm_v ]`. With `using_qmatrix == 0` the
    // §7.12.3 QM arm never fires and the neutral all-`15` fill stands.
    let mut seg_qm_level = [[15u8; MAX_SEGMENTS]; 3];
    if qp.using_qmatrix {
        for segment_id in 0..MAX_SEGMENTS {
            if !lossless[segment_id] {
                seg_qm_level[0][segment_id] = qp.qm_y;
                seg_qm_level[1][segment_id] = qp.qm_u;
                seg_qm_level[2][segment_id] = qp.qm_v;
            }
        }
    }
    let quant = QuantizerParams {
        base_q_idx: qp.base_q_idx,
        delta_q_y_dc: qp.delta_q_y_dc,
        delta_q_u_dc: qp.delta_q_u_dc,
        delta_q_u_ac: qp.delta_q_u_ac,
        delta_q_v_dc: qp.delta_q_v_dc,
        delta_q_v_ac: qp.delta_q_v_ac,
        using_qmatrix: qp.using_qmatrix,
        bit_depth: cc.bit_depth,
        delta_q_present: dq.delta_q_present,
        current_q_index: qp.base_q_idx,
        segmentation_enabled: sp.enabled,
        seg_alt_q_active,
        seg_alt_q_data,
        seg_qm_level,
    };

    // r426 — per-segment SEG_LVL_SKIP active flags: the §5.11.11
    // short-circuit derives per block from the coded segment id.
    let seg_skip: [bool; MAX_SEGMENTS] = if sp.enabled {
        core::array::from_fn(|s| sp.segment_feature_active[s][SEG_LVL_SKIP])
    } else {
        [false; MAX_SEGMENTS]
    };

    let cdef_bits = fh
        .cdef_params
        .as_ref()
        .map_or(0, |c| u32::from(c.cdef_bits));
    let tx_mode_select = matches!(
        fh.tx_mode,
        Some(crate::uncompressed_header_tail::TxMode::TxModeSelect)
    );

    let params = TileDecodeParams {
        frame_is_intra: fh.frame_is_intra,
        subsampling_x: u8::from(cc.subsampling_x),
        subsampling_y: u8::from(cc.subsampling_y),
        num_planes: cc.num_planes,
        seg_id_pre_skip: sp.seg_id_pre_skip,
        segmentation_enabled: sp.enabled,
        seg_skip,
        last_active_seg_id: sp.last_active_seg_id,
        lossless_array: &lossless,
        coded_lossless,
        enable_cdef: seq.enable_cdef,
        allow_intrabc: fh.allow_intrabc,
        cdef_bits,
        use_128x128_superblock: seq.use_128x128_superblock,
        delta_q_res: dq.delta_q_res,
        delta_lf_present: dlf.delta_lf_present,
        delta_lf_multi: dlf.delta_lf_multi,
        mono_chrome: cc.mono_chrome,
        delta_lf_res: dlf.delta_lf_res,
        allow_screen_content_tools: fh.allow_screen_content_tools,
        enable_filter_intra: seq.enable_filter_intra,
        bit_depth: cc.bit_depth,
        tx_mode_select,
        reduced_tx_set: fh.reduced_tx_set.unwrap_or(false),
        enable_intra_edge_filter: seq.enable_intra_edge_filter,
    };

    // Per-tile §8.2 byte ranges — the caller parsed the §5.11.1
    // grouping; this driver requires the full frame's tiles.
    let num_tiles = ti.tile_cols * ti.tile_rows;
    if ti.mi_col_starts.len() != (ti.tile_cols as usize) + 1
        || ti.mi_row_starts.len() != (ti.tile_rows as usize) + 1
    {
        return Err(Error::PartitionWalkOutOfRange);
    }
    if tiles.len() != num_tiles as usize {
        return Err(Error::TileGroupInvalid);
    }

    let mi_rows = fs.mi_rows;
    let mi_cols = fs.mi_cols;
    let mut walker = PartitionWalker::new(
        mi_rows,
        mi_cols,
        TileGeometry {
            mi_row_start: 0,
            mi_row_end: mi_rows,
            mi_col_start: 0,
            mi_col_end: mi_cols,
        },
    )
    .ok_or(Error::PartitionWalkOutOfRange)?;

    // ---- Inter-frame state: §5.11.18 context + §7.11.3 ref pixels. ----
    // Owned buffers first (the `InterFrameContext` borrows them), then
    // the context itself. All of it is inert on intra frames.
    let is_inter_frame = !fh.frame_is_intra;
    let sub_x_u8 = u8::from(cc.subsampling_x);
    let sub_y_u8 = u8::from(cc.subsampling_y);
    let num_planes_usize = cc.num_planes as usize;
    // §7.9 motion-field grid — the temporal-scan source. §5.9.2
    // invokes `motion_field_estimation()` only when `use_ref_frame_mvs
    // == 1`; the projection walks the §7.20-stored `SavedMvs` /
    // `SavedRefFrames` / `SavedOrderHints` of up to four reference
    // frames (LAST backwards, BWDREF / ALTREF2 / ALTREF forwards,
    // LAST2 as the stack filler).
    let mfmvs = if is_inter_frame
        && fh
            .inter_refs
            .as_ref()
            .is_some_and(|ir| ir.use_ref_frame_mvs)
    {
        motion_field_estimation(
            refs.ok_or(Error::PartitionWalkOutOfRange)?,
            fh,
            seq,
            mi_rows,
            mi_cols,
        )?
    } else {
        MotionFieldMvs::new_invalid(mi_rows, mi_cols)
    };
    // §5.9.2 `load_previous_segment_ids()` — with a primary reference
    // whose stored mi extent matches, `PrevSegmentIds` is that slot's
    // §7.20 `SavedSegmentIds`; otherwise all-zero (also the
    // `setup_past_independence()` state for PRIMARY_REF_NONE).
    let prev_segment_ids: Option<Vec<i32>> =
        if is_inter_frame && sp.enabled && fh.primary_ref_frame != PRIMARY_REF_NONE {
            let st = refs.ok_or(Error::PartitionWalkOutOfRange)?;
            let ir = fh
                .inter_refs
                .as_ref()
                .ok_or(Error::PartitionWalkOutOfRange)?;
            let slot = ir.ref_frame_idx[fh.primary_ref_frame as usize] as usize;
            match st.slots.get(slot).and_then(|s| s.as_ref()) {
                Some(prev)
                    if prev.mi_rows == mi_rows
                        && prev.mi_cols == mi_cols
                        && prev.segment_ids.len() == (mi_rows as usize) * (mi_cols as usize) =>
                {
                    Some(prev.segment_ids.clone())
                }
                _ => Some(vec![0i32; (mi_rows as usize) * (mi_cols as usize)]),
            }
        } else if is_inter_frame && sp.enabled {
            Some(vec![0i32; (mi_rows as usize) * (mi_cols as usize)])
        } else {
            None
        };
    // Never-referenced placeholder for empty §7.20 slots (a conformant
    // stream only references `RefValid` slots).
    let dummy_plane: Vec<u16> = vec![0u16; 64];
    let dummy_entry = || RefFrameStoreEntry {
        plane: &dummy_plane,
        stride: 8,
        upscaled_width: 8,
        width: 8,
        height: 8,
    };
    // Per-plane `FrameStore[ slot ]` views over the §7.20 store.
    let mut plane_stores: Vec<[RefFrameStoreEntry<'_>; NUM_REF_FRAMES as usize]> = Vec::new();
    let mut ref_frame_idx = [0u8; 7];
    let mut order_hints_by_ref = [0i32; ALTREF_FRAME + 1];
    let mut sign_bias = [0i32; 8];
    let mut is_scaled_per_ref = [false; 7];
    if is_inter_frame {
        let st = refs.ok_or(Error::PartitionWalkOutOfRange)?;
        let ir = fh
            .inter_refs
            .as_ref()
            .ok_or(Error::PartitionWalkOutOfRange)?;
        for plane in 0..num_planes_usize {
            let arr: [RefFrameStoreEntry<'_>; NUM_REF_FRAMES as usize] =
                core::array::from_fn(|slot| match st.slots[slot].as_ref() {
                    Some(s) if plane < s.planes.len() => {
                        let (w, _h) = s.plane_dims[plane];
                        // r405: the dimension fields carry the stored
                        // frame's LUMA extents (§7.11.3.3 scale ratio +
                        // §7.11.3.4/.5 clamps derive per-plane
                        // internally); only the buffer + stride are in
                        // this plane's own sample units.
                        let (lw, lh) = s.plane_dims[0];
                        RefFrameStoreEntry {
                            plane: &s.planes[plane],
                            stride: w as usize,
                            upscaled_width: lw,
                            width: lw,
                            height: lh,
                        }
                    }
                    _ => dummy_entry(),
                });
            plane_stores.push(arr);
        }
        for i in 0..7 {
            let slot = ir.ref_frame_idx[i] as usize;
            if slot >= NUM_REF_FRAMES as usize {
                return Err(Error::PartitionWalkOutOfRange);
            }
            ref_frame_idx[i] = slot as u8;
            // §5.9.2 `OrderHints[ LAST_FRAME + i ] =
            // RefOrderHint[ ref_frame_idx[ i ] ]`.
            let hint = st.info.order_hint[slot] as i32;
            order_hints_by_ref[LAST_FRAME + i] = hint;
            // §7.8 `RefFrameSignBias[ refFrame ] =
            // get_relative_dist( hint, OrderHint ) > 0`.
            if seq.enable_order_hint {
                sign_bias[LAST_FRAME + i] = i32::from(
                    get_relative_dist(hint, fh.order_hint as i32, u32::from(seq.order_hint_bits))
                        > 0,
                );
            }
            // §5.11.27 `is_scaled( refFrame )` — the literal spec body:
            //
            //   xScale = ( ( RefUpscaledWidth[ refIdx ] << REF_SCALE_SHIFT )
            //              + ( FrameWidth / 2 ) ) / FrameWidth
            //   yScale = ( ( RefFrameHeight[ refIdx ] << REF_SCALE_SHIFT )
            //              + ( FrameHeight / 2 ) ) / FrameHeight
            //   return xScale != noScale || yScale != noScale
            //
            // The divisor is the CODED `FrameWidth`/`FrameHeight`, NOT
            // `UpscaledWidth` (r408 fix): a superres frame codes at
            // `FrameWidth < UpscaledWidth` while its references store
            // `RefUpscaledWidth == UpscaledWidth`, so every reference
            // IS scaled even though the upscaled extents match. The
            // pre-r408 upscaled-vs-upscaled shortcut read the §5.11.27
            // `motion_mode` symbol where the encoder wrote `use_obmc`,
            // desynchronising the arithmetic decoder on the first
            // superres inter frame.
            let no_scale = 1u64 << crate::inter_pred::REF_SCALE_SHIFT;
            let fw = u64::from(fs.frame_width.max(1));
            let fhh = u64::from(fs.frame_height.max(1));
            let x_scale = ((u64::from(st.info.upscaled_width[slot])
                << crate::inter_pred::REF_SCALE_SHIFT)
                + fw / 2)
                / fw;
            let y_scale = ((u64::from(st.info.frame_height[slot])
                << crate::inter_pred::REF_SCALE_SHIFT)
                + fhh / 2)
                / fhh;
            is_scaled_per_ref[i] = x_scale != no_scale || y_scale != no_scale;
        }
    }
    let order_hints = FrameInterOrderHints {
        order_hint_bits: if seq.enable_order_hint {
            u32::from(seq.order_hint_bits)
        } else {
            0
        },
        current_order_hint: fh.order_hint as i32,
        order_hints_by_ref,
    };
    let plane_ref_specs: Vec<PlaneRefSpec<'_>> = (0..plane_stores.len())
        .map(|p| PlaneRefSpec {
            plane: p as u8,
            subsampling_x: if p > 0 { sub_x_u8 } else { 0 },
            subsampling_y: if p > 0 { sub_y_u8 } else { 0 },
            frame_store: &plane_stores[p],
            // r405: LUMA extents for every plane — the §7.11.3.3
            // scale ratio is luma-derived (an odd `FrameWidth` such
            // as a 213-wide resized frame has no faithful per-plane
            // representation).
            frame_width: fs.frame_width,
            frame_height: fs.frame_height,
        })
        .collect();
    let pixels = InterWalkPixels {
        ref_frame_idx,
        bit_depth: cc.bit_depth,
        plane_refs: &plane_ref_specs,
        order_hints,
    };
    let ictx: Option<InterFrameContext<'_>> = if is_inter_frame {
        let ir = fh
            .inter_refs
            .as_ref()
            .ok_or(Error::PartitionWalkOutOfRange)?;
        let mut c = InterFrameContext::identity_default(&mfmvs);
        // §5.9.14 segmentation state: the update flags, the
        // FeatureEnabled/FeatureData tables (per-block
        // `seg_feature_active` gates), and the §5.11.21
        // `PrevSegmentIds` prediction source.
        c.segmentation_update_map = sp.update_map;
        c.segmentation_temporal_update = sp.temporal_update;
        c.seg_feature_active = sp.segment_feature_active;
        c.seg_feature_data = sp.segment_feature_data;
        c.prev_segment_ids = prev_segment_ids.as_deref();
        // §5.9.22 skip-mode state: the §5.11.10 `read_skip_mode` gate
        // plus the fixed compound reference pair the skip-mode arm
        // predicts from.
        c.skip_mode_present = fh.skip_mode_present.unwrap_or(false);
        if let Some(smf) = fh.skip_mode_frame {
            c.skip_mode_frame = [i32::from(smf[0]), i32::from(smf[1])];
        }
        c.reference_select = fh.reference_select.unwrap_or(false);
        if let Some(g) = fh.global_motion_params.as_ref() {
            for r in 0..8 {
                c.gm_type[r] = g.gm_type[r] as i32;
                c.gm_params[r] = g.gm_params[r];
            }
        }
        c.ref_frame_sign_bias = sign_bias;
        c.allow_high_precision_mv = ir.allow_high_precision_mv;
        c.force_integer_mv = fh.force_integer_mv;
        c.use_ref_frame_mvs = ir.use_ref_frame_mvs;
        c.is_motion_mode_switchable = ir.is_motion_mode_switchable;
        c.allow_warped_motion = fh.allow_warped_motion.unwrap_or(false);
        c.is_scaled_per_ref = is_scaled_per_ref;
        c.enable_interintra_compound = seq.enable_interintra_compound;
        c.enable_masked_compound = seq.enable_masked_compound;
        c.enable_jnt_comp = seq.enable_jnt_comp;
        // §8.3.2 compound-idx ctx + §7.11.3.15 DISTANCE weights both
        // derive per block from the frame's OrderHint / OrderHints[].
        c.order_hints = order_hints;
        c.interpolation_filter = ir.interpolation_filter as u8;
        c.enable_dual_filter = seq.enable_dual_filter;
        c.pixels = Some(&pixels);
        Some(c)
    } else {
        None
    };

    // §8.3.1 frame-start CDF state: `load_cdfs( ref_frame_idx[
    // primary_ref_frame ] )` when a primary reference exists (the
    // §7.20-saved frame-end state of that slot, coefficient CDFs
    // included), otherwise `init_non_coeff_cdfs()` (the §9.4 defaults)
    // + the q-context-selected `init_coeff_cdfs( base_q_idx )`.
    let frame_start_cdfs: Box<TileCdfContext> = if fh.primary_ref_frame != PRIMARY_REF_NONE {
        let st = refs.ok_or(Error::PartitionWalkOutOfRange)?;
        let ir = fh
            .inter_refs
            .as_ref()
            .ok_or(Error::PartitionWalkOutOfRange)?;
        let slot = ir.ref_frame_idx[fh.primary_ref_frame as usize] as usize;
        let slot_state = st
            .slots
            .get(slot)
            .and_then(|s| s.as_ref())
            .ok_or(Error::PartitionWalkOutOfRange)?;
        let mut loaded = slot_state.cdfs.clone();
        // §6.8.21 `load_cdfs`: the symbol counts restart at zero (the
        // probabilities carry over) — the §8.3 adaptation rate depends
        // on the per-row count.
        loaded.zero_counts();
        loaded
    } else {
        let mut c = TileCdfContext::new_from_defaults();
        c.init_coeff_cdfs(qp.base_q_idx);
        Box::new(c)
    };

    // §5.11.2 decode_tile() per tile, in tile order — each tile gets a
    // fresh §8.2.2 symbol decoder and a fresh copy of the frame-start
    // CDF state (§8.2.2 `clear_above_context` etc. are per-tile inside
    // `begin_tile`), while the walker's frame-scope decode grids
    // accumulate across tiles. The §5.11.57 `read_lr` interleave runs
    // when the frame signals loop restoration. Per §8.2.4
    // `exit_symbol` / §8.4 `frame_end_update_cdf`, the tile numbered
    // `context_update_tile_id` donates its adapted CDFs as the
    // frame-end state (unless `disable_frame_end_update_cdf`, which
    // keeps the frame-start state).
    let mut end_cdfs: Option<Box<TileCdfContext>> = None;
    let t_tiles = Instant::now();
    // r475 — tile-parallel walk. §5.11 scopes every context, every
    // neighbour availability (`is_inside` is tile-relative), the
    // intra edges, the delta-q / delta-lf / loop-restoration
    // reference state and the entropy state to the tile, and tile
    // boundaries are superblock aligned — so an intra tile decoded on
    // a TILE-SIZED walker (the tile at the origin of its own frame)
    // reconstructs the samples and stamps the grids the frame walk
    // would, bit for bit; only §5.11.57 `read_lr` addresses a
    // frame-level grid (`set_lr_origin`). The inter arms read the
    // frame-level motion field / reference planes at frame positions
    // and intra block copy reads the frame-level `CurrFrame`, so
    // those frames keep the serial walk. Finished tiles are stitched
    // into the frame walker as they arrive (`import_rect_from` +
    // the planes + the restoration units), so at most `threads`
    // tile-sized states are live beyond the frame's own.
    let tile_parallel =
        threads > 1 && tiles.len() > 1 && fh.frame_is_intra && !fh.allow_intrabc && !is_inter_frame;
    if tile_parallel {
        use std::sync::atomic::{AtomicUsize, Ordering};
        let worker_count = threads.min(tiles.len());
        let next = AtomicUsize::new(0);
        let lr_walk_ref = if lr.uses_lr { Some(&lr_walk) } else { None };
        let decode_one =
            |tile_num: usize| -> Result<(PartitionWalker, Box<TileCdfContext>), Error> {
                let tile = &tiles[tile_num];
                let tile_row = (tile_num as u32) / ti.tile_cols;
                let tile_col = (tile_num as u32) % ti.tile_cols;
                let (r0, r1) = (
                    ti.mi_row_starts[tile_row as usize],
                    ti.mi_row_starts[tile_row as usize + 1],
                );
                let (c0, c1) = (
                    ti.mi_col_starts[tile_col as usize],
                    ti.mi_col_starts[tile_col as usize + 1],
                );
                let (h4, w4) = (r1 - r0, c1 - c0);
                let mut local = PartitionWalker::new(
                    h4,
                    w4,
                    TileGeometry {
                        mi_row_start: 0,
                        mi_row_end: h4,
                        mi_col_start: 0,
                        mi_col_end: w4,
                    },
                )
                .ok_or(Error::PartitionWalkOutOfRange)?;
                local.set_lr_origin(r0, c0);
                local.begin_tile(TileGeometry {
                    mi_row_start: 0,
                    mi_row_end: h4,
                    mi_col_start: 0,
                    mi_col_end: w4,
                });
                local.set_current_q_index(i32::from(qp.base_q_idx));
                let mut decoder = SymbolDecoder::init_symbol(
                    &tile.bytes,
                    tile.bytes.len(),
                    fh.disable_cdf_update,
                )?;
                let mut cdfs = frame_start_cdfs.as_ref().clone();
                local.decode_tile_syntax_with_lr(
                    &mut decoder,
                    &mut cdfs,
                    &params,
                    lr_walk_ref,
                    None,
                    &quant,
                    dq.delta_q_present,
                )?;
                Ok((local, Box::new(cdfs)))
            };
        let mut first_err: Option<Error> = None;
        std::thread::scope(|scope| {
            let (tx, rx) = std::sync::mpsc::channel();
            for _ in 0..worker_count {
                let tx = tx.clone();
                let next = &next;
                let decode_one = &decode_one;
                scope.spawn(move || loop {
                    let t = next.fetch_add(1, Ordering::SeqCst);
                    if t >= tiles.len() {
                        break;
                    }
                    let out = decode_one(t);
                    if tx.send((t, out)).is_err() {
                        break;
                    }
                });
            }
            drop(tx);
            for (t, out) in rx {
                match out {
                    Ok((local, cdfs)) => {
                        let tile_row = (t as u32) / ti.tile_cols;
                        let tile_col = (t as u32) % ti.tile_cols;
                        let (r0, r1) = (
                            ti.mi_row_starts[tile_row as usize],
                            ti.mi_row_starts[tile_row as usize + 1],
                        );
                        let (c0, c1) = (
                            ti.mi_col_starts[tile_col as usize],
                            ti.mi_col_starts[tile_col as usize + 1],
                        );
                        walker.import_rect_from(
                            &local,
                            0,
                            0,
                            r0,
                            c0,
                            r1 - r0,
                            c1 - c0,
                            sub_x_u8,
                            sub_y_u8,
                        );
                        walker.import_curr_frame_rect_from(&local, r0, c0, sub_x_u8, sub_y_u8);
                        if lr.uses_lr {
                            walker.import_lr_units_from(&local, r0, r1, c0, c1, &lr_walk);
                        }
                        if !fh.disable_frame_end_update_cdf && t as u32 == ti.context_update_tile_id
                        {
                            end_cdfs = Some(cdfs);
                        }
                    }
                    Err(e) => {
                        if first_err.is_none() {
                            first_err = Some(e);
                        }
                    }
                }
            }
        });
        if let Some(e) = first_err {
            return Err(e);
        }
    }
    for (tile_num, tile) in tiles.iter().enumerate() {
        if tile_parallel {
            break;
        }
        let tile_row = (tile_num as u32) / ti.tile_cols;
        let tile_col = (tile_num as u32) % ti.tile_cols;
        walker.begin_tile(TileGeometry {
            mi_row_start: ti.mi_row_starts[tile_row as usize],
            mi_row_end: ti.mi_row_starts[tile_row as usize + 1],
            mi_col_start: ti.mi_col_starts[tile_col as usize],
            mi_col_end: ti.mi_col_starts[tile_col as usize + 1],
        });
        // §5.11.1 per-tile prologue: `CurrentQIndex = base_q_idx` —
        // the §5.11.12 running delta-q accumulator restarts from the
        // frame quantiser at every tile (r405: it previously started
        // from the walker-construction zero, so every
        // `delta_q_present` stream dequantised against a garbage
        // base).
        walker.set_current_q_index(i32::from(qp.base_q_idx));
        let mut decoder =
            SymbolDecoder::init_symbol(&tile.bytes, tile.bytes.len(), fh.disable_cdf_update)?;
        let mut cdfs = frame_start_cdfs.as_ref().clone();
        walker.decode_tile_syntax_with_lr(
            &mut decoder,
            &mut cdfs,
            &params,
            if lr.uses_lr { Some(&lr_walk) } else { None },
            /* inter_ctx = */ ictx.as_ref(),
            &quant,
            /* read_deltas = */ dq.delta_q_present,
        )?;
        if !fh.disable_frame_end_update_cdf && tile_num as u32 == ti.context_update_tile_id {
            end_cdfs = Some(Box::new(cdfs));
        }
    }
    let end_cdfs = end_cdfs.unwrap_or(frame_start_cdfs);
    stages.tiles += t_tiles.elapsed();

    // ---- Take `CurrFrame[ plane ]` at its FULL mi-grid extent. ----
    // The spec's CurrFrame covers the padded `MiRows x MiCols` grid
    // (frames whose dimensions are not mi-aligned carry decoded padding
    // columns/rows past the crop). The §7.4 in-loop passes read that
    // padding — the §7.14 wide-filter taps, the §7.15 mi-bounded
    // availability region, and especially the §7.16 upscaler's
    // `Clip3(0, miW * MI_SIZE - 1, ..)` source clamp — so the whole
    // chain runs at the padded extent and the §7.18.2 crop to the
    // output extents happens at the very end.
    let sub_x = u8::from(cc.subsampling_x) as u32;
    let sub_y = u8::from(cc.subsampling_y) as u32;
    let num_planes = cc.num_planes as usize;
    let mut plane_bufs: Vec<Vec<i32>> = Vec::with_capacity(num_planes);
    let mut plane_dims: Vec<(u32, u32)> = Vec::with_capacity(num_planes);
    for plane in 0..num_planes {
        // r453 — MOVE the plane out of the walker (the §7.14/§7.15/
        // §7.16/§7.17 in-loop passes below operate on these caller-
        // owned buffers; nothing in the walker reads `CurrFrame`
        // after the tile walk), saving a full-frame `i32` copy.
        let (rows, cols, src) = walker
            .take_curr_frame_plane(plane)
            .ok_or(Error::PartitionWalkOutOfRange)?;
        let (pw, ph) = if plane == 0 {
            (fs.frame_width, fs.frame_height)
        } else {
            (
                (fs.frame_width + sub_x) >> sub_x,
                (fs.frame_height + sub_y) >> sub_y,
            )
        };
        if rows < ph || cols < pw {
            return Err(Error::PartitionWalkOutOfRange);
        }
        plane_bufs.push(src);
        plane_dims.push((cols, rows));
    }

    // ---- §7.4 in-loop passes: §7.14 deblock, then §7.15 CDEF. ----
    // §7.4 step 1: the loop filter is invoked ONLY when
    // `loop_filter_level[ 0 ] != 0 || loop_filter_level[ 1 ] != 0` —
    // with both luma levels zero the frame is NOT deblocked at all,
    // even though the §7.14.4 `loop_filter_delta_enabled` ref-delta
    // path could otherwise lift a per-edge strength above zero.
    let t_deblock = Instant::now();
    if let Some(lf) = fh.loop_filter_params.as_ref() {
        if (lf.loop_filter_level[0] != 0 || lf.loop_filter_level[1] != 0)
            && !coded_lossless
            && !fh.allow_intrabc
        {
            let mut bufs: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
            for (buf, &(pw, ph)) in plane_bufs.iter_mut().zip(plane_dims.iter()) {
                bufs.push(PlaneBuffer {
                    rows: ph,
                    cols: pw,
                    samples: buf,
                });
            }
            if threads > 1 {
                walker.loop_filter_frame_parallel_from_grid(
                    lf,
                    sp,
                    dlf.delta_lf_multi,
                    cc.num_planes,
                    cc.bit_depth,
                    sub_x as u8,
                    sub_y as u8,
                    fs.frame_width,
                    fs.frame_height,
                    &mut bufs,
                    threads,
                );
            } else {
                walker.loop_filter_frame_from_grid(
                    lf,
                    sp,
                    dlf.delta_lf_multi,
                    cc.num_planes,
                    cc.bit_depth,
                    sub_x as u8,
                    sub_y as u8,
                    fs.frame_width,
                    fs.frame_height,
                    &mut bufs,
                );
            }
        }
    }
    // §7.4 steps 2-5: CDEF, then loop restoration. `plane_bufs` holds
    // the post-deblock CurrFrame here; §7.17 needs BOTH that frame (the
    // `UpscaledCurrFrame` — no superres on this path) and the CDEF
    // output (`UpscaledCdefFrame`), so keep the pre-CDEF copy around
    // when restoration is active.
    stages.deblock += t_deblock.elapsed();
    let t_cdef = Instant::now();
    let cdef_active = fh.cdef_params.as_ref().is_some_and(|cdef| {
        !coded_lossless && !fh.allow_intrabc && seq.enable_cdef && !cdef.short_circuited
    });
    let superres_active = fs.use_superres && fs.upscaled_width > fs.frame_width;
    if !superres_active {
        // r475 — the band / stripe path: §7.15 CDEF runs in place over
        // a rolling window of the unfiltered rows its taps read (the
        // block ± 2, §7.15.3 `cdef_get_at` bounded by §5.11.52), and
        // §7.17 restores one stripe at a time from a copy of the
        // stripe's CDEF rows plus the pre-CDEF rows §7.17.6 reads at
        // the stripe's 2-row skirts, captured here from the deblocked
        // frame. No frame-sized copy of `CurrFrame` / `CdefFrame` is
        // made (the pre-r475 driver held three); every sample is
        // computed from the same inputs, so the output is identical.
        // The §7.16 superres arm below keeps the whole-frame path (it
        // upscales both frames).
        let boundary: Vec<crate::loop_restoration::StripeBoundaryRows> = if lr.uses_lr {
            plane_bufs
                .iter_mut()
                .zip(plane_dims.iter())
                .enumerate()
                .map(|(plane, (buf, &(pw, ph)))| {
                    let sy = if plane == 0 { 0 } else { sub_y };
                    let plane_h = (fs.frame_height + sy) >> sy;
                    let pb = PlaneBuffer {
                        rows: ph,
                        cols: pw,
                        samples: buf,
                    };
                    crate::loop_restoration::StripeBoundaryRows::capture(
                        &pb,
                        plane_h as i32 - 1,
                        sy as u8,
                    )
                })
                .collect()
        } else {
            Vec::new()
        };
        let mut bufs: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
        for (buf, &(pw, ph)) in plane_bufs.iter_mut().zip(plane_dims.iter()) {
            bufs.push(PlaneBuffer {
                rows: ph,
                cols: pw,
                samples: buf,
            });
        }
        if let (true, Some(cdef)) = (cdef_active, fh.cdef_params.as_ref()) {
            if threads > 1 {
                walker.cdef_frame_parallel_from_idx(
                    cdef,
                    cc.num_planes,
                    cc.bit_depth,
                    sub_x as u8,
                    sub_y as u8,
                    &mut bufs,
                    threads,
                );
            } else {
                walker.cdef_frame_in_place_from_idx(
                    cdef,
                    cc.num_planes,
                    cc.bit_depth,
                    sub_x as u8,
                    sub_y as u8,
                    &mut bufs,
                );
            }
        }
        stages.cdef += t_cdef.elapsed();
        let t_lr = Instant::now();
        if lr.uses_lr {
            if threads > 1 {
                walker.loop_restore_frame_parallel_from_grid(
                    lr,
                    cc.num_planes,
                    cc.bit_depth,
                    sub_x as u8,
                    sub_y as u8,
                    mi_rows,
                    mi_cols,
                    fs.frame_height,
                    fs.upscaled_width,
                    &mut bufs,
                    &boundary,
                    threads,
                );
            } else {
                walker.loop_restore_frame_in_place_from_grid(
                    lr,
                    cc.num_planes,
                    cc.bit_depth,
                    sub_x as u8,
                    sub_y as u8,
                    mi_rows,
                    mi_cols,
                    fs.frame_height,
                    fs.upscaled_width,
                    &mut bufs,
                    &boundary,
                );
            }
        }
        stages.restoration += t_lr.elapsed();
    } else {
        let mut deblocked: Option<Vec<Vec<i32>>> = if lr.uses_lr {
            Some(plane_bufs.clone())
        } else {
            None
        };
        if let Some(cdef) = fh.cdef_params.as_ref() {
            if !coded_lossless && !fh.allow_intrabc && seq.enable_cdef && !cdef.short_circuited {
                // §7.15 filters from the deblocked frame. r453 — when
                // §7.17 is active the `deblocked` copy above IS that
                // frame (CDEF only reads its source), so borrow it
                // instead of cloning a third full-frame buffer.
                let mut fallback: Option<Vec<Vec<i32>>> = None;
                let src_owned: &mut Vec<Vec<i32>> = match deblocked {
                    Some(ref mut d) => d,
                    None => fallback.insert(plane_bufs.clone()),
                };
                let mut src: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
                for (buf, &(pw, ph)) in src_owned.iter_mut().zip(plane_dims.iter()) {
                    src.push(PlaneBuffer {
                        rows: ph,
                        cols: pw,
                        samples: buf,
                    });
                }
                let mut dst: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
                for (buf, &(pw, ph)) in plane_bufs.iter_mut().zip(plane_dims.iter()) {
                    dst.push(PlaneBuffer {
                        rows: ph,
                        cols: pw,
                        samples: buf,
                    });
                }
                walker.cdef_frame_from_idx(
                    cdef,
                    cc.num_planes,
                    cc.bit_depth,
                    sub_x as u8,
                    sub_y as u8,
                    &src,
                    &mut dst,
                );
            }
        }
        stages.cdef += t_cdef.elapsed();
        // §7.4 steps 3-4 / §7.16: horizontal superres upscaling of BOTH the
        // CDEF output (`plane_bufs` → UpscaledCdefFrame) and the post-
        // deblock copy (`deblocked` → UpscaledCurrFrame), ahead of loop
        // restoration. No-op when `use_superres == 0`.
        let t_superres = Instant::now();
        if fs.use_superres && fs.upscaled_width > fs.frame_width {
            let sr_ctx = crate::superres::SuperresFrameContext {
                use_superres: true,
                frame_width: fs.frame_width,
                upscaled_width: fs.upscaled_width,
                frame_height: fs.frame_height,
                mi_cols: fs.mi_cols,
                num_planes: cc.num_planes,
                bit_depth: cc.bit_depth,
                subsampling_x: u8::from(cc.subsampling_x),
                subsampling_y: u8::from(cc.subsampling_y),
            };
            // §7.16 output extents: `upscaledPlaneW x planeH` — exact
            // (un-padded) per-plane dimensions.
            let mut new_dims: Vec<(u32, u32)> = Vec::with_capacity(num_planes);
            for plane in 0..num_planes {
                let (out_w, out_h) = if plane == 0 {
                    (fs.upscaled_width, fs.frame_height)
                } else {
                    (
                        (fs.upscaled_width + sub_x) >> sub_x,
                        (fs.frame_height + sub_y) >> sub_y,
                    )
                };
                new_dims.push((out_w, out_h));
            }
            let upscale_set = |bufs: &mut Vec<Vec<i32>>| -> Result<(), Error> {
                let mut inputs_owned = std::mem::take(bufs);
                let mut inputs: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
                for (buf, &(pw, ph)) in inputs_owned.iter_mut().zip(plane_dims.iter()) {
                    inputs.push(PlaneBuffer {
                        rows: ph,
                        cols: pw,
                        samples: buf,
                    });
                }
                let mut outputs_owned: Vec<Vec<i32>> = new_dims
                    .iter()
                    .map(|&(pw, ph)| vec![0i32; (pw as usize) * (ph as usize)])
                    .collect();
                {
                    let mut outputs: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
                    for (buf, &(pw, ph)) in outputs_owned.iter_mut().zip(new_dims.iter()) {
                        outputs.push(PlaneBuffer {
                            rows: ph,
                            cols: pw,
                            samples: buf,
                        });
                    }
                    crate::superres::upscale_frame(&sr_ctx, &inputs, &mut outputs)
                        .map_err(|_| Error::PartitionWalkOutOfRange)?;
                }
                *bufs = outputs_owned;
                Ok(())
            };
            upscale_set(&mut plane_bufs)?;
            if let Some(d) = deblocked.as_mut() {
                upscale_set(d)?;
            }
            plane_dims = new_dims;
        }

        stages.superres += t_superres.elapsed();
        // §7.4 step 5 / §7.17: loop restoration from (UpscaledCurrFrame,
        // UpscaledCdefFrame) into LrFrame. The §5.11.57 units were decoded
        // by the tile walk's `read_lr` interleave above.
        let t_lr = Instant::now();
        if let Some(mut curr_owned) = deblocked {
            let mut cdef_owned: Vec<Vec<i32>> = plane_bufs.clone();
            let mut curr: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
            for (buf, &(pw, ph)) in curr_owned.iter_mut().zip(plane_dims.iter()) {
                curr.push(PlaneBuffer {
                    rows: ph,
                    cols: pw,
                    samples: buf,
                });
            }
            let mut cdef_bufs: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
            for (buf, &(pw, ph)) in cdef_owned.iter_mut().zip(plane_dims.iter()) {
                cdef_bufs.push(PlaneBuffer {
                    rows: ph,
                    cols: pw,
                    samples: buf,
                });
            }
            let mut lr_out: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
            for (buf, &(pw, ph)) in plane_bufs.iter_mut().zip(plane_dims.iter()) {
                lr_out.push(PlaneBuffer {
                    rows: ph,
                    cols: pw,
                    samples: buf,
                });
            }
            walker.loop_restore_frame_from_grid(
                lr,
                cc.num_planes,
                cc.bit_depth,
                sub_x as u8,
                sub_y as u8,
                mi_rows,
                mi_cols,
                fs.frame_height,
                fs.upscaled_width,
                &curr,
                &cdef_bufs,
                &mut lr_out,
            );
        }
        stages.restoration += t_lr.elapsed();
    }
    // ---- §7.18.2 intermediate-output crop to the surfaced extents. ----
    // On the superres path the buffers are already exact; otherwise
    // trim the mi-grid padding down to `UpscaledWidth x FrameHeight`
    // (per-plane subsampled). §7.18.3 film grain then applies to the
    // cropped output planes.
    let t_ref = Instant::now();
    for plane in 0..num_planes {
        let (out_w, out_h) = if plane == 0 {
            (fs.upscaled_width, fs.frame_height)
        } else {
            (
                (fs.upscaled_width + sub_x) >> sub_x,
                (fs.frame_height + sub_y) >> sub_y,
            )
        };
        let (pw, ph) = plane_dims[plane];
        if (pw, ph) == (out_w, out_h) {
            continue;
        }
        if pw < out_w || ph < out_h {
            return Err(Error::PartitionWalkOutOfRange);
        }
        let src = &plane_bufs[plane];
        let mut buf = vec![0i32; (out_w as usize) * (out_h as usize)];
        for y in 0..out_h as usize {
            let row = &src[y * pw as usize..y * pw as usize + out_w as usize];
            buf[y * out_w as usize..(y + 1) * out_w as usize].copy_from_slice(row);
        }
        plane_bufs[plane] = buf;
        plane_dims[plane] = (out_w, out_h);
    }

    // ---- §7.19 motion field motion vector storage. ----
    // Filter the §5.11.5 `Mvs[]` / `RefFrames[]` grids down to the
    // `MfMvs[]` / `MfRefFrames[]` payload §7.20 stores: per cell, the
    // LAST candidate list whose reference lies in the past
    // (`get_relative_dist( RefOrderHint[ refIdx ], OrderHint ) < 0`)
    // and whose MV components sit within `REFMVS_LIMIT`.
    const REFMVS_LIMIT: i16 = (1 << 12) - 1;
    let cells = (mi_rows as usize) * (mi_cols as usize);
    let mut mf_ref_frames: Vec<i8> = vec![-1; cells];
    let mut mf_mvs: Vec<i16> = vec![0; cells * 2];
    if is_inter_frame {
        let st = refs.ok_or(Error::PartitionWalkOutOfRange)?;
        let raw_refs = walker.ref_frames();
        let raw_mvs = walker.mvs();
        let hint_bits = if seq.enable_order_hint {
            u32::from(seq.order_hint_bits)
        } else {
            0
        };
        for cell in 0..cells {
            for list in 0..2usize {
                let r = raw_refs[cell * 2 + list];
                if r > 0 {
                    let slot = ref_frame_idx[(r - 1) as usize] as usize;
                    let dist = get_relative_dist(
                        st.info.order_hint[slot] as i32,
                        fh.order_hint as i32,
                        hint_bits,
                    );
                    if dist < 0 {
                        let mv_row = raw_mvs[(cell * 2 + list) * 2];
                        let mv_col = raw_mvs[(cell * 2 + list) * 2 + 1];
                        if mv_row.abs() <= REFMVS_LIMIT && mv_col.abs() <= REFMVS_LIMIT {
                            mf_ref_frames[cell] = r;
                            mf_mvs[cell * 2] = mv_row;
                            mf_mvs[cell * 2 + 1] = mv_col;
                        }
                    }
                }
            }
        }
    }

    // §7.20 `SavedSegmentIds` payload — the walker's stamped
    // `SegmentIds[][]` grid; unwritten `-1` cells clamp to `0` (the
    // spec map is always written: `segment_id = 0` when segmentation
    // is disabled).
    let segment_ids: Vec<i32> = walker.segment_ids().iter().map(|&v| v.max(0)).collect();
    // r475 — nothing below reads the walker: release its per-mi grids
    // (tens of MB at 12 MP) before the output buffers are built.
    drop(walker);

    // ---- §7.20 store payload: pre-grain cropped planes (u16). ----
    // §7.18.3 film grain applies to the OUTPUT copy only; the §7.20
    // reference store keeps the grain-free frame (the samples later
    // frames motion-compensate against). r475 — under
    // `reduced_still_picture_header` the coded video sequence holds
    // exactly this one frame (§5.5.1: it implies `still_picture`), no
    // later frame can reference the store, and `show_existing_frame`
    // is forced off — so the planes are not materialised (36 MiB at
    // 12 MP 4:2:0); the empty store reads as the never-referenced
    // placeholder if a non-conformant stream tries.
    let store_planes = !seq.reduced_still_picture_header;
    let ref_plane_dims: Vec<(u32, u32)> = plane_dims.clone();
    let to_u16 = |buf: &[i32]| -> Vec<u16> { buf.iter().map(|&v| v.max(0) as u16).collect() };
    let max_val: i32 = (1 << cc.bit_depth) - 1;
    let narrow = |buf: Vec<i32>| -> Vec<u8> {
        if cc.bit_depth == 8 {
            buf.into_iter().map(|v| v.clamp(0, 255) as u8).collect()
        } else {
            let mut out = Vec::with_capacity(buf.len() * 2);
            for v in buf {
                let v = v.clamp(0, max_val) as u16;
                out.extend_from_slice(&v.to_le_bytes());
            }
            out
        }
    };

    // ---- §7.18.3 film grain. ----
    // §5.9.30 `update_grain == 0` predicted path: `load_grain_params(
    // film_grain_params_ref_idx )` — every field except `grain_seed`
    // comes from the referenced slot's §7.20-saved state (the
    // `tempGrainSeed` dance keeps the newly-read seed). A slot with
    // no saved grain state resolves to `reset_grain_params()`.
    let effective_grain: Option<crate::uncompressed_header_tail::FilmGrainParams> =
        match fh.film_grain_params.as_ref() {
            Some(fg) if fg.apply_grain && fg.predicted => {
                let slot = fg.film_grain_params_ref_idx as usize;
                refs.and_then(|st| st.slots.get(slot))
                    .and_then(|s| s.as_ref())
                    .and_then(|s| s.grain_params.clone())
                    .map(|mut g| {
                        g.grain_seed = fg.grain_seed;
                        g.apply_grain = true;
                        g
                    })
            }
            Some(fg) if fg.apply_grain => Some(fg.clone()),
            _ => None,
        };
    let (ref_planes, planes): (Vec<Vec<u16>>, Vec<Vec<u8>>) = match effective_grain.as_ref() {
        Some(fg) if fg.apply_grain => {
            // Grain needs every plane at once (chroma scaling reads
            // luma): store first, synthesise, then narrow.
            let ref_planes: Vec<Vec<u16>> = if store_planes {
                plane_bufs.iter().map(|b| to_u16(b)).collect()
            } else {
                Vec::new()
            };
            stages.ref_store += t_ref.elapsed();
            let t_grain = Instant::now();
            let mut bufs: Vec<PlaneBuffer<'_>> = Vec::with_capacity(num_planes);
            for (buf, &(pw, ph)) in plane_bufs.iter_mut().zip(plane_dims.iter()) {
                bufs.push(PlaneBuffer {
                    rows: ph,
                    cols: pw,
                    samples: buf,
                });
            }
            film_grain_synthesis(
                fg,
                cc.bit_depth,
                cc.num_planes,
                u8::from(cc.subsampling_x),
                u8::from(cc.subsampling_y),
                cc.matrix_coefficients,
                &mut bufs,
            );
            stages.grain += t_grain.elapsed();
            let t_out = Instant::now();
            let planes: Vec<Vec<u8>> = plane_bufs.into_iter().map(narrow).collect();
            stages.output += t_out.elapsed();
            (ref_planes, planes)
        }
        _ => {
            // r475 — no grain: one plane at a time, the `i32` plane
            // released as soon as its `u16` store copy and output bytes
            // exist (the peak holds one plane three ways, not all of
            // them).
            let t_out = Instant::now();
            let mut ref_planes: Vec<Vec<u16>> = Vec::with_capacity(num_planes);
            let mut planes: Vec<Vec<u8>> = Vec::with_capacity(num_planes);
            for buf in plane_bufs {
                if store_planes {
                    ref_planes.push(to_u16(&buf));
                }
                planes.push(narrow(buf));
            }
            stages.ref_store += t_ref.elapsed();
            stages.output += t_out.elapsed();
            (ref_planes, planes)
        }
    };

    Ok(DecodedFrameInternal {
        frame: SpecFrame {
            // Post-§7.16 the surfaced luma width is `UpscaledWidth`
            // (`== FrameWidth` when superres is off).
            width: fs.upscaled_width,
            height: fs.frame_height,
            planes,
            plane_dims,
            bit_depth: cc.bit_depth,
        },
        ref_planes,
        ref_plane_dims,
        mf_mvs,
        mf_ref_frames,
        order_hints_by_ref,
        end_cdfs,
        mi_rows,
        mi_cols,
        segment_ids,
        grain_params: effective_grain,
    })
}

/// §7.9 `motion_field_estimation()` — adapter over the shared
/// [`crate::inter_pred::motion_field_estimation_core`] (the encoder's
/// write mirror runs the SAME core, so both sides project identical
/// `MotionFieldMvs`): builds one [`MotionFieldSlot`] view per stored
/// §7.20 slot plus the current frame's §5.9.2 scalars.
fn motion_field_estimation(
    refs: &SpecRefState,
    fh: &FrameHeader,
    seq: &SequenceHeader,
    mi_rows: u32,
    mi_cols: u32,
) -> Result<MotionFieldMvs, Error> {
    use crate::inter_pred::{motion_field_estimation_core, MotionFieldSlot};
    let ir = fh
        .inter_refs
        .as_ref()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    let hint_bits = if seq.enable_order_hint {
        u32::from(seq.order_hint_bits)
    } else {
        0
    };
    let order_hint = fh.order_hint as i32;
    // §5.9.2 `OrderHints[ LAST_FRAME + i ]`.
    let mut order_hints = [0i32; ALTREF_FRAME + 1];
    for i in 0..7 {
        order_hints[LAST_FRAME + i] = refs.info.order_hint[ir.ref_frame_idx[i] as usize] as i32;
    }
    let mut slots: [Option<MotionFieldSlot<'_>>; 8] = [None; 8];
    for (i, slot) in refs.slots.iter().enumerate() {
        slots[i] = slot.as_ref().map(|st| MotionFieldSlot {
            mf_mvs: &st.mf_mvs,
            mf_ref_frames: &st.mf_ref_frames,
            saved_order_hints: st.saved_order_hints,
            mi_rows: st.mi_rows,
            mi_cols: st.mi_cols,
            frame_is_intra: st.frame_is_intra,
        });
    }
    Ok(motion_field_estimation_core(
        &slots,
        &ir.ref_frame_idx,
        &order_hints,
        order_hint,
        hint_bits,
        mi_rows,
        mi_cols,
    ))
}

/// Decode an AV1 IVF v0 buffer through the spec-faithful frame driver.
///
/// Walks the IVF frame records, and within each temporal unit walks the
/// §5.2 OBU sequence: `OBU_TEMPORAL_DELIMITER` / `OBU_SEQUENCE_HEADER`
/// / `OBU_FRAME_HEADER` + `OBU_TILE_GROUP` / the combined `OBU_FRAME`
/// (§5.10: frame header + `byte_alignment()` + tile group in one OBU,
/// split via [`FrameHeader::bits_consumed`]). Padding and metadata OBUs
/// are skipped.
pub fn decode_av1_spec(input: &[u8]) -> Result<Vec<SpecFrame>, Error> {
    SpecDecodeSession::new().decode_ivf(input)
}

/// r452 — default picture-size ceiling (luma samples,
/// `UpscaledWidth * FrameHeight`) a session accepts before reserving
/// frame-sized storage.
///
/// Annex A §A.3 bounds every DEFINED level's picture by `MaxPicSize`;
/// the largest defined value (levels 6.0–6.3) is 35 651 584 samples
/// (16384 × 2176, or 8192 × 4352). For `seq_level_idx = 31`
/// ("maximum parameters") the spec's note leaves the bound to the
/// decoder maker, and reserved level indices carry no table row at
/// all — this crate applies the largest defined `MaxPicSize` uniformly
/// as its documented ceiling rather than trusting the header's level
/// field (encoders routinely under-declare it). A header past the
/// ceiling surfaces [`Error::PictureSizeExceedsLimit`] before any
/// `MiRows * MiCols` grid or plane buffer is allocated.
pub const MAX_PICTURE_SIZE: u32 = 35_651_584;

/// The Annex A picture-size gate: `UpscaledWidth * FrameHeight` must
/// not exceed `cap`. Runs on the parsed header before any frame-sized
/// reservation.
fn check_picture_size(fh: &FrameHeader, cap: u32) -> Result<(), Error> {
    if let Some(fs) = fh.frame_size.as_ref() {
        let samples = u64::from(fs.upscaled_width) * u64::from(fs.frame_height);
        if samples > u64::from(cap) {
            return Err(Error::PictureSizeExceedsLimit);
        }
    }
    Ok(())
}

/// r430 — [`decode_av1_spec`] at an externally selected operating
/// point (§6.7.5): the §5.3.1 `drop_obu()` rule skips every OBU
/// outside the selected point's temporal/spatial layer set, so a
/// temporally scalable stream decodes to exactly the shown frames of
/// the surviving layers. `operating_point = 0` (the list's preferred
/// entry) reproduces [`decode_av1_spec`] byte for byte.
///
/// ## Errors
///
/// * [`Error::OperatingPointOutOfRange`] — `operating_point` exceeds
///   the sequence header's `operating_points_cnt_minus_1`.
/// * Every [`decode_av1_spec`] error surface otherwise.
pub fn decode_av1_spec_at_operating_point(
    input: &[u8],
    operating_point: u8,
) -> Result<Vec<SpecFrame>, Error> {
    let mut session = SpecDecodeSession::new();
    session.set_operating_point(operating_point)?;
    session.decode_ivf(input)
}

/// Cross-packet decode session — the §7.20 reference-frame store, the
/// cached sequence header, and the per-slot CDF / motion-field /
/// segment-id state, held across successive temporal units so a
/// container demuxer can feed one §7.5 temporal unit (or one IVF
/// record payload) per packet and decode whole GOPs.
///
/// [`decode_av1_spec`] is the one-shot IVF convenience wrapper over
/// this type; the `oxideav-core` registry decoder drives it directly.
#[derive(Debug)]
pub struct SpecDecodeSession {
    seq: Option<SequenceHeader>,
    refs: SpecRefState,
    /// r430 — the operating point selected by external means (the
    /// §6.7.5 `choose_operating_point()` return value). Defaults to
    /// `0` — the earliest (preferred) entry of the sequence header's
    /// operating-point list.
    chosen_op: u8,
    /// §5.5.1 `OperatingPointIdc = operating_point_idc[
    /// operatingPoint ]` — refreshed at every sequence-header parse.
    /// `0` = scalability not in use (the §5.3.1 `drop_obu()` arm
    /// never fires).
    op_idc: u16,
    /// r452 — the picture-size ceiling (luma samples) enforced on
    /// every parsed frame header before frame-sized storage is
    /// reserved. Defaults to [`MAX_PICTURE_SIZE`].
    max_picture_size: u32,
    /// r475 — per-stage wall time of the last `decode_temporal_unit`.
    stages: DecodeStageTimings,
    /// r475 — the worker budget (`1` = serial).
    threads: usize,
}

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

impl SpecDecodeSession {
    /// Fresh session: no sequence header, every §7.20 slot empty.
    #[must_use]
    pub fn new() -> Self {
        Self {
            seq: None,
            refs: SpecRefState::new(),
            chosen_op: 0,
            op_idc: 0,
            max_picture_size: MAX_PICTURE_SIZE,
            stages: DecodeStageTimings::default(),
            threads: 1,
        }
    }

    /// r475 — grant the session a worker budget: a multi-tile frame
    /// decodes its tiles `threads`-wide (the post-filters by row
    /// band), byte-identical to the serial decode for any budget.
    /// `0` / `1` = serial.
    pub fn set_threads(&mut self, threads: usize) {
        self.threads = threads.max(1);
    }

    /// The worker budget granted through [`Self::set_threads`].
    #[must_use]
    pub fn threads(&self) -> usize {
        self.threads
    }

    /// r452 — lower (or raise) the picture-size ceiling
    /// ([`MAX_PICTURE_SIZE`] by default) applied to every frame
    /// header this session parses: a header whose
    /// `UpscaledWidth * FrameHeight` exceeds `max_samples` is rejected
    /// with [`Error::PictureSizeExceedsLimit`] before any frame-sized
    /// buffer is reserved. Memory-constrained hosts (and the fuzz
    /// harness, which runs under a hard RSS limit) bound their
    /// worst-case reservation here.
    pub fn set_max_picture_size(&mut self, max_samples: u32) {
        self.max_picture_size = max_samples;
    }

    /// The session's current picture-size ceiling (luma samples).
    #[must_use]
    pub fn max_picture_size(&self) -> u32 {
        self.max_picture_size
    }

    /// Decode a whole IVF v0 buffer through this session — the
    /// one-shot [`decode_av1_spec`] walk (every IVF record is one §7.5
    /// temporal unit) over a caller-configured session, so the
    /// operating point and picture-size ceiling apply.
    ///
    /// ## Errors
    ///
    /// [`Error::UnexpectedEnd`] on a malformed IVF wrapper, plus every
    /// [`Self::decode_temporal_unit`] error surface.
    pub fn decode_ivf(&mut self, input: &[u8]) -> Result<Vec<SpecFrame>, Error> {
        let reader = IvfReader::new(input).map_err(|_| Error::UnexpectedEnd)?;
        let records = reader.read_all().map_err(|_| Error::UnexpectedEnd)?;
        let mut out = Vec::new();
        for record in records {
            out.extend(self.decode_temporal_unit(&record.payload)?);
        }
        Ok(out)
    }

    /// r430 — select the operating point (§6.7.5) by external means.
    ///
    /// `operating_point` is an index into the sequence header's
    /// operating-point list (`0..=operating_points_cnt_minus_1`; `0`
    /// is the list's preferred entry and the session default). The
    /// §5.5.1 `OperatingPointIdc` derivation re-runs at every
    /// sequence-header parse, so the selection naturally applies to
    /// each new coded video sequence; when a sequence header has
    /// already been parsed the selection is re-derived immediately
    /// against it.
    ///
    /// With a non-zero `OperatingPointIdc`, every OBU that carries an
    /// extension header and lies outside the operating point's
    /// temporal/spatial layer set is dropped per the §5.3.1
    /// `drop_obu()` rule — decoding a temporally scalable stream at a
    /// reduced operating point yields exactly the shown frames of the
    /// selected layer subset.
    ///
    /// ## Errors
    ///
    /// [`Error::OperatingPointOutOfRange`] when a sequence header is
    /// cached and `operating_point` exceeds its
    /// `operating_points_cnt_minus_1` (the same reject surfaces from
    /// [`Self::decode_temporal_unit`] at the next sequence-header
    /// parse otherwise).
    pub fn set_operating_point(&mut self, operating_point: u8) -> Result<(), Error> {
        self.chosen_op = operating_point;
        if let Some(s) = self.seq.as_ref() {
            let idx = usize::from(operating_point);
            if idx >= s.operating_points.len() {
                return Err(Error::OperatingPointOutOfRange);
            }
            self.op_idc = s.operating_points[idx].operating_point_idc;
        }
        Ok(())
    }

    /// The current §5.5.1 `OperatingPointIdc` (`0` until a sequence
    /// header with a non-zero `operating_point_idc` for the selected
    /// operating point is parsed).
    #[must_use]
    pub fn operating_point_idc(&self) -> u16 {
        self.op_idc
    }

    /// Decode one §7.5 temporal-unit body (a low-overhead OBU
    /// bytestream: temporal delimiter / sequence header / frame
    /// header / tile group / frame OBUs), returning every SHOWN frame
    /// in output order and updating the session's reference state.
    ///
    /// ## Errors
    ///
    /// Every [`decode_av1_spec`] error surface — malformed OBUs, a
    /// frame arriving before any sequence header, reference slots the
    /// stream names but never filled, or feature shapes outside the
    /// decoder's scope.
    pub fn decode_temporal_unit(&mut self, payload: &[u8]) -> Result<Vec<SpecFrame>, Error> {
        let mut out = Vec::new();
        self.stages = DecodeStageTimings::default();
        decode_temporal_unit_spec(
            payload,
            &mut self.seq,
            &mut self.refs,
            &mut out,
            self.chosen_op,
            &mut self.op_idc,
            self.max_picture_size,
            &mut self.stages,
            self.threads,
        )?;
        Ok(out)
    }

    /// r475 — wall time per decode stage of the last
    /// [`Self::decode_temporal_unit`] call (summed over the unit's
    /// frames; all-zero before the first call).
    #[doc(hidden)]
    #[must_use]
    pub fn last_stage_timings(&self) -> DecodeStageTimings {
        self.stages
    }

    /// Drop the §7.20 reference store (a seek discontinuity) while
    /// keeping the cached sequence header — the next temporal unit
    /// must start a new coded video sequence (a KEY frame), which
    /// rebuilds every slot before anything can reference it.
    pub fn reset_references(&mut self) {
        self.refs = SpecRefState::new();
    }
}

/// §7.20 `reference_frame_update()` — store the just-decoded frame
/// into every slot `refresh_frame_flags` selects, updating the
/// §5.9.2 `RefInfo` arrays in lockstep.
fn reference_frame_update(
    refs: &mut SpecRefState,
    fh: &FrameHeader,
    decoded: DecodedFrameInternal,
) -> Result<SpecFrame, Error> {
    if fh.refresh_frame_flags == 0 {
        return Ok(decoded.frame);
    }
    let fs = fh
        .frame_size
        .as_ref()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    // §7.20 per-slot payload: pixels + §7.19 motion-field grids +
    // `SavedOrderHints` + `save_cdfs` + `SavedGmParams` +
    // `save_loop_filter_params`.
    let mut gm_params = crate::uncompressed_header_tail::prev_gm_params_default();
    if let Some(g) = fh.global_motion_params.as_ref() {
        gm_params = g.gm_params;
    }
    let (lf_ref_deltas, lf_mode_deltas) = fh
        .loop_filter_params
        .as_ref()
        .map(|lf| (lf.loop_filter_ref_deltas, lf.loop_filter_mode_deltas))
        .unwrap_or((
            crate::uncompressed_header_tail::LOOP_FILTER_REF_DELTAS_DEFAULT,
            [0i8; 2],
        ));
    // §7.20 `save_segmentation_params( i )` — the current frame's
    // FeatureEnabled / FeatureData (post-§5.9.14, including the
    // `segmentation_update_data == 0` loaded values).
    let (seg_active, seg_data) = fh
        .segmentation_params
        .as_ref()
        .map(|sp| (sp.segment_feature_active, sp.segment_feature_data))
        .unwrap_or((
            [[false; crate::uncompressed_header_tail::SEG_LVL_MAX]
                as [bool; crate::uncompressed_header_tail::SEG_LVL_MAX];
                crate::uncompressed_header_tail::MAX_SEGMENTS],
            [[0i16; crate::uncompressed_header_tail::SEG_LVL_MAX];
                crate::uncompressed_header_tail::MAX_SEGMENTS],
        ));
    // r453 — the payload MOVES out of the decode result (the surfaced
    // output frame is a separate buffer) and is Arc-shared across the
    // selected slots.
    let DecodedFrameInternal {
        frame,
        ref_planes,
        ref_plane_dims,
        mf_mvs,
        mf_ref_frames,
        order_hints_by_ref,
        end_cdfs,
        mi_rows,
        mi_cols,
        segment_ids,
        grain_params,
    } = decoded;
    let payload = Arc::new(SpecRefSlot {
        planes: ref_planes,
        plane_dims: ref_plane_dims,
        mf_mvs,
        mf_ref_frames,
        saved_order_hints: order_hints_by_ref,
        mi_rows,
        mi_cols,
        frame_is_intra: fh.frame_is_intra,
        frame_type_is_key: matches!(fh.frame_type, crate::frame_header::FrameType::Key),
        bit_depth: frame.bit_depth,
        cdfs: end_cdfs,
        gm_params,
        lf_ref_deltas,
        lf_mode_deltas,
        segment_ids,
        grain_params,
    });
    for i in 0..NUM_REF_FRAMES as usize {
        if (fh.refresh_frame_flags >> i) & 1 != 0 {
            refs.info.valid[i] = true;
            refs.info.order_hint[i] = fh.order_hint;
            refs.info.frame_id[i] = fh.current_frame_id;
            refs.info.upscaled_width[i] = fs.upscaled_width;
            refs.info.frame_height[i] = fs.frame_height;
            refs.info.render_width[i] = fs.render_width;
            refs.info.render_height[i] = fs.render_height;
            refs.info.frame_type_is_key[i] = payload.frame_type_is_key;
            refs.info.saved_gm_params[i] = payload.gm_params;
            refs.info.saved_lf_ref_deltas[i] = payload.lf_ref_deltas;
            refs.info.saved_lf_mode_deltas[i] = payload.lf_mode_deltas;
            refs.info.saved_seg_feature_active[i] = seg_active;
            refs.info.saved_seg_feature_data[i] = seg_data;
            refs.slots[i] = Some(Arc::clone(&payload));
        }
    }
    Ok(frame)
}

/// §7.21-adjacent `show_existing_frame` output: surface the stored
/// slot's planes as a [`SpecFrame`], applying §7.18.3 film grain from
/// the slot's §7.20-saved grain state (§5.9.2 `show_existing_frame`
/// runs `load_grain_params( frame_to_show_map_idx )` when
/// `film_grain_params_present == 1`; the stored planes themselves stay
/// grain-free).
fn output_existing_frame(
    refs: &SpecRefState,
    fh: &FrameHeader,
    seq: &SequenceHeader,
) -> Result<SpecFrame, Error> {
    let idx = fh
        .frame_to_show_map_idx
        .ok_or(Error::PartitionWalkOutOfRange)? as usize;
    if idx >= NUM_REF_FRAMES as usize {
        return Err(Error::PartitionWalkOutOfRange);
    }
    let slot = refs.slots[idx]
        .as_ref()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    // §7.18.3 on the OUTPUT copy only.
    let mut grain_bufs: Option<Vec<Vec<i32>>> = None;
    if seq.film_grain_params_present {
        if let Some(fg) = slot.grain_params.as_ref() {
            if fg.apply_grain {
                let mut bufs: Vec<Vec<i32>> = slot
                    .planes
                    .iter()
                    .map(|p| p.iter().map(|&v| v as i32).collect())
                    .collect();
                let cc = &seq.color_config;
                let mut views: Vec<PlaneBuffer<'_>> = Vec::with_capacity(bufs.len());
                for (buf, &(pw, ph)) in bufs.iter_mut().zip(slot.plane_dims.iter()) {
                    views.push(PlaneBuffer {
                        rows: ph,
                        cols: pw,
                        samples: buf,
                    });
                }
                film_grain_synthesis(
                    fg,
                    slot.bit_depth,
                    cc.num_planes,
                    u8::from(cc.subsampling_x),
                    u8::from(cc.subsampling_y),
                    cc.matrix_coefficients,
                    &mut views,
                );
                grain_bufs = Some(bufs);
            }
        }
    }
    let max_val: i32 = (1i32 << slot.bit_depth) - 1;
    let planes: Vec<Vec<u8>> = match grain_bufs {
        Some(bufs) => bufs
            .into_iter()
            .map(|p| {
                if slot.bit_depth == 8 {
                    p.into_iter().map(|v| v.clamp(0, 255) as u8).collect()
                } else {
                    let mut out = Vec::with_capacity(p.len() * 2);
                    for v in p {
                        out.extend_from_slice(&(v.clamp(0, max_val) as u16).to_le_bytes());
                    }
                    out
                }
            })
            .collect(),
        None => slot
            .planes
            .iter()
            .map(|p| {
                if slot.bit_depth == 8 {
                    p.iter().map(|&v| v.min(255) as u8).collect()
                } else {
                    let mut out = Vec::with_capacity(p.len() * 2);
                    for &v in p {
                        out.extend_from_slice(&v.to_le_bytes());
                    }
                    out
                }
            })
            .collect(),
    };
    let (w, h) = *slot
        .plane_dims
        .first()
        .ok_or(Error::PartitionWalkOutOfRange)?;
    Ok(SpecFrame {
        width: w,
        height: h,
        planes,
        plane_dims: slot.plane_dims.clone(),
        bit_depth: slot.bit_depth,
    })
}

/// Decode one §7.5 temporal-unit body, appending every SHOWN frame to
/// `out` (`show_frame == 1` coded frames and `show_existing_frame`
/// outputs — the §7.4 output discipline) and updating the cached
/// sequence header + the §7.20 reference state.
#[allow(clippy::too_many_arguments)]
fn decode_temporal_unit_spec(
    payload: &[u8],
    seq: &mut Option<SequenceHeader>,
    refs: &mut SpecRefState,
    out: &mut Vec<SpecFrame>,
    chosen_op: u8,
    op_idc: &mut u16,
    max_picture_size: u32,
    stages: &mut DecodeStageTimings,
    threads: usize,
) -> Result<(), Error> {
    // §5.9.1 `SeenFrameHeader` state: `Some` between a frame's
    // `OBU_FRAME_HEADER` and the tile group whose `tg_end ==
    // NumTiles - 1` (which per §5.11.1 resets `SeenFrameHeader = 0`).
    // r433 — a frame's tiles may arrive across SEVERAL tile-group
    // OBUs; §6.10.1 requires each group's `tg_start` to equal the
    // running `TileNum`, so the accumulator tracks the next expected
    // index and concatenation reconstructs tile raster order.
    struct PendingFrame {
        fh: FrameHeader,
        /// The original `OBU_FRAME_HEADER` payload bytes — the §6.8.1
        /// `frame_header_copy` rule says every
        /// `OBU_REDUNDANT_FRAME_HEADER` for this frame must carry
        /// identical contents.
        fh_payload: Vec<u8>,
        /// Accumulated §8.2 per-tile payloads, tile raster order.
        tiles: Vec<TilePayload>,
        /// §6.10.1: the `TileNum` the next tile group must start at.
        next_tg_start: u32,
    }
    let mut pending: Option<PendingFrame> = None;
    for desc in ObuIter::new(payload) {
        let desc = desc?;
        // §5.3.1 `drop_obu()`: with a non-zero OperatingPointIdc,
        // every OBU other than a sequence header or temporal
        // delimiter that carries an extension header and lies outside
        // the selected operating point's layer set is dropped before
        // any payload parse:
        //
        //   inTemporalLayer = (OperatingPointIdc >> temporal_id) & 1
        //   inSpatialLayer  = (OperatingPointIdc >> (spatial_id + 8)) & 1
        if *op_idc != 0
            && desc.extension_flag
            && !matches!(
                desc.obu_type,
                ObuType::SequenceHeader | ObuType::TemporalDelimiter
            )
        {
            let in_temporal = (*op_idc >> desc.temporal_id) & 1 != 0;
            let in_spatial = (*op_idc >> (desc.spatial_id + 8)) & 1 != 0;
            if !in_temporal || !in_spatial {
                continue;
            }
        }
        match desc.obu_type {
            ObuType::TemporalDelimiter | ObuType::Padding | ObuType::Metadata => {}
            // §7.3.1: tile-list decoding is a separate operating mode
            // whose AnchorFrames array exists only by external means
            // ("a decoder is recommended to support decoding of tile
            // list OBUs, but this is not a requirement for decoder
            // conformance") — the general driver skips them; the
            // dedicated [`crate::tile_list::decode_tile_list`] entry
            // decodes them against a caller-supplied anchor set.
            ObuType::TileList => {}
            ObuType::SequenceHeader => {
                let sh = parse_sequence_header(desc.payload)?;
                // §5.5.1: `operatingPoint = choose_operating_point()`
                // (the session's externally-selected index, default
                // 0); `OperatingPointIdc = operating_point_idc[
                // operatingPoint ]`. §6.7.5: an index outside
                // `0..=operating_points_cnt_minus_1` abandons the
                // decoding process.
                let idx = usize::from(chosen_op);
                if idx >= sh.operating_points.len() {
                    return Err(Error::OperatingPointOutOfRange);
                }
                *op_idc = sh.operating_points[idx].operating_point_idc;
                *seq = Some(sh);
            }
            ObuType::FrameHeader => {
                // §6.8.1: `OBU_FRAME_HEADER` requires `SeenFrameHeader
                // == 0` — a header arriving after this frame's tile
                // groups started must use the REDUNDANT type. (A
                // pending header with NO tiles yet is tolerated as a
                // replacement, preserving the historical driver's
                // acceptance.)
                if pending.as_ref().is_some_and(|p| !p.tiles.is_empty()) {
                    return Err(Error::TileGroupInvalid);
                }
                let s = seq.as_ref().ok_or(Error::PartitionWalkOutOfRange)?;
                let t_parse = Instant::now();
                let fh = parse_frame_header_with_refs(desc.payload, s, &refs.info)?;
                stages.parse += t_parse.elapsed();
                check_picture_size(&fh, max_picture_size)?;
                if fh.show_existing_frame {
                    out.push(output_existing_frame(refs, &fh, s)?);
                    // §7.4 / §7.21: a shown KEY frame re-loads the
                    // stored frame state and re-stores it into every
                    // slot (`refresh_frame_flags == allFrames` per the
                    // §5.9.2 show_existing arm). The wholesale slot
                    // clone IS the §7.21 load followed by the §7.20
                    // store — pixels, §7.19 grids, `SavedOrderHints`,
                    // CDFs, gm params and loop-filter deltas all ride
                    // the payload.
                    if fh.refresh_frame_flags != 0 {
                        let idx = fh
                            .frame_to_show_map_idx
                            .ok_or(Error::PartitionWalkOutOfRange)?
                            as usize;
                        let payload = refs.slots[idx]
                            .as_ref()
                            .cloned()
                            .ok_or(Error::PartitionWalkOutOfRange)?;
                        let loaded_order_hint = refs.info.order_hint[idx];
                        let loaded_frame_id = refs.info.frame_id[idx];
                        let loaded_uw = refs.info.upscaled_width[idx];
                        let loaded_fh = refs.info.frame_height[idx];
                        let loaded_rw = refs.info.render_width[idx];
                        let loaded_rh = refs.info.render_height[idx];
                        for i in 0..NUM_REF_FRAMES as usize {
                            if (fh.refresh_frame_flags >> i) & 1 != 0 {
                                refs.info.valid[i] = true;
                                refs.info.order_hint[i] = loaded_order_hint;
                                refs.info.frame_id[i] = loaded_frame_id;
                                refs.info.upscaled_width[i] = loaded_uw;
                                refs.info.frame_height[i] = loaded_fh;
                                refs.info.render_width[i] = loaded_rw;
                                refs.info.render_height[i] = loaded_rh;
                                refs.info.frame_type_is_key[i] = payload.frame_type_is_key;
                                refs.info.saved_gm_params[i] = payload.gm_params;
                                refs.info.saved_lf_ref_deltas[i] = payload.lf_ref_deltas;
                                refs.info.saved_lf_mode_deltas[i] = payload.lf_mode_deltas;
                                // §7.21 load + §7.20 re-store: the
                                // segmentation params ride the RefInfo
                                // copy from the loaded slot.
                                refs.info.saved_seg_feature_active[i] =
                                    refs.info.saved_seg_feature_active[idx];
                                refs.info.saved_seg_feature_data[i] =
                                    refs.info.saved_seg_feature_data[idx];
                                refs.slots[i] = Some(payload.clone());
                            }
                        }
                    }
                    pending = None;
                } else {
                    pending = Some(PendingFrame {
                        fh,
                        fh_payload: desc.payload.to_vec(),
                        tiles: Vec::new(),
                        next_tg_start: 0,
                    });
                }
            }
            ObuType::RedundantFrameHeader => {
                // §5.9.1 `frame_header_copy()`: only legal while
                // `SeenFrameHeader == 1` (§6.8.1), and the copy must
                // carry contents identical to the original
                // `OBU_FRAME_HEADER`. Nothing re-parses — the byte
                // comparison IS the identity check.
                let p = pending.as_ref().ok_or(Error::TileGroupInvalid)?;
                if desc.payload != &p.fh_payload[..] {
                    return Err(Error::TileGroupInvalid);
                }
            }
            ObuType::TileGroup => {
                let s = seq.as_ref().ok_or(Error::PartitionWalkOutOfRange)?;
                let p = pending.as_mut().ok_or(Error::TileGroupInvalid)?;
                let ti =
                    p.fh.tile_info
                        .as_ref()
                        .ok_or(Error::PartitionWalkOutOfRange)?;
                let num_tiles = ti.tile_cols * ti.tile_rows;
                let t_parse = Instant::now();
                let parsed = parse_tile_group_body_with_ti(ti, desc.payload)?;
                stages.parse += t_parse.elapsed();
                // §6.10.1: `tg_start` equals the running `TileNum`.
                if parsed.tg_start != p.next_tg_start {
                    return Err(Error::TileGroupInvalid);
                }
                p.next_tg_start = parsed.tg_end + 1;
                p.tiles.extend(parsed.tiles);
                // §5.11.1: the frame decodes (frame_end_update_cdf +
                // decode_frame_wrapup, SeenFrameHeader = 0) only once
                // `tg_end == NumTiles - 1`.
                if parsed.tg_end + 1 == num_tiles {
                    let pf = pending.take().expect("pending frame checked above");
                    let decoded =
                        decode_frame_spec_tiles(s, &pf.fh, &pf.tiles, Some(refs), stages, threads)?;
                    let frame = reference_frame_update(refs, &pf.fh, decoded)?;
                    if pf.fh.show_frame {
                        out.push(frame);
                    }
                }
            }
            ObuType::Frame => {
                // §5.10 frame_obu: frame_header_obu() + byte_alignment()
                // + tile_group_obu(). The tile group starts at the next
                // byte boundary after the frame header, and §6.10.1
                // requires `tile_start_and_end_present_flag == 0` — the
                // embedded group always covers the whole frame.
                if pending.as_ref().is_some_and(|p| !p.tiles.is_empty()) {
                    return Err(Error::TileGroupInvalid);
                }
                pending = None;
                let s = seq.as_ref().ok_or(Error::PartitionWalkOutOfRange)?;
                let t_parse = Instant::now();
                let fh = parse_frame_header_with_refs(desc.payload, s, &refs.info)?;
                check_picture_size(&fh, max_picture_size)?;
                let tg_offset = fh.bits_consumed.div_ceil(8);
                if tg_offset > desc.payload.len() {
                    return Err(Error::UnexpectedEnd);
                }
                let tg_body = &desc.payload[tg_offset..];
                let ti = fh
                    .tile_info
                    .as_ref()
                    .ok_or(Error::PartitionWalkOutOfRange)?;
                let tiles = parse_whole_frame_tile_group(ti, tg_body, true)?;
                stages.parse += t_parse.elapsed();
                let decoded = decode_frame_spec_tiles(s, &fh, &tiles, Some(refs), stages, threads)?;
                let frame = reference_frame_update(refs, &fh, decoded)?;
                if fh.show_frame {
                    out.push(frame);
                }
            }
            _ => return Err(Error::PartitionWalkOutOfRange),
        }
    }
    // §7.5 / §6.10.1: a coded frame's tile groups all live inside its
    // temporal unit — the last group (`tg_end == NumTiles - 1`) must
    // have arrived before the unit ends.
    if pending.as_ref().is_some_and(|p| !p.tiles.is_empty()) {
        return Err(Error::TileGroupInvalid);
    }
    Ok(())
}