ladybird/Userland/Libraries/LibVideo/VP9/Parser.cpp

/*
 * Copyright (c) 2021, Hunter Salyer <thefalsehonesty@gmail.com>
 * Copyright (c) 2022, Gregory Bertilson <zaggy1024@gmail.com>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/String.h>
#include <LibGfx/Point.h>
#include <LibGfx/Size.h>

#include "Decoder.h"
#include "Parser.h"
#include "Utilities.h"

namespace Video::VP9 {

#define TRY_READ(expression) DECODER_TRY(DecoderErrorCategory::Corrupted, expression)

Parser::Parser(Decoder& decoder)
    : m_probability_tables(make<ProbabilityTables>())
    , m_tree_parser(make<TreeParser>(*this))
    , m_decoder(decoder)
{
}

Parser::~Parser()
{
}

Vector<size_t> Parser::parse_superframe_sizes(ReadonlyBytes frame_data)
{
    if (frame_data.size() < 1)
        return {};

    // The decoder determines the presence of a superframe by:
    // 1. parsing the final byte of the chunk and checking that the superframe_marker equals 0b110,

    // If the checks in steps 1 and 3 both pass, then the chunk is determined to contain a superframe and each
    // frame in the superframe is passed to the decoding process in turn.
    // Otherwise, the chunk is determined to not contain a superframe, and the whole chunk is passed to the
    // decoding process.

    // NOTE: Reading from span data will be quicker than spinning up a BitStream.
    u8 superframe_byte = frame_data[frame_data.size() - 1];

    // NOTE: We have to read out of the byte from the little end first, hence the padding bits in the masks below.
    u8 superframe_marker = superframe_byte & 0b1110'0000;
    if (superframe_marker == 0b1100'0000) {
        u8 bytes_per_framesize = ((superframe_byte >> 3) & 0b11) + 1;
        u8 frames_in_superframe = (superframe_byte & 0b111) + 1;
        // 2. setting the total size of the superframe_index SzIndex equal to 2 + NumFrames * SzBytes,
        size_t index_size = 2 + bytes_per_framesize * frames_in_superframe;

        if (index_size > frame_data.size())
            return {};

        auto superframe_header_data = frame_data.data() + frame_data.size() - index_size;

        u8 start_superframe_byte = *(superframe_header_data++);
        // 3. checking that the first byte of the superframe_index matches the final byte.
        if (superframe_byte != start_superframe_byte)
            return {};

        Vector<size_t> result;
        for (u8 i = 0; i < frames_in_superframe; i++) {
            size_t frame_size = 0;
            for (u8 j = 0; j < bytes_per_framesize; j++)
                frame_size |= (static_cast<size_t>(*(superframe_header_data++)) << (j * 8));
            result.append(frame_size);
        }
        return result;
    }

    return {};
}

/* (6.1) */
DecoderErrorOr<void> Parser::parse_frame(ReadonlyBytes frame_data)
{
    m_bit_stream = make<BitStream>(frame_data.data(), frame_data.size());
    m_syntax_element_counter = make<SyntaxElementCounter>();

    TRY(uncompressed_header());
    if (!trailing_bits())
        return DecoderError::corrupted("Trailing bits were non-zero"sv);
    if (m_header_size_in_bytes == 0)
        return DecoderError::corrupted("Frame header is zero-sized"sv);
    m_probability_tables->load_probs(m_frame_context_idx);
    m_probability_tables->load_probs2(m_frame_context_idx);
    m_syntax_element_counter->clear_counts();

    TRY_READ(m_bit_stream->init_bool(m_header_size_in_bytes));
    TRY(compressed_header());
    TRY_READ(m_bit_stream->exit_bool());

    TRY(m_decoder.allocate_buffers());

    TRY(decode_tiles());
    TRY(refresh_probs());

    return {};
}

bool Parser::trailing_bits()
{
    while (m_bit_stream->bits_remaining() & 7u) {
        if (MUST(m_bit_stream->read_bit()))
            return false;
    }
    return true;
}

DecoderErrorOr<void> Parser::refresh_probs()
{
    if (!m_error_resilient_mode && !m_frame_parallel_decoding_mode) {
        m_probability_tables->load_probs(m_frame_context_idx);
        TRY(m_decoder.adapt_coef_probs());
        if (!m_frame_is_intra) {
            m_probability_tables->load_probs2(m_frame_context_idx);
            TRY(m_decoder.adapt_non_coef_probs());
        }
    }
    if (m_refresh_frame_context)
        m_probability_tables->save_probs(m_frame_context_idx);
    return {};
}

DecoderErrorOr<FrameType> Parser::read_frame_type()
{
    if (TRY_READ(m_bit_stream->read_bit()))
        return NonKeyFrame;
    return KeyFrame;
}

DecoderErrorOr<ColorRange> Parser::read_color_range()
{
    if (TRY_READ(m_bit_stream->read_bit()))
        return ColorRange::Full;
    return ColorRange::Studio;
}

/* (6.2) */
DecoderErrorOr<void> Parser::uncompressed_header()
{
    auto frame_marker = TRY_READ(m_bit_stream->read_bits(2));
    if (frame_marker != 2)
        return DecoderError::corrupted("uncompressed_header: Frame marker must be 2"sv);
    auto profile_low_bit = TRY_READ(m_bit_stream->read_bit());
    auto profile_high_bit = TRY_READ(m_bit_stream->read_bit());
    m_profile = (profile_high_bit << 1u) + profile_low_bit;
    if (m_profile == 3 && TRY_READ(m_bit_stream->read_bit()))
        return DecoderError::corrupted("uncompressed_header: Profile 3 reserved bit was non-zero"sv);
    m_show_existing_frame = TRY_READ(m_bit_stream->read_bit());
    if (m_show_existing_frame) {
        m_frame_to_show_map_index = TRY_READ(m_bit_stream->read_bits(3));
        m_header_size_in_bytes = 0;
        m_refresh_frame_flags = 0;
        m_loop_filter_level = 0;
        return {};
    }

    m_last_frame_type = m_frame_type;
    m_frame_type = TRY(read_frame_type());
    m_show_frame = TRY_READ(m_bit_stream->read_bit());
    m_error_resilient_mode = TRY_READ(m_bit_stream->read_bit());

    if (m_frame_type == KeyFrame) {
        TRY(frame_sync_code());
        TRY(color_config());
        m_frame_size = TRY(frame_size());
        m_render_size = TRY(render_size(m_frame_size));
        m_refresh_frame_flags = 0xFF;
        m_frame_is_intra = true;
    } else {
        m_frame_is_intra = !m_show_frame && TRY_READ(m_bit_stream->read_bit());

        if (!m_error_resilient_mode) {
            m_reset_frame_context = TRY_READ(m_bit_stream->read_bits(2));
        } else {
            m_reset_frame_context = 0;
        }

        if (m_frame_is_intra) {
            TRY(frame_sync_code());
            if (m_profile > 0) {
                TRY(color_config());
            } else {
                m_color_space = Bt601;
                m_subsampling_x = true;
                m_subsampling_y = true;
                m_bit_depth = 8;
            }

            m_refresh_frame_flags = TRY_READ(m_bit_stream->read_f8());
            m_frame_size = TRY(frame_size());
            m_render_size = TRY(render_size(m_frame_size));
        } else {
            m_refresh_frame_flags = TRY_READ(m_bit_stream->read_f8());
            for (auto i = 0; i < 3; i++) {
                m_ref_frame_idx[i] = TRY_READ(m_bit_stream->read_bits(3));
                m_ref_frame_sign_bias[LastFrame + i] = TRY_READ(m_bit_stream->read_bit());
            }
            m_frame_size = TRY(frame_size_with_refs());
            m_render_size = TRY(render_size(m_frame_size));
            m_allow_high_precision_mv = TRY_READ(m_bit_stream->read_bit());
            TRY(read_interpolation_filter());
        }
    }

    compute_image_size();

    if (!m_error_resilient_mode) {
        m_refresh_frame_context = TRY_READ(m_bit_stream->read_bit());
        m_frame_parallel_decoding_mode = TRY_READ(m_bit_stream->read_bit());
    } else {
        m_refresh_frame_context = false;
        m_frame_parallel_decoding_mode = true;
    }

    m_frame_context_idx = TRY_READ(m_bit_stream->read_bits(2));
    if (m_frame_is_intra || m_error_resilient_mode) {
        setup_past_independence();
        if (m_frame_type == KeyFrame || m_error_resilient_mode || m_reset_frame_context == 3) {
            for (auto i = 0; i < 4; i++) {
                m_probability_tables->save_probs(i);
            }
        } else if (m_reset_frame_context == 2) {
            m_probability_tables->save_probs(m_frame_context_idx);
        }
        m_frame_context_idx = 0;
    }

    TRY(loop_filter_params());
    TRY(quantization_params());
    TRY(segmentation_params());
    TRY(tile_info());

    m_header_size_in_bytes = TRY_READ(m_bit_stream->read_f16());

    return {};
}

DecoderErrorOr<void> Parser::frame_sync_code()
{
    if (TRY_READ(m_bit_stream->read_f8()) != 0x49)
        return DecoderError::corrupted("frame_sync_code: Byte 0 was not 0x49."sv);
    if (TRY_READ(m_bit_stream->read_f8()) != 0x83)
        return DecoderError::corrupted("frame_sync_code: Byte 1 was not 0x83."sv);
    if (TRY_READ(m_bit_stream->read_f8()) != 0x42)
        return DecoderError::corrupted("frame_sync_code: Byte 2 was not 0x42."sv);
    return {};
}

DecoderErrorOr<void> Parser::color_config()
{
    if (m_profile >= 2) {
        m_bit_depth = TRY_READ(m_bit_stream->read_bit()) ? 12 : 10;
    } else {
        m_bit_depth = 8;
    }

    auto color_space = TRY_READ(m_bit_stream->read_bits(3));
    VERIFY(color_space <= RGB);
    m_color_space = static_cast<ColorSpace>(color_space);

    if (color_space != RGB) {
        m_color_range = TRY(read_color_range());
        if (m_profile == 1 || m_profile == 3) {
            m_subsampling_x = TRY_READ(m_bit_stream->read_bit());
            m_subsampling_y = TRY_READ(m_bit_stream->read_bit());
            if (TRY_READ(m_bit_stream->read_bit()))
                return DecoderError::corrupted("color_config: Subsampling reserved zero was set"sv);
        } else {
            m_subsampling_x = true;
            m_subsampling_y = true;
        }
    } else {
        m_color_range = ColorRange::Full;
        if (m_profile == 1 || m_profile == 3) {
            m_subsampling_x = false;
            m_subsampling_y = false;
            if (TRY_READ(m_bit_stream->read_bit()))
                return DecoderError::corrupted("color_config: RGB reserved zero was set"sv);
        }
    }
    return {};
}

DecoderErrorOr<Gfx::Size<u32>> Parser::frame_size()
{
    return Gfx::Size<u32> { TRY_READ(m_bit_stream->read_f16()) + 1, TRY_READ(m_bit_stream->read_f16()) + 1 };
}

DecoderErrorOr<Gfx::Size<u32>> Parser::render_size(Gfx::Size<u32> frame_size)
{
    if (!TRY_READ(m_bit_stream->read_bit()))
        return frame_size;
    return Gfx::Size<u32> { TRY_READ(m_bit_stream->read_f16()) + 1, TRY_READ(m_bit_stream->read_f16()) + 1 };
}

DecoderErrorOr<Gfx::Size<u32>> Parser::frame_size_with_refs()
{
    Optional<Gfx::Size<u32>> size;
    for (auto frame_index : m_ref_frame_idx) {
        if (TRY_READ(m_bit_stream->read_bit())) {
            size.emplace(m_ref_frame_size[frame_index]);
            break;
        }
    }

    if (size.has_value())
        return size.value();

    return TRY(frame_size());
}

void Parser::compute_image_size()
{
    auto new_cols = (m_frame_size.width() + 7u) >> 3u;
    auto new_rows = (m_frame_size.height() + 7u) >> 3u;

    // 7.2.6 Compute image size semantics
    // When compute_image_size is invoked, the following ordered steps occur:
    // 1. If this is the first time compute_image_size is invoked, or if either FrameWidth or FrameHeight have
    // changed in value compared to the previous time this function was invoked, then the segmentation map is
    // cleared to all zeros by setting SegmentId[ row ][ col ] equal to 0 for row = 0..MiRows-1 and col =
    // 0..MiCols-1.
    bool first_invoke = !m_mi_cols && !m_mi_rows;
    bool same_size = m_mi_cols == new_cols && m_mi_rows == new_rows;
    if (first_invoke || !same_size) {
        // m_segment_ids will be resized from decode_tiles() later.
        m_segment_ids.clear_with_capacity();
    }

    // 2. The variable UsePrevFrameMvs is set equal to 1 if all of the following conditions are true:
    // a. This is not the first time compute_image_size is invoked.
    // b. Both FrameWidth and FrameHeight have the same value compared to the previous time this function
    // was invoked.
    // c. show_frame was equal to 1 the previous time this function was invoked.
    // d. error_resilient_mode is equal to 0.
    // e. FrameIsIntra is equal to 0.
    // Otherwise, UsePrevFrameMvs is set equal to 0.
    m_use_prev_frame_mvs = !first_invoke && same_size && m_prev_show_frame && !m_error_resilient_mode && !m_frame_is_intra;
    m_prev_show_frame = m_show_frame;

    m_mi_cols = new_cols;
    m_mi_rows = new_rows;
    m_sb64_cols = (m_mi_cols + 7u) >> 3u;
    m_sb64_rows = (m_mi_rows + 7u) >> 3u;
}

DecoderErrorOr<void> Parser::read_interpolation_filter()
{
    if (TRY_READ(m_bit_stream->read_bit())) {
        m_interpolation_filter = Switchable;
    } else {
        m_interpolation_filter = literal_to_type[TRY_READ(m_bit_stream->read_bits(2))];
    }
    return {};
}

DecoderErrorOr<void> Parser::loop_filter_params()
{
    m_loop_filter_level = TRY_READ(m_bit_stream->read_bits(6));
    m_loop_filter_sharpness = TRY_READ(m_bit_stream->read_bits(3));
    m_loop_filter_delta_enabled = TRY_READ(m_bit_stream->read_bit());
    if (m_loop_filter_delta_enabled) {
        if (TRY_READ(m_bit_stream->read_bit())) {
            for (auto& loop_filter_ref_delta : m_loop_filter_ref_deltas) {
                if (TRY_READ(m_bit_stream->read_bit()))
                    loop_filter_ref_delta = TRY_READ(m_bit_stream->read_s(6));
            }
            for (auto& loop_filter_mode_delta : m_loop_filter_mode_deltas) {
                if (TRY_READ(m_bit_stream->read_bit()))
                    loop_filter_mode_delta = TRY_READ(m_bit_stream->read_s(6));
            }
        }
    }

    return {};
}

DecoderErrorOr<void> Parser::quantization_params()
{
    m_base_q_idx = TRY_READ(m_bit_stream->read_f8());
    m_delta_q_y_dc = TRY(read_delta_q());
    m_delta_q_uv_dc = TRY(read_delta_q());
    m_delta_q_uv_ac = TRY(read_delta_q());
    m_lossless = m_base_q_idx == 0 && m_delta_q_y_dc == 0 && m_delta_q_uv_dc == 0 && m_delta_q_uv_ac == 0;
    return {};
}

DecoderErrorOr<i8> Parser::read_delta_q()
{
    if (TRY_READ(m_bit_stream->read_bit()))
        return TRY_READ(m_bit_stream->read_s(4));
    return 0;
}

DecoderErrorOr<void> Parser::segmentation_params()
{
    m_segmentation_enabled = TRY_READ(m_bit_stream->read_bit());
    if (!m_segmentation_enabled)
        return {};

    m_segmentation_update_map = TRY_READ(m_bit_stream->read_bit());
    if (m_segmentation_update_map) {
        for (auto& segmentation_tree_prob : m_segmentation_tree_probs)
            segmentation_tree_prob = TRY(read_prob());
        m_segmentation_temporal_update = TRY_READ(m_bit_stream->read_bit());
        for (auto& segmentation_pred_prob : m_segmentation_pred_prob)
            segmentation_pred_prob = m_segmentation_temporal_update ? TRY(read_prob()) : 255;
    }

    auto segmentation_update_data = (TRY_READ(m_bit_stream->read_bit()));

    if (!segmentation_update_data)
        return {};

    m_segmentation_abs_or_delta_update = TRY_READ(m_bit_stream->read_bit());
    for (auto i = 0; i < MAX_SEGMENTS; i++) {
        for (auto j = 0; j < SEG_LVL_MAX; j++) {
            auto feature_value = 0;
            auto feature_enabled = TRY_READ(m_bit_stream->read_bit());
            m_feature_enabled[i][j] = feature_enabled;
            if (feature_enabled) {
                auto bits_to_read = segmentation_feature_bits[j];
                feature_value = TRY_READ(m_bit_stream->read_bits(bits_to_read));
                if (segmentation_feature_signed[j]) {
                    if (TRY_READ(m_bit_stream->read_bit()))
                        feature_value = -feature_value;
                }
            }
            m_feature_data[i][j] = feature_value;
        }
    }

    return {};
}

DecoderErrorOr<u8> Parser::read_prob()
{
    if (TRY_READ(m_bit_stream->read_bit()))
        return TRY_READ(m_bit_stream->read_f8());
    return 255;
}

DecoderErrorOr<void> Parser::tile_info()
{
    auto min_log2_tile_cols = calc_min_log2_tile_cols();
    auto max_log2_tile_cols = calc_max_log2_tile_cols();
    m_tile_cols_log2 = min_log2_tile_cols;
    while (m_tile_cols_log2 < max_log2_tile_cols) {
        if (TRY_READ(m_bit_stream->read_bit()))
            m_tile_cols_log2++;
        else
            break;
    }
    m_tile_rows_log2 = TRY_READ(m_bit_stream->read_bit());
    if (m_tile_rows_log2) {
        m_tile_rows_log2 += TRY_READ(m_bit_stream->read_bit());
    }
    return {};
}

u16 Parser::calc_min_log2_tile_cols()
{
    auto min_log_2 = 0u;
    while ((u32)(MAX_TILE_WIDTH_B64 << min_log_2) < m_sb64_cols)
        min_log_2++;
    return min_log_2;
}

u16 Parser::calc_max_log2_tile_cols()
{
    u16 max_log_2 = 1;
    while ((m_sb64_cols >> max_log_2) >= MIN_TILE_WIDTH_B64)
        max_log_2++;
    return max_log_2 - 1;
}

void Parser::setup_past_independence()
{
    for (auto i = 0; i < 8; i++) {
        for (auto j = 0; j < 4; j++) {
            m_feature_data[i][j] = 0;
            m_feature_enabled[i][j] = false;
        }
    }
    m_segmentation_abs_or_delta_update = false;
    m_prev_segment_ids.clear_with_capacity();
    m_prev_segment_ids.resize_and_keep_capacity(m_mi_rows * m_mi_cols);
    m_loop_filter_delta_enabled = true;
    m_loop_filter_ref_deltas[IntraFrame] = 1;
    m_loop_filter_ref_deltas[LastFrame] = 0;
    m_loop_filter_ref_deltas[GoldenFrame] = -1;
    m_loop_filter_ref_deltas[AltRefFrame] = -1;
    for (auto& loop_filter_mode_delta : m_loop_filter_mode_deltas)
        loop_filter_mode_delta = 0;
    m_probability_tables->reset_probs();
}

DecoderErrorOr<void> Parser::compressed_header()
{
    TRY(read_tx_mode());
    if (m_tx_mode == TXModeSelect)
        TRY(tx_mode_probs());
    TRY(read_coef_probs());
    TRY(read_skip_prob());
    if (!m_frame_is_intra) {
        TRY(read_inter_mode_probs());
        if (m_interpolation_filter == Switchable)
            TRY(read_interp_filter_probs());
        TRY(read_is_inter_probs());
        TRY(frame_reference_mode());
        TRY(frame_reference_mode_probs());
        TRY(read_y_mode_probs());
        TRY(read_partition_probs());
        TRY(mv_probs());
    }
    return {};
}

DecoderErrorOr<void> Parser::read_tx_mode()
{
    if (m_lossless) {
        m_tx_mode = Only_4x4;
    } else {
        auto tx_mode = TRY_READ(m_bit_stream->read_literal(2));
        if (tx_mode == Allow_32x32)
            tx_mode += TRY_READ(m_bit_stream->read_literal(1));
        m_tx_mode = static_cast<TXMode>(tx_mode);
    }
    return {};
}

DecoderErrorOr<void> Parser::tx_mode_probs()
{
    auto& tx_probs = m_probability_tables->tx_probs();
    for (auto i = 0; i < TX_SIZE_CONTEXTS; i++) {
        for (auto j = 0; j < TX_SIZES - 3; j++)
            tx_probs[TX_8x8][i][j] = TRY(diff_update_prob(tx_probs[TX_8x8][i][j]));
    }
    for (auto i = 0; i < TX_SIZE_CONTEXTS; i++) {
        for (auto j = 0; j < TX_SIZES - 2; j++)
            tx_probs[TX_16x16][i][j] = TRY(diff_update_prob(tx_probs[TX_16x16][i][j]));
    }
    for (auto i = 0; i < TX_SIZE_CONTEXTS; i++) {
        for (auto j = 0; j < TX_SIZES - 1; j++)
            tx_probs[TX_32x32][i][j] = TRY(diff_update_prob(tx_probs[TX_32x32][i][j]));
    }
    return {};
}

DecoderErrorOr<u8> Parser::diff_update_prob(u8 prob)
{
    auto update_prob = TRY_READ(m_bit_stream->read_bool(252));
    if (update_prob) {
        auto delta_prob = TRY(decode_term_subexp());
        prob = inv_remap_prob(delta_prob, prob);
    }
    return prob;
}

DecoderErrorOr<u8> Parser::decode_term_subexp()
{
    if (TRY_READ(m_bit_stream->read_literal(1)) == 0)
        return TRY_READ(m_bit_stream->read_literal(4));
    if (TRY_READ(m_bit_stream->read_literal(1)) == 0)
        return TRY_READ(m_bit_stream->read_literal(4)) + 16;
    if (TRY_READ(m_bit_stream->read_literal(1)) == 0)
        return TRY_READ(m_bit_stream->read_literal(5)) + 32;

    auto v = TRY_READ(m_bit_stream->read_literal(7));
    if (v < 65)
        return v + 64;
    return (v << 1u) - 1 + TRY_READ(m_bit_stream->read_literal(1));
}

u8 Parser::inv_remap_prob(u8 delta_prob, u8 prob)
{
    u8 m = prob - 1;
    auto v = inv_map_table[delta_prob];
    if ((m << 1u) <= 255)
        return 1 + inv_recenter_nonneg(v, m);
    return 255 - inv_recenter_nonneg(v, 254 - m);
}

u8 Parser::inv_recenter_nonneg(u8 v, u8 m)
{
    if (v > 2 * m)
        return v;
    if (v & 1u)
        return m - ((v + 1u) >> 1u);
    return m + (v >> 1u);
}

DecoderErrorOr<void> Parser::read_coef_probs()
{
    m_max_tx_size = tx_mode_to_biggest_tx_size[m_tx_mode];
    for (u8 tx_size = 0; tx_size <= m_max_tx_size; tx_size++) {
        auto update_probs = TRY_READ(m_bit_stream->read_literal(1));
        if (update_probs == 1) {
            for (auto i = 0; i < 2; i++) {
                for (auto j = 0; j < 2; j++) {
                    for (auto k = 0; k < 6; k++) {
                        auto max_l = (k == 0) ? 3 : 6;
                        for (auto l = 0; l < max_l; l++) {
                            for (auto m = 0; m < 3; m++) {
                                auto& prob = m_probability_tables->coef_probs()[tx_size][i][j][k][l][m];
                                prob = TRY(diff_update_prob(prob));
                            }
                        }
                    }
                }
            }
        }
    }
    return {};
}

DecoderErrorOr<void> Parser::read_skip_prob()
{
    for (auto i = 0; i < SKIP_CONTEXTS; i++)
        m_probability_tables->skip_prob()[i] = TRY(diff_update_prob(m_probability_tables->skip_prob()[i]));
    return {};
}

DecoderErrorOr<void> Parser::read_inter_mode_probs()
{
    for (auto i = 0; i < INTER_MODE_CONTEXTS; i++) {
        for (auto j = 0; j < INTER_MODES - 1; j++)
            m_probability_tables->inter_mode_probs()[i][j] = TRY(diff_update_prob(m_probability_tables->inter_mode_probs()[i][j]));
    }
    return {};
}

DecoderErrorOr<void> Parser::read_interp_filter_probs()
{
    for (auto i = 0; i < INTERP_FILTER_CONTEXTS; i++) {
        for (auto j = 0; j < SWITCHABLE_FILTERS - 1; j++)
            m_probability_tables->interp_filter_probs()[i][j] = TRY(diff_update_prob(m_probability_tables->interp_filter_probs()[i][j]));
    }
    return {};
}

DecoderErrorOr<void> Parser::read_is_inter_probs()
{
    for (auto i = 0; i < IS_INTER_CONTEXTS; i++)
        m_probability_tables->is_inter_prob()[i] = TRY(diff_update_prob(m_probability_tables->is_inter_prob()[i]));
    return {};
}

DecoderErrorOr<void> Parser::frame_reference_mode()
{
    auto compound_reference_allowed = false;
    for (size_t i = 2; i <= REFS_PER_FRAME; i++) {
        if (m_ref_frame_sign_bias[i] != m_ref_frame_sign_bias[1])
            compound_reference_allowed = true;
    }
    if (compound_reference_allowed) {
        auto non_single_reference = TRY_READ(m_bit_stream->read_literal(1));
        if (non_single_reference == 0) {
            m_reference_mode = SingleReference;
        } else {
            auto reference_select = TRY_READ(m_bit_stream->read_literal(1));
            if (reference_select == 0)
                m_reference_mode = CompoundReference;
            else
                m_reference_mode = ReferenceModeSelect;
            setup_compound_reference_mode();
        }
    } else {
        m_reference_mode = SingleReference;
    }
    return {};
}

DecoderErrorOr<void> Parser::frame_reference_mode_probs()
{
    if (m_reference_mode == ReferenceModeSelect) {
        for (auto i = 0; i < COMP_MODE_CONTEXTS; i++) {
            auto& comp_mode_prob = m_probability_tables->comp_mode_prob();
            comp_mode_prob[i] = TRY(diff_update_prob(comp_mode_prob[i]));
        }
    }
    if (m_reference_mode != CompoundReference) {
        for (auto i = 0; i < REF_CONTEXTS; i++) {
            auto& single_ref_prob = m_probability_tables->single_ref_prob();
            single_ref_prob[i][0] = TRY(diff_update_prob(single_ref_prob[i][0]));
            single_ref_prob[i][1] = TRY(diff_update_prob(single_ref_prob[i][1]));
        }
    }
    if (m_reference_mode != SingleReference) {
        for (auto i = 0; i < REF_CONTEXTS; i++) {
            auto& comp_ref_prob = m_probability_tables->comp_ref_prob();
            comp_ref_prob[i] = TRY(diff_update_prob(comp_ref_prob[i]));
        }
    }
    return {};
}

DecoderErrorOr<void> Parser::read_y_mode_probs()
{
    for (auto i = 0; i < BLOCK_SIZE_GROUPS; i++) {
        for (auto j = 0; j < INTRA_MODES - 1; j++) {
            auto& y_mode_probs = m_probability_tables->y_mode_probs();
            y_mode_probs[i][j] = TRY(diff_update_prob(y_mode_probs[i][j]));
        }
    }
    return {};
}

DecoderErrorOr<void> Parser::read_partition_probs()
{
    for (auto i = 0; i < PARTITION_CONTEXTS; i++) {
        for (auto j = 0; j < PARTITION_TYPES - 1; j++) {
            auto& partition_probs = m_probability_tables->partition_probs();
            partition_probs[i][j] = TRY(diff_update_prob(partition_probs[i][j]));
        }
    }
    return {};
}

DecoderErrorOr<void> Parser::mv_probs()
{
    for (auto j = 0; j < MV_JOINTS - 1; j++) {
        auto& mv_joint_probs = m_probability_tables->mv_joint_probs();
        mv_joint_probs[j] = TRY(update_mv_prob(mv_joint_probs[j]));
    }

    for (auto i = 0; i < 2; i++) {
        auto& mv_sign_prob = m_probability_tables->mv_sign_prob();
        mv_sign_prob[i] = TRY(update_mv_prob(mv_sign_prob[i]));
        for (auto j = 0; j < MV_CLASSES - 1; j++) {
            auto& mv_class_probs = m_probability_tables->mv_class_probs();
            mv_class_probs[i][j] = TRY(update_mv_prob(mv_class_probs[i][j]));
        }
        auto& mv_class0_bit_prob = m_probability_tables->mv_class0_bit_prob();
        mv_class0_bit_prob[i] = TRY(update_mv_prob(mv_class0_bit_prob[i]));
        for (auto j = 0; j < MV_OFFSET_BITS; j++) {
            auto& mv_bits_prob = m_probability_tables->mv_bits_prob();
            mv_bits_prob[i][j] = TRY(update_mv_prob(mv_bits_prob[i][j]));
        }
    }

    for (auto i = 0; i < 2; i++) {
        for (auto j = 0; j < CLASS0_SIZE; j++) {
            for (auto k = 0; k < MV_FR_SIZE - 1; k++) {
                auto& mv_class0_fr_probs = m_probability_tables->mv_class0_fr_probs();
                mv_class0_fr_probs[i][j][k] = TRY(update_mv_prob(mv_class0_fr_probs[i][j][k]));
            }
        }
        for (auto k = 0; k < MV_FR_SIZE - 1; k++) {
            auto& mv_fr_probs = m_probability_tables->mv_fr_probs();
            mv_fr_probs[i][k] = TRY(update_mv_prob(mv_fr_probs[i][k]));
        }
    }

    if (m_allow_high_precision_mv) {
        for (auto i = 0; i < 2; i++) {
            auto& mv_class0_hp_prob = m_probability_tables->mv_class0_hp_prob();
            auto& mv_hp_prob = m_probability_tables->mv_hp_prob();
            mv_class0_hp_prob[i] = TRY(update_mv_prob(mv_class0_hp_prob[i]));
            mv_hp_prob[i] = TRY(update_mv_prob(mv_hp_prob[i]));
        }
    }

    return {};
}

DecoderErrorOr<u8> Parser::update_mv_prob(u8 prob)
{
    if (TRY_READ(m_bit_stream->read_bool(252))) {
        return (TRY_READ(m_bit_stream->read_literal(7)) << 1u) | 1u;
    }
    return prob;
}

void Parser::setup_compound_reference_mode()
{
    if (m_ref_frame_sign_bias[LastFrame] == m_ref_frame_sign_bias[GoldenFrame]) {
        m_comp_fixed_ref = AltRefFrame;
        m_comp_var_ref[0] = LastFrame;
        m_comp_var_ref[1] = GoldenFrame;
    } else if (m_ref_frame_sign_bias[LastFrame] == m_ref_frame_sign_bias[AltRefFrame]) {
        m_comp_fixed_ref = GoldenFrame;
        m_comp_var_ref[0] = LastFrame;
        m_comp_var_ref[1] = AltRefFrame;
    } else {
        m_comp_fixed_ref = LastFrame;
        m_comp_var_ref[0] = GoldenFrame;
        m_comp_var_ref[1] = AltRefFrame;
    }
}

void Parser::cleanup_tile_allocations()
{
    // FIXME: Is this necessary? Data should be truncated and
    //        overwritten by the next tile.
    m_skips.clear_with_capacity();
    m_tx_sizes.clear_with_capacity();
    m_mi_sizes.clear_with_capacity();
    m_y_modes.clear_with_capacity();
    m_segment_ids.clear_with_capacity();
    m_ref_frames.clear_with_capacity();
    m_interp_filters.clear_with_capacity();
    m_mvs.clear_with_capacity();
    m_sub_mvs.clear_with_capacity();
    m_sub_modes.clear_with_capacity();
}

DecoderErrorOr<void> Parser::allocate_tile_data()
{
    auto dimensions = m_mi_rows * m_mi_cols;
    cleanup_tile_allocations();
    DECODER_TRY_ALLOC(m_skips.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_tx_sizes.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_mi_sizes.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_y_modes.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_segment_ids.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_ref_frames.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_interp_filters.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_mvs.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_sub_mvs.try_resize_and_keep_capacity(dimensions));
    DECODER_TRY_ALLOC(m_sub_modes.try_resize_and_keep_capacity(dimensions));
    return {};
}

DecoderErrorOr<void> Parser::decode_tiles()
{
    auto tile_cols = 1 << m_tile_cols_log2;
    auto tile_rows = 1 << m_tile_rows_log2;
    TRY(allocate_tile_data());
    clear_above_context();
    for (auto tile_row = 0; tile_row < tile_rows; tile_row++) {
        for (auto tile_col = 0; tile_col < tile_cols; tile_col++) {
            auto last_tile = (tile_row == tile_rows - 1) && (tile_col == tile_cols - 1);
            u64 tile_size;
            if (last_tile)
                tile_size = m_bit_stream->bytes_remaining();
            else
                tile_size = TRY_READ(m_bit_stream->read_bits(32));

            m_mi_row_start = get_tile_offset(tile_row, m_mi_rows, m_tile_rows_log2);
            m_mi_row_end = get_tile_offset(tile_row + 1, m_mi_rows, m_tile_rows_log2);
            m_mi_col_start = get_tile_offset(tile_col, m_mi_cols, m_tile_cols_log2);
            m_mi_col_end = get_tile_offset(tile_col + 1, m_mi_cols, m_tile_cols_log2);
            TRY_READ(m_bit_stream->init_bool(tile_size));
            TRY(decode_tile());
            TRY_READ(m_bit_stream->exit_bool());
        }
    }
    return {};
}

template<typename T>
void Parser::clear_context(Vector<T>& context, size_t size)
{
    context.resize_and_keep_capacity(size);
    __builtin_memset(context.data(), 0, sizeof(T) * size);
}

template<typename T>
void Parser::clear_context(Vector<Vector<T>>& context, size_t outer_size, size_t inner_size)
{
    if (context.size() < outer_size)
        context.resize(outer_size);
    for (auto& sub_vector : context)
        clear_context(sub_vector, inner_size);
}

void Parser::clear_above_context()
{
    for (auto i = 0u; i < m_above_nonzero_context.size(); i++)
        clear_context(m_above_nonzero_context[i], 2 * m_mi_cols);
    clear_context(m_above_seg_pred_context, m_mi_cols);
    clear_context(m_above_partition_context, m_sb64_cols * 8);
}

u32 Parser::get_tile_offset(u32 tile_num, u32 mis, u32 tile_size_log2)
{
    u32 super_blocks = (mis + 7) >> 3u;
    u32 offset = ((tile_num * super_blocks) >> tile_size_log2) << 3u;
    return min(offset, mis);
}

DecoderErrorOr<void> Parser::decode_tile()
{
    for (auto row = m_mi_row_start; row < m_mi_row_end; row += 8) {
        clear_left_context();
        for (auto col = m_mi_col_start; col < m_mi_col_end; col += 8) {
            TRY(decode_partition(row, col, Block_64x64));
        }
    }
    return {};
}

void Parser::clear_left_context()
{
    for (auto i = 0u; i < m_left_nonzero_context.size(); i++)
        clear_context(m_left_nonzero_context[i], 2 * m_mi_rows);
    clear_context(m_left_seg_pred_context, m_mi_rows);
    clear_context(m_left_partition_context, m_sb64_rows * 8);
}

DecoderErrorOr<void> Parser::decode_partition(u32 row, u32 col, BlockSubsize block_subsize)
{
    if (row >= m_mi_rows || col >= m_mi_cols)
        return {};
    m_block_subsize = block_subsize;
    m_num_8x8 = num_8x8_blocks_wide_lookup[block_subsize];
    auto half_block_8x8 = m_num_8x8 >> 1;
    m_has_rows = (row + half_block_8x8) < m_mi_rows;
    m_has_cols = (col + half_block_8x8) < m_mi_cols;
    m_row = row;
    m_col = col;
    auto partition = TRY_READ(TreeParser::parse_partition(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, m_has_rows, m_has_cols, m_block_subsize, m_num_8x8, m_above_partition_context, m_left_partition_context, row, col, m_frame_is_intra));

    auto subsize = subsize_lookup[partition][block_subsize];
    if (subsize < Block_8x8 || partition == PartitionNone) {
        TRY(decode_block(row, col, subsize));
    } else if (partition == PartitionHorizontal) {
        TRY(decode_block(row, col, subsize));
        if (m_has_rows)
            TRY(decode_block(row + half_block_8x8, col, subsize));
    } else if (partition == PartitionVertical) {
        TRY(decode_block(row, col, subsize));
        if (m_has_cols)
            TRY(decode_block(row, col + half_block_8x8, subsize));
    } else {
        TRY(decode_partition(row, col, subsize));
        TRY(decode_partition(row, col + half_block_8x8, subsize));
        TRY(decode_partition(row + half_block_8x8, col, subsize));
        TRY(decode_partition(row + half_block_8x8, col + half_block_8x8, subsize));
    }
    if (block_subsize == Block_8x8 || partition != PartitionSplit) {
        auto above_context = 15 >> b_width_log2_lookup[subsize];
        auto left_context = 15 >> b_height_log2_lookup[subsize];
        for (size_t i = 0; i < m_num_8x8; i++) {
            m_above_partition_context[col + i] = above_context;
            m_left_partition_context[row + i] = left_context;
        }
    }
    return {};
}

size_t Parser::get_image_index(u32 row, u32 column)
{
    VERIFY(row < m_mi_rows && column < m_mi_cols);
    return row * m_mi_cols + column;
}

DecoderErrorOr<void> Parser::decode_block(u32 row, u32 col, BlockSubsize subsize)
{
    m_mi_row = row;
    m_mi_col = col;
    m_mi_size = subsize;
    m_available_u = row > 0;
    m_available_l = col > m_mi_col_start;
    TRY(mode_info());
    m_eob_total = 0;
    TRY(residual());
    if (m_is_inter && subsize >= Block_8x8 && m_eob_total == 0)
        m_skip = true;

    // Spec doesn't specify whether it might index outside the frame here, but it seems that it can. Ensure that we don't
    // write out of bounds. This check seems consistent with libvpx.
    // See here:
    // https://github.com/webmproject/libvpx/blob/705bf9de8c96cfe5301451f1d7e5c90a41c64e5f/vp9/decoder/vp9_decodeframe.c#L917
    auto maximum_block_y = min<u32>(num_8x8_blocks_high_lookup[subsize], m_mi_rows - row);
    auto maximum_block_x = min<u32>(num_8x8_blocks_wide_lookup[subsize], m_mi_cols - col);

    for (size_t y = 0; y < maximum_block_y; y++) {
        for (size_t x = 0; x < maximum_block_x; x++) {
            auto pos = get_image_index(row + y, col + x);
            m_skips[pos] = m_skip;
            m_tx_sizes[pos] = m_tx_size;
            m_mi_sizes[pos] = m_mi_size;
            m_y_modes[pos] = m_y_mode;
            m_segment_ids[pos] = m_segment_id;
            for (size_t ref_list = 0; ref_list < 2; ref_list++)
                m_ref_frames[pos][ref_list] = m_ref_frame[ref_list];
            if (m_is_inter) {
                m_interp_filters[pos] = m_interp_filter;
                for (size_t ref_list = 0; ref_list < 2; ref_list++) {
                    // FIXME: Can we just store all the sub_mvs and then look up
                    //        the main one by index 3?
                    m_mvs[pos][ref_list] = m_block_mvs[ref_list][3];
                    for (size_t b = 0; b < 4; b++)
                        m_sub_mvs[pos][ref_list][b] = m_block_mvs[ref_list][b];
                }
            } else {
                for (size_t b = 0; b < 4; b++)
                    m_sub_modes[pos][b] = static_cast<PredictionMode>(m_block_sub_modes[b]);
            }
        }
    }
    return {};
}

DecoderErrorOr<void> Parser::mode_info()
{
    if (m_frame_is_intra)
        TRY(intra_frame_mode_info());
    else
        TRY(inter_frame_mode_info());
    return {};
}

DecoderErrorOr<void> Parser::intra_frame_mode_info()
{
    TRY(intra_segment_id());
    TRY(read_skip());
    TRY(read_tx_size(true));
    m_ref_frame[0] = IntraFrame;
    m_ref_frame[1] = None;
    m_is_inter = false;
    // FIXME: This if statement is also present in parse_default_intra_mode. The selection of parameters for
    //        the probability table lookup should be inlined here.
    if (m_mi_size >= Block_8x8) {
        // FIXME: This context should be available in the block setup. Make a struct to store the context
        //        that is needed to call the tree parses and set it in decode_block().
        auto above_context = Optional<Array<PredictionMode, 4> const&>();
        auto left_context = Optional<Array<PredictionMode, 4> const&>();
        if (m_available_u)
            above_context = m_sub_modes[get_image_index(m_mi_row - 1, m_mi_col)];
        if (m_available_l)
            left_context = m_sub_modes[get_image_index(m_mi_row, m_mi_col - 1)];
        m_default_intra_mode = TRY_READ(TreeParser::parse_default_intra_mode(*m_bit_stream, *m_probability_tables, m_mi_size, above_context, left_context, m_block_sub_modes, 0, 0));

        m_y_mode = m_default_intra_mode;
        for (auto& block_sub_mode : m_block_sub_modes)
            block_sub_mode = m_y_mode;
    } else {
        m_num_4x4_w = num_4x4_blocks_wide_lookup[m_mi_size];
        m_num_4x4_h = num_4x4_blocks_high_lookup[m_mi_size];
        for (auto idy = 0; idy < 2; idy += m_num_4x4_h) {
            for (auto idx = 0; idx < 2; idx += m_num_4x4_w) {
                // FIXME: See the FIXME above.
                auto above_context = Optional<Array<PredictionMode, 4> const&>();
                auto left_context = Optional<Array<PredictionMode, 4> const&>();
                if (m_available_u)
                    above_context = m_sub_modes[get_image_index(m_mi_row - 1, m_mi_col)];
                if (m_available_l)
                    left_context = m_sub_modes[get_image_index(m_mi_row, m_mi_col - 1)];
                m_default_intra_mode = TRY_READ(TreeParser::parse_default_intra_mode(*m_bit_stream, *m_probability_tables, m_mi_size, above_context, left_context, m_block_sub_modes, idx, idy));

                for (auto y = 0; y < m_num_4x4_h; y++) {
                    for (auto x = 0; x < m_num_4x4_w; x++) {
                        auto index = (idy + y) * 2 + idx + x;
                        m_block_sub_modes[index] = m_default_intra_mode;
                    }
                }
            }
        }
        m_y_mode = m_default_intra_mode;
    }
    m_uv_mode = TRY_READ(TreeParser::parse_default_uv_mode(*m_bit_stream, *m_probability_tables, m_y_mode));
    return {};
}

DecoderErrorOr<void> Parser::intra_segment_id()
{
    if (m_segmentation_enabled && m_segmentation_update_map)
        m_segment_id = TRY_READ(TreeParser::parse_segment_id(*m_bit_stream, m_segmentation_tree_probs));
    else
        m_segment_id = 0;
    return {};
}

DecoderErrorOr<void> Parser::read_skip()
{
    if (seg_feature_active(SEG_LVL_SKIP)) {
        m_skip = true;
    } else {
        Optional<bool> above_skip = m_available_u ? m_skips[get_image_index(m_mi_row - 1, m_mi_col)] : Optional<bool>();
        Optional<bool> left_skip = m_available_l ? m_skips[get_image_index(m_mi_row, m_mi_col - 1)] : Optional<bool>();
        m_skip = TRY_READ(TreeParser::parse_skip(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, above_skip, left_skip));
    }
    return {};
}

bool Parser::seg_feature_active(u8 feature)
{
    return m_segmentation_enabled && m_feature_enabled[m_segment_id][feature];
}

DecoderErrorOr<void> Parser::read_tx_size(bool allow_select)
{
    m_max_tx_size = max_txsize_lookup[m_mi_size];
    if (allow_select && m_tx_mode == TXModeSelect && m_mi_size >= Block_8x8) {
        Optional<bool> above_skip = m_available_u ? m_skips[get_image_index(m_mi_row - 1, m_mi_col)] : Optional<bool>();
        Optional<bool> left_skip = m_available_l ? m_skips[get_image_index(m_mi_row, m_mi_col - 1)] : Optional<bool>();
        Optional<TXSize> above_tx_size = m_available_u ? m_tx_sizes[get_image_index(m_mi_row - 1, m_mi_col)] : Optional<TXSize>();
        Optional<TXSize> left_tx_size = m_available_l ? m_tx_sizes[get_image_index(m_mi_row, m_mi_col - 1)] : Optional<TXSize>();
        m_tx_size = TRY_READ(TreeParser::parse_tx_size(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, m_max_tx_size, above_skip, left_skip, above_tx_size, left_tx_size));
    } else {
        m_tx_size = min(m_max_tx_size, tx_mode_to_biggest_tx_size[m_tx_mode]);
    }
    return {};
}

DecoderErrorOr<void> Parser::inter_frame_mode_info()
{
    m_left_ref_frame[0] = m_available_l ? m_ref_frames[get_image_index(m_mi_row, m_mi_col - 1)][0] : IntraFrame;
    m_above_ref_frame[0] = m_available_u ? m_ref_frames[get_image_index(m_mi_row - 1, m_mi_col)][0] : IntraFrame;
    m_left_ref_frame[1] = m_available_l ? m_ref_frames[get_image_index(m_mi_row, m_mi_col - 1)][1] : None;
    m_above_ref_frame[1] = m_available_u ? m_ref_frames[get_image_index(m_mi_row - 1, m_mi_col)][1] : None;
    m_left_intra = m_left_ref_frame[0] <= IntraFrame;
    m_above_intra = m_above_ref_frame[0] <= IntraFrame;
    m_left_single = m_left_ref_frame[1] <= None;
    m_above_single = m_above_ref_frame[1] <= None;
    TRY(inter_segment_id());
    TRY(read_skip());
    TRY(read_is_inter());
    TRY(read_tx_size(!m_skip || !m_is_inter));
    if (m_is_inter) {
        TRY(inter_block_mode_info());
    } else {
        TRY(intra_block_mode_info());
    }
    return {};
}

DecoderErrorOr<void> Parser::inter_segment_id()
{
    if (!m_segmentation_enabled) {
        m_segment_id = 0;
        return {};
    }
    auto predicted_segment_id = get_segment_id();
    if (!m_segmentation_update_map) {
        m_segment_id = predicted_segment_id;
        return {};
    }
    if (!m_segmentation_temporal_update) {
        m_segment_id = TRY_READ(TreeParser::parse_segment_id(*m_bit_stream, m_segmentation_tree_probs));
        return {};
    }

    auto seg_id_predicted = TRY_READ(TreeParser::parse_segment_id_predicted(*m_bit_stream, m_segmentation_pred_prob, m_left_seg_pred_context[m_mi_row], m_above_seg_pred_context[m_mi_col]));
    if (seg_id_predicted)
        m_segment_id = predicted_segment_id;
    else
        m_segment_id = TRY_READ(TreeParser::parse_segment_id(*m_bit_stream, m_segmentation_tree_probs));

    for (size_t i = 0; i < num_8x8_blocks_wide_lookup[m_mi_size]; i++) {
        auto index = m_mi_col + i;
        // (7.4.1) AboveSegPredContext[ i ] only needs to be set to 0 for i = 0..MiCols-1.
        if (index < m_above_seg_pred_context.size())
            m_above_seg_pred_context[index] = seg_id_predicted;
    }
    for (size_t i = 0; i < num_8x8_blocks_high_lookup[m_mi_size]; i++) {
        auto index = m_mi_row + i;
        // (7.4.1) LeftSegPredContext[ i ] only needs to be set to 0 for i = 0..MiRows-1.
        if (index < m_above_seg_pred_context.size())
            m_left_seg_pred_context[m_mi_row + i] = seg_id_predicted;
    }
    return {};
}

u8 Parser::get_segment_id()
{
    auto bw = num_8x8_blocks_wide_lookup[m_mi_size];
    auto bh = num_8x8_blocks_high_lookup[m_mi_size];
    auto xmis = min(m_mi_cols - m_mi_col, (u32)bw);
    auto ymis = min(m_mi_rows - m_mi_row, (u32)bh);
    u8 segment = 7;
    for (size_t y = 0; y < ymis; y++) {
        for (size_t x = 0; x < xmis; x++) {
            segment = min(segment, m_prev_segment_ids[(m_mi_row + y) + (m_mi_col + x)]);
        }
    }
    return segment;
}

DecoderErrorOr<void> Parser::read_is_inter()
{
    if (seg_feature_active(SEG_LVL_REF_FRAME)) {
        m_is_inter = m_feature_data[m_segment_id][SEG_LVL_REF_FRAME] != IntraFrame;
    } else {
        Optional<bool> above_intra = m_available_u ? m_above_intra : Optional<bool>();
        Optional<bool> left_intra = m_available_l ? m_left_intra : Optional<bool>();
        m_is_inter = TRY_READ(TreeParser::parse_is_inter(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, above_intra, left_intra));
    }
    return {};
}

DecoderErrorOr<void> Parser::intra_block_mode_info()
{
    m_ref_frame[0] = IntraFrame;
    m_ref_frame[1] = None;
    if (m_mi_size >= Block_8x8) {
        m_y_mode = TRY_READ(TreeParser::parse_intra_mode(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, m_mi_size));
        for (auto& block_sub_mode : m_block_sub_modes)
            block_sub_mode = m_y_mode;
    } else {
        m_num_4x4_w = num_4x4_blocks_wide_lookup[m_mi_size];
        m_num_4x4_h = num_4x4_blocks_high_lookup[m_mi_size];
        PredictionMode sub_intra_mode;
        for (auto idy = 0; idy < 2; idy += m_num_4x4_h) {
            for (auto idx = 0; idx < 2; idx += m_num_4x4_w) {
                sub_intra_mode = TRY_READ(TreeParser::parse_sub_intra_mode(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter));
                for (auto y = 0; y < m_num_4x4_h; y++) {
                    for (auto x = 0; x < m_num_4x4_w; x++)
                        m_block_sub_modes[(idy + y) * 2 + idx + x] = sub_intra_mode;
                }
            }
        }
        m_y_mode = sub_intra_mode;
    }
    m_uv_mode = TRY_READ(TreeParser::parse_uv_mode(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, m_y_mode));
    return {};
}

DecoderErrorOr<void> Parser::inter_block_mode_info()
{
    TRY(read_ref_frames());
    for (auto j = 0; j < 2; j++) {
        if (m_ref_frame[j] > IntraFrame) {
            find_mv_refs(m_ref_frame[j], -1);
            find_best_ref_mvs(j);
        }
    }
    auto is_compound = m_ref_frame[1] > IntraFrame;
    if (seg_feature_active(SEG_LVL_SKIP)) {
        m_y_mode = PredictionMode::ZeroMv;
    } else if (m_mi_size >= Block_8x8) {
        m_y_mode = TRY_READ(TreeParser::parse_inter_mode(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, m_mode_context[m_ref_frame[0]]));
    }
    if (m_interpolation_filter == Switchable) {
        Optional<ReferenceFrameType> above_ref_frame = m_available_u ? m_ref_frames[get_image_index(m_mi_row - 1, m_mi_col)][0] : Optional<ReferenceFrameType>();
        Optional<ReferenceFrameType> left_ref_frame = m_available_l ? m_ref_frames[get_image_index(m_mi_row, m_mi_col - 1)][0] : Optional<ReferenceFrameType>();
        Optional<InterpolationFilter> above_interpolation_filter = m_available_u ? m_interp_filters[get_image_index(m_mi_row - 1, m_mi_col)] : Optional<InterpolationFilter>();
        Optional<InterpolationFilter> left_interpolation_filter = m_available_l ? m_interp_filters[get_image_index(m_mi_row, m_mi_col - 1)] : Optional<InterpolationFilter>();
        m_interp_filter = TRY_READ(TreeParser::parse_interpolation_filter(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, above_ref_frame, left_ref_frame, above_interpolation_filter, left_interpolation_filter));
    } else {
        m_interp_filter = m_interpolation_filter;
    }
    if (m_mi_size < Block_8x8) {
        m_num_4x4_w = num_4x4_blocks_wide_lookup[m_mi_size];
        m_num_4x4_h = num_4x4_blocks_high_lookup[m_mi_size];
        for (auto idy = 0; idy < 2; idy += m_num_4x4_h) {
            for (auto idx = 0; idx < 2; idx += m_num_4x4_w) {
                m_y_mode = TRY_READ(TreeParser::parse_inter_mode(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, m_mode_context[m_ref_frame[0]]));
                if (m_y_mode == PredictionMode::NearestMv || m_y_mode == PredictionMode::NearMv) {
                    for (auto j = 0; j < 1 + is_compound; j++)
                        append_sub8x8_mvs(idy * 2 + idx, j);
                }
                TRY(assign_mv(is_compound));
                for (auto y = 0; y < m_num_4x4_h; y++) {
                    for (auto x = 0; x < m_num_4x4_w; x++) {
                        auto block = (idy + y) * 2 + idx + x;
                        for (auto ref_list = 0; ref_list < 1 + is_compound; ref_list++) {
                            m_block_mvs[ref_list][block] = m_mv[ref_list];
                        }
                    }
                }
            }
        }
        return {};
    }
    TRY(assign_mv(is_compound));
    for (auto ref_list = 0; ref_list < 1 + is_compound; ref_list++) {
        for (auto block = 0; block < 4; block++) {
            m_block_mvs[ref_list][block] = m_mv[ref_list];
        }
    }
    return {};
}

DecoderErrorOr<void> Parser::read_ref_frames()
{
    if (seg_feature_active(SEG_LVL_REF_FRAME)) {
        m_ref_frame[0] = static_cast<ReferenceFrameType>(m_feature_data[m_segment_id][SEG_LVL_REF_FRAME]);
        m_ref_frame[1] = None;
        return {};
    }
    ReferenceMode comp_mode;
    Optional<bool> above_single = m_available_u ? m_above_single : Optional<bool>();
    Optional<bool> left_single = m_available_l ? m_left_single : Optional<bool>();
    Optional<bool> above_intra = m_available_u ? m_above_intra : Optional<bool>();
    Optional<bool> left_intra = m_available_l ? m_left_intra : Optional<bool>();
    Optional<ReferenceFrameType> above_ref_frame_0 = m_available_u ? m_above_ref_frame[0] : Optional<ReferenceFrameType>();
    Optional<ReferenceFrameType> left_ref_frame_0 = m_available_l ? m_left_ref_frame[0] : Optional<ReferenceFrameType>();
    Optional<ReferenceFramePair> above_ref_frame = m_available_u ? m_above_ref_frame : Optional<ReferenceFramePair>();
    Optional<ReferenceFramePair> left_ref_frame = m_available_l ? m_left_ref_frame : Optional<ReferenceFramePair>();
    if (m_reference_mode == ReferenceModeSelect) {
        comp_mode = TRY_READ(TreeParser::parse_comp_mode(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, m_comp_fixed_ref, above_single, left_single, above_intra, left_intra, above_ref_frame_0, left_ref_frame_0));
    } else {
        comp_mode = m_reference_mode;
    }
    if (comp_mode == CompoundReference) {
        auto biased_reference_index = m_ref_frame_sign_bias[m_comp_fixed_ref];
        auto inverse_biased_reference_index = biased_reference_index == 0 ? 1 : 0;

        Optional<ReferenceFrameType> above_ref_frame_biased = m_available_u ? m_above_ref_frame[inverse_biased_reference_index] : Optional<ReferenceFrameType>();
        Optional<ReferenceFrameType> left_ref_frame_biased = m_available_l ? m_left_ref_frame[inverse_biased_reference_index] : Optional<ReferenceFrameType>();
        // FIXME: Create an enum for compound frame references using names Primary and Secondary.
        auto comp_ref = TRY_READ(TreeParser::parse_comp_ref(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, m_comp_fixed_ref, m_comp_var_ref, above_single, left_single, above_intra, left_intra, above_ref_frame_0, left_ref_frame_0, above_ref_frame_biased, left_ref_frame_biased));

        m_ref_frame[biased_reference_index] = m_comp_fixed_ref;
        m_ref_frame[inverse_biased_reference_index] = m_comp_var_ref[comp_ref];
        return {};
    }
    // FIXME: Maybe consolidate this into a tree. Context is different between part 1 and 2 but still, it would look nice here.
    auto single_ref_p1 = TRY_READ(TreeParser::parse_single_ref_part_1(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, above_single, left_single, above_intra, left_intra, above_ref_frame, left_ref_frame));
    if (single_ref_p1) {
        auto single_ref_p2 = TRY_READ(TreeParser::parse_single_ref_part_2(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, above_single, left_single, above_intra, left_intra, above_ref_frame, left_ref_frame));
        m_ref_frame[0] = single_ref_p2 ? AltRefFrame : GoldenFrame;
    } else {
        m_ref_frame[0] = LastFrame;
    }
    m_ref_frame[1] = None;
    return {};
}

DecoderErrorOr<void> Parser::assign_mv(bool is_compound)
{
    m_mv[1] = {};
    for (auto i = 0; i < 1 + is_compound; i++) {
        if (m_y_mode == PredictionMode::NewMv) {
            TRY(read_mv(i));
        } else if (m_y_mode == PredictionMode::NearestMv) {
            m_mv[i] = m_nearest_mv[i];
        } else if (m_y_mode == PredictionMode::NearMv) {
            m_mv[i] = m_near_mv[i];
        } else {
            m_mv[i] = {};
        }
    }
    return {};
}

DecoderErrorOr<void> Parser::read_mv(u8 ref)
{
    m_use_hp = m_allow_high_precision_mv && use_mv_hp(m_best_mv[ref]);
    MotionVector diff_mv;
    auto mv_joint = TRY_READ(TreeParser::parse_motion_vector_joint(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter));
    if (mv_joint == MvJointHzvnz || mv_joint == MvJointHnzvnz)
        diff_mv.set_row(TRY(read_mv_component(0)));
    if (mv_joint == MvJointHnzvz || mv_joint == MvJointHnzvnz)
        diff_mv.set_column(TRY(read_mv_component(1)));

    // FIXME: We probably don't need to assign MVs to a field, these can just
    //        be returned and assigned where they are requested.
    m_mv[ref] = m_best_mv[ref] + diff_mv;
    return {};
}

DecoderErrorOr<i32> Parser::read_mv_component(u8 component)
{
    auto mv_sign = TRY_READ(TreeParser::parse_motion_vector_sign(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, component));
    auto mv_class = TRY_READ(TreeParser::parse_motion_vector_class(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, component));
    u32 magnitude;
    if (mv_class == MvClass0) {
        auto mv_class0_bit = TRY_READ(TreeParser::parse_motion_vector_class0_bit(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, component));
        auto mv_class0_fr = TRY_READ(TreeParser::parse_motion_vector_class0_fr(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, component, mv_class0_bit));
        auto mv_class0_hp = TRY_READ(TreeParser::parse_motion_vector_class0_hp(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, component, m_use_hp));
        magnitude = ((mv_class0_bit << 3) | (mv_class0_fr << 1) | mv_class0_hp) + 1;
    } else {
        u32 bits = 0;
        for (u8 i = 0; i < mv_class; i++) {
            auto mv_bit = TRY_READ(TreeParser::parse_motion_vector_bit(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, component, i));
            bits |= mv_bit << i;
        }
        magnitude = CLASS0_SIZE << (mv_class + 2);
        auto mv_fr = TRY_READ(TreeParser::parse_motion_vector_fr(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, component));
        auto mv_hp = TRY_READ(TreeParser::parse_motion_vector_hp(*m_bit_stream, *m_probability_tables, *m_syntax_element_counter, component, m_use_hp));
        magnitude += ((bits << 3) | (mv_fr << 1) | mv_hp) + 1;
    }
    return (mv_sign ? -1 : 1) * static_cast<i32>(magnitude);
}

Gfx::Point<size_t> Parser::get_decoded_point_for_plane(u32 column, u32 row, u8 plane)
{
    if (plane == 0)
        return { column * 8, row * 8 };
    return { (column * 8) >> m_subsampling_x, (row * 8) >> m_subsampling_y };
}

Gfx::Size<size_t> Parser::get_decoded_size_for_plane(u8 plane)
{
    auto point = get_decoded_point_for_plane(m_mi_cols, m_mi_rows, plane);
    return { point.x(), point.y() };
}

DecoderErrorOr<void> Parser::residual()
{
    auto block_size = m_mi_size < Block_8x8 ? Block_8x8 : static_cast<BlockSubsize>(m_mi_size);
    for (u8 plane = 0; plane < 3; plane++) {
        auto tx_size = (plane > 0) ? get_uv_tx_size() : m_tx_size;
        auto step = 1 << tx_size;
        auto plane_size = get_plane_block_size(block_size, plane);
        auto num_4x4_w = num_4x4_blocks_wide_lookup[plane_size];
        auto num_4x4_h = num_4x4_blocks_high_lookup[plane_size];
        auto sub_x = (plane > 0) ? m_subsampling_x : 0;
        auto sub_y = (plane > 0) ? m_subsampling_y : 0;
        auto base_x = (m_mi_col * 8) >> sub_x;
        auto base_y = (m_mi_row * 8) >> sub_y;
        if (m_is_inter) {
            if (m_mi_size < Block_8x8) {
                for (auto y = 0; y < num_4x4_h; y++) {
                    for (auto x = 0; x < num_4x4_w; x++) {
                        TRY(m_decoder.predict_inter(plane, base_x + (4 * x), base_y + (4 * y), 4, 4, (y * num_4x4_w) + x));
                    }
                }
            } else {
                TRY(m_decoder.predict_inter(plane, base_x, base_y, num_4x4_w * 4, num_4x4_h * 4, 0));
            }
        }
        auto max_x = (m_mi_cols * 8) >> sub_x;
        auto max_y = (m_mi_rows * 8) >> sub_y;
        auto block_index = 0;
        for (auto y = 0; y < num_4x4_h; y += step) {
            for (auto x = 0; x < num_4x4_w; x += step) {
                auto start_x = base_x + (4 * x);
                auto start_y = base_y + (4 * y);
                auto non_zero = false;
                if (start_x < max_x && start_y < max_y) {
                    if (!m_is_inter)
                        TRY(m_decoder.predict_intra(plane, start_x, start_y, m_available_l || x > 0, m_available_u || y > 0, (x + step) < num_4x4_w, tx_size, block_index));
                    if (!m_skip) {
                        non_zero = TRY(tokens(plane, start_x, start_y, tx_size, block_index));
                        TRY(m_decoder.reconstruct(plane, start_x, start_y, tx_size));
                    }
                }

                auto& above_sub_context = m_above_nonzero_context[plane];
                auto above_sub_context_index = start_x >> 2;
                auto above_sub_context_end = min(above_sub_context_index + step, above_sub_context.size());
                for (; above_sub_context_index < above_sub_context_end; above_sub_context_index++)
                    above_sub_context[above_sub_context_index] = non_zero;

                auto& left_sub_context = m_left_nonzero_context[plane];
                auto left_sub_context_index = start_y >> 2;
                auto left_sub_context_end = min(left_sub_context_index + step, left_sub_context.size());
                for (; left_sub_context_index < left_sub_context_end; left_sub_context_index++)
                    left_sub_context[left_sub_context_index] = non_zero;

                block_index++;
            }
        }
    }
    return {};
}

TXSize Parser::get_uv_tx_size()
{
    if (m_mi_size < Block_8x8)
        return TX_4x4;
    return min(m_tx_size, max_txsize_lookup[get_plane_block_size(m_mi_size, 1)]);
}

BlockSubsize Parser::get_plane_block_size(u32 subsize, u8 plane)
{
    auto sub_x = (plane > 0) ? m_subsampling_x : 0;
    auto sub_y = (plane > 0) ? m_subsampling_y : 0;
    return ss_size_lookup[subsize][sub_x][sub_y];
}

DecoderErrorOr<bool> Parser::tokens(size_t plane, u32 start_x, u32 start_y, TXSize tx_size, u32 block_index)
{
    m_tree_parser->set_start_x_and_y(start_x, start_y);
    size_t segment_eob = 16 << (tx_size << 1);
    auto scan = get_scan(plane, tx_size, block_index);
    auto check_eob = true;
    size_t c = 0;
    for (; c < segment_eob; c++) {
        auto pos = scan[c];
        auto band = (tx_size == TX_4x4) ? coefband_4x4[c] : coefband_8x8plus[c];
        m_tree_parser->set_tokens_variables(band, c, plane, tx_size, pos);
        if (check_eob) {
            auto more_coefs = TRY_READ(m_tree_parser->parse_tree<bool>(SyntaxElementType::MoreCoefs));
            if (!more_coefs)
                break;
        }
        auto token = TRY_READ(m_tree_parser->parse_tree<Token>(SyntaxElementType::Token));
        m_token_cache[pos] = energy_class[token];
        if (token == ZeroToken) {
            m_tokens[pos] = 0;
            check_eob = false;
        } else {
            i32 coef = TRY(read_coef(token));
            auto sign_bit = TRY_READ(m_bit_stream->read_literal(1));
            m_tokens[pos] = sign_bit ? -coef : coef;
            check_eob = true;
        }
    }
    auto non_zero = c > 0;
    m_eob_total += non_zero;
    for (size_t i = c; i < segment_eob; i++)
        m_tokens[scan[i]] = 0;
    return non_zero;
}

u32 const* Parser::get_scan(size_t plane, TXSize tx_size, u32 block_index)
{
    if (plane > 0 || tx_size == TX_32x32) {
        m_tx_type = DCT_DCT;
    } else if (tx_size == TX_4x4) {
        if (m_lossless || m_is_inter)
            m_tx_type = DCT_DCT;
        else
            m_tx_type = mode_to_txfm_map[to_underlying(m_mi_size < Block_8x8 ? m_block_sub_modes[block_index] : m_y_mode)];
    } else {
        m_tx_type = mode_to_txfm_map[to_underlying(m_y_mode)];
    }
    if (tx_size == TX_4x4) {
        if (m_tx_type == ADST_DCT)
            return row_scan_4x4;
        if (m_tx_type == DCT_ADST)
            return col_scan_4x4;
        return default_scan_4x4;
    }
    if (tx_size == TX_8x8) {
        if (m_tx_type == ADST_DCT)
            return row_scan_8x8;
        if (m_tx_type == DCT_ADST)
            return col_scan_8x8;
        return default_scan_8x8;
    }
    if (tx_size == TX_16x16) {
        if (m_tx_type == ADST_DCT)
            return row_scan_16x16;
        if (m_tx_type == DCT_ADST)
            return col_scan_16x16;
        return default_scan_16x16;
    }
    return default_scan_32x32;
}

DecoderErrorOr<i32> Parser::read_coef(Token token)
{
    auto cat = extra_bits[token][0];
    auto num_extra = extra_bits[token][1];
    u32 coef = extra_bits[token][2];
    if (token == DctValCat6) {
        for (size_t e = 0; e < (u8)(m_bit_depth - 8); e++) {
            auto high_bit = TRY_READ(m_bit_stream->read_bool(255));
            coef += high_bit << (5 + m_bit_depth - e);
        }
    }
    for (size_t e = 0; e < num_extra; e++) {
        auto coef_bit = TRY_READ(m_bit_stream->read_bool(cat_probs[cat][e]));
        coef += coef_bit << (num_extra - 1 - e);
    }
    return coef;
}

bool Parser::is_inside(i32 row, i32 column)
{
    if (row < 0)
        return false;
    if (column < 0)
        return false;
    u32 row_positive = row;
    u32 column_positive = column;
    return row_positive < m_mi_rows && column_positive >= m_mi_col_start && column_positive < m_mi_col_end;
}

void Parser::add_mv_ref_list(u8 ref_list)
{
    if (m_ref_mv_count >= 2)
        return;
    if (m_ref_mv_count > 0 && m_candidate_mv[ref_list] == m_ref_list_mv[0])
        return;

    m_ref_list_mv[m_ref_mv_count] = m_candidate_mv[ref_list];
    m_ref_mv_count++;
}

void Parser::get_block_mv(u32 candidate_row, u32 candidate_column, u8 ref_list, bool use_prev)
{
    auto index = get_image_index(candidate_row, candidate_column);
    if (use_prev) {
        m_candidate_mv[ref_list] = m_prev_mvs[index][ref_list];
        m_candidate_frame[ref_list] = m_prev_ref_frames[index][ref_list];
    } else {
        m_candidate_mv[ref_list] = m_mvs[index][ref_list];
        m_candidate_frame[ref_list] = m_ref_frames[index][ref_list];
    }
}

void Parser::if_same_ref_frame_add_mv(u32 candidate_row, u32 candidate_column, ReferenceFrameType ref_frame, bool use_prev)
{
    for (auto ref_list = 0u; ref_list < 2; ref_list++) {
        get_block_mv(candidate_row, candidate_column, ref_list, use_prev);
        if (m_candidate_frame[ref_list] == ref_frame) {
            add_mv_ref_list(ref_list);
            return;
        }
    }
}

void Parser::scale_mv(u8 ref_list, ReferenceFrameType ref_frame)
{
    auto candidate_frame = m_candidate_frame[ref_list];
    if (m_ref_frame_sign_bias[candidate_frame] != m_ref_frame_sign_bias[ref_frame])
        m_candidate_mv[ref_list] *= -1;
}

void Parser::if_diff_ref_frame_add_mv(u32 candidate_row, u32 candidate_column, ReferenceFrameType ref_frame, bool use_prev)
{
    for (auto ref_list = 0u; ref_list < 2; ref_list++)
        get_block_mv(candidate_row, candidate_column, ref_list, use_prev);
    auto mvs_are_same = m_candidate_mv[0] == m_candidate_mv[1];
    if (m_candidate_frame[0] > ReferenceFrameType::IntraFrame && m_candidate_frame[0] != ref_frame) {
        scale_mv(0, ref_frame);
        add_mv_ref_list(0);
    }
    if (m_candidate_frame[1] > ReferenceFrameType::IntraFrame && m_candidate_frame[1] != ref_frame && !mvs_are_same) {
        scale_mv(1, ref_frame);
        add_mv_ref_list(1);
    }
}

MotionVector Parser::clamp_mv(MotionVector vector, i32 border)
{
    i32 blocks_high = num_8x8_blocks_high_lookup[m_mi_size];
    // Casts must be done here to prevent subtraction underflow from wrapping the values.
    i32 mb_to_top_edge = -8 * (static_cast<i32>(m_mi_row) * MI_SIZE);
    i32 mb_to_bottom_edge = 8 * ((static_cast<i32>(m_mi_rows) - blocks_high - static_cast<i32>(m_mi_row)) * MI_SIZE);

    i32 blocks_wide = num_8x8_blocks_wide_lookup[m_mi_size];
    i32 mb_to_left_edge = -8 * (static_cast<i32>(m_mi_col) * MI_SIZE);
    i32 mb_to_right_edge = 8 * ((static_cast<i32>(m_mi_cols) - blocks_wide - static_cast<i32>(m_mi_col)) * MI_SIZE);

    return {
        clip_3(mb_to_top_edge - border, mb_to_bottom_edge + border, vector.row()),
        clip_3(mb_to_left_edge - border, mb_to_right_edge + border, vector.column())
    };
}

void Parser::clamp_mv_ref(u8 i)
{
    MotionVector& vector = m_ref_list_mv[i];
    vector = clamp_mv(vector, MV_BORDER);
}

// 6.5.1 Find MV refs syntax
void Parser::find_mv_refs(ReferenceFrameType reference_frame, i32 block)
{
    m_ref_mv_count = 0;
    bool different_ref_found = false;
    u8 context_counter = 0;

    m_ref_list_mv[0] = {};
    m_ref_list_mv[1] = {};

    MotionVector base_coordinates = MotionVector(m_mi_row, m_mi_col);

    for (auto i = 0u; i < 2; i++) {
        auto offset_vector = mv_ref_blocks[m_mi_size][i];
        auto candidate = base_coordinates + offset_vector;

        if (is_inside(candidate.row(), candidate.column())) {
            auto candidate_index = get_image_index(candidate.row(), candidate.column());
            auto index = get_image_index(candidate.row(), candidate.column());
            different_ref_found = true;
            context_counter += mode_2_counter[to_underlying(m_y_modes[index])];

            for (auto ref_list = 0u; ref_list < 2; ref_list++) {
                if (m_ref_frames[candidate_index][ref_list] == reference_frame) {
                    // This section up until add_mv_ref_list() is defined in spec as get_sub_block_mv().
                    constexpr u8 idx_n_column_to_subblock[4][2] = {
                        { 1, 2 },
                        { 1, 3 },
                        { 3, 2 },
                        { 3, 3 }
                    };
                    auto index = block >= 0 ? idx_n_column_to_subblock[block][offset_vector.column() == 0] : 3;
                    m_candidate_mv[ref_list] = m_sub_mvs[candidate_index][ref_list][index];

                    add_mv_ref_list(ref_list);
                    break;
                }
            }
        }
    }

    for (auto i = 2u; i < MVREF_NEIGHBOURS; i++) {
        MotionVector candidate = base_coordinates + mv_ref_blocks[m_mi_size][i];
        if (is_inside(candidate.row(), candidate.column())) {
            different_ref_found = true;
            if_same_ref_frame_add_mv(candidate.row(), candidate.column(), reference_frame, false);
        }
    }
    if (m_use_prev_frame_mvs)
        if_same_ref_frame_add_mv(m_mi_row, m_mi_col, reference_frame, true);

    if (different_ref_found) {
        for (auto i = 0u; i < MVREF_NEIGHBOURS; i++) {
            MotionVector candidate = base_coordinates + mv_ref_blocks[m_mi_size][i];
            if (is_inside(candidate.row(), candidate.column()))
                if_diff_ref_frame_add_mv(candidate.row(), candidate.column(), reference_frame, false);
        }
    }
    if (m_use_prev_frame_mvs)
        if_diff_ref_frame_add_mv(m_mi_row, m_mi_col, reference_frame, true);

    m_mode_context[reference_frame] = counter_to_context[context_counter];
    for (auto i = 0u; i < MAX_MV_REF_CANDIDATES; i++)
        clamp_mv_ref(i);
}

bool Parser::use_mv_hp(MotionVector const& vector)
{
    return (abs(vector.row()) >> 3) < COMPANDED_MVREF_THRESH && (abs(vector.column()) >> 3) < COMPANDED_MVREF_THRESH;
}

void Parser::find_best_ref_mvs(u8 ref_list)
{
    for (auto i = 0u; i < MAX_MV_REF_CANDIDATES; i++) {
        auto delta = m_ref_list_mv[i];
        auto delta_row = delta.row();
        auto delta_column = delta.column();
        if (!m_allow_high_precision_mv || !use_mv_hp(delta)) {
            if (delta_row & 1)
                delta_row += delta_row > 0 ? -1 : 1;
            if (delta_column & 1)
                delta_column += delta_column > 0 ? -1 : 1;
        }
        delta = { delta_row, delta_column };
        m_ref_list_mv[i] = clamp_mv(delta, (BORDERINPIXELS - INTERP_EXTEND) << 3);
    }

    m_nearest_mv[ref_list] = m_ref_list_mv[0];
    m_near_mv[ref_list] = m_ref_list_mv[1];
    m_best_mv[ref_list] = m_ref_list_mv[0];
}

void Parser::append_sub8x8_mvs(i32 block, u8 ref_list)
{
    MotionVector sub_8x8_mvs[2];
    find_mv_refs(m_ref_frame[ref_list], block);
    auto destination_index = 0;
    if (block == 0) {
        for (auto i = 0u; i < 2; i++)
            sub_8x8_mvs[destination_index++] = m_ref_list_mv[i];
    } else if (block <= 2) {
        sub_8x8_mvs[destination_index++] = m_block_mvs[ref_list][0];
    } else {
        sub_8x8_mvs[destination_index++] = m_block_mvs[ref_list][2];
        for (auto index = 1; index >= 0 && destination_index < 2; index--) {
            auto block_vector = m_block_mvs[ref_list][index];
            if (block_vector != sub_8x8_mvs[0])
                sub_8x8_mvs[destination_index++] = block_vector;
        }
    }

    for (auto n = 0u; n < 2 && destination_index < 2; n++) {
        auto ref_list_vector = m_ref_list_mv[n];
        if (ref_list_vector != sub_8x8_mvs[0])
            sub_8x8_mvs[destination_index++] = ref_list_vector;
    }

    if (destination_index < 2)
        sub_8x8_mvs[destination_index++] = {};
    m_nearest_mv[ref_list] = sub_8x8_mvs[0];
    m_near_mv[ref_list] = sub_8x8_mvs[1];
}

void Parser::dump_info()
{
    outln("Frame dimensions: {}x{}", m_frame_size.width(), m_frame_size.height());
    outln("Render dimensions: {}x{}", m_render_size.width(), m_render_size.height());
    outln("Bit depth: {}", m_bit_depth);
    outln("Show frame: {}", m_show_frame);
}

}