codec/

delta.rs

//! Delta snapshot encoding/decoding.

use std::cmp::Ordering;

use bitstream::{BitReader, BitWriter};
use schema::{schema_hash, ChangePolicy, ComponentDef, ComponentId, FieldDef};
use wire::{decode_packet, encode_header, SectionTag, WirePacket};

use crate::baseline::BaselineStore;
use crate::error::{CodecError, CodecResult, LimitKind, MaskKind, MaskReason, ValueReason};
use crate::limits::CodecLimits;
use crate::scratch::CodecScratch;
use crate::snapshot::{
    ensure_known_components, read_field_value, read_field_value_sparse, read_mask, required_bits,
    write_field_value, write_field_value_sparse, write_section, ComponentSnapshot, EntitySnapshot,
    FieldValue, Snapshot,
};
use crate::types::{EntityId, SnapshotTick};

/// Selects the latest baseline tick at or before the ack tick.
#[must_use]
pub fn select_baseline_tick<T>(
    store: &BaselineStore<T>,
    ack_tick: SnapshotTick,
) -> Option<SnapshotTick> {
    store.latest_at_or_before(ack_tick).map(|(tick, _)| tick)
}

/// Encodes a delta snapshot into the provided output buffer.
///
/// This is the scan-based path and is kept as a convenience/fallback. For
/// production engines with dirty/change lists, prefer `encode_delta_from_changes`.
/// Baseline and current snapshots must have entities sorted by `EntityId`.
pub fn encode_delta_snapshot(
    schema: &schema::Schema,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    out: &mut [u8],
) -> CodecResult<usize> {
    let mut scratch = CodecScratch::default();
    encode_delta_snapshot_with_scratch(
        schema,
        tick,
        baseline_tick,
        baseline,
        current,
        limits,
        &mut scratch,
        out,
    )
}

/// Encodes a delta snapshot using reusable scratch buffers.
#[allow(clippy::too_many_arguments)]
pub fn encode_delta_snapshot_with_scratch(
    schema: &schema::Schema,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    out: &mut [u8],
) -> CodecResult<usize> {
    encode_delta_snapshot_with_scratch_mode(
        schema,
        tick,
        baseline_tick,
        baseline,
        current,
        limits,
        scratch,
        out,
        EncodeUpdateMode::Auto,
    )
}

/// Encodes a delta snapshot for a client-specific view.
///
/// This assumes `baseline` and `current` are already filtered for the client interest set.
/// Updates are encoded in sparse mode to optimize for small per-client packets.
pub fn encode_delta_snapshot_for_client(
    schema: &schema::Schema,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    out: &mut [u8],
) -> CodecResult<usize> {
    let mut scratch = CodecScratch::default();
    encode_delta_snapshot_for_client_with_scratch(
        schema,
        tick,
        baseline_tick,
        baseline,
        current,
        limits,
        &mut scratch,
        out,
    )
}

/// Encoder context for delta packets that use caller-provided change lists.
///
/// This avoids scanning baseline/current snapshots and is preferred for
/// production engines that already track dirty state.
pub struct SessionEncoder<'a> {
    schema: &'a schema::Schema,
    limits: &'a CodecLimits,
    #[allow(dead_code)]
    scratch: CodecScratch,
}

impl<'a> SessionEncoder<'a> {
    #[must_use]
    pub fn new(schema: &'a schema::Schema, limits: &'a CodecLimits) -> Self {
        Self {
            schema,
            limits,
            scratch: CodecScratch::default(),
        }
    }

    #[must_use]
    pub fn schema(&self) -> &'a schema::Schema {
        self.schema
    }

    #[must_use]
    pub fn limits(&self) -> &'a CodecLimits {
        self.limits
    }
}

/// Encodes a delta snapshot from precomputed change lists.
///
/// Preferred for production engines (dirty/change-list driven). The scan-based
/// encode paths remain as a convenience/fallback.
pub fn encode_delta_from_changes(
    session: &mut SessionEncoder<'_>,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    creates: &[EntitySnapshot],
    destroys: &[EntityId],
    updates: &[DeltaUpdateEntity],
    out: &mut [u8],
) -> CodecResult<usize> {
    encode_delta_snapshot_from_updates(
        session.schema,
        tick,
        baseline_tick,
        destroys,
        creates,
        updates,
        session.limits,
        out,
    )
}

/// Encodes a delta snapshot using precomputed change lists.
///
/// This is a lower-level helper. Prefer `encode_delta_from_changes` with a
/// `SessionEncoder` for production use.
#[allow(clippy::too_many_arguments)]
pub fn encode_delta_snapshot_from_updates(
    schema: &schema::Schema,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    destroys: &[EntityId],
    creates: &[EntitySnapshot],
    updates: &[DeltaUpdateEntity],
    limits: &CodecLimits,
    out: &mut [u8],
) -> CodecResult<usize> {
    if out.len() < wire::HEADER_SIZE {
        return Err(CodecError::OutputTooSmall {
            needed: wire::HEADER_SIZE,
            available: out.len(),
        });
    }
    let payload_len = encode_delta_payload_from_updates(
        schema,
        destroys,
        creates,
        updates,
        limits,
        &mut out[wire::HEADER_SIZE..],
    )?;
    let header = wire::PacketHeader::delta_snapshot(
        schema_hash(schema),
        tick.raw(),
        baseline_tick.raw(),
        payload_len as u32,
    );
    encode_header(&header, &mut out[..wire::HEADER_SIZE]).map_err(|_| {
        CodecError::OutputTooSmall {
            needed: wire::HEADER_SIZE,
            available: out.len(),
        }
    })?;
    Ok(wire::HEADER_SIZE + payload_len)
}

/// Encodes a client delta snapshot using a compact session header.
#[allow(clippy::too_many_arguments)]
pub fn encode_delta_snapshot_for_client_session(
    schema: &schema::Schema,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    last_tick: &mut SnapshotTick,
    out: &mut [u8],
) -> CodecResult<usize> {
    let mut scratch = CodecScratch::default();
    encode_delta_snapshot_for_client_session_with_scratch(
        schema,
        tick,
        baseline_tick,
        baseline,
        current,
        limits,
        &mut scratch,
        last_tick,
        out,
    )
}

/// Encodes a delta snapshot for a client-specific view using reusable scratch buffers.
#[allow(clippy::too_many_arguments)]
pub fn encode_delta_snapshot_for_client_with_scratch(
    schema: &schema::Schema,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    out: &mut [u8],
) -> CodecResult<usize> {
    encode_delta_snapshot_with_scratch_mode(
        schema,
        tick,
        baseline_tick,
        baseline,
        current,
        limits,
        scratch,
        out,
        EncodeUpdateMode::Sparse,
    )
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum EncodeUpdateMode {
    Auto,
    Sparse,
}

/// Encodes a client delta snapshot using a compact session header.
#[allow(clippy::too_many_arguments)]
pub fn encode_delta_snapshot_for_client_session_with_scratch(
    schema: &schema::Schema,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    last_tick: &mut SnapshotTick,
    out: &mut [u8],
) -> CodecResult<usize> {
    let max_header = wire::SESSION_MAX_HEADER_SIZE;
    if out.len() < max_header {
        return Err(CodecError::OutputTooSmall {
            needed: max_header,
            available: out.len(),
        });
    }
    if tick.raw() <= last_tick.raw() {
        return Err(CodecError::InvalidEntityOrder {
            previous: last_tick.raw(),
            current: tick.raw(),
        });
    }
    if baseline_tick.raw() > tick.raw() {
        return Err(CodecError::BaselineTickMismatch {
            expected: baseline_tick.raw(),
            found: tick.raw(),
        });
    }
    let payload_len = encode_delta_payload_with_mode(
        schema,
        baseline_tick,
        baseline,
        current,
        limits,
        scratch,
        &mut out[max_header..],
        EncodeUpdateMode::Sparse,
    )?;
    let tick_delta = tick.raw() - last_tick.raw();
    let baseline_delta = tick.raw() - baseline_tick.raw();
    let header_len = wire::encode_session_header(
        &mut out[..max_header],
        wire::SessionFlags::delta_snapshot(),
        tick_delta,
        baseline_delta,
        payload_len as u32,
    )
    .map_err(|_| CodecError::OutputTooSmall {
        needed: max_header,
        available: out.len(),
    })?;
    if header_len < max_header {
        let payload_start = max_header;
        let payload_end = max_header + payload_len;
        out.copy_within(payload_start..payload_end, header_len);
    }
    *last_tick = tick;
    Ok(header_len + payload_len)
}

#[allow(clippy::too_many_arguments)]
fn encode_delta_snapshot_with_scratch_mode(
    schema: &schema::Schema,
    tick: SnapshotTick,
    baseline_tick: SnapshotTick,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    out: &mut [u8],
    mode: EncodeUpdateMode,
) -> CodecResult<usize> {
    if out.len() < wire::HEADER_SIZE {
        return Err(CodecError::OutputTooSmall {
            needed: wire::HEADER_SIZE,
            available: out.len(),
        });
    }
    let payload_len = encode_delta_payload_with_mode(
        schema,
        baseline_tick,
        baseline,
        current,
        limits,
        scratch,
        &mut out[wire::HEADER_SIZE..],
        mode,
    )?;
    let header = wire::PacketHeader::delta_snapshot(
        schema_hash(schema),
        tick.raw(),
        baseline_tick.raw(),
        payload_len as u32,
    );
    encode_header(&header, &mut out[..wire::HEADER_SIZE]).map_err(|_| {
        CodecError::OutputTooSmall {
            needed: wire::HEADER_SIZE,
            available: out.len(),
        }
    })?;

    Ok(wire::HEADER_SIZE + payload_len)
}

#[allow(clippy::too_many_arguments)]
fn encode_delta_payload_with_mode(
    schema: &schema::Schema,
    baseline_tick: SnapshotTick,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    out: &mut [u8],
    mode: EncodeUpdateMode,
) -> CodecResult<usize> {
    if baseline.tick != baseline_tick {
        return Err(CodecError::BaselineTickMismatch {
            expected: baseline.tick.raw(),
            found: baseline_tick.raw(),
        });
    }

    ensure_entities_sorted(&baseline.entities)?;
    ensure_entities_sorted(&current.entities)?;

    let mut counts = DiffCounts::default();
    diff_counts(schema, baseline, current, limits, &mut counts)?;

    if counts.creates > limits.max_entities_create {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesCreate,
            limit: limits.max_entities_create,
            actual: counts.creates,
        });
    }
    if counts.updates > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: counts.updates,
        });
    }
    if counts.destroys > limits.max_entities_destroy {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesDestroy,
            limit: limits.max_entities_destroy,
            actual: counts.destroys,
        });
    }

    let mut offset = 0;
    if counts.destroys > 0 {
        let written = write_section(
            SectionTag::EntityDestroy,
            &mut out[offset..],
            limits,
            |writer| encode_destroy_body(baseline, current, counts.destroys, limits, writer),
        )?;
        offset += written;
    }
    if counts.creates > 0 {
        let written = write_section(
            SectionTag::EntityCreate,
            &mut out[offset..],
            limits,
            |writer| encode_create_body(schema, baseline, current, counts.creates, limits, writer),
        )?;
        offset += written;
    }
    if counts.updates > 0 {
        let update_encoding = match mode {
            EncodeUpdateMode::Auto => {
                select_update_encoding(schema, baseline, current, limits, scratch)?
            }
            EncodeUpdateMode::Sparse => UpdateEncoding::Sparse,
        };
        let section_tag = match update_encoding {
            UpdateEncoding::Masked => SectionTag::EntityUpdate,
            UpdateEncoding::Sparse => SectionTag::EntityUpdateSparsePacked,
        };
        let written =
            write_section(
                section_tag,
                &mut out[offset..],
                limits,
                |writer| match update_encoding {
                    UpdateEncoding::Masked => encode_update_body_masked(
                        schema,
                        baseline,
                        current,
                        counts.updates,
                        limits,
                        scratch,
                        writer,
                    ),
                    UpdateEncoding::Sparse => encode_update_body_sparse_packed(
                        schema,
                        baseline,
                        current,
                        counts.updates,
                        limits,
                        scratch,
                        writer,
                    ),
                },
            )?;
        offset += written;
    }

    Ok(offset)
}

#[allow(clippy::too_many_arguments)]
fn encode_delta_payload_from_updates(
    schema: &schema::Schema,
    destroys: &[EntityId],
    creates: &[EntitySnapshot],
    updates: &[DeltaUpdateEntity],
    limits: &CodecLimits,
    out: &mut [u8],
) -> CodecResult<usize> {
    if destroys.len() > limits.max_entities_destroy {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesDestroy,
            limit: limits.max_entities_destroy,
            actual: destroys.len(),
        });
    }
    if creates.len() > limits.max_entities_create {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesCreate,
            limit: limits.max_entities_create,
            actual: creates.len(),
        });
    }
    if updates.len() > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: updates.len(),
        });
    }

    ensure_entity_ids_sorted(destroys)?;
    ensure_entities_sorted(creates)?;
    let lookup = build_component_lookup(schema);
    validate_updates_for_encoding(schema, updates, limits, &lookup)?;

    let mut offset = 0;
    if !destroys.is_empty() {
        let written = write_section(
            SectionTag::EntityDestroy,
            &mut out[offset..],
            limits,
            |writer| encode_destroy_body_from_list(destroys, limits, writer),
        )?;
        offset += written;
    }
    if !creates.is_empty() {
        let written = write_section(
            SectionTag::EntityCreate,
            &mut out[offset..],
            limits,
            |writer| encode_create_body_from_list(schema, creates, limits, writer),
        )?;
        offset += written;
    }
    if !updates.is_empty() {
        let written = write_section(
            SectionTag::EntityUpdateSparsePacked,
            &mut out[offset..],
            limits,
            |writer| {
                encode_update_body_sparse_packed_from_updates(
                    schema, updates, limits, &lookup, writer,
                )
            },
        )?;
        offset += written;
    }

    Ok(offset)
}

/// Applies a delta snapshot to a baseline snapshot.
pub fn apply_delta_snapshot(
    schema: &schema::Schema,
    baseline: &Snapshot,
    bytes: &[u8],
    wire_limits: &wire::Limits,
    limits: &CodecLimits,
) -> CodecResult<Snapshot> {
    let packet = decode_packet(bytes, wire_limits)?;
    apply_delta_snapshot_from_packet(schema, baseline, &packet, limits)
}

/// Applies a delta snapshot from a parsed wire packet.
pub fn apply_delta_snapshot_from_packet(
    schema: &schema::Schema,
    baseline: &Snapshot,
    packet: &WirePacket<'_>,
    limits: &CodecLimits,
) -> CodecResult<Snapshot> {
    let header = packet.header;
    if !header.flags.is_delta_snapshot() {
        return Err(CodecError::Wire(wire::DecodeError::InvalidFlags {
            flags: header.flags.raw(),
        }));
    }
    if header.baseline_tick == 0 {
        return Err(CodecError::Wire(wire::DecodeError::InvalidBaselineTick {
            baseline_tick: header.baseline_tick,
            flags: header.flags.raw(),
        }));
    }
    if header.baseline_tick != baseline.tick.raw() {
        return Err(CodecError::BaselineTickMismatch {
            expected: baseline.tick.raw(),
            found: header.baseline_tick,
        });
    }

    let expected_hash = schema_hash(schema);
    if header.schema_hash != expected_hash {
        return Err(CodecError::SchemaMismatch {
            expected: expected_hash,
            found: header.schema_hash,
        });
    }

    let (destroys, creates, updates) = decode_delta_sections(schema, packet, limits)?;

    ensure_entities_sorted(&baseline.entities)?;
    ensure_entities_sorted(&creates)?;

    let mut remaining = apply_destroys(&baseline.entities, &destroys)?;
    remaining = apply_creates(remaining, creates)?;
    if remaining.len() > limits.max_total_entities_after_apply {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::TotalEntitiesAfterApply,
            limit: limits.max_total_entities_after_apply,
            actual: remaining.len(),
        });
    }
    apply_updates(&mut remaining, &updates)?;

    Ok(Snapshot {
        tick: SnapshotTick::new(header.tick),
        entities: remaining,
    })
}

/// Decodes a delta packet without applying it to a baseline.
pub fn decode_delta_packet(
    schema: &schema::Schema,
    packet: &WirePacket<'_>,
    limits: &CodecLimits,
) -> CodecResult<DeltaDecoded> {
    let header = packet.header;
    if !header.flags.is_delta_snapshot() {
        return Err(CodecError::Wire(wire::DecodeError::InvalidFlags {
            flags: header.flags.raw(),
        }));
    }
    if header.baseline_tick == 0 {
        return Err(CodecError::Wire(wire::DecodeError::InvalidBaselineTick {
            baseline_tick: header.baseline_tick,
            flags: header.flags.raw(),
        }));
    }
    let expected_hash = schema_hash(schema);
    if header.schema_hash != expected_hash {
        return Err(CodecError::SchemaMismatch {
            expected: expected_hash,
            found: header.schema_hash,
        });
    }

    let (destroys, creates, updates) = decode_delta_sections(schema, packet, limits)?;

    Ok(DeltaDecoded {
        tick: SnapshotTick::new(header.tick),
        baseline_tick: SnapshotTick::new(header.baseline_tick),
        destroys,
        creates,
        updates,
    })
}

#[derive(Default)]
struct DiffCounts {
    creates: usize,
    updates: usize,
    destroys: usize,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum UpdateEncoding {
    Masked,
    Sparse,
}

fn diff_counts(
    schema: &schema::Schema,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    counts: &mut DiffCounts,
) -> CodecResult<()> {
    let mut i = 0usize;
    let mut j = 0usize;
    while i < baseline.entities.len() || j < current.entities.len() {
        let base = baseline.entities.get(i);
        let curr = current.entities.get(j);
        match (base, curr) {
            (Some(b), Some(c)) => {
                if b.id.raw() < c.id.raw() {
                    counts.destroys += 1;
                    i += 1;
                } else if b.id.raw() > c.id.raw() {
                    counts.creates += 1;
                    j += 1;
                } else {
                    if entity_has_updates(schema, b, c, limits)? {
                        counts.updates += 1;
                    }
                    i += 1;
                    j += 1;
                }
            }
            (Some(_), None) => {
                counts.destroys += 1;
                i += 1;
            }
            (None, Some(_)) => {
                counts.creates += 1;
                j += 1;
            }
            (None, None) => break,
        }
    }
    Ok(())
}

fn select_update_encoding(
    schema: &schema::Schema,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
) -> CodecResult<UpdateEncoding> {
    let component_count = schema.components.len();
    let mut mask_bits = 0usize;
    let mut sparse_bits = 0usize;
    let mut baseline_iter = baseline.entities.iter();
    let mut current_iter = current.entities.iter();
    let mut baseline_next = baseline_iter.next();
    let mut current_next = current_iter.next();

    while let (Some(base), Some(curr)) = (baseline_next, current_next) {
        match base.id.cmp(&curr.id) {
            Ordering::Less => {
                baseline_next = baseline_iter.next();
            }
            Ordering::Greater => {
                current_next = current_iter.next();
            }
            Ordering::Equal => {
                for component in &schema.components {
                    let base_component = find_component(base, component.id);
                    let curr_component = find_component(curr, component.id);
                    if base_component.is_some() != curr_component.is_some() {
                        return Err(CodecError::InvalidMask {
                            kind: MaskKind::ComponentMask,
                            reason: MaskReason::ComponentPresenceMismatch {
                                component: component.id,
                            },
                        });
                    }
                    if let (Some(base_component), Some(curr_component)) =
                        (base_component, curr_component)
                    {
                        if base_component.fields.len() != component.fields.len()
                            || curr_component.fields.len() != component.fields.len()
                        {
                            return Err(CodecError::InvalidMask {
                                kind: MaskKind::FieldMask {
                                    component: component.id,
                                },
                                reason: MaskReason::FieldCountMismatch {
                                    expected: component.fields.len(),
                                    actual: base_component
                                        .fields
                                        .len()
                                        .max(curr_component.fields.len()),
                                },
                            });
                        }
                        if component.fields.len() > limits.max_fields_per_component {
                            return Err(CodecError::LimitsExceeded {
                                kind: LimitKind::FieldsPerComponent,
                                limit: limits.max_fields_per_component,
                                actual: component.fields.len(),
                            });
                        }
                        let (_, field_mask) = scratch
                            .component_and_field_masks_mut(component_count, component.fields.len());
                        // Use the same change predicate as masked/sparse encoding.
                        let field_mask = compute_field_mask_into(
                            component,
                            base_component,
                            curr_component,
                            field_mask,
                        )?;
                        let changed = field_mask.iter().filter(|bit| **bit).count();
                        if changed > 0 {
                            let field_count = component.fields.len();
                            let index_bits =
                                required_bits(field_count.saturating_sub(1) as u64) as usize;
                            // Compare per-component mask bits vs packed sparse index bits.
                            // Values are sent in both encodings, so we only estimate index/mask + id overhead.
                            mask_bits += field_count;
                            sparse_bits += index_bits * changed;
                            sparse_bits += varu32_len(curr.id.raw()) * 8;
                            sparse_bits += varu32_len(component.id.get() as u32) * 8;
                            sparse_bits += varu32_len(changed as u32) * 8;
                        }
                    }
                }

                baseline_next = baseline_iter.next();
                current_next = current_iter.next();
            }
        }
    }

    if mask_bits == 0 {
        return Ok(UpdateEncoding::Masked);
    }

    if sparse_bits <= mask_bits {
        Ok(UpdateEncoding::Sparse)
    } else {
        Ok(UpdateEncoding::Masked)
    }
}

fn varu32_len(value: u32) -> usize {
    if value < (1 << 7) {
        1
    } else if value < (1 << 14) {
        2
    } else if value < (1 << 21) {
        3
    } else if value < (1 << 28) {
        4
    } else {
        5
    }
}

fn ensure_entity_ids_sorted(ids: &[EntityId]) -> CodecResult<()> {
    let mut prev: Option<u32> = None;
    for id in ids {
        let raw = id.raw();
        if let Some(prev_id) = prev {
            if raw <= prev_id {
                return Err(CodecError::InvalidEntityOrder {
                    previous: prev_id,
                    current: raw,
                });
            }
        }
        prev = Some(raw);
    }
    Ok(())
}

fn encode_destroy_body_from_list(
    destroys: &[EntityId],
    limits: &CodecLimits,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if destroys.len() > limits.max_entities_destroy {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesDestroy,
            limit: limits.max_entities_destroy,
            actual: destroys.len(),
        });
    }
    writer.align_to_byte()?;
    writer.write_varu32(destroys.len() as u32)?;
    for id in destroys {
        writer.align_to_byte()?;
        writer.write_u32_aligned(id.raw())?;
    }
    writer.align_to_byte()?;
    Ok(())
}

fn encode_create_body_from_list(
    schema: &schema::Schema,
    creates: &[EntitySnapshot],
    limits: &CodecLimits,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if creates.len() > limits.max_entities_create {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesCreate,
            limit: limits.max_entities_create,
            actual: creates.len(),
        });
    }
    writer.align_to_byte()?;
    writer.write_varu32(creates.len() as u32)?;
    for entity in creates {
        write_create_entity(schema, entity, limits, writer)?;
    }
    writer.align_to_byte()?;
    Ok(())
}

fn validate_updates_for_encoding(
    schema: &schema::Schema,
    updates: &[DeltaUpdateEntity],
    limits: &CodecLimits,
    lookup: &ComponentLookup,
) -> CodecResult<()> {
    let mut prev: Option<u32> = None;
    for entity_update in updates {
        let id = entity_update.id.raw();
        if let Some(prev_id) = prev {
            if id <= prev_id {
                return Err(CodecError::InvalidEntityOrder {
                    previous: prev_id,
                    current: id,
                });
            }
        }
        prev = Some(id);
        if entity_update.components.len() > limits.max_components_per_entity {
            return Err(CodecError::LimitsExceeded {
                kind: LimitKind::ComponentsPerEntity,
                limit: limits.max_components_per_entity,
                actual: entity_update.components.len(),
            });
        }
        for component_update in &entity_update.components {
            let component = lookup.component(schema, component_update.id)?;
            if component_update.fields.is_empty() {
                return Err(CodecError::InvalidMask {
                    kind: MaskKind::FieldMask {
                        component: component_update.id,
                    },
                    reason: MaskReason::EmptyFieldMask {
                        component: component_update.id,
                    },
                });
            }
            if component_update.fields.len() > limits.max_fields_per_component {
                return Err(CodecError::LimitsExceeded {
                    kind: LimitKind::FieldsPerComponent,
                    limit: limits.max_fields_per_component,
                    actual: component_update.fields.len(),
                });
            }
            let max_index = component.fields.len().saturating_sub(1);
            for (field_idx, _value) in &component_update.fields {
                if *field_idx >= component.fields.len() {
                    return Err(CodecError::InvalidMask {
                        kind: MaskKind::FieldMask {
                            component: component_update.id,
                        },
                        reason: MaskReason::InvalidFieldIndex {
                            field_index: *field_idx,
                            max: max_index,
                        },
                    });
                }
            }
        }
    }
    Ok(())
}

fn encode_update_body_sparse_packed_from_updates(
    schema: &schema::Schema,
    updates: &[DeltaUpdateEntity],
    limits: &CodecLimits,
    lookup: &ComponentLookup,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if updates.len() > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: updates.len(),
        });
    }
    let entry_count: usize = updates.iter().map(|entity| entity.components.len()).sum();
    let entry_limit = limits
        .max_entities_update
        .saturating_mul(limits.max_components_per_entity);
    if entry_count > entry_limit {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: entry_limit,
            actual: entry_count,
        });
    }
    writer.align_to_byte()?;
    writer.write_varu32(entry_count as u32)?;
    for entity_update in updates {
        let entity_id = entity_update.id.raw();
        for component_update in &entity_update.components {
            let component = lookup.component(schema, component_update.id)?;
            writer.align_to_byte()?;
            writer.write_varu32(entity_id)?;
            writer.write_varu32(component.id.get() as u32)?;
            writer.write_varu32(component_update.fields.len() as u32)?;
            let index_bits = lookup.index_bits(component.id);
            for (field_idx, value) in &component_update.fields {
                if index_bits > 0 {
                    writer.write_bits(*field_idx as u64, index_bits)?;
                }
                write_field_value_sparse(
                    component.id,
                    component.fields[*field_idx],
                    *value,
                    writer,
                )?;
            }
        }
    }
    writer.align_to_byte()?;
    Ok(())
}

struct ComponentLookup {
    index: Vec<Option<usize>>,
    index_bits: Vec<u8>,
}

impl ComponentLookup {
    fn component<'a>(
        &self,
        schema: &'a schema::Schema,
        id: ComponentId,
    ) -> CodecResult<&'a ComponentDef> {
        let idx = id.get() as usize;
        let Some(Some(component_index)) = self.index.get(idx) else {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::ComponentMask,
                reason: MaskReason::UnknownComponent { component: id },
            });
        };
        Ok(&schema.components[*component_index])
    }

    fn index_bits(&self, id: ComponentId) -> u8 {
        let idx = id.get() as usize;
        let Some(bits) = self.index_bits.get(idx).copied() else {
            return 0;
        };
        bits
    }
}

fn build_component_lookup(schema: &schema::Schema) -> ComponentLookup {
    let max_id = schema
        .components
        .iter()
        .map(|component| component.id.get() as usize)
        .max()
        .unwrap_or(0);
    let mut index = vec![None; max_id + 1];
    let mut index_bits = vec![0u8; max_id + 1];
    for (component_index, component) in schema.components.iter().enumerate() {
        let id = component.id.get() as usize;
        index[id] = Some(component_index);
        let bits = required_bits(component.fields.len().saturating_sub(1) as u64);
        index_bits[id] = bits;
    }
    ComponentLookup { index, index_bits }
}

fn encode_destroy_body(
    baseline: &Snapshot,
    current: &Snapshot,
    destroy_count: usize,
    limits: &CodecLimits,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if destroy_count > limits.max_entities_destroy {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesDestroy,
            limit: limits.max_entities_destroy,
            actual: destroy_count,
        });
    }

    writer.align_to_byte()?;
    writer.write_varu32(destroy_count as u32)?;

    let mut i = 0usize;
    let mut j = 0usize;
    while i < baseline.entities.len() || j < current.entities.len() {
        let base = baseline.entities.get(i);
        let curr = current.entities.get(j);
        match (base, curr) {
            (Some(b), Some(c)) => {
                if b.id.raw() < c.id.raw() {
                    writer.align_to_byte()?;
                    writer.write_u32_aligned(b.id.raw())?;
                    i += 1;
                } else if b.id.raw() > c.id.raw() {
                    j += 1;
                } else {
                    i += 1;
                    j += 1;
                }
            }
            (Some(b), None) => {
                writer.align_to_byte()?;
                writer.write_u32_aligned(b.id.raw())?;
                i += 1;
            }
            (None, Some(_)) => {
                j += 1;
            }
            (None, None) => break,
        }
    }

    writer.align_to_byte()?;
    Ok(())
}

fn encode_create_body(
    schema: &schema::Schema,
    baseline: &Snapshot,
    current: &Snapshot,
    create_count: usize,
    limits: &CodecLimits,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if create_count > limits.max_entities_create {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesCreate,
            limit: limits.max_entities_create,
            actual: create_count,
        });
    }

    writer.align_to_byte()?;
    writer.write_varu32(create_count as u32)?;

    let mut i = 0usize;
    let mut j = 0usize;
    while i < baseline.entities.len() || j < current.entities.len() {
        let base = baseline.entities.get(i);
        let curr = current.entities.get(j);
        match (base, curr) {
            (Some(b), Some(c)) => {
                if b.id.raw() < c.id.raw() {
                    i += 1;
                } else if b.id.raw() > c.id.raw() {
                    write_create_entity(schema, c, limits, writer)?;
                    j += 1;
                } else {
                    i += 1;
                    j += 1;
                }
            }
            (Some(_), None) => {
                i += 1;
            }
            (None, Some(c)) => {
                write_create_entity(schema, c, limits, writer)?;
                j += 1;
            }
            (None, None) => break,
        }
    }

    writer.align_to_byte()?;
    Ok(())
}

fn encode_update_body_masked(
    schema: &schema::Schema,
    baseline: &Snapshot,
    current: &Snapshot,
    update_count: usize,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if update_count > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: update_count,
        });
    }

    writer.align_to_byte()?;
    writer.write_varu32(update_count as u32)?;

    let mut i = 0usize;
    let mut j = 0usize;
    while i < baseline.entities.len() || j < current.entities.len() {
        let base = baseline.entities.get(i);
        let curr = current.entities.get(j);
        match (base, curr) {
            (Some(b), Some(c)) => {
                if b.id.raw() < c.id.raw() {
                    i += 1;
                } else if b.id.raw() > c.id.raw() {
                    j += 1;
                } else {
                    if entity_has_updates(schema, b, c, limits)? {
                        writer.align_to_byte()?;
                        writer.write_u32_aligned(c.id.raw())?;
                        ensure_component_presence_matches(schema, b, c)?;
                        write_update_components(schema, b, c, limits, scratch, writer)?;
                    }
                    i += 1;
                    j += 1;
                }
            }
            (Some(_), None) => i += 1,
            (None, Some(_)) => j += 1,
            (None, None) => break,
        }
    }

    writer.align_to_byte()?;
    Ok(())
}

#[allow(dead_code)]
fn encode_update_body_sparse_varint(
    schema: &schema::Schema,
    baseline: &Snapshot,
    current: &Snapshot,
    update_count: usize,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if update_count > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: update_count,
        });
    }

    let entry_count = count_sparse_update_entries(schema, baseline, current, limits, scratch)?;
    let entry_limit = limits
        .max_entities_update
        .saturating_mul(limits.max_components_per_entity);
    if entry_count > entry_limit {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: entry_limit,
            actual: entry_count,
        });
    }

    writer.align_to_byte()?;
    writer.write_varu32(entry_count as u32)?;

    let mut baseline_iter = baseline.entities.iter();
    let mut current_iter = current.entities.iter();
    let mut baseline_next = baseline_iter.next();
    let mut current_next = current_iter.next();
    let component_count = schema.components.len();

    while let (Some(base), Some(curr)) = (baseline_next, current_next) {
        match base.id.cmp(&curr.id) {
            Ordering::Less => {
                baseline_next = baseline_iter.next();
            }
            Ordering::Greater => {
                current_next = current_iter.next();
            }
            Ordering::Equal => {
                if entity_has_updates(schema, base, curr, limits)? {
                    for component in &schema.components {
                        let base_component = find_component(base, component.id);
                        let curr_component = find_component(curr, component.id);
                        if base_component.is_some() != curr_component.is_some() {
                            return Err(CodecError::InvalidMask {
                                kind: MaskKind::ComponentMask,
                                reason: MaskReason::ComponentPresenceMismatch {
                                    component: component.id,
                                },
                            });
                        }
                        if let (Some(base_component), Some(curr_component)) =
                            (base_component, curr_component)
                        {
                            if base_component.fields.len() != component.fields.len()
                                || curr_component.fields.len() != component.fields.len()
                            {
                                return Err(CodecError::InvalidMask {
                                    kind: MaskKind::FieldMask {
                                        component: component.id,
                                    },
                                    reason: MaskReason::FieldCountMismatch {
                                        expected: component.fields.len(),
                                        actual: base_component
                                            .fields
                                            .len()
                                            .max(curr_component.fields.len()),
                                    },
                                });
                            }
                            if component.fields.len() > limits.max_fields_per_component {
                                return Err(CodecError::LimitsExceeded {
                                    kind: LimitKind::FieldsPerComponent,
                                    limit: limits.max_fields_per_component,
                                    actual: component.fields.len(),
                                });
                            }
                            let (_, field_mask) = scratch.component_and_field_masks_mut(
                                component_count,
                                component.fields.len(),
                            );
                            let field_mask = compute_field_mask_into(
                                component,
                                base_component,
                                curr_component,
                                field_mask,
                            )?;
                            let changed_fields = field_mask.iter().filter(|bit| **bit).count();
                            if changed_fields > 0 {
                                writer.align_to_byte()?;
                                writer.write_u32_aligned(curr.id.raw())?;
                                writer.write_u16_aligned(component.id.get())?;
                                writer.write_varu32(changed_fields as u32)?;
                                for (idx, field) in component.fields.iter().enumerate() {
                                    if field_mask[idx] {
                                        writer.align_to_byte()?;
                                        writer.write_varu32(idx as u32)?;
                                        write_field_value_sparse(
                                            component.id,
                                            *field,
                                            curr_component.fields[idx],
                                            writer,
                                        )?;
                                    }
                                }
                            }
                        }
                    }
                }
                baseline_next = baseline_iter.next();
                current_next = current_iter.next();
            }
        }
    }

    writer.align_to_byte()?;
    Ok(())
}

fn encode_update_body_sparse_packed(
    schema: &schema::Schema,
    baseline: &Snapshot,
    current: &Snapshot,
    update_count: usize,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if update_count > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: update_count,
        });
    }

    let entry_count = count_sparse_update_entries(schema, baseline, current, limits, scratch)?;
    let entry_limit = limits
        .max_entities_update
        .saturating_mul(limits.max_components_per_entity);
    if entry_count > entry_limit {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: entry_limit,
            actual: entry_count,
        });
    }

    writer.align_to_byte()?;
    writer.write_varu32(entry_count as u32)?;

    let mut baseline_iter = baseline.entities.iter();
    let mut current_iter = current.entities.iter();
    let mut baseline_next = baseline_iter.next();
    let mut current_next = current_iter.next();
    let component_count = schema.components.len();

    while let (Some(base), Some(curr)) = (baseline_next, current_next) {
        match base.id.cmp(&curr.id) {
            Ordering::Less => {
                baseline_next = baseline_iter.next();
            }
            Ordering::Greater => {
                current_next = current_iter.next();
            }
            Ordering::Equal => {
                if entity_has_updates(schema, base, curr, limits)? {
                    for component in &schema.components {
                        let base_component = find_component(base, component.id);
                        let curr_component = find_component(curr, component.id);
                        if base_component.is_some() != curr_component.is_some() {
                            return Err(CodecError::InvalidMask {
                                kind: MaskKind::ComponentMask,
                                reason: MaskReason::ComponentPresenceMismatch {
                                    component: component.id,
                                },
                            });
                        }
                        if let (Some(base_component), Some(curr_component)) =
                            (base_component, curr_component)
                        {
                            if base_component.fields.len() != component.fields.len()
                                || curr_component.fields.len() != component.fields.len()
                            {
                                return Err(CodecError::InvalidMask {
                                    kind: MaskKind::FieldMask {
                                        component: component.id,
                                    },
                                    reason: MaskReason::FieldCountMismatch {
                                        expected: component.fields.len(),
                                        actual: base_component
                                            .fields
                                            .len()
                                            .max(curr_component.fields.len()),
                                    },
                                });
                            }
                            if component.fields.len() > limits.max_fields_per_component {
                                return Err(CodecError::LimitsExceeded {
                                    kind: LimitKind::FieldsPerComponent,
                                    limit: limits.max_fields_per_component,
                                    actual: component.fields.len(),
                                });
                            }
                            let (_, field_mask) = scratch.component_and_field_masks_mut(
                                component_count,
                                component.fields.len(),
                            );
                            let field_mask = compute_field_mask_into(
                                component,
                                base_component,
                                curr_component,
                                field_mask,
                            )?;
                            let changed_fields = field_mask.iter().filter(|bit| **bit).count();
                            if changed_fields > 0 {
                                writer.align_to_byte()?;
                                writer.write_varu32(curr.id.raw())?;
                                writer.write_varu32(component.id.get() as u32)?;
                                writer.write_varu32(changed_fields as u32)?;
                                let index_bits =
                                    required_bits(component.fields.len().saturating_sub(1) as u64);
                                for (idx, field) in component.fields.iter().enumerate() {
                                    if field_mask[idx] {
                                        if index_bits > 0 {
                                            writer.write_bits(idx as u64, index_bits)?;
                                        }
                                        write_field_value_sparse(
                                            component.id,
                                            *field,
                                            curr_component.fields[idx],
                                            writer,
                                        )?;
                                    }
                                }
                            }
                        }
                    }
                }
                baseline_next = baseline_iter.next();
                current_next = current_iter.next();
            }
        }
    }

    writer.align_to_byte()?;
    Ok(())
}

fn count_sparse_update_entries(
    schema: &schema::Schema,
    baseline: &Snapshot,
    current: &Snapshot,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
) -> CodecResult<usize> {
    let component_count = schema.components.len();
    let mut count = 0usize;
    let mut baseline_iter = baseline.entities.iter();
    let mut current_iter = current.entities.iter();
    let mut baseline_next = baseline_iter.next();
    let mut current_next = current_iter.next();

    while let (Some(base), Some(curr)) = (baseline_next, current_next) {
        match base.id.cmp(&curr.id) {
            Ordering::Less => baseline_next = baseline_iter.next(),
            Ordering::Greater => current_next = current_iter.next(),
            Ordering::Equal => {
                if entity_has_updates(schema, base, curr, limits)? {
                    for component in &schema.components {
                        let base_component = find_component(base, component.id);
                        let curr_component = find_component(curr, component.id);
                        if base_component.is_some() != curr_component.is_some() {
                            return Err(CodecError::InvalidMask {
                                kind: MaskKind::ComponentMask,
                                reason: MaskReason::ComponentPresenceMismatch {
                                    component: component.id,
                                },
                            });
                        }
                        if let (Some(base_component), Some(curr_component)) =
                            (base_component, curr_component)
                        {
                            if base_component.fields.len() != component.fields.len()
                                || curr_component.fields.len() != component.fields.len()
                            {
                                return Err(CodecError::InvalidMask {
                                    kind: MaskKind::FieldMask {
                                        component: component.id,
                                    },
                                    reason: MaskReason::FieldCountMismatch {
                                        expected: component.fields.len(),
                                        actual: base_component
                                            .fields
                                            .len()
                                            .max(curr_component.fields.len()),
                                    },
                                });
                            }
                            if component.fields.len() > limits.max_fields_per_component {
                                return Err(CodecError::LimitsExceeded {
                                    kind: LimitKind::FieldsPerComponent,
                                    limit: limits.max_fields_per_component,
                                    actual: component.fields.len(),
                                });
                            }
                            let (_, field_mask) = scratch.component_and_field_masks_mut(
                                component_count,
                                component.fields.len(),
                            );
                            let field_mask = compute_field_mask_into(
                                component,
                                base_component,
                                curr_component,
                                field_mask,
                            )?;
                            if field_mask.iter().any(|bit| *bit) {
                                count += 1;
                            }
                        }
                    }
                }
                baseline_next = baseline_iter.next();
                current_next = current_iter.next();
            }
        }
    }

    Ok(count)
}

fn write_create_entity(
    schema: &schema::Schema,
    entity: &EntitySnapshot,
    limits: &CodecLimits,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    writer.align_to_byte()?;
    writer.write_u32_aligned(entity.id.raw())?;
    ensure_known_components(schema, entity)?;
    write_component_mask(schema, entity, writer)?;
    for component in schema.components.iter() {
        if let Some(snapshot) = find_component(entity, component.id) {
            write_full_component(component, snapshot, limits, writer)?;
        }
    }
    Ok(())
}

fn decode_destroy_section(body: &[u8], limits: &CodecLimits) -> CodecResult<Vec<EntityId>> {
    if body.len() > limits.max_section_bytes {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::SectionBytes,
            limit: limits.max_section_bytes,
            actual: body.len(),
        });
    }

    let mut reader = BitReader::new(body);
    reader.align_to_byte()?;
    let count = reader.read_varu32()? as usize;
    if count > limits.max_entities_destroy {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesDestroy,
            limit: limits.max_entities_destroy,
            actual: count,
        });
    }

    let mut ids = Vec::with_capacity(count);
    let mut prev: Option<u32> = None;
    for _ in 0..count {
        reader.align_to_byte()?;
        let id = reader.read_u32_aligned()?;
        if let Some(prev_id) = prev {
            if id <= prev_id {
                return Err(CodecError::InvalidEntityOrder {
                    previous: prev_id,
                    current: id,
                });
            }
        }
        prev = Some(id);
        ids.push(EntityId::new(id));
    }
    reader.align_to_byte()?;
    if reader.bits_remaining() != 0 {
        return Err(CodecError::TrailingSectionData {
            section: SectionTag::EntityDestroy,
            remaining_bits: reader.bits_remaining(),
        });
    }
    Ok(ids)
}

fn decode_create_section(
    schema: &schema::Schema,
    body: &[u8],
    limits: &CodecLimits,
) -> CodecResult<Vec<EntitySnapshot>> {
    if body.len() > limits.max_section_bytes {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::SectionBytes,
            limit: limits.max_section_bytes,
            actual: body.len(),
        });
    }

    let mut reader = BitReader::new(body);
    reader.align_to_byte()?;
    let count = reader.read_varu32()? as usize;
    if count > limits.max_entities_create {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesCreate,
            limit: limits.max_entities_create,
            actual: count,
        });
    }

    let mut entities = Vec::with_capacity(count);
    let mut prev: Option<u32> = None;
    for _ in 0..count {
        reader.align_to_byte()?;
        let id = reader.read_u32_aligned()?;
        if let Some(prev_id) = prev {
            if id <= prev_id {
                return Err(CodecError::InvalidEntityOrder {
                    previous: prev_id,
                    current: id,
                });
            }
        }
        prev = Some(id);

        let component_mask = read_mask(
            &mut reader,
            schema.components.len(),
            MaskKind::ComponentMask,
        )?;

        let mut components = Vec::new();
        for (idx, component) in schema.components.iter().enumerate() {
            if component_mask[idx] {
                let fields = decode_full_component(component, &mut reader, limits)?;
                components.push(ComponentSnapshot {
                    id: component.id,
                    fields,
                });
            }
        }

        let entity = EntitySnapshot {
            id: EntityId::new(id),
            components,
        };
        ensure_known_components(schema, &entity)?;
        entities.push(entity);
    }

    reader.align_to_byte()?;
    if reader.bits_remaining() != 0 {
        return Err(CodecError::TrailingSectionData {
            section: SectionTag::EntityCreate,
            remaining_bits: reader.bits_remaining(),
        });
    }
    Ok(entities)
}

fn decode_update_section_masked(
    schema: &schema::Schema,
    body: &[u8],
    limits: &CodecLimits,
) -> CodecResult<Vec<DeltaUpdateEntity>> {
    if body.len() > limits.max_section_bytes {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::SectionBytes,
            limit: limits.max_section_bytes,
            actual: body.len(),
        });
    }

    let mut reader = BitReader::new(body);
    reader.align_to_byte()?;
    let count = reader.read_varu32()? as usize;
    if count > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: count,
        });
    }

    let mut updates = Vec::with_capacity(count);
    let mut prev: Option<u32> = None;
    for _ in 0..count {
        reader.align_to_byte()?;
        let id = reader.read_u32_aligned()?;
        if let Some(prev_id) = prev {
            if id <= prev_id {
                return Err(CodecError::InvalidEntityOrder {
                    previous: prev_id,
                    current: id,
                });
            }
        }
        prev = Some(id);

        let component_mask = read_mask(
            &mut reader,
            schema.components.len(),
            MaskKind::ComponentMask,
        )?;
        let mut components = Vec::new();
        for (idx, component) in schema.components.iter().enumerate() {
            if component_mask[idx] {
                let fields = decode_update_component(component, &mut reader, limits)?;
                components.push(DeltaUpdateComponent {
                    id: component.id,
                    fields,
                });
            }
        }

        updates.push(DeltaUpdateEntity {
            id: EntityId::new(id),
            components,
        });
    }

    reader.align_to_byte()?;
    if reader.bits_remaining() != 0 {
        return Err(CodecError::TrailingSectionData {
            section: SectionTag::EntityUpdate,
            remaining_bits: reader.bits_remaining(),
        });
    }
    Ok(updates)
}

fn decode_update_section_sparse_varint(
    schema: &schema::Schema,
    body: &[u8],
    limits: &CodecLimits,
) -> CodecResult<Vec<DeltaUpdateEntity>> {
    if body.len() > limits.max_section_bytes {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::SectionBytes,
            limit: limits.max_section_bytes,
            actual: body.len(),
        });
    }

    let mut reader = BitReader::new(body);
    reader.align_to_byte()?;
    let entry_count = reader.read_varu32()? as usize;
    let entry_limit = limits
        .max_entities_update
        .saturating_mul(limits.max_components_per_entity);
    if entry_count > entry_limit {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: entry_limit,
            actual: entry_count,
        });
    }

    let mut updates: Vec<DeltaUpdateEntity> = Vec::new();
    let mut prev_entity: Option<u32> = None;
    let mut prev_component: Option<u16> = None;
    for _ in 0..entry_count {
        reader.align_to_byte()?;
        let entity_id = reader.read_u32_aligned()?;
        let component_raw = reader.read_u16_aligned()?;
        let component_id = ComponentId::new(component_raw).ok_or(CodecError::InvalidMask {
            kind: MaskKind::ComponentMask,
            reason: MaskReason::InvalidComponentId { raw: component_raw },
        })?;
        if let Some(prev) = prev_entity {
            if entity_id < prev {
                return Err(CodecError::InvalidEntityOrder {
                    previous: prev,
                    current: entity_id,
                });
            }
            if entity_id == prev {
                if let Some(prev_component) = prev_component {
                    if component_raw <= prev_component {
                        return Err(CodecError::InvalidEntityOrder {
                            previous: prev,
                            current: entity_id,
                        });
                    }
                }
            }
        }
        prev_entity = Some(entity_id);
        prev_component = Some(component_raw);

        let component = schema
            .components
            .iter()
            .find(|component| component.id == component_id)
            .ok_or(CodecError::InvalidMask {
                kind: MaskKind::ComponentMask,
                reason: MaskReason::UnknownComponent {
                    component: component_id,
                },
            })?;

        let field_count = reader.read_varu32()? as usize;
        if field_count == 0 {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::FieldMask {
                    component: component.id,
                },
                reason: MaskReason::EmptyFieldMask {
                    component: component.id,
                },
            });
        }
        if field_count > limits.max_fields_per_component {
            return Err(CodecError::LimitsExceeded {
                kind: LimitKind::FieldsPerComponent,
                limit: limits.max_fields_per_component,
                actual: field_count,
            });
        }
        if field_count > component.fields.len() {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::FieldMask {
                    component: component.id,
                },
                reason: MaskReason::FieldCountMismatch {
                    expected: component.fields.len(),
                    actual: field_count,
                },
            });
        }

        let mut fields = Vec::with_capacity(field_count);
        let mut prev_index: Option<usize> = None;
        for _ in 0..field_count {
            reader.align_to_byte()?;
            let field_index = reader.read_varu32()? as usize;
            if field_index >= component.fields.len() {
                return Err(CodecError::InvalidMask {
                    kind: MaskKind::FieldMask {
                        component: component.id,
                    },
                    reason: MaskReason::InvalidFieldIndex {
                        field_index,
                        max: component.fields.len(),
                    },
                });
            }
            if let Some(prev_index) = prev_index {
                if field_index <= prev_index {
                    return Err(CodecError::InvalidMask {
                        kind: MaskKind::FieldMask {
                            component: component.id,
                        },
                        reason: MaskReason::InvalidFieldIndex {
                            field_index,
                            max: component.fields.len(),
                        },
                    });
                }
            }
            prev_index = Some(field_index);
            let field = component.fields[field_index];
            let value = read_field_value_sparse(component.id, field, &mut reader)?;
            fields.push((field_index, value));
        }

        if let Some(last) = updates.last_mut() {
            if last.id.raw() == entity_id {
                last.components.push(DeltaUpdateComponent {
                    id: component.id,
                    fields,
                });
                continue;
            }
        }

        updates.push(DeltaUpdateEntity {
            id: EntityId::new(entity_id),
            components: vec![DeltaUpdateComponent {
                id: component.id,
                fields,
            }],
        });
    }

    reader.align_to_byte()?;
    if reader.bits_remaining() != 0 {
        return Err(CodecError::TrailingSectionData {
            section: SectionTag::EntityUpdateSparse,
            remaining_bits: reader.bits_remaining(),
        });
    }

    let unique_entities = updates.len();
    if unique_entities > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: unique_entities,
        });
    }

    Ok(updates)
}

fn decode_update_section_sparse_packed(
    schema: &schema::Schema,
    body: &[u8],
    limits: &CodecLimits,
) -> CodecResult<Vec<DeltaUpdateEntity>> {
    if body.len() > limits.max_section_bytes {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::SectionBytes,
            limit: limits.max_section_bytes,
            actual: body.len(),
        });
    }

    let mut reader = BitReader::new(body);
    reader.align_to_byte()?;
    let entry_count = reader.read_varu32()? as usize;
    let entry_limit = limits
        .max_entities_update
        .saturating_mul(limits.max_components_per_entity);
    if entry_count > entry_limit {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: entry_limit,
            actual: entry_count,
        });
    }

    let mut updates: Vec<DeltaUpdateEntity> = Vec::new();
    let mut prev_entity: Option<u32> = None;
    let mut prev_component: Option<u16> = None;
    for _ in 0..entry_count {
        reader.align_to_byte()?;
        let entity_id = reader.read_varu32()?;
        let component_raw = reader.read_varu32()?;
        if component_raw > u16::MAX as u32 {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::ComponentMask,
                reason: MaskReason::InvalidComponentId { raw: u16::MAX },
            });
        }
        let component_raw = component_raw as u16;
        let component_id = ComponentId::new(component_raw).ok_or(CodecError::InvalidMask {
            kind: MaskKind::ComponentMask,
            reason: MaskReason::InvalidComponentId { raw: component_raw },
        })?;
        if let Some(prev) = prev_entity {
            if entity_id < prev {
                return Err(CodecError::InvalidEntityOrder {
                    previous: prev,
                    current: entity_id,
                });
            }
            if entity_id == prev {
                if let Some(prev_component) = prev_component {
                    if component_raw <= prev_component {
                        return Err(CodecError::InvalidEntityOrder {
                            previous: prev,
                            current: entity_id,
                        });
                    }
                }
            }
        }
        prev_entity = Some(entity_id);
        prev_component = Some(component_raw);

        let component = schema
            .components
            .iter()
            .find(|component| component.id == component_id)
            .ok_or(CodecError::InvalidMask {
                kind: MaskKind::ComponentMask,
                reason: MaskReason::UnknownComponent {
                    component: component_id,
                },
            })?;

        let field_count = reader.read_varu32()? as usize;
        if field_count == 0 {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::FieldMask {
                    component: component.id,
                },
                reason: MaskReason::EmptyFieldMask {
                    component: component.id,
                },
            });
        }
        if field_count > limits.max_fields_per_component {
            return Err(CodecError::LimitsExceeded {
                kind: LimitKind::FieldsPerComponent,
                limit: limits.max_fields_per_component,
                actual: field_count,
            });
        }
        if field_count > component.fields.len() {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::FieldMask {
                    component: component.id,
                },
                reason: MaskReason::FieldCountMismatch {
                    expected: component.fields.len(),
                    actual: field_count,
                },
            });
        }

        let index_bits = required_bits(component.fields.len().saturating_sub(1) as u64);
        let mut fields = Vec::with_capacity(field_count);
        let mut prev_index: Option<usize> = None;
        for _ in 0..field_count {
            let field_index = if index_bits == 0 {
                0
            } else {
                reader.read_bits(index_bits)? as usize
            };
            if field_index >= component.fields.len() {
                return Err(CodecError::InvalidMask {
                    kind: MaskKind::FieldMask {
                        component: component.id,
                    },
                    reason: MaskReason::InvalidFieldIndex {
                        field_index,
                        max: component.fields.len(),
                    },
                });
            }
            if let Some(prev_index) = prev_index {
                if field_index <= prev_index {
                    return Err(CodecError::InvalidMask {
                        kind: MaskKind::FieldMask {
                            component: component.id,
                        },
                        reason: MaskReason::InvalidFieldIndex {
                            field_index,
                            max: component.fields.len(),
                        },
                    });
                }
            }
            prev_index = Some(field_index);
            let field = component.fields[field_index];
            let value = read_field_value_sparse(component.id, field, &mut reader)?;
            fields.push((field_index, value));
        }

        if let Some(last) = updates.last_mut() {
            if last.id.raw() == entity_id {
                last.components.push(DeltaUpdateComponent {
                    id: component.id,
                    fields,
                });
                continue;
            }
        }

        updates.push(DeltaUpdateEntity {
            id: EntityId::new(entity_id),
            components: vec![DeltaUpdateComponent {
                id: component.id,
                fields,
            }],
        });
    }

    reader.align_to_byte()?;
    if reader.bits_remaining() != 0 {
        return Err(CodecError::TrailingSectionData {
            section: SectionTag::EntityUpdateSparsePacked,
            remaining_bits: reader.bits_remaining(),
        });
    }

    let unique_entities = updates.len();
    if unique_entities > limits.max_entities_update {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::EntitiesUpdate,
            limit: limits.max_entities_update,
            actual: unique_entities,
        });
    }

    Ok(updates)
}

fn decode_delta_sections(
    schema: &schema::Schema,
    packet: &WirePacket<'_>,
    limits: &CodecLimits,
) -> CodecResult<(Vec<EntityId>, Vec<EntitySnapshot>, Vec<DeltaUpdateEntity>)> {
    let mut destroys: Option<Vec<EntityId>> = None;
    let mut creates: Option<Vec<EntitySnapshot>> = None;
    let mut updates_masked: Option<Vec<DeltaUpdateEntity>> = None;
    let mut updates_sparse: Option<Vec<DeltaUpdateEntity>> = None;

    for section in &packet.sections {
        match section.tag {
            SectionTag::EntityDestroy => {
                if destroys.is_some() {
                    return Err(CodecError::DuplicateSection {
                        section: section.tag,
                    });
                }
                destroys = Some(decode_destroy_section(section.body, limits)?);
            }
            SectionTag::EntityCreate => {
                if creates.is_some() {
                    return Err(CodecError::DuplicateSection {
                        section: section.tag,
                    });
                }
                creates = Some(decode_create_section(schema, section.body, limits)?);
            }
            SectionTag::EntityUpdate => {
                if updates_masked.is_some() {
                    return Err(CodecError::DuplicateSection {
                        section: section.tag,
                    });
                }
                updates_masked = Some(decode_update_section_masked(schema, section.body, limits)?);
            }
            SectionTag::EntityUpdateSparse => {
                if updates_sparse.is_some() {
                    return Err(CodecError::DuplicateSection {
                        section: section.tag,
                    });
                }
                updates_sparse = Some(decode_update_section_sparse_varint(
                    schema,
                    section.body,
                    limits,
                )?);
            }
            SectionTag::EntityUpdateSparsePacked => {
                if updates_sparse.is_some() {
                    return Err(CodecError::DuplicateSection {
                        section: section.tag,
                    });
                }
                updates_sparse = Some(decode_update_section_sparse_packed(
                    schema,
                    section.body,
                    limits,
                )?);
            }
            _ => {
                return Err(CodecError::UnexpectedSection {
                    section: section.tag,
                });
            }
        }
    }

    Ok((
        destroys.unwrap_or_default(),
        creates.unwrap_or_default(),
        match (updates_masked, updates_sparse) {
            (Some(_), Some(_)) => return Err(CodecError::DuplicateUpdateEncoding),
            (Some(updates), None) => updates,
            (None, Some(updates)) => updates,
            (None, None) => Vec::new(),
        },
    ))
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct DeltaDecoded {
    pub tick: SnapshotTick,
    pub baseline_tick: SnapshotTick,
    pub destroys: Vec<EntityId>,
    pub creates: Vec<EntitySnapshot>,
    pub updates: Vec<DeltaUpdateEntity>,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct DeltaUpdateEntity {
    pub id: EntityId,
    pub components: Vec<DeltaUpdateComponent>,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct DeltaUpdateComponent {
    pub id: ComponentId,
    pub fields: Vec<(usize, FieldValue)>,
}

fn apply_destroys(
    baseline: &[EntitySnapshot],
    destroys: &[EntityId],
) -> CodecResult<Vec<EntitySnapshot>> {
    let mut result = Vec::with_capacity(baseline.len());
    let mut i = 0usize;
    let mut j = 0usize;
    while i < baseline.len() || j < destroys.len() {
        let base = baseline.get(i);
        let destroy = destroys.get(j);
        match (base, destroy) {
            (Some(b), Some(d)) => {
                if b.id.raw() < d.raw() {
                    result.push(b.clone());
                    i += 1;
                } else if b.id.raw() > d.raw() {
                    return Err(CodecError::EntityNotFound { entity_id: d.raw() });
                } else {
                    i += 1;
                    j += 1;
                }
            }
            (Some(b), None) => {
                result.push(b.clone());
                i += 1;
            }
            (None, Some(d)) => {
                return Err(CodecError::EntityNotFound { entity_id: d.raw() });
            }
            (None, None) => break,
        }
    }
    Ok(result)
}

fn apply_creates(
    baseline: Vec<EntitySnapshot>,
    creates: Vec<EntitySnapshot>,
) -> CodecResult<Vec<EntitySnapshot>> {
    let mut result = Vec::with_capacity(baseline.len() + creates.len());
    let mut i = 0usize;
    let mut j = 0usize;
    while i < baseline.len() || j < creates.len() {
        let base = baseline.get(i);
        let create = creates.get(j);
        match (base, create) {
            (Some(b), Some(c)) => {
                if b.id.raw() < c.id.raw() {
                    result.push(b.clone());
                    i += 1;
                } else if b.id.raw() > c.id.raw() {
                    result.push(c.clone());
                    j += 1;
                } else {
                    return Err(CodecError::EntityAlreadyExists {
                        entity_id: c.id.raw(),
                    });
                }
            }
            (Some(b), None) => {
                result.push(b.clone());
                i += 1;
            }
            (None, Some(c)) => {
                result.push(c.clone());
                j += 1;
            }
            (None, None) => break,
        }
    }
    Ok(result)
}

fn apply_updates(
    entities: &mut [EntitySnapshot],
    updates: &[DeltaUpdateEntity],
) -> CodecResult<()> {
    for update in updates {
        let idx = entities
            .binary_search_by_key(&update.id.raw(), |e| e.id.raw())
            .map_err(|_| CodecError::EntityNotFound {
                entity_id: update.id.raw(),
            })?;
        let entity = &mut entities[idx];
        for component_update in &update.components {
            let component = entity
                .components
                .iter_mut()
                .find(|c| c.id == component_update.id)
                .ok_or_else(|| CodecError::ComponentNotFound {
                    entity_id: update.id.raw(),
                    component_id: component_update.id.get(),
                })?;
            for (field_idx, value) in &component_update.fields {
                if *field_idx >= component.fields.len() {
                    return Err(CodecError::InvalidMask {
                        kind: MaskKind::FieldMask {
                            component: component_update.id,
                        },
                        reason: MaskReason::FieldCountMismatch {
                            expected: component.fields.len(),
                            actual: *field_idx + 1,
                        },
                    });
                }
                component.fields[*field_idx] = *value;
            }
        }
    }
    Ok(())
}

fn ensure_entities_sorted(entities: &[EntitySnapshot]) -> CodecResult<()> {
    let mut prev: Option<u32> = None;
    for entity in entities {
        if let Some(prev_id) = prev {
            if entity.id.raw() <= prev_id {
                return Err(CodecError::InvalidEntityOrder {
                    previous: prev_id,
                    current: entity.id.raw(),
                });
            }
        }
        prev = Some(entity.id.raw());
    }
    Ok(())
}

fn write_component_mask(
    schema: &schema::Schema,
    entity: &EntitySnapshot,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    for component in &schema.components {
        let present = find_component(entity, component.id).is_some();
        writer.write_bit(present)?;
    }
    Ok(())
}

fn write_full_component(
    component: &ComponentDef,
    snapshot: &ComponentSnapshot,
    limits: &CodecLimits,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    if snapshot.fields.len() != component.fields.len() {
        return Err(CodecError::InvalidMask {
            kind: MaskKind::FieldMask {
                component: component.id,
            },
            reason: MaskReason::FieldCountMismatch {
                expected: component.fields.len(),
                actual: snapshot.fields.len(),
            },
        });
    }
    if snapshot.fields.len() > limits.max_fields_per_component {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::FieldsPerComponent,
            limit: limits.max_fields_per_component,
            actual: snapshot.fields.len(),
        });
    }

    for _ in &component.fields {
        writer.write_bit(true)?;
    }
    for (field, value) in component.fields.iter().zip(snapshot.fields.iter()) {
        write_field_value(component.id, *field, *value, writer)?;
    }
    Ok(())
}

fn decode_full_component(
    component: &ComponentDef,
    reader: &mut BitReader<'_>,
    limits: &CodecLimits,
) -> CodecResult<Vec<FieldValue>> {
    if component.fields.len() > limits.max_fields_per_component {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::FieldsPerComponent,
            limit: limits.max_fields_per_component,
            actual: component.fields.len(),
        });
    }

    let mask = read_mask(
        reader,
        component.fields.len(),
        MaskKind::FieldMask {
            component: component.id,
        },
    )?;
    let mut values = Vec::with_capacity(component.fields.len());
    for (idx, field) in component.fields.iter().enumerate() {
        if !mask[idx] {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::FieldMask {
                    component: component.id,
                },
                reason: MaskReason::MissingField { field: field.id },
            });
        }
        values.push(read_field_value(component.id, *field, reader)?);
    }
    Ok(values)
}

fn write_update_components(
    schema: &schema::Schema,
    baseline: &EntitySnapshot,
    current: &EntitySnapshot,
    limits: &CodecLimits,
    scratch: &mut CodecScratch,
    writer: &mut BitWriter<'_>,
) -> CodecResult<()> {
    let component_count = schema.components.len();
    let (component_changed, _) = scratch.component_and_field_masks_mut(component_count, 0);
    component_changed.fill(false);
    for (idx, component) in schema.components.iter().enumerate() {
        let base = find_component(baseline, component.id);
        let curr = find_component(current, component.id);
        if base.is_some() != curr.is_some() {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::ComponentMask,
                reason: MaskReason::ComponentPresenceMismatch {
                    component: component.id,
                },
            });
        }
        if let (Some(base), Some(curr)) = (base, curr) {
            if base.fields.len() != component.fields.len()
                || curr.fields.len() != component.fields.len()
            {
                return Err(CodecError::InvalidMask {
                    kind: MaskKind::FieldMask {
                        component: component.id,
                    },
                    reason: MaskReason::FieldCountMismatch {
                        expected: component.fields.len(),
                        actual: base.fields.len().max(curr.fields.len()),
                    },
                });
            }
            if component.fields.len() > limits.max_fields_per_component {
                return Err(CodecError::LimitsExceeded {
                    kind: LimitKind::FieldsPerComponent,
                    limit: limits.max_fields_per_component,
                    actual: component.fields.len(),
                });
            }
            let (component_changed, field_mask) =
                scratch.component_and_field_masks_mut(component_count, component.fields.len());
            let any_changed = compute_field_mask_into(component, base, curr, field_mask)?
                .iter()
                .any(|b| *b);
            writer.write_bit(any_changed)?;
            if any_changed {
                component_changed[idx] = true;
            }
        } else {
            writer.write_bit(false)?;
        }
    }

    for (idx, component) in schema.components.iter().enumerate() {
        let (base, curr) = match (
            find_component(baseline, component.id),
            find_component(current, component.id),
        ) {
            (Some(base), Some(curr)) => (base, curr),
            _ => continue,
        };
        if component.fields.len() > limits.max_fields_per_component {
            return Err(CodecError::LimitsExceeded {
                kind: LimitKind::FieldsPerComponent,
                limit: limits.max_fields_per_component,
                actual: component.fields.len(),
            });
        }
        let (component_changed, field_mask) =
            scratch.component_and_field_masks_mut(component_count, component.fields.len());
        if component_changed[idx] {
            let field_mask = compute_field_mask_into(component, base, curr, field_mask)?;
            for bit in field_mask {
                writer.write_bit(*bit)?;
            }
            for (((field, _base_val), curr_val), changed) in component
                .fields
                .iter()
                .zip(base.fields.iter())
                .zip(curr.fields.iter())
                .zip(field_mask.iter())
            {
                if *changed {
                    write_field_value(component.id, *field, *curr_val, writer)?;
                }
            }
        }
    }
    Ok(())
}

fn decode_update_component(
    component: &ComponentDef,
    reader: &mut BitReader<'_>,
    limits: &CodecLimits,
) -> CodecResult<Vec<(usize, FieldValue)>> {
    if component.fields.len() > limits.max_fields_per_component {
        return Err(CodecError::LimitsExceeded {
            kind: LimitKind::FieldsPerComponent,
            limit: limits.max_fields_per_component,
            actual: component.fields.len(),
        });
    }
    let mask = read_mask(
        reader,
        component.fields.len(),
        MaskKind::FieldMask {
            component: component.id,
        },
    )?;
    if !mask.iter().any(|b| *b) {
        return Err(CodecError::InvalidMask {
            kind: MaskKind::FieldMask {
                component: component.id,
            },
            reason: MaskReason::EmptyFieldMask {
                component: component.id,
            },
        });
    }
    let mut fields = Vec::new();
    for (idx, field) in component.fields.iter().enumerate() {
        if mask[idx] {
            let value = read_field_value(component.id, *field, reader)?;
            fields.push((idx, value));
        }
    }
    Ok(fields)
}

fn compute_field_mask_into<'a>(
    component: &ComponentDef,
    baseline: &ComponentSnapshot,
    current: &ComponentSnapshot,
    field_mask: &'a mut [bool],
) -> CodecResult<&'a [bool]> {
    for (((field, base_val), curr_val), slot) in component
        .fields
        .iter()
        .zip(baseline.fields.iter())
        .zip(current.fields.iter())
        .zip(field_mask.iter_mut())
    {
        *slot = field_changed(component.id, *field, *base_val, *curr_val)?;
    }
    Ok(field_mask)
}

fn field_changed(
    component_id: ComponentId,
    field: FieldDef,
    baseline: FieldValue,
    current: FieldValue,
) -> CodecResult<bool> {
    match field.change {
        ChangePolicy::Always => field_differs(component_id, field, baseline, current),
        ChangePolicy::Threshold { threshold_q } => {
            field_exceeds_threshold(component_id, field, baseline, current, threshold_q)
        }
    }
}

fn field_differs(
    component_id: ComponentId,
    field: FieldDef,
    baseline: FieldValue,
    current: FieldValue,
) -> CodecResult<bool> {
    match (baseline, current) {
        (FieldValue::Bool(a), FieldValue::Bool(b)) => Ok(a != b),
        (FieldValue::UInt(a), FieldValue::UInt(b)) => Ok(a != b),
        (FieldValue::SInt(a), FieldValue::SInt(b)) => Ok(a != b),
        (FieldValue::VarUInt(a), FieldValue::VarUInt(b)) => Ok(a != b),
        (FieldValue::VarSInt(a), FieldValue::VarSInt(b)) => Ok(a != b),
        (FieldValue::FixedPoint(a), FieldValue::FixedPoint(b)) => Ok(a != b),
        _ => Err(CodecError::InvalidValue {
            component: component_id,
            field: field.id,
            reason: ValueReason::TypeMismatch {
                expected: codec_name(field.codec),
                found: value_name(current),
            },
        }),
    }
}

fn field_exceeds_threshold(
    component_id: ComponentId,
    field: FieldDef,
    baseline: FieldValue,
    current: FieldValue,
    threshold_q: u32,
) -> CodecResult<bool> {
    let threshold_q = threshold_q as u64;
    match (baseline, current) {
        (FieldValue::FixedPoint(a), FieldValue::FixedPoint(b)) => {
            Ok((a - b).unsigned_abs() > threshold_q)
        }
        (FieldValue::UInt(a), FieldValue::UInt(b)) => Ok(a.abs_diff(b) > threshold_q),
        (FieldValue::SInt(a), FieldValue::SInt(b)) => Ok((a - b).unsigned_abs() > threshold_q),
        (FieldValue::VarUInt(a), FieldValue::VarUInt(b)) => Ok(a.abs_diff(b) > threshold_q),
        (FieldValue::VarSInt(a), FieldValue::VarSInt(b)) => {
            Ok((a - b).unsigned_abs() > threshold_q)
        }
        (FieldValue::Bool(a), FieldValue::Bool(b)) => Ok(a != b),
        _ => Err(CodecError::InvalidValue {
            component: component_id,
            field: field.id,
            reason: ValueReason::TypeMismatch {
                expected: codec_name(field.codec),
                found: value_name(current),
            },
        }),
    }
}

fn entity_has_updates(
    schema: &schema::Schema,
    baseline: &EntitySnapshot,
    current: &EntitySnapshot,
    limits: &CodecLimits,
) -> CodecResult<bool> {
    ensure_component_presence_matches(schema, baseline, current)?;
    for component in &schema.components {
        let base = find_component(baseline, component.id);
        let curr = find_component(current, component.id);
        if let (Some(base), Some(curr)) = (base, curr) {
            if base.fields.len() != component.fields.len()
                || curr.fields.len() != component.fields.len()
            {
                return Err(CodecError::InvalidMask {
                    kind: MaskKind::FieldMask {
                        component: component.id,
                    },
                    reason: MaskReason::FieldCountMismatch {
                        expected: component.fields.len(),
                        actual: base.fields.len().max(curr.fields.len()),
                    },
                });
            }
            if component.fields.len() > limits.max_fields_per_component {
                return Err(CodecError::LimitsExceeded {
                    kind: LimitKind::FieldsPerComponent,
                    limit: limits.max_fields_per_component,
                    actual: component.fields.len(),
                });
            }
            for ((field, base_val), curr_val) in component
                .fields
                .iter()
                .zip(base.fields.iter())
                .zip(curr.fields.iter())
            {
                if field_changed(component.id, *field, *base_val, *curr_val)? {
                    return Ok(true);
                }
            }
        }
    }
    Ok(false)
}

fn ensure_component_presence_matches(
    schema: &schema::Schema,
    baseline: &EntitySnapshot,
    current: &EntitySnapshot,
) -> CodecResult<()> {
    // In this version, component presence is stable across an entity's lifetime.
    for component in &schema.components {
        let base = find_component(baseline, component.id).is_some();
        let curr = find_component(current, component.id).is_some();
        if base != curr {
            return Err(CodecError::InvalidMask {
                kind: MaskKind::ComponentMask,
                reason: MaskReason::ComponentPresenceMismatch {
                    component: component.id,
                },
            });
        }
    }
    Ok(())
}

fn find_component(entity: &EntitySnapshot, id: ComponentId) -> Option<&ComponentSnapshot> {
    entity.components.iter().find(|c| c.id == id)
}

fn codec_name(codec: schema::FieldCodec) -> &'static str {
    match codec {
        schema::FieldCodec::Bool => "bool",
        schema::FieldCodec::UInt { .. } => "uint",
        schema::FieldCodec::SInt { .. } => "sint",
        schema::FieldCodec::VarUInt => "varuint",
        schema::FieldCodec::VarSInt => "varsint",
        schema::FieldCodec::FixedPoint(_) => "fixed-point",
    }
}

fn value_name(value: FieldValue) -> &'static str {
    match value {
        FieldValue::Bool(_) => "bool",
        FieldValue::UInt(_) => "uint",
        FieldValue::SInt(_) => "sint",
        FieldValue::VarUInt(_) => "varuint",
        FieldValue::VarSInt(_) => "varsint",
        FieldValue::FixedPoint(_) => "fixed-point",
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use schema::{ComponentDef, FieldCodec, FieldDef, FieldId, Schema};

    fn schema_one_bool() -> Schema {
        let component = ComponentDef::new(ComponentId::new(1).unwrap())
            .field(FieldDef::new(FieldId::new(1).unwrap(), FieldCodec::bool()));
        Schema::new(vec![component]).unwrap()
    }

    fn schema_uint_threshold(threshold_q: u32) -> Schema {
        let field = FieldDef::new(FieldId::new(1).unwrap(), FieldCodec::uint(8))
            .change(ChangePolicy::Threshold { threshold_q });
        let component = ComponentDef::new(ComponentId::new(1).unwrap()).field(field);
        Schema::new(vec![component]).unwrap()
    }

    fn schema_two_components() -> Schema {
        let c1 = ComponentDef::new(ComponentId::new(1).unwrap())
            .field(FieldDef::new(FieldId::new(1).unwrap(), FieldCodec::bool()));
        let c2 = ComponentDef::new(ComponentId::new(2).unwrap())
            .field(FieldDef::new(FieldId::new(1).unwrap(), FieldCodec::bool()));
        Schema::new(vec![c1, c2]).unwrap()
    }

    fn baseline_snapshot() -> Snapshot {
        Snapshot {
            tick: SnapshotTick::new(10),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![ComponentSnapshot {
                    id: ComponentId::new(1).unwrap(),
                    fields: vec![FieldValue::Bool(false)],
                }],
            }],
        }
    }

    #[test]
    fn no_op_delta_is_empty() {
        let schema = schema_one_bool();
        let baseline = baseline_snapshot();
        let current = baseline.clone();
        let mut buf = [0u8; 128];
        let bytes = encode_delta_snapshot(
            &schema,
            SnapshotTick::new(11),
            baseline.tick,
            &baseline,
            &current,
            &CodecLimits::for_testing(),
            &mut buf,
        )
        .unwrap();
        let header =
            wire::PacketHeader::delta_snapshot(schema_hash(&schema), 11, baseline.tick.raw(), 0);
        let mut expected = [0u8; wire::HEADER_SIZE];
        encode_header(&header, &mut expected).unwrap();
        assert_eq!(&buf[..bytes], expected.as_slice());
    }

    #[test]
    fn delta_roundtrip_single_update() {
        let schema = schema_one_bool();
        let baseline = baseline_snapshot();
        let current = Snapshot {
            tick: SnapshotTick::new(11),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![ComponentSnapshot {
                    id: ComponentId::new(1).unwrap(),
                    fields: vec![FieldValue::Bool(true)],
                }],
            }],
        };

        let mut buf = [0u8; 128];
        let bytes = encode_delta_snapshot(
            &schema,
            current.tick,
            baseline.tick,
            &baseline,
            &current,
            &CodecLimits::for_testing(),
            &mut buf,
        )
        .unwrap();
        let applied = apply_delta_snapshot(
            &schema,
            &baseline,
            &buf[..bytes],
            &wire::Limits::for_testing(),
            &CodecLimits::for_testing(),
        )
        .unwrap();
        assert_eq!(applied.entities, current.entities);
    }

    #[test]
    fn delta_roundtrip_reuse_scratch() {
        let schema = schema_one_bool();
        let baseline = baseline_snapshot();
        let current_one = Snapshot {
            tick: SnapshotTick::new(11),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![ComponentSnapshot {
                    id: ComponentId::new(1).unwrap(),
                    fields: vec![FieldValue::Bool(true)],
                }],
            }],
        };
        let current_two = Snapshot {
            tick: SnapshotTick::new(12),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![ComponentSnapshot {
                    id: ComponentId::new(1).unwrap(),
                    fields: vec![FieldValue::Bool(false)],
                }],
            }],
        };

        let mut scratch = CodecScratch::default();
        let mut buf_one = [0u8; 128];
        let mut buf_two = [0u8; 128];

        let bytes_one = encode_delta_snapshot_with_scratch(
            &schema,
            current_one.tick,
            baseline.tick,
            &baseline,
            &current_one,
            &CodecLimits::for_testing(),
            &mut scratch,
            &mut buf_one,
        )
        .unwrap();
        let applied_one = apply_delta_snapshot(
            &schema,
            &baseline,
            &buf_one[..bytes_one],
            &wire::Limits::for_testing(),
            &CodecLimits::for_testing(),
        )
        .unwrap();
        assert_eq!(applied_one.entities, current_one.entities);

        let bytes_two = encode_delta_snapshot_with_scratch(
            &schema,
            current_two.tick,
            baseline.tick,
            &baseline,
            &current_two,
            &CodecLimits::for_testing(),
            &mut scratch,
            &mut buf_two,
        )
        .unwrap();
        let applied_two = apply_delta_snapshot(
            &schema,
            &baseline,
            &buf_two[..bytes_two],
            &wire::Limits::for_testing(),
            &CodecLimits::for_testing(),
        )
        .unwrap();
        assert_eq!(applied_two.entities, current_two.entities);
    }

    #[test]
    fn delta_rejects_both_update_encodings() {
        let schema = schema_one_bool();
        let baseline = baseline_snapshot();
        let update_body = [0u8; 1];
        let mut update_section = [0u8; 8];
        let update_len =
            wire::encode_section(SectionTag::EntityUpdate, &update_body, &mut update_section)
                .unwrap();

        let mut sparse_section = [0u8; 8];
        let sparse_len = wire::encode_section(
            SectionTag::EntityUpdateSparse,
            &update_body,
            &mut sparse_section,
        )
        .unwrap();

        let payload_len = (update_len + sparse_len) as u32;
        let header = wire::PacketHeader::delta_snapshot(
            schema_hash(&schema),
            baseline.tick.raw() + 1,
            baseline.tick.raw(),
            payload_len,
        );
        let mut buf = vec![0u8; wire::HEADER_SIZE + payload_len as usize];
        wire::encode_header(&header, &mut buf[..wire::HEADER_SIZE]).unwrap();
        buf[wire::HEADER_SIZE..wire::HEADER_SIZE + update_len]
            .copy_from_slice(&update_section[..update_len]);
        buf[wire::HEADER_SIZE + update_len..wire::HEADER_SIZE + update_len + sparse_len]
            .copy_from_slice(&sparse_section[..sparse_len]);

        let packet = wire::decode_packet(&buf, &wire::Limits::for_testing()).unwrap();
        let err = decode_delta_packet(&schema, &packet, &CodecLimits::for_testing()).unwrap_err();
        assert!(matches!(err, CodecError::DuplicateUpdateEncoding));

        let err = apply_delta_snapshot_from_packet(
            &schema,
            &baseline,
            &packet,
            &CodecLimits::for_testing(),
        )
        .unwrap_err();
        assert!(matches!(err, CodecError::DuplicateUpdateEncoding));
    }

    #[test]
    fn delta_roundtrip_create_destroy_update() {
        let schema = schema_one_bool();
        let baseline = Snapshot {
            tick: SnapshotTick::new(10),
            entities: vec![
                EntitySnapshot {
                    id: EntityId::new(1),
                    components: vec![ComponentSnapshot {
                        id: ComponentId::new(1).unwrap(),
                        fields: vec![FieldValue::Bool(false)],
                    }],
                },
                EntitySnapshot {
                    id: EntityId::new(2),
                    components: vec![ComponentSnapshot {
                        id: ComponentId::new(1).unwrap(),
                        fields: vec![FieldValue::Bool(false)],
                    }],
                },
            ],
        };
        let current = Snapshot {
            tick: SnapshotTick::new(11),
            entities: vec![
                EntitySnapshot {
                    id: EntityId::new(2),
                    components: vec![ComponentSnapshot {
                        id: ComponentId::new(1).unwrap(),
                        fields: vec![FieldValue::Bool(true)],
                    }],
                },
                EntitySnapshot {
                    id: EntityId::new(3),
                    components: vec![ComponentSnapshot {
                        id: ComponentId::new(1).unwrap(),
                        fields: vec![FieldValue::Bool(true)],
                    }],
                },
            ],
        };

        let mut buf = [0u8; 256];
        let bytes = encode_delta_snapshot(
            &schema,
            current.tick,
            baseline.tick,
            &baseline,
            &current,
            &CodecLimits::for_testing(),
            &mut buf,
        )
        .unwrap();
        let applied = apply_delta_snapshot(
            &schema,
            &baseline,
            &buf[..bytes],
            &wire::Limits::for_testing(),
            &CodecLimits::for_testing(),
        )
        .unwrap();
        assert_eq!(applied.entities, current.entities);
    }

    #[test]
    fn delta_session_header_matches_payload() {
        let schema = schema_one_bool();
        let baseline = baseline_snapshot();
        let current = Snapshot {
            tick: SnapshotTick::new(11),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![ComponentSnapshot {
                    id: ComponentId::new(1).unwrap(),
                    fields: vec![FieldValue::Bool(true)],
                }],
            }],
        };

        let mut full_buf = [0u8; 128];
        let full_bytes = encode_delta_snapshot_for_client(
            &schema,
            current.tick,
            baseline.tick,
            &baseline,
            &current,
            &CodecLimits::for_testing(),
            &mut full_buf,
        )
        .unwrap();
        let full_payload = &full_buf[wire::HEADER_SIZE..full_bytes];

        let mut session_buf = [0u8; 128];
        let mut last_tick = baseline.tick;
        let session_bytes = encode_delta_snapshot_for_client_session_with_scratch(
            &schema,
            current.tick,
            baseline.tick,
            &baseline,
            &current,
            &CodecLimits::for_testing(),
            &mut CodecScratch::default(),
            &mut last_tick,
            &mut session_buf,
        )
        .unwrap();

        let session_header =
            wire::decode_session_header(&session_buf[..session_bytes], baseline.tick.raw())
                .unwrap();
        let payload_start = session_header.header_len;
        let payload_end = payload_start + session_header.payload_len as usize;
        let session_payload = &session_buf[payload_start..payload_end];
        assert_eq!(full_payload, session_payload);
    }

    #[test]
    fn delta_roundtrip_single_component_change() {
        let schema = schema_two_components();
        let baseline = Snapshot {
            tick: SnapshotTick::new(10),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![
                    ComponentSnapshot {
                        id: ComponentId::new(1).unwrap(),
                        fields: vec![FieldValue::Bool(false)],
                    },
                    ComponentSnapshot {
                        id: ComponentId::new(2).unwrap(),
                        fields: vec![FieldValue::Bool(false)],
                    },
                ],
            }],
        };
        let current = Snapshot {
            tick: SnapshotTick::new(11),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![
                    ComponentSnapshot {
                        id: ComponentId::new(1).unwrap(),
                        fields: vec![FieldValue::Bool(true)],
                    },
                    ComponentSnapshot {
                        id: ComponentId::new(2).unwrap(),
                        fields: vec![FieldValue::Bool(false)],
                    },
                ],
            }],
        };

        let mut buf = [0u8; 256];
        let bytes = encode_delta_snapshot(
            &schema,
            current.tick,
            baseline.tick,
            &baseline,
            &current,
            &CodecLimits::for_testing(),
            &mut buf,
        )
        .unwrap();
        let applied = apply_delta_snapshot(
            &schema,
            &baseline,
            &buf[..bytes],
            &wire::Limits::for_testing(),
            &CodecLimits::for_testing(),
        )
        .unwrap();
        assert_eq!(applied.entities, current.entities);
    }

    #[test]
    fn baseline_tick_mismatch_is_error() {
        let schema = schema_one_bool();
        let baseline = baseline_snapshot();
        let current = baseline.clone();
        let mut buf = [0u8; 128];
        let bytes = encode_delta_snapshot(
            &schema,
            SnapshotTick::new(11),
            baseline.tick,
            &baseline,
            &current,
            &CodecLimits::for_testing(),
            &mut buf,
        )
        .unwrap();
        let mut packet = wire::decode_packet(&buf[..bytes], &wire::Limits::for_testing()).unwrap();
        packet.header.baseline_tick = 999;
        wire::encode_header(&packet.header, &mut buf[..wire::HEADER_SIZE]).unwrap();
        let err = apply_delta_snapshot(
            &schema,
            &baseline,
            &buf[..bytes],
            &wire::Limits::for_testing(),
            &CodecLimits::for_testing(),
        )
        .unwrap_err();
        assert!(matches!(err, CodecError::BaselineTickMismatch { .. }));
    }

    #[test]
    fn threshold_suppresses_small_change() {
        let schema = schema_uint_threshold(5);
        let baseline = Snapshot {
            tick: SnapshotTick::new(10),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![ComponentSnapshot {
                    id: ComponentId::new(1).unwrap(),
                    fields: vec![FieldValue::UInt(10)],
                }],
            }],
        };
        let current = Snapshot {
            tick: SnapshotTick::new(11),
            entities: vec![EntitySnapshot {
                id: EntityId::new(1),
                components: vec![ComponentSnapshot {
                    id: ComponentId::new(1).unwrap(),
                    fields: vec![FieldValue::UInt(12)],
                }],
            }],
        };

        let mut buf = [0u8; 128];
        let bytes = encode_delta_snapshot(
            &schema,
            current.tick,
            baseline.tick,
            &baseline,
            &current,
            &CodecLimits::for_testing(),
            &mut buf,
        )
        .unwrap();

        let packet = wire::decode_packet(&buf[..bytes], &wire::Limits::for_testing()).unwrap();
        assert_eq!(packet.sections.len(), 0);

        let applied = apply_delta_snapshot(
            &schema,
            &baseline,
            &buf[..bytes],
            &wire::Limits::for_testing(),
            &CodecLimits::for_testing(),
        )
        .unwrap();
        assert_eq!(applied.entities, baseline.entities);
    }
}