power_play/src/pp/pp_ent.c

/* Id magic number constants (to be used in conjunction with ent ids in deterministic id combinations) */
#define SIM_ENT_CONTACT_BASIS_Uid (UID(0x6a2a5d2dbecf534f, 0x0a8ca7c372a015af))
#define SIM_ENT_COLLISION_DEBUG_BASIS_Uid (UID(0x302c01182013bb02, 0x570bd270399d11a5))
#define SIM_ENT_TILE_CHUNK_BASIS_Uid (UID(0x3ce42de071dd226b, 0x9b566f7df30c813a))

internal u32 index_from_ent(Snapshot *ss, Entity *ent)
{
    return ent - ss->ents;
}

internal Entity *ent_from_index(Snapshot *ss, u32 index)
{
    if (index > 0 && index < ss->num_ents_reserved) {
        return &ss->ents[index];
    } else {
        return sim_ent_nil();
    }
}

/* ========================== *
 * Entity allocation
 * ========================== */

Entity *sim_ent_alloc_raw(Snapshot *ss, Entity *parent, EntityId id)
{
    Assert(parent->valid);
    Assert(ss->valid);
    Assert(ss == parent->ss);
    Entity *ent;
    if (ss->first_free_ent > 0 && ss->first_free_ent < ss->num_ents_reserved) {
        /* Reuse from free list */
        ent = &ss->ents[ss->first_free_ent];
        ss->first_free_ent = ent->next_free;
    } else {
        /* Make new */
        ent = PushStructNoZero(ss->ents_arena, Entity);
        ++ss->num_ents_reserved;
    }
    *ent = *sim_ent_nil();
    ent->ss = ss;
    ent->valid = 1;
    ent->owner = ss->client->player_id;
    ent->_is_xform_dirty = 1;
    ++ss->num_ents_allocated;

    sim_ent_set_id(ent, id);
    sim_ent_link_parent(ent, parent);

    return ent;
}

/* Allocates a new entity that will not sync */
Entity *sim_ent_alloc_local(Entity *parent)
{
    Snapshot *ss = parent->ss;
    Entity *e = sim_ent_alloc_raw(ss, parent, sim_ent_random_id());
    e->owner = ss->local_player;
    return e;
}

Entity *sim_ent_alloc_local_with_id(Entity *parent, EntityId id)
{
    Snapshot *ss = parent->ss;
    Entity *e = sim_ent_alloc_raw(ss, parent, id);
    e->owner = ss->local_player;
    return e;
}

/* Allocates a new entity to be synced to clients */
Entity *sim_ent_alloc_sync_src(Entity *parent)
{
    Snapshot *ss = parent->ss;
    Entity *e = sim_ent_alloc_raw(ss, parent, sim_ent_random_id());
    sim_ent_enable_prop(e, SEPROP_SYNC_SRC);
    e->owner = ss->local_player;
    return e;
}

Entity *sim_ent_alloc_sync_src_with_id(Entity *parent, EntityId id)
{
    Snapshot *ss = parent->ss;
    Entity *e = sim_ent_alloc_raw(ss, parent, id);
    sim_ent_enable_prop(e, SEPROP_SYNC_SRC);
    e->owner = ss->local_player;
    return e;
}

/* Allocates a new entity that will sync with incoming net src ents containing id, and coming from the specified owner */
Entity *sim_ent_alloc_sync_dst(Entity *parent, EntityId ent_id, EntityId owner_id)
{
    Snapshot *ss = parent->ss;
    Entity *e = sim_ent_alloc_raw(ss, parent, ent_id);
    sim_ent_enable_prop(e, SEPROP_SYNC_DST);
    e->owner = owner_id;
    return e;
}

void sim_ent_release_raw(Entity *ent)
{
    Snapshot *ss = ent->ss;
    /* Release children */
    Entity *child = sim_ent_from_id(ss, ent->first);
    while (child->valid) {
        Entity *next = sim_ent_from_id(ss, child->next);
        sim_ent_release_raw(child);
        child = next;
    }

    /* Release uid */
    sim_ent_set_id(ent, SIM_ENT_NIL_ID);

    /* Release */
    ent->valid = 0;
    ent->next_free = ss->first_free_ent;
    ss->first_free_ent = index_from_ent(ss, ent);
    --ss->num_ents_allocated;
}

void sim_ent_release(Entity *ent)
{
    Snapshot *ss = ent->ss;
    Entity *parent = sim_ent_from_id(ss, ent->parent);
    if (parent->valid) {
        sim_ent_unlink_from_parent(ent);
    }
    sim_ent_release_raw(ent);
}

void sim_ent_release_all_with_prop(Snapshot *ss, EntProp prop)
{
    TempArena scratch = BeginScratchNoConflict();

    Entity **ents_to_release = PushDry(scratch.arena, Entity *);
    u64 ents_to_release_count = 0;
    for (u64 ent_index = 0; ent_index < ss->num_ents_reserved; ++ent_index) {
        Entity *ent = &ss->ents[ent_index];
        if (ent->valid && sim_ent_has_prop(ent, prop)) {
            *PushStructNoZero(scratch.arena, Entity *) = ent;
            ++ents_to_release_count;
        }
    }

    /* Release from snapshot */
    /* TODO: Breadth first iteration to only release parent entities (since
     * child entities will be released along with parent anyway) */
    for (u64 i = 0; i < ents_to_release_count; ++i) {
        Entity *ent = ents_to_release[i];
        if (ent->valid && !ent->is_root && !sim_ent_has_prop(ent, SEPROP_CMD) && !sim_ent_has_prop(ent, SEPROP_PLAYER)) {
            sim_ent_release(ent);
        }
    }

    EndScratch(scratch);
}

/* ========================== *
 * Activate
 * ========================== */

void sim_ent_activate(Entity *ent, u64 current_tick)
{
    sim_ent_enable_prop(ent, SEPROP_ACTIVE);
    ent->activation_tick = current_tick;
    ++ent->continuity_gen;
}

/* ========================== *
 * Entity id
 * ========================== */

internal EntBin *bin_from_id(Snapshot *ss, EntityId id)
{
    return &ss->id_bins[id.uid.lo % ss->num_id_bins];
}

/* NOTE: This should only really happen during ent allocation (it doesn't make sense for an allocated ent's id to change) */
void sim_ent_set_id(Entity *ent, EntityId id)
{
    Snapshot *ss = ent->ss;
    EntityId old_id = ent->id;
    if (!sim_ent_id_eq(old_id, id)) {
        /* Release old from lookup */
        if (!sim_ent_id_is_nil(old_id)) {
            EntBin *bin = bin_from_id(ss, old_id);
            u32 prev_index = 0;
            u32 next_index = 0;
            u32 search_index = bin->first;
            Entity *prev = sim_ent_nil();
            Entity *next = sim_ent_nil();
            Entity *search = ent_from_index(ss, search_index);
            while (search->valid) {
                next_index = search->next_in_id_bin;
                next = ent_from_index(ss, next_index);
                if (sim_ent_id_eq(search->id, old_id)) {
                    break;
                }
                prev_index = search_index;
                prev = search;
                search_index = next_index;
                search = next;
            }

            /* Old id not in bin, this should be impossible. */
            Assert(search->valid);

            if (prev->valid) {
                prev->next_in_id_bin = next_index;
            } else {
                bin->first = next_index;
            }

            if (next->valid) {
                next->prev_in_id_bin = prev_index;
            } else {
                bin->last = prev_index;
            }
        }

        /* Insert new id into lookup */
        if (!sim_ent_id_is_nil(id)) {
#if RtcIsEnabled
            {
                Entity *existing = sim_ent_from_id(ss, id);
                /* Collision should be extremely unlikely under normal circumstances, there's probably a logic error somewhere. */
                Assert(!existing->valid);
            }
#endif

            EntBin *bin = bin_from_id(ss, id);
            u32 ent_index = index_from_ent(ss, ent);
            Entity *last = ent_from_index(ss, bin->last);
            if (last->valid) {
                last->next_in_id_bin = ent_index;
                ent->prev_in_id_bin = bin->last;
            } else {
                bin->first = ent_index;
                ent->prev_in_id_bin = 0;
            }
            bin->last = ent_index;
        }

        ent->id = id;
    }

}

Entity *sim_ent_from_id(Snapshot *ss, EntityId id)
{
    Entity *result = sim_ent_nil();
    if (!sim_ent_id_is_nil(id) && ss->valid) {
        EntBin *bin = bin_from_id(ss, id);
        for (Entity *e = ent_from_index(ss, bin->first); e->valid; e = ent_from_index(ss, e->next_in_id_bin)) {
            if (sim_ent_id_eq(e->id, id)) {
                result = e;
                break;
            }
        }
    }
    return result;
}

EntityId sim_ent_random_id(void)
{
    EntityId result = ZI;
    result.uid = UidFromTrueRand();
    return result;
}

/* Returns the deterministic id of the contact constraint ent id that should be produced from e0 & e1 colliding */
EntityId sim_ent_contact_constraint_id_from_contacting_ids(EntityId player_id, EntityId id0, EntityId id1)
{
    EntityId result = ZI;
    result.uid = SIM_ENT_CONTACT_BASIS_Uid;
    result.uid = CombineUid(result.uid, player_id.uid);
    result.uid = CombineUid(result.uid, id0.uid);
    result.uid = CombineUid(result.uid, id1.uid);
    return result;
}

/* Returns the deterministic id of the debug contact constraint ent id that should be produced from e0 & e1 colliding */
EntityId sim_ent_collision_debug_id_from_ids(EntityId player_id, EntityId id0, EntityId id1)
{
    EntityId result = ZI;
    result.uid = SIM_ENT_COLLISION_DEBUG_BASIS_Uid;
    result.uid = CombineUid(result.uid, player_id.uid);
    result.uid = CombineUid(result.uid, id0.uid);
    result.uid = CombineUid(result.uid, id1.uid);
    return result;
}

/* Returns the deterministic id of the tile chunk that should be produced at chunk pos */
EntityId sim_ent_tile_chunk_id_from_tile_chunk_index(Vec2I32 chunk_index)
{
    EntityId result = ZI;
    result.uid = SIM_ENT_TILE_CHUNK_BASIS_Uid;
    result.uid = CombineUid(result.uid, UID(RandU64FromSeed(chunk_index.x), RandU64FromSeed(chunk_index.y)));
    return result;
}

/* ========================== *
 * Entity query
 * ==========================  */

Entity *sim_ent_find_first_match_one(Snapshot *ss, EntProp prop)
{
    u64 count = ss->num_ents_reserved;
    Entity *entities = ss->ents;
    for (u64 ent_index = 0; ent_index < count; ++ent_index) {
        Entity *ent = &entities[ent_index];
        if (ent->valid && sim_ent_has_prop(ent, prop)) {
            return ent;
        }
    }
    return sim_ent_nil();
}

Entity *sim_ent_find_first_match_all(Snapshot *ss, EntPropArray props)
{
    u64 count = ss->num_ents_reserved;
    Entity *entities = ss->ents;
    for (u64 ent_index = 0; ent_index < count; ++ent_index) {
        Entity *ent = &entities[ent_index];
        if (ent->valid) {
            b32 all = 1;
            for (u64 i = 0; i < props.count; ++i) {
                if (!sim_ent_has_prop(ent, props.props[i])) {
                    all = 0;
                    break;
                }
            }
            if (all) {
                return ent;
            }
        }
    }
    return sim_ent_nil();
}

/* ========================== *
 * Entity tree
 * ========================== */

void sim_ent_link_parent(Entity *ent, Entity *parent)
{
    Snapshot *ss = ent->ss;

    Entity *old_parent = sim_ent_from_id(ss, ent->parent);
    if (old_parent->valid) {
        /* Unlink from current parent */
        sim_ent_unlink_from_parent(ent);
    }

    EntityId ent_id = ent->id;
    EntityId last_child_id = parent->last;
    Entity *last_child = sim_ent_from_id(ss, last_child_id);
    if (last_child->valid) {
        ent->prev = last_child_id;
        last_child->next = ent_id;
    } else {
        parent->first = ent_id;
    }
    parent->last = ent_id;

    if (parent->is_root) {
        ent->is_top = 1;
        ent->top = ent_id;
    } else {
        ent->top = parent->top;
    }

    ent->parent = parent->id;
}

/* NOTE: Entity will be dangling after calling this, should re-link to root ent. */
void sim_ent_unlink_from_parent(Entity *ent)
{
    Snapshot *ss = ent->ss;

    EntityId parent_id = ent->parent;
    Entity *parent = sim_ent_from_id(ss, parent_id);
    Entity *prev = sim_ent_from_id(ss, ent->prev);
    Entity *next = sim_ent_from_id(ss, ent->next);

    /* Unlink from parent & siblings */
    if (prev->valid) {
        prev->next = next->id;
    } else {
        parent->first = next->id;
    }
    if (next->valid) {
        next->prev = prev->id;
    } else {
        parent->last = prev->id;
    }
    ent->prev = SIM_ENT_NIL_ID;
    ent->next = SIM_ENT_NIL_ID;
}

/* ========================== *
 * Entity xform
 * ========================== */

internal void sim_ent_mark_child_xforms_dirty(Snapshot *ss, Entity *ent)
{
    for (Entity *child = sim_ent_from_id(ss, ent->first); child->valid; child = sim_ent_from_id(ss, child->next)) {
        if (child->_is_xform_dirty) {
            break;
        } else {
            child->_is_xform_dirty = 1;
            sim_ent_mark_child_xforms_dirty(ss, child);
        }
    }
}

internal Xform sim_ent_get_xform_internal(Snapshot *ss, Entity *ent)
{
    Xform xf;
    if (ent->_is_xform_dirty) {
        if (ent->is_top) {
            xf = ent->_local_xform;
        } else {
            Entity *parent = sim_ent_from_id(ss, ent->parent);
            xf = sim_ent_get_xform_internal(ss, parent);
            xf = MulXform(xf, ent->_local_xform);
            ent->_xform = xf;
            ent->_is_xform_dirty = 0;
        }
        ent->_xform = xf;
        ent->_is_xform_dirty = 0;
    } else {
        xf = ent->_xform;
    }
    return xf;
}

Xform sim_ent_get_xform(Entity *ent)
{
    Xform xf;
    if (ent->_is_xform_dirty) {
        if (ent->is_top) {
            xf = ent->_local_xform;
        } else {
            Snapshot *ss = ent->ss;
            Entity *parent = sim_ent_from_id(ss, ent->parent);
            xf = sim_ent_get_xform_internal(ss, parent);
            xf = MulXform(xf, ent->_local_xform);
            ent->_xform = xf;
            ent->_is_xform_dirty = 0;
        }
        ent->_xform = xf;
        ent->_is_xform_dirty = 0;
    } else {
        xf = ent->_xform;
    }
    return xf;
}

Xform sim_ent_get_local_xform(Entity *ent)
{
    return ent->_local_xform;
}

void sim_ent_set_xform(Entity *ent, Xform xf)
{
    if (!EqXform(xf, ent->_xform)) {
        Snapshot *ss = ent->ss;
        /* Update local xform */
        if (ent->is_top) {
            ent->_local_xform = xf;
        } else {
            Entity *parent = sim_ent_from_id(ss, ent->parent);
            Xform parent_global = sim_ent_get_xform_internal(ss, parent);
            ent->_local_xform = MulXform(InvertXform(parent_global), xf);
        }
        ent->_xform = xf;
        ent->_is_xform_dirty = 0;
        sim_ent_mark_child_xforms_dirty(ss, ent);
    }
}

void sim_ent_set_local_xform(Entity *ent, Xform xf)
{
    if (!EqXform(xf, ent->_local_xform)) {
        ent->_local_xform = xf;
        ent->_is_xform_dirty = 1;
        sim_ent_mark_child_xforms_dirty(ent->ss, ent);
    }
}

/* ========================== *
 * Entity movement
 * ========================== */

void sim_ent_set_linear_velocity(Entity *ent, Vec2 velocity)
{
    if (sim_ent_has_prop(ent, SEPROP_KINEMATIC) || sim_ent_has_prop(ent, SEPROP_DYNAMIC)) {
        ent->linear_velocity = ClampVec2Len(velocity, SIM_MAX_LINEAR_VELOCITY);
    }
}

void sim_ent_set_angular_velocity(Entity *ent, f32 velocity)
{
    if (sim_ent_has_prop(ent, SEPROP_KINEMATIC) || sim_ent_has_prop(ent, SEPROP_DYNAMIC)) {
        ent->angular_velocity = ClampF32(velocity, -SIM_MAX_ANGULAR_VELOCITY, SIM_MAX_ANGULAR_VELOCITY);
    }
}

void sim_ent_apply_linear_impulse(Entity *ent, Vec2 impulse, Vec2 point)
{
    if (sim_ent_has_prop(ent, SEPROP_DYNAMIC)) {
        Xform xf = sim_ent_get_xform(ent);
        Vec2 center = xf.og;
        f32 scale = AbsF32(DeterminantFromXform(xf));
        f32 inv_mass = 1.f / (ent->mass_unscaled * scale);
        f32 inv_inertia = 1.f / (ent->inertia_unscaled * scale);

        Vec2 vcp = SubVec2(point, center);
        sim_ent_set_linear_velocity(ent, AddVec2(ent->linear_velocity, MulVec2(impulse, inv_mass)));
        sim_ent_set_angular_velocity(ent, WedgeVec2(vcp, impulse) * inv_inertia);
    }
}

void sim_ent_apply_linear_impulse_to_center(Entity *ent, Vec2 impulse)
{
    if (sim_ent_has_prop(ent, SEPROP_DYNAMIC)) {
        Xform xf = sim_ent_get_xform(ent);
        f32 scale = AbsF32(DeterminantFromXform(xf));
        f32 inv_mass = 1.f / (ent->mass_unscaled * scale);

        sim_ent_set_linear_velocity(ent, AddVec2(ent->linear_velocity, MulVec2(impulse, inv_mass)));
    }
}

void sim_ent_apply_force_to_center(Entity *ent, Vec2 force)
{
    if (sim_ent_has_prop(ent, SEPROP_DYNAMIC)) {
        ent->force = AddVec2(ent->force, force);
    }
}

void sim_ent_apply_angular_impulse(Entity *ent, f32 impulse)
{
    if (sim_ent_has_prop(ent, SEPROP_DYNAMIC)) {
        Xform xf = sim_ent_get_xform(ent);
        f32 scale = AbsF32(DeterminantFromXform(xf));
        f32 inv_inertia = 1.f / (ent->inertia_unscaled * scale);
        sim_ent_set_angular_velocity(ent, ent->angular_velocity + impulse * inv_inertia);
    }
}

void sim_ent_apply_torque(Entity *ent, f32 torque)
{
    if (sim_ent_has_prop(ent, SEPROP_DYNAMIC)) {
        ent->torque += torque;
    }
}

/* ========================== *
 * Tile
 * ========================== */

Entity *sim_tile_chunk_from_chunk_index(Snapshot *ss, Vec2I32 chunk_index)
{
    EntityId chunk_id = sim_ent_tile_chunk_id_from_tile_chunk_index(chunk_index);
    Entity *chunk_ent = sim_ent_from_id(ss, chunk_id);
    return chunk_ent;
}

Entity *sim_tile_chunk_from_world_tile_index(Snapshot *ss, Vec2I32 world_tile_index)
{
    Vec2I32 chunk_index = sim_tile_chunk_index_from_world_tile_index(world_tile_index);
    Entity *chunk_ent = sim_tile_chunk_from_chunk_index(ss, chunk_index);
    return chunk_ent;
}

TileKind sim_get_chunk_tile(Entity *chunk_ent, Vec2I32 local_tile_index)
{
    TileKind result = chunk_ent->tile_chunk_tiles[local_tile_index.x + (local_tile_index.y * SIM_TILES_PER_CHUNK_SQRT)];
    return result;
}

/* ========================== *
 * Entity lerp
 * ========================== */

void sim_ent_lerp(Entity *e, Entity *e0, Entity *e1, f64 blend)
{
    if (sim_ent_is_valid_and_active(e0) && sim_ent_is_valid_and_active(e1)
        && sim_ent_id_eq(e0->id, e1->id)
        && e0->continuity_gen == e1->continuity_gen) {
        e->_local_xform = LerpXform(e0->_local_xform, e1->_local_xform, blend);

        if (e->is_top) {
            /* TODO: Cache parent & child xforms in sim */
            Xform e0_xf = sim_ent_get_xform(e0);
            Xform e1_xf = sim_ent_get_xform(e1);
            sim_ent_set_xform(e, LerpXform(e0_xf, e1_xf, blend));
        }

        e->control_force = LerpF32(e0->control_force, e1->control_force, blend);
        e->control_torque = LerpF32(e0->control_torque, e1->control_torque, blend);

        e->linear_velocity = LerpVec2(e0->linear_velocity, e1->linear_velocity, blend);
        e->angular_velocity = LerpAngleF32(e0->angular_velocity, e1->angular_velocity, blend);

        e->control.move = LerpVec2(e0->control.move, e1->control.move, blend);
        e->control.focus = LerpVec2(e0->control.focus, e1->control.focus, blend);

        e->sprite_local_xform = LerpXform(e0->sprite_local_xform, e1->sprite_local_xform, blend);
        e->animation_last_frame_change_time_ns = LerpI64(e0->animation_last_frame_change_time_ns, e1->animation_last_frame_change_time_ns, (f64)blend);
        e->animation_frame = (u32)RoundF32ToI32(LerpF32(e0->animation_frame, e1->animation_frame, blend));

        e->camera_quad_xform = LerpXform(e0->camera_quad_xform, e1->camera_quad_xform, blend);
        e->camera_xform_target = LerpXform(e0->camera_xform_target, e1->camera_xform_target, blend);
        e->shake = LerpF32(e0->shake, e1->shake, blend);

        e->tracer_gradient_start = LerpVec2(e0->tracer_gradient_start, e1->tracer_gradient_start, blend);
        e->tracer_gradient_end = LerpVec2(e0->tracer_gradient_end, e1->tracer_gradient_end, blend);
    }
}

/* ========================== *
 * Entity sync
 * ========================== */

 /* Walks a local & remote ent tree and allocates any missing net dst ents from remote src ents */
void sim_ent_sync_alloc_tree(Entity *local_parent, Entity *remote, EntityId remote_player)
{
    __prof;
    if (sim_ent_has_prop(remote, SEPROP_SYNC_SRC)) {
        Snapshot *local_ss = local_parent->ss;
        Snapshot *remote_ss = remote->ss;

        EntityId id = remote->id;
        Entity *local_ent = sim_ent_from_id(local_ss, id);
        if (!local_ent->valid) {
            local_ent = sim_ent_alloc_sync_dst(local_parent, id, remote_player);
        }
        for (Entity *remote_child = sim_ent_from_id(remote_ss, remote->first); remote_child->valid; remote_child = sim_ent_from_id(remote_ss, remote_child->next)) {
            sim_ent_sync_alloc_tree(local_ent, remote_child, remote_player);
        }
    }
}

/* Copies data between two synced entities */
void sim_ent_sync(Entity *local, Entity *remote)
{
    Entity old = *local;
    CopyStruct(local, remote);

    /* Why would 2 ents w/ different uids ever be synced? */
    Assert(sim_ent_id_eq(local->id, old.id));

    local->ss = old.ss;
    local->id = old.id;

    /* Keep local tree */
    local->parent = old.parent;
    local->prev = old.prev;
    local->next = old.next;
    local->first = old.first;
    local->last = old.last;
    local->top = old.top;
    local->owner = old.owner;

    /* Keep indices */
    local->next_in_id_bin = old.next_in_id_bin;
    local->prev_in_id_bin = old.prev_in_id_bin;
    local->next_free = old.next_free;

    sim_ent_disable_prop(local, SEPROP_SYNC_SRC);
    sim_ent_enable_prop(local, SEPROP_SYNC_DST);
}










#if 1

/* ========================== *
 * Entity encode
 * ========================== */

void sim_ent_encode(BB_Writer *bw, Entity *e0, Entity *e1)
{
    Snapshot *ss = e1->ss;
    /* FIXME: Things like xforms need to be retreived manually rather than memcopied. */

     /* TODO: Granular delta encoding */

    u64 pos = 0;
    e1->ss = e0->ss;
    while (pos < sizeof(*e1)) {
        u64 chunk_size = MinU64(pos + 8, sizeof(*e1)) - pos;
        u8 *chunk0 = (u8 *)e0 + pos;
        u8 *chunk1 = (u8 *)e1 + pos;
        if (BB_WriteBit(bw, !EqBytes(chunk0, chunk1, chunk_size))) {
            u64 bits = 0;
            CopyBytes(&bits, chunk1, chunk_size);
            BB_WriteUBits(bw, bits, 64);
        }
        pos += 8;
    }
    e1->ss = ss;
}

/* ========================== *
 * Entity decode
 * ========================== */

void sim_ent_decode(BB_Reader *br, Entity *e)
{
    Snapshot *old_ss = e->ss;
    {
        u64 pos = 0;
        while (pos < sizeof(*e)) {
            u8 *chunk = (u8 *)e + pos;
            if (BB_ReadBit(br)) {
                u64 chunk_size = MinU64(pos + 8, sizeof(*e)) - pos;
                u64 bits = BB_ReadUBits(br, 64);
                CopyBytes(chunk, &bits, chunk_size);
            }
            pos += 8;
        }
    }
    e->ss = old_ss;

}

#else

/* ========================== *
 * Entity encode
 * ========================== */

void sim_ent_encode(BB_Writer *bw, Entity *e0, Entity *e1)
{
    Snapshot *ss = e1->ss;


    /* FIXME: Things like xforms need to be retreived manually rather than memcopied.
     * This will also be true for things like ent handles once uids are implemented. */

    /* TODO: Granular delta encoding */

    u64 pos = 0;
    e1->ss = e0->ss;
    while (pos < sizeof(*e1)) {
        u64 chunk_size = MinU64(pos + 8, sizeof(*e1)) - pos;
        u8 *chunk0 = (u8 *)e0 + pos;
        u8 *chunk1 = (u8 *)e1 + pos;
        if (EqBytes(chunk0, chunk1, chunk_size)) {
            BB_WriteBit(bw, 0);
        } else {
            BB_WriteBit(bw, 1);
            u64 bits = 0;
            CopyBytes(&bits, chunk1, chunk_size);
            BB_WriteUBits(bw, bits, 64);
        }
        pos += 8;
    }
    e1->ss = ss;
}

/* ========================== *
 * Entity decode
 * ========================== */

void sim_ent_decode(BB_Reader *br, Entity *e)
{
    Entity decoded = *e;
    {
        u64 pos = 0;
        while (pos < sizeof(decoded)) {
            u8 *chunk = (u8 *)&decoded + pos;
            if (BB_ReadBit(br)) {
                u64 chunk_size = MinU64(pos + 8, sizeof(decoded)) - pos;
                u64 bits = BB_ReadUBits(br, 64);
                CopyBytes(chunk, &bits, chunk_size);
            }
            pos += 8;
        }
    }
    decoded.ss = e->ss;

    EntityId old_id = e->id;
    EntityId new_id = decoded.id;
    CopyStruct(e, &decoded);
    e->id = old_id;
    if (!sim_ent_id_eq(old_id, new_id)) {
        sim_ent_set_id(e, new_id);
    }
}
#endif