Unit — Entities Core

01why

Identity in ECS is set membership, and set membership is an address. State the direct consequence for mutating an entity's shape.

1. aMutating shape is free because entities are just numbers
2. bChanging an entity's component set changes its archetype and therefore its physical location — every add/remove/create/destroy is a data relocation (structural change)
3. cOnly adding components costs anything; removing is free
4. dShape changes are batched away by the compiler at build time

02how

A query WithAll<Enemy>().WithNone<Frozen>() is evaluated against millions of entities. What is actually iterated?

1. aEvery entity, checking two conditions each
2. bOnly the chunks of archetypes that contain Enemy and lack Frozen — matching is decided per-archetype and cached, then those chunks are walked
3. cA precomputed flat list of matching entity IDs, rebuilt each frame
4. dThe first matching chunk only, until it's exhausted

03why

Why is 16 KB a defensible chunk size in terms of the memory hierarchy?

1. aIt's the largest block the allocator can return without fragmentation
2. bIt's small enough that a chunk's streaming working set stays in L1/L2 during a linear walk, yet large enough to amortize per-chunk header and iteration overhead
3. cIt equals the CPU's cache line size times a fixed constant
4. dBurst rejects chunks larger than 16 KB

04how

A shared component's cardinality (number of distinct values) determines what?

1. aHow many bytes each entity spends on it
2. bThe number of chunk partitions — each distinct value gets its own set of chunks, so high cardinality fragments the archetype into sparse chunks
3. cNothing measurable; it's purely cosmetic
4. dThe iteration order of the query

05why

You write SetComponent on every entity every frame and your change filter stops skipping anything. What's the mechanism, and the fix?

1. aChange filters are unreliable; remove it
2. bAny write to a component type bumps that chunk's change version, so unconditional writes mark every chunk changed every frame; fix by writing only when the value actually changes
3. cThe filter must be re-registered each frame; call SetChangedVersionFilter again
4. dChange versions reset to zero each frame, so the filter can never match

06howmeasure

Build the density probe from the lab: an archetype summing ~48 bytes/entity. Read Archetype.ChunkCapacity and enter the value.

07howmeasure

Add a 128-byte cold component to that archetype and re-read ChunkCapacity. Enter capacity_lean / capacity_fat.