# Terrain Quality Mesh Cache

## Metadata

| Field | Value |
|---|---|
| Status | Implemented extraction |
| Target/work package | M03 / `M03-RND-TERRAIN-CACHE-SERVICE-001` |
| Owners | Full-quality terrain revisit Mesh retention and LRU policy |
| Last verified | Worktree `work/sindo-main-codex/m03-terrain-quality-cache`, 2026-07-16 |
| Profiles/capabilities | Performance/Balanced/High/Custom capacity; session-memory only |

## Purpose

Retain eligible full-quality terrain Mesh references after ADT tile state unload,
so a revisit can restore quality without flashing to a lower-quality fallback,
while bounding retained references with the existing least-recently-used policy.

## Non-goals

- Parse/build terrain, own tile state or schedule quality-upgrade tasks.
- Create/finalize Nodes or RIDs, mutate materials or choose terrain LOD.
- Persist cache entries or change baked/control-splat/splat format versions.
- Cache control-splat/splat sources whose active radius has separate ownership.
- Provide a generic cache framework or change profile capacity defaults.

## Context and boundaries

```mermaid
flowchart LR
    Upgrade[Full-quality baked upgrade result] --> Loader[StreamingWorldLoader adapter]
    Loader --> Cache[TerrainQualityMeshCache]
    Cache -->|retained Mesh reference| Revisit[Reloaded tile state]
    Revisit --> Finalize[Existing tile LOD/finalization path]
    Splat[Control/splat quality sources] -. rejected .-> Cache
```

Allowed dependencies: `Mesh`, Dictionary/Array scalar containers and the loader
adapter. Forbidden dependencies: Node/SceneTree/RID, tasks/mutexes, ADT parsing,
gameplay/network/editor UI, files and persistent Resource cache ownership.

## Public API

| Symbol | Kind | Purpose | Thread/lifetime | Errors |
|---|---|---|---|---|
| `TerrainQualityMeshCache.store(tile_key, mesh, source, capacity)` | Command | Admit/touch a Mesh and trim oldest entries | Owning renderer thread/session | Returns false for rejected input or immediate zero-capacity eviction |
| `TerrainQualityMeshCache.restore(tile_key)` | Query/touch | Return a fresh Mesh/source record and mark newest | Owning renderer thread | Empty on miss; invalid record is removed |
| `TerrainQualityMeshCache.clear()` | Lifecycle command | Release all retained Mesh references and LRU keys | Reset/shutdown | Idempotent |
| `TerrainQualityMeshCache.diagnostic_snapshot()` | Read model | Detached count and oldest-to-newest key/source list | Caller-owned result | Never exposes Mesh references |

## Inputs and outputs

| Direction | Contract/data | Producer | Consumer | Ownership | Thread/lifetime |
|---|---|---|---|---|---|
| Input | Tile key | Streamer tile catalog/state | Cache index | Copied string | Map session |
| Input | Full-quality Mesh | Baked upgrade/fallback path | Cache | Retained shared reference | Until eviction/clear |
| Input | Quality source | Streamer quality adapter | Admission/restore | Copied string | Entry lifetime |
| Input | Current capacity | Active renderer profile | Store trim | Scalar copy | One store |
| Output | Fresh `{mesh, source}` record | Cache | Loader state adapter | Caller dictionary; shared Mesh reference | One restore |
| Output | Detached diagnostics | Cache | Tests/future metrics | Caller-owned scalars | One query |

Side effects are limited to retained reference and LRU container mutation. There
is no scene, GPU, task, file, database, network or persistent-cache mutation.

## Data flow

```mermaid
flowchart TD
    Store[store tile/mesh/source/capacity] --> Admit{Valid key + Mesh + source?}
    Admit -->|no| Reject[Return false]
    Admit -->|yes| Touch[Insert/replace and touch newest]
    Touch --> Trim[Evict oldest until count <= capacity]
    Restore[restore tile] --> Hit{Valid admitted entry?}
    Hit -->|no| Miss[Remove invalid / return empty]
    Hit -->|yes| TouchRestore[Touch newest and return fresh record]
    Clear[world reset/shutdown] --> Empty[Release all references and keys]
```

## Lifecycle/state

```mermaid
stateDiagram-v2
    [*] --> Empty
    Empty --> Populated: admitted store with capacity > 0
    Populated --> Populated: store / restore touch / bounded eviction
    Populated --> Empty: capacity zero store evicts all
    Populated --> Empty: clear on reset/shutdown
    Empty --> Empty: rejected store / miss / clear
    Empty --> [*]
    Populated --> [*]: owner releases cache
```

## Main sequence

```mermaid
sequenceDiagram
    participant Upgrade as Terrain upgrade result
    participant Loader as StreamingWorldLoader
    participant Cache as TerrainQualityMeshCache
    Upgrade->>Loader: full Mesh + baked_full source
    Loader->>Cache: store(key, mesh, source, profile capacity)
    Note over Loader,Cache: tile state may later unload
    Loader->>Cache: restore(key) on tile creation/revisit
    Cache-->>Loader: fresh record or empty
    Loader->>Loader: apply Mesh/source to existing tile state
    Loader->>Cache: clear() on world reset/shutdown
```

## Ownership, threading and resources

- The cache owns its dictionary, ordered tile keys and retained Mesh references.
- The loader retains tile state, async tasks/results, quality selection and all
  Node/RID/material finalization.
- Returned records are new dictionaries; their Mesh is a shared reference.
- Calls currently occur on the main renderer thread. The service has no lock and
  must not be mutated concurrently.
- `clear` deterministically drops references during the existing world reset path.

## Errors, cancellation and recovery

| Failure | Detection | Behavior | Diagnostic | Recovery |
|---|---|---|---|---|
| Empty key/null Mesh | Store guard | Reject unchanged | Boolean/test | Fix caller result |
| Splat/control source | Admission list | Reject unchanged | Boolean/test | Keep source in active-radius ownership |
| Zero/negative capacity | Clamp and trim | Entry immediately evicted; cache empty | Snapshot/count | Increase profile capacity |
| Miss/invalid entry | Restore validation | Remove invalid and return empty | Empty record | Existing loader fallback/rebuild |
| World reset/shutdown | Loader lifecycle | Clear every reference/key | Shutdown regression | Refill during next session |

There is no asynchronous work or cancellation token inside this service.

## Configuration and capabilities

| Setting/capability | Default | Profile | Runtime mutable | Effect |
|---|---|---|---|---|
| `terrain_quality_mesh_cache_limit` | Existing loader value | `0/24/48` in current presets | Read on every store | Maximum retained entries |
| Admitted source | Non-splat (currently `baked_full`) | All | No hidden override | Revisit retention |
| Excluded sources | `control_splat_cache`, `splat_cache`, `splat` | All | No | Preserve active quality ownership |

## Persistence, cache and migration

- This is an in-memory reference cache, not a serialized cache format.
- `REQUIRED_BAKED_TILE_FORMAT_VERSION=5`, control splat v3 and splat v1 remain
  loader/resource validation contracts and are unchanged.
- No migration or rebuild is required. Entry keys retain the existing tile-key form.

## Diagnostics and observability

- Detached diagnostics report count and oldest-to-newest tile key/source entries.
- Mesh, material, Node and RID references are deliberately absent.
- Performance is bounded dictionary lookup plus one ordered-array erase/append;
  no new optimization or scale claim is made.

## Verification

- `verify_terrain_quality_mesh_cache.gd`: admission, exclusions, Mesh identity,
  source, LRU touch/eviction, detached diagnostics, negative capacity and clear.
- Loader source boundary proves cache composition/delegation and removal of its
  former LRU array/touch/trim implementation.
- Existing shutdown, facade, internal-access, manifest, planner and scheduler
  regressions remain required.
- Fidelity: source rules, capacities and three loader adapter sites are unchanged;
  no visible terrain or build-12340 parity change is claimed.

## Extension points

- Expose scalar cache counts through facade metrics when needed.
- Move additional terrain responsibilities only as separate bounded services.
- Replace ordered-array LRU only after measured scale evidence.

## Capability status

| Capability | Status | Evidence | Gap/next step |
|---|---|---|---|
| Bounded quality Mesh retention | Implemented extraction | Contract/LRU fixtures | Asset-backed memory/revisit timing pending |
| Deterministic reset ownership | Implemented | Clear path plus shutdown regression | Full terrain service still monolithic |
| Persistent terrain cache formats | Unchanged | Loader version constants | Separate acceptance audit remains |

## Known gaps and risks

- Ordered-array touch is linear in entry count, matching prior behavior.
- Mesh memory size is not measured; capacity remains entry-count based.
- Only revisit Mesh ownership moved; build/tasks/state/finalization remain loader-owned.
- Asset-backed flash/revisit timing and p95/p99 evidence remain pending.

## Source map

| Path | Responsibility |
|---|---|
| `src/render/terrain/terrain_quality_mesh_cache.gd` | Session cache entries, LRU and admission |
| `src/scenes/streaming/streaming_world_loader.gd` | Store/restore state adapter and reset lifecycle |
| `src/tools/verify_terrain_quality_mesh_cache.gd` | Scene-free cache and loader-boundary regression |

## Related decisions and references

- [`world-renderer.md`](world-renderer.md)
- [`terrain-query.md`](terrain-query.md)
- [`../../RENDER.md`](../../RENDER.md)
- [`../../targets/roadmap/02-rendering-and-graphics.md`](../../targets/roadmap/02-rendering-and-graphics.md)