## Summary
Phase #3b (final roadmap item), stacked on #2470. Compacts the index trailer from 20 to 8 bytes/chunk.
Trammel: 19.2 MB → 17.9 MB. Roadmap total: 565 MB → 17.9 MB (−96.8%).
## Details
- Trailer stores `{ u32 packedKey = (ChunkX << 16) | ChunkY, u32 recordLength }` per chunk, in record write order; the file offset is dropped and reconstructed by cumulative recordLength from HeaderSize.
- No record reordering, no varint; fixed-stride, TryReadChunk unchanged.
- Also simplifies the accumulated `.swb` code comments across the stack.
- Format v8; v7 files rejected and re-baked once.
## Tests
v8 multi-chunk round-trip (cumulative offset reconstruction) + the v6/v7 suite; full pathfinding suite green; Release build clean.
## Summary
Phase #3a, stacked on #2469. Compresses each chunk record independently with libdeflate (random access preserved).
Trammel: 124.7 MB → 19.2 MB (−85%).
## Details
- Whole-record framing `[u32 UncompressedLen][payload]`; records that don't shrink (tiny Uniform) are stored raw, detected as payload length == UncompressedLen.
- Codec chosen by full-Trammel spike: libdeflate VeryHigh (16.5 MB, 1.83 µs/chunk decompress) over zstd L19/22 (17.4 MB) and managed Brotli q11 (16.4 MB) — best native ratio, fastest decompress, already the repo's packet codec (no new dependency).
- Reuses cached thread-static bindings: `Deflate.Maximum` for bake, `Deflate.Standard` for reads.
- Compression is bake-time only; decompression is one-time per chunk (LRU-cached).
- Format v7; v6 files rejected and re-baked once.
## Tests
v7 unit tests + the v6 suite run through the compression path; full pathfinding suite green; Release build clean.
## Summary
Phase #2 of the `.swb` step-cache size-reduction roadmap (after #2465, v5 uniform elision). Stores the 16 base directional Z arrays as masked residuals against each cell's own SourceZ and omits any array that matches its prediction. Lossless, byte-identical reconstruction.
Trammel: 231.9 MB → 124.7 MB (−46%).
## Details
- Predictor: `predict = mask bit ? SourceZ : 0` (matches the baker's 0 on unwalkable directions); residual `Z − predict` via unchecked two's-complement (byte-exact for all inputs); reconstruct `Z = predict + residual`.
- A `u16 ZArrayMask` flags which of the 16 base arrays differ from prediction; matching arrays are omitted and synthesized from mask + SourceZ at read.
- Serializer-layer only: StepChunk, the cache, the algorithm, and the baker are unchanged.
- Format v6; v5 files rejected and re-baked once.
## Tests
21 v6 unit tests; full pathfinding suite green; Release build clean.
## Summary
Sub-project #1 of the `.swb` step-cache size-reduction roadmap (`dev-docs/pathfinding.md` § Future work). Adds **uniform-chunk elision** to the `StepCacheFile` format, bumping it **v4 → v5**.
A fully-uniform 16×16 chunk — no strata, **no swim layer**, all 19 base arrays constant (open ocean, Green Acres, void) — serializes to a **~28-byte record** (`KindUniform`) instead of ~5,393, and reconstructs **byte-identically** via `Array.Fill`. Non-uniform chunks use the existing v4 body (`KindFull`) with the swim-layer and strata trailers **fully preserved** — the Kind byte is just prepended.
## Calibrated result (measured, not projected)
Baked Trammel via `SaveToFile`:
| | |
|---|---:|
| Chunks | 114,688 |
| Uniform (swim-aware) → elided | 62.7% |
| Swim-layer chunks (stay Full) | 8.9% |
| Strata chunks (stay Full) | 1.8% |
| Baseline (full records) | 592.2 MB |
| **Actual v5 `.swb`** | **231.9 MB (−61%)** |
The residual is ~150 MB of non-uniform land Z-blocks (targeted by #2 predictive-Z) + ~81 MB of swim-layer trailers (#2/#3). #2 and #3 are separate follow-up PRs.
## Implementation
- `StepChunk.IsUniform()` — false if it has strata **or a swim layer**, else true only when all 19 base arrays are constant (the "all-same" check uses the SIMD-accelerated `ContainsAnyExcept`).
- `StepCacheFile` v5 — `Kind` byte (`KindFull=0`/`KindUniform=2`, 1 reserved); uniform write/read; `FormatVersion`/`MinSupportedVersion` → 5 (v4 files rejected on open and re-baked). No `StepCache`/algorithm/index changes; fingerprint logic untouched.
## Tests
7 `StepCacheFileV5Tests` (uniform round-trip + `<200 B` compactness, varied-full, swim-layer-full, strata-full, swim+strata combined, v4 version-gate rejection) + the existing StepCache/pathfinding suite — **70 pass**, including the prior `SwimLayer_RoundTrips`. An independent review verified write/read symmetry, cast round-tripping, swim/strata preservation, and the version gate (READY TO MERGE).
## Summary
Adds `dev-docs/pathfinding.md` — a reference for how creature pathfinding works in ModernUO, written so a future contributor (human or AI) can reason about it without re-deriving it from the code.
Covers:
- **The stack** end to end: `ApproachTarget` → `PathFollower` → `MovementPath` → `BitmapAStarAlgorithm` → `StepCache` → `MovementImpl` slow path (and that the removed FastAStar survives as the slow path).
- **Windowed-A* limits** (`AreaSize=38`, `MaxSearchNodes`, Z planes) and what they mean (2D-adjacent-but-obstacle-separated / a-floor-up goals are unsolvable by design).
- **StepCache**: second-touch warming, LRU-bounded memory, lazy `.swb` backing stores — with **measured** disk sizes (~565 MB/Trammel; ~1.5–2 GB all facets).
- **Four config levers** in a table: `pathfinding.enable`, `bitmap_pathfinding_cache`, `pathfinding.maxResidentChunks`, `pathfinding.maxSearchNodes`.
- **Small/crappy-hardware shard spectrum** (cache off ≈ FastAStar at ~1× with zero warming memory, up through baked `.swb`).
- **Diagnostics & tooling** (`[PathCacheStats`/`[PathRecord`/`[PathBake`, the MapDump tool, the benchmark suite) and the Debug/Release test note.
- **Future work**: background-thread bake, long-traverse BDN scenario, swim `SourceZ` bake, and the `.swb` size-reduction roadmap.
## Note on scope
This is docs-only. It documents the *complete* pathfinding system, so it references a couple of pieces that ride in separate PRs (the `ApproachTarget` AI fix and the `pathfinding.maxSearchNodes` setting). If those havent merged yet, sequence this after them so the doc doesnt describe unshipped code. The StepCache/`.swb`/provider material it documents is already on `main`.
