The pathing and step-cache comments had accumulated as development notes rather
than documentation: internal phase jargon ("Tier 4", "the Phase-2 synthesizer"),
change narration aimed at a reviewer ("which the old ComputeStandingZ anchor
missed", "legacy behavior"), benchmark anecdotes ("benchmarked as near-optimal",
"a ~20 ns lookup"), and multi-paragraph blocks restating what the code says.
Rewritten to keep the rationale a reader cannot derive from the code - why the
source-Z guard cannot be loosened, why multis fall through, why the promotion gate
counts Finds instead of calls, why the fingerprint hashes files rather than the
live tile tables - and to drop the history that got us there.
Also corrects comments that had gone stale:
- CacheEvictionTimer and CacheStats documented a class named
StaticWalkabilityCache, which no longer exists; it is StepCache.
- StepCacheFile's header said "File layout v8" while FormatVersion is 9, and the
body called the current record layout "the v6 layout" throughout. The layout
descriptions are now unversioned, since they describe whatever FormatVersion
currently is.
- StepProbe.ComputeStandingZ claimed StepCache uses it to bake SourceZ. It hasn't
since the baker moved to the clearance-aware ComputeStandableSurfaceZs; only a
parity test calls it now.
Two small code changes came along with the comment work, both behavior-preserving:
_lazyReaders now uses a collection expression like its neighbours, and
TryLoadFromLazyReader collapses to an expression body once its inline comment moved
to the doc comment.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
## 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
Replaces `FastAStarAlgorithm` with `BitmapAStarAlgorithm`: one cache lookup per cell expansion (8-direction mask + per-direction destination Z) instead of 8 separate `MovementImpl.CheckMovement` calls. Adds the supporting cache infrastructure to back it.
Public API unchanged — `MovementPath` / `Mobile.Move` / `CalcMoves.Find` return the same shapes; the algorithm swap is internal.
## What's in this PR
- **`BitmapAStarAlgorithm`** — A* that issues one `StepCache.TryGetMask` call per cell expansion. Inline fallthrough to the per-cell slow path for multi-Z, off-map, source-Z mismatch, and non-default walkers.
- **`StepCache`** — singleton chunk store keyed by `(mapId, chunkX, chunkY)`. Lazily built on first query, invalidated by `Sector.MultisVersion` mismatch, memory-bounded by sampled probabilistic LRU.
- **`StepProbe`** — computes static-only walkability for a single cell, mirroring `MovementImpl.Check` minus the item / mobile collision phases.
- **`StepMask` / `StepChunk`** — value / storage types for the per-cell results.
- **`CacheEvictionTimer`** — periodic cap backstop (60s interval; early-returns when not over cap).
- **`Map.Sector.MultisVersion`** promoted to `public` so the cache can detect dynamic-static invalidations cheaply.
## Eviction strategy
Sampled probabilistic LRU (Redis-style). Per eviction, sample 5 random keys from a parallel `List<long>` kept in lockstep with the chunk dictionary; evict the oldest of the sample via swap-and-pop. O(1) per eviction regardless of resident count, so sustained cap pressure has no perpetual perf hit.
## Capability handling (interim)
Non-default walkers (non-GM players, creatures with `CanSwim` / `CanFly` / `CanOpenDoors` / `CanMoveOverObstacles`) route entirely through the per-cell slow path via `BitmapAStarAlgorithm.GetSuccessorsSlowPath`. The 2-pass design (cache + capability overlay + dynamic-obstacle pass) lands in the follow-up PR.
