After 1b, triangulating both polygons on every pair was the largest
remaining overlap cost (27% of main-thread samples on the corpus job).
PartOverlapChecker now triangulates each part at most once per check,
lazily after the bounding-box gate, and passes the triangles to a new
internal Collision.HasOverlap overload that runs the unchanged
OverlapRegions body. Triangles are only read by clipping and hole
subtraction, so reuse gives identical verdicts.
Verification:
- 49,000 seeded decisions with reused triangles match LegacyCollision;
triangles stay bit-identical to a fresh triangulation afterwards.
- Debug PolygonTriangulations: 246 -> 40 and 64 -> 36 per grid check;
sharing triangles per Program instead fails 23 tests.
- Corpus job (169 parts, --engines Default --parallel 1): median
13,398 -> 12,702 ms over 4+4 alternating runs vs 1b, identical
outcomes; serialized layout byte-identical to the base.
Also records the Follow-up B' (Slices 1a, 1b, 2a) measurements in
docs/performance/fill-performance.md.
Collision.HasOverlap only needs the verdict, but it went through Check,
which also collected crossing points. Triangulation, clipping and hole
subtraction now live in one private OverlapRegions method shared by Check
and HasOverlap, so verdict arithmetic stays single-sourced; Check output is
unchanged.
Tests: a frozen copy of the previous Collision is the oracle. 50,000 seeded
HasOverlap verdicts and 2,400 bitwise Check results match it, plus
containment, contact, hole and input-immutability cases. A Debug-only
PerfCounters.CrossingPointScans counter proves HasOverlap no longer scans.
Malformed polygons with null outer vertices still throw when the bounding
boxes overlap (now ArgumentNullException from triangulation rather than
NullReferenceException from ToLines).
Measured (Release, same harness in both trees): about 44% less time per
overlap-only polygon check, allocations 10.0 -> 7.9 MB per 155-pair sweep.
The 169-part serialized corpus layout is byte-identical.
aj