FA-88679 / Mesh topology invariants / Member archive

One-ring overwrites a repeated spoke · case 04

Two faces leaving v along the same edge (a flipped neighbour) are merged into a plausible ring.

Member previewVariant 4 · 3 implementations · 7 checks per implementation

Case contract

Input [faces, v] with consistently oriented polygons. Each face containing v maps its next corner p to its previous corner q (counter-clockwise order around v). A repeated p is "nonmanifold"; no incident face is "isolated". If exactly one p has no predecessor the ring is a boundary fan starting there; more than one is "nonmanifold"; otherwise start at the smallest neighbour. Walk successors; a walk that does not visit every neighbour is "nonmanifold". Return [ring, "boundary"|"interior"].

Why this case matters

Mesh processing pipelines (remeshing, export, simulation, printing) trust these topological counts and adjacency answers; a wrong invariant silently accepts broken meshes or rejects valid ones.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
same spoke twice	`[[1, 3], "boundary"]`	`"nonmanifold"`	Failed

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