FA-70827 / GIS polygon topology / Member archive

Missing node (T-junction) detection in a coverage: distinct vertex walk · case 02

The first vertex of a ring is reported twice.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Input: polygons as closed integer rings forming a coverage. For each polygon i and each distinct vertex v (closing duplicate excluded), and each other polygon j, report [i, v, j] when v lies exactly on an edge of j (zero cross product, inside the closed segment box) but is not one of that edge endpoints. Return the reports sorted.

Why this case matters

Coverage cleaning requires shared boundaries to have identical vertices; T-junctions cause gaps after simplification and must be noded first.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression #4	`[[0, [4, 4], 1], [1, [2, 4], 0], [1, [2, 4], 0]]`	`[[0, [4, 4], 1], [1, [2, 4], 0]]`	Failed

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