FA-70583 / GIS polygon topology / Member archive

Grid snapping with ring collapse handling: hole collapse policy · case 03

A polygon disappears because one tiny hole collapsed.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Input [rings, g]: polygon rings (exterior first), closed, numeric coordinates; grid size g > 0 anchored at 0. Each coordinate snaps to floor(v/g + 0.5)*g (halves round toward +infinity). Consecutive identical snapped positions are merged. A ring left with fewer than 4 positions has collapsed: if it is the exterior return None, otherwise drop that hole. Return the snapped rings.

Why this case matters

Precision reduction for storage, tiling and topology cleaning snaps vertices to a grid; collapse rules decide whether tiny holes vanish or the polygon is discarded.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
regression #6	`null`	`[[[0, 0], [20, 0], [20, 20], [0, 20], [0, 0]]]`	Failed

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