FA-70577 / GIS polygon topology / Member archive

Grid snapping with ring collapse handling: collapse threshold · case 02

Rings collapsed to a doubled segment are kept.

Member previewVariant 2 · 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
boundary #20	`[[[0, 0], [9, 0], [9, 9], [0, 9], [0, 0]], [[3, 3], [5, 3], [3, 3]]]`	`[[[0, 0], [9, 0], [9, 9], [0, 9], [0, 0]]]`	Failed

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