FA-70468 / GIS polygon topology / Member archive

TopoJSON arc stitching into rings: reversed arc index · case 03

Reversed arcs pull in the neighbouring arc.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Input [arcs, transform, ring]. Each arc is a list of quantized delta positions (the first is absolute, later ones add to the running position within that arc). transform [sx, sy, tx, ty] maps q to q*s + t per axis; a null transform is the identity [1, 1, 0, 0]. A ring lists arc indexes; a negative index i means arc ~i (bitwise complement) traversed backwards. Arcs after the first contribute all but their first position, which duplicates the previous arc end. Return the stitched list of [x, y].

Why this case matters

Shared-boundary topology formats store each border once; decoding errors break rings, reverse borders or duplicate junction vertices.

One recorded failure

Sample boundary fixture

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

Boundary fixture	Actual	Expected	Outcome
control #4	`[[100.0, 200.0], [105.0, 200.0], [105.0, 202.5], [107.5, 200.0], [105.0, 203.75], [100.0, 203.75], [100.0, 202.5]]`	`[[100.0, 200.0], [105.0, 200.0], [105.0, 202.5], [107.5, 202.5], [105.0, 202.5], [107.5, 200.0]]`	Failed

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