topology.AsTopoJSON(TopoGeometry)

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Function to output TopoJSON from TopoGeometry. ​https://github.com/mbostock/topojson/

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Example outputs:

{"type": "LineString", "arcs": [42, 43]}
{"type": "Polygon", "arcs": [[42, 43], [44]]}

The numbers in square brackets (array elements) are "arc identifiers", pretty much like our edge identifiers except they are (0-based) indices into an array, which we'll need to build somehow.

Also, backward arcs/edges are expressed with -(INDEX+1)

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Format specification: ​https://github.com/mbostock/topojson/wiki/Specification

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r11183 implements this

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r11187 reduces the time it takes to output a polygonal TopoJSON to 6% of previous time, and fixes some bugs. Lineal geometries are still output in an expensive way.

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Reopening because according to the specs a MultiPolygon must be represented as an array of Polygons. The current code is instead using a single array of _rings_ whereas ring orientation tells if a ring is an hole or a shell but nothing tells which hole belongs to which shell.

I've been thinking a way to figure out what's an hole and what's a shell: while walking a ring, the code can keep track of which faces are being bound; at the end of ring any still-not-visited edge having one of the visited faces on any side will surely be an hole belonging to the previous shell.

Holes within an embedded face will need to be tested, like this:

 TopoGeometry formed by (F1,F2)
 +----------+
 |    F1    |
 | +------+ |
 | |  F2  | |
 | | +--+ | |
 | | |F3| | |
 | | +--+ | |
 | +------+ |
 +----------+

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Note that in order for the above test to work the code will need to be extended to also walk around the shell of F2, which is currently skipped.

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r11193 fixes the grouping of polygon rings, including the test above. It's still fast

As part of the change all polygons are reported as MultiPolygon, for consistency (this is the same we do for TopoGeometry→Geometry.

Before closing this the same thing needs to be done for linear TopoGeometries.

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r11196 fixes it for linear topogeometries.