Hi,

I've been trying to make a TopoJSON parser with C# and I've read the specification at least 10-20 times now. There's these lines which I can't understand:

If more than one arc is referenced to construct a LineString or LinearRing, the first position of a subsequent arc must be equal to the last position of the previous arc. Then, when reconstructing the geometry, the first position of each arc except the first may be dropped; equivalently, the last position of each arc except the last may be dropped.

I've found this example which I found quite illustrative and simple:

{ "type": "Topology", "objects": { "collection": { "type": "GeometryCollection", "geometries": [ { "type": "Polygon", "properties": { "name": "Left_Polygon" }, "arcs": [ [ 0, 1 ] ] }, { "type": "Polygon", "properties": { "name": "Right_Polygon" }, "arcs": [ [ 2, -1 ] ] } ] } }, "arcs": [ [ [ 1, 2 ], [ 0, -2 ] ], [ [ 1, 0 ], [ -1, 0 ], [ 0, 2 ], [ 1, 0 ] ], [ [ 1, 2 ], [ 1, 0 ], [ 0, -2 ], [ -1, 0 ] ] ], "transform": { "scale": [ 1.1957244926770118e-8, 1.4241991634471996e-8 ], "translate": [ 1.9563164719471948, 41.72678319291929 ] }, "bbox": [ 1.9563164719471948, 41.72678319291929, 1.9564360324392176, 41.726925598593645 ] }

from here: https://en.wikipedia.org/wiki/GeoJSON#TopoJSON_Schema.

What I don't undestand is when a LinearRing has more than one arc, the first index except the first can be dropped. This means that for the Left_Polygon second arc first index [1,0] is dropped. Does the first index from the first arc of the second LinearRing (Right_Polygon) [1,2] get dropped too? Do the reversed indexes get dropped too? When the -1 arc from Right_Polygon starts [1,0] for example.

I also don't understand this line:

equivalently, the last position of each arc except the last may be dropped.

This means that the (n-1) index of every arch except the last one gets dropped? If so there's no possible way I can reconstruct the original 2 LineRings...

Sorry for asking so many questions.