Non-destructive topology creation. #4
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This fixes #2, changing topojson.topology so that it no longer destroys the input objects. It also fixes #3 by removing type reduction: for example, an empty MultiLineString geometry on input is now an empty MultiLineString geometry on output, rather than a null geometry. Most of the internal implementation is still destructive. This works by changing the geometry method to make a shallow copy of the input, and by changing the prequantize method to make a copy of the coordinates. I’d like to improve this so that prequantize operates on the extracted coordinates, so that we can avoid the additional copy. This should be possible in a future commit. (It would also be nice if the intermediate representation used two typed arrays rather than an array of objects, but I think that will be more difficult to implement.)
Since topojson.topology was previously destructive, it would delete the coordinates array and assign a new arcs array. Now that it is non-destructive, it makes sense to initialize the arcs array when making the geometry object.
However, per the spec, each arc must retain at least two positions, even if the first and last point are coincident.
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This fixes #2, changing topojson.topology so that it no longer destroys the input objects; instead, shallow copies are made as needed.
This also fixes #3 by removing type reduction: for example, an input MultiPolygon geometry with only a single polygon is no longer reduced to a Polygon, and an input empty MultiLineString geometry is no longer reduced to a null geometry. The output type of a geometry object is always the same as its input type. You can remove null geometries using topojson.filter.
This will be a major version change, released as 3.0.
For the other changes in 3.0, see: