Mass importing many maps

For https://github.com/dabreegster/abstreet/issues/326, I'm starting to figure out how to import hundreds of maps into A/B Street. There are many issues with scaling up the number of supported maps. This document just focuses on importing.

The current approach

https://download.bbbike.org/ conveniently has 200 OSM extracts for major cities world-wide. The data/bbike.sh script downloads these. Then data/mass_import.sh attempts to import them into A/B Street.

The bbike extracts, however, cover huge areas surrounding major cities. Importing such large areas is slow, and the result is too large to work well in A/B Street or the OSM viewer. Ideally, we want just the area concentrated around the "core" of each city.

https://github.com/dabreegster/abstreet/blob/master/convert_osm/src/bin/extract_cities.rs transforms a huge .osm file into smaller pieces, each focusing on one city core. This tool looks for administrative boundary relations tagged as cities, produces a clipping polygon covering the city, and uses osmconvert to produce a smaller .osm file. The tool has two strategies for generating clipping polygons. One is to locate the admin_centre or label node for the region, then generate a circle of fixed radius around that point. Usually this node is located in the city core, so it works reasonably, except for "narrow" cities along a coast. The other strategy glues together the relation's multipolygon boundary, then simplifies the shape (usually with thousands of points) using a convex hull. This strategy tends to produce results that're too large, because city limits are often really huge.

Problems

• Outside the US, administrative boundaries don't always have a "city" defined. In Tokyo in particular, this name isn't used. I'm not sure which boundary level to use yet.
• The tool assumes driving on the right everywhere. OSM has https://wiki.openstreetmap.org/wiki/Key:driving_side, but this is usually tagged at the country level, which isn't included in the bbike extracts.
• The resulting maps are all "flattened" in A/B Street's list, so you can't see any hierarchy of areas. Two cities with the same name from different areas will arbitrarily collide.