From OSM to RawMap (`convert_osm` crate)

The first phase of map building reads in data from OSM files and a few others, producing a serialized RawMap.

Only major steps are described; see the code for the rest.

extract.rs

Read .osm, extracting the points for road-like ways, buildings, and areas

Areas usually come from a relation of multiple ways, with the points out of order. Gluing all the points together fails when the .osm has some ways clipped out. In that case, try to trace along the map boundary if the partial area intersects the boundary in a clear way. Otherwise, just use a straight line to try to close off the polygon.
Also read traffic signal locations and turn restrictions between OSM ways

Split OSM ways into road segments

OSM ways cross many intersections, so treat points with multiple ways and the points at the beginning and end of a way as intersections, then split the way into road segments between two intersections.
This phase remembers which road segment is the beginning and end of the OSM way, for per-lane turn restrictions later
Apply turn restrictions between roads here. Since OSM ways cross many intersections, the turn restrictions only apply to one particular road segment that gets created from the way. Make sure the destination of the restriction is actually incident to a particular source road.

Clip the map to the boundary polygon

osmium options preserve ways that cross the boundary
Trim roads that cross the boundary. There may be cases where a road dips out of bounds, then immediately comes back in. Disconnecting it isn't ideal, but it's better to manually tune the boundary polygon when this happens than try to preserve lots of out-of-bounds geometry.
Area polygons are intersected with the boundary polygon using the clipping crate