Live edits

A key feature of A/B Street is the player editing the map and seeing how traffic responds. The possible edits include:

• Change lane types (driving, bus, bike, parking -- sidewalks are fixed)
• Change speed limits
• Reverse a lane
• Change a stop sign policy (which roads have a stop sign and which have priority)
• Change a traffic signal policy

The map conversion process outlined above takes a few minutes, so reusing this process directly to compute a map with edits wouldn't work at all for real gameplay. Instead, the process for applying edits is incremental:

• Figure out the actual diff between edits and the current map
  • This is necessary for correctness, but also speeds up a sequence of edits made in the UI -- only one or two lanes or intersections actually changes each time. Of course when loading some saved edits, lots of things might change.
• For any changed roads, make sure any bus stop on it have a good pointer to their equivalent driving position for the bus.
• For any modified intersections, recompute turns and the default intersection policies
• Recompute all the CHs for cars, buses, and bikes -- note sidewalks and bus stops never change
  • This is the slowest step. Critically, the fast_paths crate lets a previous node ordering be reused. If just a few edge weights change, then recomputing is much faster than starting from scratch.
  • While making edits in the UI, we don't actually need to recompute the CH after every little tweak. When the player exits edit mode, only then do we recompute everything.

A list of lanes and intersections actually modified is then returned to the drawing layer, which uploads new geometry to the GPU accordingly.