Add a tool to convert maps from JSON back to binary, to enable experiments with modifying parking per buildings. Except... apparently we can't read back in JSON maps?!

book/src/dev/api.md

@@ -108,6 +108,20 @@ cargo run --bin dump_map data/system/seattle/maps/montlake.bin > montlake.json
 See some example code that
 [reads this JSON and finds buildings](https://github.com/dabreegster/abstreet/blob/master/headless/examples/generate_traffic.py).
 
+You could also edit the map JSON, convert it back to binary, and use it in the
+simulation. This isn't recommended generally, but one possible use case could be
+tuning the amount of offstreet parking per building. The map JSON has a list
+called `buildings`, and each object there has a field `parking`. You coud set
+this object to `{ "Private": [100, false] }` to indicate 100 parking spots, for
+a building not explicitly designated in OpenStreetMap as a garage. After editing
+the JSON, you have to convert it back to the binary format:
+
+```
+cargo run --bin json_to_binary_map -- --input=montlake.json out=data/system/seattle/maps/montlake_modified.bin`
+```
+
+... Except this tool doesn't seem to work yet!
+
 The format of the map isn't well-documented yet. See the
 [generated API docs](https://dabreegster.github.io/abstreet/rustdoc/map_model/index.html)
 and [the map model docs](../map/index.md) in the meantime.

importer/src/bin/json_to_binary_map.rs

@@ -0,0 +1,11 @@
+use abstutil::{CmdArgs, Timer};
+use map_model::Map;
+
+fn main() {
+    let mut args = CmdArgs::new();
+    // TODO This can't handle the output of dump_map! What?!
+    let mut map: Map = abstutil::read_json(args.required("--input"), &mut Timer::throwaway());
+    map.map_loaded_directly();
+    abstutil::write_binary(args.required("--output"), &map);
+    args.done();
+}