refactors mapgen into chained builders

src/map_builders/room_based_spawner.rs

@@ -0,0 +1,27 @@
+use super::{spawner, BuilderMap, MetaMapBuilder};
+use rltk::RandomNumberGenerator;
+
+pub struct RoomBasedSpawner {}
+
+impl MetaMapBuilder for RoomBasedSpawner {
+    fn build_map(&mut self, rng: &mut rltk::RandomNumberGenerator, build_data: &mut BuilderMap) {
+        self.build(rng, build_data);
+    }
+}
+
+impl RoomBasedSpawner {
+    #[allow(dead_code)]
+    pub fn new() -> Box<RoomBasedSpawner> {
+        Box::new(RoomBasedSpawner {})
+    }
+
+    fn build(&mut self, rng: &mut RandomNumberGenerator, build_data: &mut BuilderMap) {
+        if let Some(rooms) = &build_data.rooms {
+            for room in rooms.iter().skip(1) {
+                spawner::spawn_room(&build_data.map, rng, room, build_data.map.depth, &mut build_data.spawn_list);
+            }
+        } else {
+            panic!("RoomBasedSpawner only works after rooms have been created");
+        }
+    }
+}