refactors mapgen into chained builders

src/map_builders/room_based_stairs.rs

@@ -0,0 +1,28 @@
+use super::{BuilderMap, MetaMapBuilder, TileType};
+use rltk::RandomNumberGenerator;
+
+pub struct RoomBasedStairs {}
+
+impl MetaMapBuilder for RoomBasedStairs {
+    fn build_map(&mut self, rng: &mut rltk::RandomNumberGenerator, build_data: &mut BuilderMap) {
+        self.build(rng, build_data);
+    }
+}
+
+impl RoomBasedStairs {
+    #[allow(dead_code)]
+    pub fn new() -> Box<RoomBasedStairs> {
+        Box::new(RoomBasedStairs {})
+    }
+
+    fn build(&mut self, _rng: &mut RandomNumberGenerator, build_data: &mut BuilderMap) {
+        if let Some(rooms) = &build_data.rooms {
+            let stairs_position = rooms[rooms.len() - 1].centre();
+            let stairs_idx = build_data.map.xy_idx(stairs_position.0, stairs_position.1);
+            build_data.map.tiles[stairs_idx] = TileType::DownStair;
+            build_data.take_snapshot();
+        } else {
+            panic!("RoomBasedStairs only works after rooms have been created");
+        }
+    }
+}