You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: cull_unreachable.rs
    path of file: ./repos/rustrogueliketutorial/chapter-59-caverns2/src/map_builders
    
    the code of the file until where you have to start completion: use super::{MetaMapBuilder, BuilderMap, TileType};
use rltk::RandomNumberGenerator;

pub struct CullUnreachable {}

impl MetaMapBuilder for CullUnreachable {
    fn build_map(&mut self, rng: &mut rltk::RandomNumberGenerator, build_data : &mut BuilderMap)  {
        self.build(rng, build
    
    fn build(&mut self, _rng : &mut RandomNumberGenerator, build_data : &mut BuilderMap) {
        let starting_pos = build_data.starting_position.as_ref().unwrap().clone();
        let start_idx = build_data.map.xy_idx(
            starting_pos.x,
            starting_pos.y
        );
        build_data.map.populate_blocked();
        let map_starts : Vec<usize> = vec![start_idx];
        let dijkstra_map = rltk::DijkstraMap::new(build_data.map.width as usize, build_data.map.height as usize, &map_starts , &build_data.map, 1000.0);
        for (i, tile) in build_data.map.tiles.iter_mut().enumerate() {
            if *tile == TileType::Floor {
                let distance_to_start = dijkstra_map.map[i];
                // We can't get to this tile - so we'll make it a wall
                if distance_to_start == std::f32::MAX {
                    *tile = TileType::Wall;
                }
            }
        }
    }