https://git.spwbk.site/swatson/rs-rl/raw/master/src/visibility_system.rs
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use specs::prelude::*;
use super::{Viewshed, Position, Map, Player};
use rltk::{field_of_view, Point};

pub struct VisibilitySystem {}

impl<'a> System<'a> for VisibilitySystem {
   type SystemData = ( WriteExpect<'a, Map>,
                       Entities<'a>,
                       WriteStorage<'a, Viewshed>,
                       WriteStorage<'a, Position>,
                       ReadStorage<'a, Player>);

   fn run(&mut self, data : Self::SystemData) {
       let (mut map, entities, mut viewshed, pos, player) = data;

       for (ent,viewshed,pos) in (&entities, &mut viewshed, &pos).join() {
           if viewshed.dirty {
               viewshed.dirty = false;
               viewshed.visible_tiles.clear();
               viewshed.visible_tiles = field_of_view(Point::new(pos.x, pos.y), viewshed.range, &*map);
               viewshed.visible_tiles.retain(|p| p.x >= 0 && p.x < map.width && p.y >= 0 && p.y < map.height );

               // If this is the player, reveal what they can see
               let _p : Option<&Player> = player.get(ent);
               if let Some(_p) = _p {
                   for t in map.visible_tiles.iter_mut() { *t = false };
                   for vis in viewshed.visible_tiles.iter() {
                       let idx = map.xy_idx(vis.x, vis.y);
                       map.revealed_tiles[idx] = true;
                       map.visible_tiles[idx] = true;
                   }
               }
           }
       }
   }
}