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

pub struct MonsterAI {}

impl<'a> System<'a> for MonsterAI {
   #[allow(clippy::type_complexity)]
   type SystemData = ( WriteExpect<'a, Map>,
                       ReadExpect<'a, Point>,
                       ReadExpect<'a, Entity>,
                       ReadExpect<'a, RunState>,
                       Entities<'a>,
                       WriteStorage<'a, Viewshed>,
                       ReadStorage<'a, Monster>,
                       WriteStorage<'a, Position>,
                       WriteStorage<'a, WantsToMelee>);

   fn run(&mut self, data : Self::SystemData) {
       let (mut map, player_pos, player_entity, runstate, entities, mut viewshed, monster, mut position, mut wants_to_melee) = data;
       if *runstate != RunState::MonsterTurn { return; }
       for (entity, mut viewshed,_monster,mut pos) in (&entities, &mut viewshed, &monster, &mut position).join() {
           let distance = rltk::DistanceAlg::Pythagoras.distance2d(Point::new(pos.x, pos.y), *player_pos);
           if distance < 1.5 {
               wants_to_melee.insert(entity, WantsToMelee{ target: *player_entity }).expect("Unable to insert attack");
           }
           else if viewshed.visible_tiles.contains(&*player_pos) {
               // Path to the player
               let path = rltk::a_star_search(
                   map.xy_idx(pos.x, pos.y),
                   map.xy_idx(player_pos.x, player_pos.y),
                   &mut *map
               );
               if path.success && path.steps.len()>1 {
                   let mut idx = map.xy_idx(pos.x, pos.y);
                   map.blocked[idx] = false;
                   pos.x = path.steps[1] as i32 % map.width;
                   pos.y = path.steps[1] as i32 / map.width;
                   idx = map.xy_idx(pos.x, pos.y);
                   map.blocked[idx] = true;
                   viewshed.dirty = true;
               }
           }
       }
   }
}