Don't remove previous agent position

It might still be used by check_move

src/agent.rs

@@ -1,5 +1,6 @@
 pub type Distance = isize;
 
+#[derive(Clone)]
 pub struct Position {
   pub x: Distance,
   pub y: Distance,

src/world.rs

@@ -87,6 +87,7 @@ impl ActualWorld {
         self.check_move(id, &mv);
       }
       let mut new_pos = None;
+      let pos = self.state.agent_positions.get(&id).unwrap();
       match mv {
         Move::Noop => {}
         Move::TryTag(other_id) => {
@@ -94,14 +95,12 @@ impl ActualWorld {
           new_state.tagged_by = Some(id);
         }
         Move::TryMove(dir) => {
-          let pos = self.state.agent_positions.get(&id).unwrap();
           new_pos = Some((pos.x + dir.x, pos.y + dir.y).into());
         }
       }
-      new_state.agent_positions.insert(
-        id,
-        new_pos.unwrap_or_else(|| self.state.agent_positions.remove(&id).unwrap()),
-      );
+      new_state
+        .agent_positions
+        .insert(id, new_pos.unwrap_or_else(|| pos.clone()));
     }
     self.state = new_state;
   }