Managing a Zombie Horde

If we were to create an apocalypse game, we'd want to make sure we don't cause an apocalypse on the computer's resources by controlling the size of the hordes. Using an Object Pool can help us improve the performance and manage the total resources at the time time!

Design Considerations

• This example does not have a mechanism for removing zombies when their health drops to 0, but this could be achieved with a callback handler that automatically recycles the zombie instance.

The Zombie Class

public class Zombie
{
   private const float START_HEALTH = 100.0f;

   private float health;

   // ... More variables for the zombie:

   public Zombie()
   {
       this.Clean();
   }

   // This seems only slightly ironic:
   public void IsAlive()
   {
       return health > 0.0f;
   }

   public void TakeDamage(float damage)
   {
       health -= damage;
   }

   // This resets the zombie to a "factory new" setting:
   public void Clean()
   {
       this.health = START_HEALTH;
   }

}

The Zombie Pool

using System.Collections.Generic;

public class ZombiePool
{
   protected List<Zombie> activeList;
   protected List<Zombie> reserveList;

   int numberActive;
   int numberReserved;
   int spawnCap;

   public ZombiePool(int reserve = 5, int spawnCap = 20)
   {
       this.activeList = new List<Zombie>();
       this.reserveList = new List<Zombie>();

       this.numberActive = 0;
       this.numberReserved = 0;
       this.spawnCap = spawnCap;

       InitializeReserve(reserve);
   }

   private void InitializeReserve(int reserve)
   {
       for(int i = 0; i < reserve; i++)
       {
           Zombie zomb = new Zombie();
           this.reserveList.Add(zomb);
           this.numberReserved++;
       }
   }

   public bool CanSpawn()
   {
       return this.numberActive < this.spawnCap;
   }

   public Zombie GetZombie()
   {
       /*
           NOTICE:
           In your code, make sure to check the spawn limit.
           you can either:
               1. Throw an exception
               2. Return null
               3. Spawn a new zombie anyways.

               We'll return null.
       */

       if(this.numberActive >= spawnCap)
       {
           return null;
       }

       if(this.numberReserved == 0)
       {
           Zombie tmp = new Zombie();
           this.reserveList.Add(tmp);
           this.numberReserved++;
       }

       Zombie zomb = this.reserveList[0];
       this.reserveList.RemoveAt(0);
       this.numberReserved--;

       this.activeList.Insert(0, zomb);
       this.numberActive++;

       zomb.Clean();

       return zomb;

   }

   public void ReturnZombie(Zombie zomb)
   {
       if(this.activeList.Remove(zomb))
       {
           this.numberActive--;

           this.reserveList.Insert(0, zomb);
           this.numberReserved++;
       }
   }

}