Boid.prototype.moveWith = function(boids, distance) {
      if(boids.length < 1) return
  
          // calculate the average velocity of the other boids
          var avgX = 0;
          var avgY = 0;
          for(var i = 0; i < boids.length; i++) {
                  var boid = boids[i];
                  if(boid.x == this.x && boid.y == this.y) continue;
                  if(this.distance(boid) > distance) continue;
                  
                  avgX += boid.xVelocity;
                  avgY += boid.yVelocity;
                  boid = null;
          }
          avgX /= boids.length;
          avgY /= boids.length;
  
          distance = Math.sqrt((avgX * avgX) + (avgY * avgY)) * 1.0
          if(distance == 0) return;
  
          this.xVelocity= Math.min(this.xVelocity + (avgX / distance) * 0.05, maxVelocity)
          this.yVelocity = Math.min(this.yVelocity + (avgY / distance) * 0.05, maxVelocity)                                                
  }
  
  var random = function(maxNum) {
          return Math.ceil(Math.random() * maxNum);
  }