code: function (diffX, diffY) {
          if (diffX === 0 && diffY === -1) return EasyStar.TOP
          else if (diffX === 1 && diffY === -1) return EasyStar.TOP_RIGHT
          else if (diffX === 1 && diffY === 0) return EasyStar.RIGHT
          else if (diffX === 1 && diffY === 1) return EasyStar.BOTTOM_RIGHT
          else if (diffX === 0 && diffY === 1) return EasyStar.BOTTOM
          else if (diffX === -1 && diffY === 1) return EasyStar.BOTTOM_LEFT
          else if (diffX === -1 && diffY === 0) return EasyStar.LEFT
          else if (diffX === -1 && diffY === -1) return EasyStar.TOP_LEFT
          throw new Error('These differences are not valid: ' + diffX + ', ' + diffY)
      }
docs: -1, -1 | 0, -1  | 1, -1
  -1,  0 | SOURCE | 1,  0
  -1,  1 | 0,  1  | 1,  1