;;; A grid is is an abstract object that holds `items` at each
;;; `coordinate`. In particular, n x m grid consists of lattice points
;;; 	(x,y) where 0 ≤ x < n and 0 ≤ y < m.
;;; At each coordinate (x,y) contains an object.

(define (make-grid n m)
  ;; add a test whether n ≥ 1 and m ≥ 1
  (let ([grid (make-vector (* m n) '())])

    (define (position->index i j)
      (+ j (* i m)))

    (define (get-coordinates i j)
      (vector-ref grid (position->index i j)))

    (define (get-self) grid)

    (define (populate-with f)
      (for* ([i n]
             [j m])
        (vector-set! grid (position->index i j) (f i j))))

    (define (set-coordinates i j val)
      (vector-set! grid (position->index i j) val))

    (case-lambda
      [(cmd)
       (cond
        [(eq? cmd 'get) grid]
        [else (error "make-grid, unknown command")])]
      ;;
      [(i j)
       (cond
        [(and (integer? i) (integer? j)) (get-coordinates i j)]
        [(and (eq? i 'populate-using) (procedure? j)) (populate-with j)]
        [(eq? i 'populate-constant) (populate-with (lambda (x y) j))])]

      [(i j val)
       (cond
        [(and (integer? i) (integer? j)) (set-coordinates i j val)])]
      )))