(defpackage inventory
  (:use :cl)
  (:export :new-inventory
		   :filled
		   :add-item
		   :remove-item))

(in-package :inventory)


(defclass inventory ()
  ((size :initarg :size
		 :reader size)
   (filled :initform 0
		   :accessor filled)
   (items :initform '()
    	  :accessor items)))

(defun new-inventory (size)
  (when (> size 0)
	(make-instance 'inventory :size size)))

(defmethod find-item ((inv inventory) name)
  (with-slots (items) inv
	(find-if
	 #'(lambda (i)
		 (equal (commodities:name (car i)) name))
	 items)))

(defmethod delete-item ((inv inventory) item)
  ;; removes <item> from inventory list and substracts
  ;; its stored quantity from the filled attribute
  (with-slots (filled items) inv
	(setq items (remove item items))
	(setq filled (- filled (cdr item)))))

(defmethod update-item ((inv inventory) item qty old-item old-qty)
  (with-slots (filled items) inv
	(delete-item inv (cons old-item old-qty))
	(let ((total-quantity (+ qty old-qty)))
	  (push (cons item total-quantity) items)
	  (setq filled (+ filled total-quantity)))))

(defmethod add-item ((inv inventory) item quantity)
  ;; add <quantity> of <item> to inventory
  (when (> quantity 0)
	(with-slots (size filled items) inv
	  (when (> size (+ filled quantity))
		(let ((old-item (find-item inv (commodities:name item))))
		  (if old-item
			  (update-item inv item quantity
						   (car old-item) (cdr old-item))
			  (prog1 (setq filled (+ filled quantity))
				(push (cons item quantity) items))))))))

(defmethod remove-item ((inv inventory) name quantity)
  ;; remove <quantity> of <item> from inventory
  ;; quantity must be positive or zero
  (when (> quantity 0)
	(with-slots (filled items) inv
	  (let ((item (find-item inv name)))
		(when item
		  (delete-item inv item)
	      (if (< quantity (cdr item))
			  (progn
				(add-item inv
						  (car item)
						  (- (cdr item) quantity))
				(cons (car item) quantity))
			  item))))))