added functional version.

commonlisp/gilded-rose-functional.lisp

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+;; Hi and welcome to team Gilded Rose. As you know, we are a small inn
+;; with a prime location in a prominent city ran by a friendly
+;; innkeeper named Allison. We also buy and sell only the finest goods.
+;; Unfortunately, our goods are constantly degrading in quality as they
+;; approach their sell by date. We have a system in place that updates
+;; our inventory for us. It was developed by a no-nonsense type named
+;; Leeroy, who has moved on to new adventures. Your task is to add the
+;; new feature to our system so that we can begin selling a new
+;; category of items.
+
+;; First an introduction to our system:
+;; All items have a SellIn value which denotes the number of days we have to sell the item
+;; All items have a Quality value which denotes how valuable the item is
+;; At the end of each day our system lowers both values for every item
+;; Pretty simple, right? Well this is where it gets interesting:
+;; Once the sell by date has passed, Quality degrades twice as fast
+;; The Quality of an item is never negative
+;; "Aged Brie" actually increases in Quality the older it gets
+;; The Quality of an item is never more than 50
+;; "Sulfuras", being a legendary item, never has to be sold or decreases in Quality
+;; "Backstage passes", like aged brie, increases in Quality as it's
+;;   SellIn value approaches; Quality increases by 2 when there are 10
+;;   days or less and by 3 when there are 5 days or less but Quality
+;;   drops to 0 after the concert
+
+;; We have recently signed a supplier of conjured items. This requires an update to our system:
+;; "Conjured" items degrade in Quality twice as fast as normal items
+;; Feel free to make any changes to the UpdateQuality method and add
+;; any new code as long as everything still works correctly. However,
+;; do not alter the Item class or Items property as those belong to the
+;; goblin in the corner who will insta-rage and one-shot you as he
+;; doesn't believe in shared code ownership (you can make the
+;; UpdateQuality method and Items property static if you like, we'll
+;; cover for you).
+;; Just for clarification, an item can never have its Quality increase
+;; above 50, however "Sulfuras" is a legendary item and as such its
+;; Quality is 80 and it never alters.
+
+;; https://github.com/emilybache/GildedRose-Refactoring-Kata
+
+;; Common Lisp functional version: Manfred Bergmann, 2022
+
+;;; ================================================================
+;;; Code
+
+(defpackage :gilded-rose
+  (:use :cl))
+
+(in-package :gilded-rose)
+
+
+;;; Class ITEM
+
+(defstruct item
+  (name "" :type string :read-only t)
+  (sell-in 0 :type integer)
+  (quality 0 :type integer))
+
+(defun update-quality (items)
+  (mapcar #'update-item items))
+
+(defun update-item (item)
+  (let ((new-item (copy-structure item)))
+    (with-slots (name quality sell-in) new-item
+      (if (and (not (equalp name "Aged Brie"))
+               (not (equalp name "Backstage passes to a TAFKAL80ETC concert")))
+          (if (> quality 0)
+              (if (not (equalp name "Sulfuras, Hand of Ragnaros"))
+                  (setf quality (- quality 1))))
+          (when (< quality 50)
+            (setf quality (+ quality 1))
+            (when (equalp name "Backstage passes to a TAFKAL80ETC concert")
+              (if (< sell-in 11)
+                  (if (< quality 50)
+                      (setf quality (+ quality 1))))
+              (if (< sell-in 6)
+                  (if (< quality 50)
+                      (setf quality (+ quality 1)))))))
+
+      (if (not (equalp name "Sulfuras, Hand of Ragnaros"))
+          (setf sell-in (- sell-in 1)))
+
+      (if (< sell-in 0)
+          (if (not (equalp name "Aged Brie"))
+              (if (not (equalp name "Backstage passes to a TAFKAL80ETC concert"))
+                  (if (> quality 0)
+                      (if (not (equalp name "Sulfuras, Hand of Ragnaros"))
+                          (setf quality (- quality 1))))
+                  (setf quality (- quality quality)))
+              (if (< quality 50)
+                  (setf quality (+ quality 1))))))
+    new-item))
+
+;;; Example
+
+(defun to-string (item)
+  (with-slots (name quality sell-in) item
+    (format nil "~a, ~a, ~a" name sell-in quality)))
+
+(defun print-day (day items)
+  (format t "-------- day ~a --------~%" day)
+  (format t "name, sell-in, quality~%")
+  (dolist (item items)
+    (write-line (to-string item)))
+  (terpri))
+
+(defun run-gilded-rose (days)
+  (write-line "OMGHAI!")
+  (let* ((descriptions '(("+5 Dexterity Vest"                         10 20)
+                         ("Aged Brie"                                  2  0)
+                         ("Elixir of the Mongoose"                     5  7)
+                         ("Sulfuras, Hand of Ragnaros"                 0 80)
+                         ("Sulfuras, Hand of Ragnaros"                -1 80)
+                         ("Backstage passes to a TAFKAL80ETC concert" 15 20)
+                         ("Backstage passes to a TAFKAL80ETC concert" 10 49)
+                         ("Backstage passes to a TAFKAL80ETC concert"  5 49)
+                         ;; this conjured item does not work properly yet
+                         ("Conjured Mana Cake"                         3  6)))
+         (items (loop :for (name sell-in quality) :in descriptions
+                      :collect (make-item :name name
+                                          :sell-in sell-in
+                                          :quality quality))))
+    (loop :for day :upto days
+          :with the-items = items
+          :do (progn
+                (print-day day the-items)
+                (setf the-items (update-quality the-items))))))
+
+;;; ================================================================