Add Gilded Rose exercise

ocaml/lib/dune

@@ -1,2 +1,3 @@
 (library
+ (preprocess (pps ppx_deriving.show))
  (name gilded_rose))

ocaml/lib/gilded_rose.ml

@@ -0,0 +1,54 @@
+module Item = struct
+  type t = { name : string; sell_in : int; quality : int } [@@deriving show]
+
+  let v name sell_in quality = { name; sell_in; quality }
+end
+
+module Items = struct
+  type items = Item.t list [@@deriving show]
+
+  let v ?(items = []) () = items
+  let show items : string = show_items items
+
+  let update_quality items =
+    let update_quality_items ({ name; sell_in; quality } as item : Item.t) =
+      let quality' =
+        if
+          name <> "Aged Brie"
+          && name <> "Backstage passes to a TAFKAL80ETC concert"
+        then
+          if quality > 0 then
+            if name <> "Sulfuras, Hand of Ragnaros" then quality - 1
+            else quality
+          else quality
+        else if quality < 50 then
+          quality + 1
+          +
+          if name = "Backstage passes to a TAFKAL80ETC concert" then
+            if sell_in < 11 then
+              if quality < 49 then
+                1 + if sell_in < 6 then if quality < 48 then 1 else 0 else 0
+              else 0
+            else 0
+          else 0
+        else quality
+      in
+      let sell_in' =
+        if name <> "Sulfuras, Hand of Ragnaros" then sell_in - 1 else sell_in
+      in
+      if sell_in' < 0 then
+        if name <> "Aged Brie" then
+          if name <> "Backstage passes to a TAFKAL80ETC concert" then
+            if quality' > 0 then
+              if name <> "Sulfuras, Hand of Ragnaros" then
+                { item with sell_in = sell_in'; quality = quality' - 1 }
+              else { item with sell_in = sell_in'; quality = quality' }
+            else { item with sell_in = sell_in'; quality = quality' }
+          else { item with sell_in = sell_in'; quality = quality' - quality' }
+        else if quality' < 50 then
+          { item with sell_in = sell_in'; quality = quality' + 1 }
+        else { item with sell_in = sell_in'; quality = quality' }
+      else { item with sell_in = sell_in'; quality = quality' }
+    in
+    List.map update_quality_items items
+end