Trading mod: shops as data and the trade window

src/TradeLedger.cs

@@ -0,0 +1,64 @@
+using Cthangover.Core.Items;
+
+namespace Cthangover.Mods.Trading
+{
+    /// <summary>
+    /// The arithmetic of a deal, apart from the window so it can be tested and
+    /// reused by a scenario action later. A deal is all or nothing: it is checked
+    /// first (offer exists, positive quantity, enough money or goods) and only
+    /// then both stacks change, so a refused deal leaves the inventory untouched.
+    /// </summary>
+    public static class TradeLedger
+    {
+        /// <summary>The player's money in the shop's currency.</summary>
+        public static int Money(IInventory inventory, ShopInfo shop)
+        {
+            return inventory == null || shop == null ? 0 : inventory.CheckCount(shop.Currency);
+        }
+
+        /// <summary>How many units of <paramref name="itemId"/> the player can afford; 0 when the shop does not sell it.</summary>
+        public static int MaxBuy(IInventory inventory, ShopInfo shop, string itemId)
+        {
+            var offer = shop?.FindSell(itemId);
+            return offer == null ? 0 : Money(inventory, shop) / offer.Price;
+        }
+
+        /// <summary>How many units of <paramref name="itemId"/> the player can sell; 0 when the merchant does not buy it.</summary>
+        public static int MaxSell(IInventory inventory, ShopInfo shop, string itemId)
+        {
+            if (inventory == null || shop?.FindBuy(itemId) == null || itemId == shop.Currency)
+                return 0;
+            return inventory.CheckCount(itemId);
+        }
+
+        /// <summary>
+        /// Buys <paramref name="count"/> units: takes their price in money and adds
+        /// the items. Returns <c>false</c> and changes nothing when the shop does not
+        /// sell the item, the count is not positive or the money is short.
+        /// </summary>
+        public static bool Buy(IInventory inventory, ShopInfo shop, string itemId, int count)
+        {
+            if (count <= 0 || count > MaxBuy(inventory, shop, itemId))
+                return false;
+            if (!inventory.Remove(shop.Currency, shop.FindSell(itemId).Price * count))
+                return false;
+            inventory.Add(itemId, count);
+            return true;
+        }
+
+        /// <summary>
+        /// Sells <paramref name="count"/> units: removes the items and pays their
+        /// price. Returns <c>false</c> and changes nothing when the merchant does not
+        /// buy the item, the count is not positive or the player has fewer units.
+        /// </summary>
+        public static bool Sell(IInventory inventory, ShopInfo shop, string itemId, int count)
+        {
+            if (count <= 0 || count > MaxSell(inventory, shop, itemId))
+                return false;
+            if (!inventory.Remove(itemId, count))
+                return false;
+            inventory.Add(shop.Currency, shop.FindBuy(itemId).Price * count);
+            return true;
+        }
+    }
+}